1. INTRODUCTION AND BACKGROUND Aerobic heterotrophic bacteria play by dfsiopmhy6

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Aerobic heterotrophic bacteria play a dominant role in the global carbon cycle by
returning CO2 to the atmosphere via decomposition of the organic fixed carbon in
plant and animal remains and excretory products, industrial, municipal and
agricultural wastes etc. Where oxygen is absent and anaerobic conditions prevail,
anaerobic bacteria carry out an alternative degradation process, resulting in the
generation of methane (CH4) and CO2, rather than CO2 and H2O, as end-products of
anaerobic decomposition.

The generation of methane by anaerobic decomposition disturbs the operation of the
global carbon cycle since methane, once released to the atmosphere, cannot be
reutilised by photosynthetic carbon fixation. Methane is an extremely potent
greenhouse gas and its increasing concentration in the atmosphere is a significant
contributor to global warming.

Anaerobic decomposition processes can, however, be beneficially harnessed for the
treatment of point sources of agricultural, municipal and industrial organic wastes.
The methane produced represents a renewable source of energy that can be utilised
for electricity generation, space heating, steam generation for industry, as a vehicle
fuel etc. The process whereby anaerobic bacteria are utilised for organic waste and
wastewater treatment and renewable energy generation is referred to as anaerobic

1.1     The Anaerobic Digestion Process
Anaerobic digestion (AD) refers to the microbiological conversion of organic matter
to biogas in the absence of oxygen or other alternative inorganic electron acceptors
(sulphate, nitrate, etc.). Biogas consists of a mixture of methane (60-80%), CO2 (20-
40%) and trace amounts of H2S, NH3, N2 and volatile organic compounds. As
produced, biogas is also saturated with water vapour. The conversion of diverse
organic compounds to biogas is mediated by a consortium of different bacterial
trophic groups, acting in a sequential and highly co-operative manner (Fig. 1.1). The
microbiology and biochemistry of this complex process is now well understood
(Colleran, 1992; Ferry, 1993; Stams, 1994) and will not be reviewed further in this

Approximately 70-80% of the chemical energy of the waste organic substrates is
conserved in the methane (CH4) product. Since anaerobic processes support low
levels of biological growth, the biomass produced (0.05-0.15 kg microbial volatile
solids (VSS) per kg COD removed) is approximately one-third to one-twentieth of the
waste microbial biomass produced during aerobic treatment (van Handel & Lettinga,
1994). Consequently, AD technology can be viewed as an energy recycling process
that produces a usable fuel (CH4), while generating low quantities of waste biomass
needing subsequent, safe and often costly disposal. Because of the low energy gain to
the participant microbial groups, anaerobic digestion takes place at slow rates under
ambient temperatures in cold and temperate environments. Consequently, AD
reactors are generally heated to either mesophilic (20-40°C) or thermophilic (50-
60°C) temperatures.



                  1        FERMENTATION                                      ,
                                                                           CH4 CO 2
    ORGANIC                                         2           3

                  1                                                   4b

                                                    H /CO
                                                        2   2

Figure 1.1 Carbon flow in anaerobic digesters: 1 = hydrolytic/fermentative bacteria; 2
= obligate hydrogen producing bacteria; 3 = homoacetogenic bacteria; 4a =
acetoclastic methanogens; 4b = hydrogenophilic methanogens; 5 = fatty acid
synthesising bacteria.

1.2    Historical application of AD for waste and wastewater treatment

1.2.1 Urban wastewater sludges
Anaerobic digestion was first applied at full-scale in the late 1890s for the treatment
of domestic wastewater from the city of Exeter in the UK, with the produced biogas
being used for street lighting and heating of the treatment works (cited by Callander
& Barford, 1983). From the 1920s onwards, AD was widely applied in sewage
treatment works for the treatment and stabilisation of sewage sludge (McCarty, 1982).
Early digester designs consisted of large, unheated and unmixed tank systems within
which solids settlement and floating scum layer formation posed operational problems
(Fig. 1.2). By the mid 1950s, heating and mixing of the digester contents had
considerably improved the efficiency of the process (Fig. 1.2) and the inclusion of
heated, high-rate digesters in large sewage treatment works became the norm in most
industrialised countries. The majority of sewage sludge digesters are operated at
mesophilic temperatures (25°C - 37°C) with sewage sludge retention times within the
digesters of c. 20-25 days and biogas productivities of c. 0.5 - 1 m3 per m3 digester
volume per day. In some countries, thermophilic digestion (52° - 55°C) has been
favoured in order to reduce sludge retention times and the digester volume required,
improve volumetric biogas productivities, and increase pathogen kill.

The first sewage sludge digester in Ireland was commissioned at the Tullamore
sewage treatment works in 1986 (Killilea et al., 1999). As a result of the
implementation of the EU Urban Wastewater Directive (91/271/EEC), four more
sewage treatment works have recently installed sewage sludge digesters (Buncrana,
Greystones, Clonmel and Tralee); digesters are currently being commissioned or
started up at three further sewage treatment works (Dundalk, Drogheda and Roscrea),
and the feasibility of including sewage sludge digestion in a number of other existing
or new sewage treatment plants is under consideration (Killilea et al., 1999). Plate 1
depicts the digesters installed at Clonmel sewage treatment works. Both digesters
(800 m3 each) are operated at mesophilic temperatures (30°-38°C). The first was
commissioned in 1998 and the second a year later. All sewage sludge digesters
installed or proposed in Ireland are mesophilic.

                    BIOGAS                                      BIOGAS



                 ACTIVE ZONE


                   (A)                                          (B)

                             SLUDGE RECYCLE


Figure 1.2 Early anaerobic digester designs: (a) unheated, unmixed sewage digester;
(b) conventional heated and mixed digester (continuously stirred tank reactors –
CSTRs); (c) anaerobic contact process.

       Plate 1 The two mesophilic sewage sludge digesters at Clonmel sewage
                                 treatment works

1.2.2 Agricultural wastes
The installation of anaerobic digesters for cattle, pig and poultry manure/slurry
treatment at individual farm level dates from the early 1970s. The majority of early
on-farm digesters resembled sewage sludge digesters (although at a smaller scale) and
were heated, mixed and either intermittently or continuously fed (Fig. 1.2b). Manure
retention times varied from 10-30 days with biogas productivities of a maximum of 1
m3 biogas per m3 reactor per day (Colleran, 1992). Plug-flow digesters were also
utilised, particularly in Switzerland and Germany (Demuynck et al., 1984). Although
a number of pilot trials were carried out, only one full-scale digester was installed in
the 1980s in Ireland at farm level (Bandon, Co. Cork). The availability of financial
incentives in other EU countries resulted in significant installation of on-farm AD
plants in the late 1970s and early 1980s. A 1982 survey within the EU and
Switzerland identified a total of 378 full-scale and 42 pilot-scale on-farm plants, of
which 144 were treating pig slurry and 134 either dairy or beef cattle manure
(Demuynck et al., 1984). In recent years, interest in application of AD technology at
farm level has increased in Ireland. Currently, three new on-farm plants have been
constructed and a number of others are at the planning stage (Section 3.12.2).

1.2.3 Industrial wastewaters
The application of anaerobic digestion to the treatment of industrial wastewaters
required the development of different reactor designs which would facilitate shorter
retention times, reduction in digester volume, higher loading rates and increased
volumetric biogas productivities. This was achieved by maintaining the degradative
microbial population within the digester independent of the wastewater flow
(Colleran, 1992). In the 1950s, the anaerobic contact process (Fig. 1.2) was
developed. This design mimicked the aerobic activated sludge system by settlement
of microbial biomass from the digester effluent and its return to the digester to
maintain high reactor biomass concentrations. The anaerobic contact process allowed
reduction of the wastewater retention time to 6-10 days. Subsequently, digester
designs based on biomass retention as a biofilm on inert support materials arranged in
random or modular configuration within the digester were developed (Fig. 1.3).
Expanded-bed and fluidized bed reactor designs were also developed whereby sand or
granular activated carbon particles acted as the support material for biofilm formation
and retention (Fig. 1.3). An alternative series of high-rate digester designs were based
on the natural tendency of the anaerobic consortium to form dense granules which
could be retained for long periods within the digester by the utilisation of efficient
gas/solids/liquid separation devices (i.e. the Upflow Anaerobic Sludge Bed digester
(UASB))(Fig. 1.3).

The retention of high concentrations of active anaerobic biomass in these new
digester designs allowed reduction of the wastewater retention time to 1-2 days and,
in some cases, to less than 10 hours, resulting in greatly decreased digester volume
requirements, lower initial capital costs and increased volumetric biogas
productivities of up to more than 30 m3 biogas per m3 reactor per day (Lettinga,
1995). More recent modifications, involving effluent recirculation, higher upflow
velocity flow-rates and compartmentalisation within the reactor have resulted in new
reactor designs, such as the Internal Circulation (IC) reactor, that allow even higher
loading rates and biogas productivities (Habets et al., 1997).

Anaerobic digestion of high-strength industrial wastewaters is usually followed by an
aerobic polishing stage prior to discharge to receiving waterbodies. This is achieved
by installation, on site, of an appropriately sized aerobic activated sludge plant or
discharge of the AD effluent to sewer for treatment in an off-site aerobic municipal
sewage treatment plant.

A 1996 survey (Habets, 1996) of the industrial application of AD worldwide
identified a total of 914 full-scale operational plants. As illustrated in Figure 1.4, the
majority of these plants were located in Europe (38%) and Asia (36%). Of the plants
surveyed, 67% were of the UASB design. Food processing (38%), brewery (25%)
and distillery (15%) wastewaters represented 78% of the industrial effluents treated
by anaerobic digestion (Fig. 1.5).

Four industrial plants in Ireland have installed anaerobic digesters as the first stage of
their wastewater treatment processes. Kerry Ingredients (Listowel, Co. Kerry)
commissioned a 3,200 m3 downflow, fixed bed digester in the late 1980s for initial
treatment of its milk-processing wastewater on a seasonal basis (February to
October). The AD plant consistently achieves >75% COD removal efficiency and
has recently (1998) been subjected to a comprehensive overhaul and refitting. ADM-
Ringaskiddy (Co. Cork) initially installed a 7,400 m3 upflow, fixed bed digester for
treatment of its citric acid production wastewater. An IC reactor was commissioned
in early 1999 in order to cater for increased wastewater volumes. Carbery Milk
Products (Ballineen, Co. Cork) has also utilised AD for treatment of its industrial
ethanol production wastewater since the late 1980s. The reactor design initially
installed was converted to a hybrid reactor in the early 1990s. More recently, two IC
reactors have been commissioned. The first of these commenced operation in early
1999 and the second has since been commissioned. Plate 2 depicts one of the IC
reactors at Carbery Milk Products. The anaerobic digestion system located at the
Irish Sugar Plant in Carlow has been operational for the past three years. It consists
of a two-phase hydrolysis/acidification CSTR followed by a methanogenic CSTR. At
peak capacity, it treats 3,840m3 of wastewater per day, resulting in the generation of
c. 20,000m3 of biogas (70% CH4) per day.

                    BIOGAS                                     BIOGAS



                                   (A)                                      (B)

         INFLUENT       EFFLUENT                                   EFFLUENT

            BIOGAS                                                      BIOGAS

                      EFFLUENT                                                    EFFLUENT

                        (C)              BIOGAS                                     (D)


                INFLUENT                                                   INFLUENT



Figure 1.3 Digester designs based on biomass retention: (a) anaerobic filter/fixed bed
reactor; (b) downflow stationary fixed-film reactor; (c) expanded bed/fluidised bed
reactor; (d) upflow anaerobic sludge blanket reactor; (e) hybrid sludge bed/fixed bed

                                                                       Asia 36%
           Europe 38%

                                                              Australia 1%
                Africa 2%

                    South America 10%              North America 13%

Figure 1.4 Distribution of full-scale industrial anaerobic digesters worldwide
(Habets, 1996).

                                                                 Brewery 25%

        Food 38%

                                                                         Chemical 6%

                                                                 Various 7%

             Pulp & Paper 9%
                                                   Distillery 15%

                            (100% = 914 plants until June '96)
Figure 1.5 Sectoral distribution of full-scale industrial anaerobic digesters worldwide
(Habets, 1996).

        Plate 2 One of the two IC reactors installed at Carbery Milk Products
                                (Ballineen, Co. Cork)

1.2.4 The organic fraction of municipal solid waste
In many countries, there is an increasing obligatory requirement to recycle/re-use
MSW constituents and to reduce dependency on landfill disposal. Source separation
of MSW at household level or in a centralised municipal recycling facility (MRF)
allows separate treatment of the organic (putrescible) fraction of MSW (OFMSW). A
variety of digester designs have been developed specifically for OFMSW digestion in
recent years and their installation at full-scale was highlighted in a survey of 23
industrialised countries carried out in 1994 by the International Energy Agency (IEA,
1994). A total of 38 full-scale plants were investigated, with 11 of these at the
commissioning or start-up stage. The digester designs utilised range from novel high
solids (20-30% VSS) thermophilic reactors to two-stage systems involving high solids
acidogenesis in the first reactor, followed by high rate UASB digestion of the acid
leachate in the second reactor. Addition of water to the macerated OFMSW fraction
in order to allow digestion in continuously stirred tank reactors (CSTRs) is also
practiced at full-scale in many countries.

Utilisation of anaerobic digesters in existing Sewage Treatment Works (S.T.W.s) for
co-digestion of OFMSW is currently being practiced in many countries. For example,
the source-separated fraction of MSW in the Grinsted region of Denmark is delivered
to the city STW where it is co-digested with primary and secondary sewage sludge in
a 2,800m3 mesophilic anaerobic digester located on site. The biogas produced is used
in a CHP plant and the digestate is separated into a solids fraction which is composted
and a liquid fraction which is recirculated back to the plant influent point and treated
prior to discharge. An aerial view of the Grinsted S.T.W. is illustrated in Plate 3.
        Plate 3: Aerial view of Grinsted Kommune Sewage Treatment Works

1.3     Application of Centralised Anaerobic Digestion (CAD) technology
Until recently, the application of AD technology for treatment of organic wastes and
wastewaters was typically an on-site process, dealing with a single waste or
wastewater, and with both the design and size of the digester tailored to meet the
needs of an individual industry, farm or sewage treatment works. CAD plants differ
fundamentally in being centrally-located and in being designed and scaled to allow
centralised co-digestion of agricultural and other organic waste arisings from defined
geographical areas. The initial objective of CAD plant installation was to centrally
treat the animal manure and slurry arisings from adjacent farms, with resultant
economy of scale, better process control, and greater operational efficiency. It was
subsequently realised that CAD plants could also be used for co-digestion of animal
wastes with other organic wastes generated within the immediate region of the CAD
plant (sewage sludge, food-processing wastes and wastewaters, OFMSW, industrial
organic wastes). Although these wastes may only represent 20-30% of the CAD
influent (on a dry volatile solids basis), they significantly increase volumetric biogas
productivity and enhance overall commercial viability. Animal manures and slurries
continue to be the primary substrates for CAD plants. This reflects the fact that the
quantity of animal wastes in the majority of EU countries greatly exceeds that of other
organic wastes arisings. The ratio of animal to other organic wastes in CAD plant
influents is also dictated by the availability of these co-digestion substrates in areas of
intensive livestock production, transport costs, etc. In addition, the majority of
industry and sewage treatment works in developed countries have existing on-site
systems for treatment of their waste/wastewater arisings and may, consequently, be
reluctant to subscribe to centralised AD systems unless there are clear financial
benefits for doing so. However, the current restrictions on organic waste disposal in
landfill sites is making available a variety of food-processing and food-use wastes that
are potential co-digestion substrates for CAD and on-farm AD plants.


The primary objective of this project was to evaluate the potential of CAD technology
for treatment of organic waste and wastewater arisings in Ireland and for renewable
energy generation (methane) on a centralised basis in sensitive catchment areas.
Based on experience in other EU countries (Section 1.3), it was assumed that the
primary feedstock for proposed CAD plants in Ireland would be animal
manures/slurries from intensive livestock production units and over-wintering of
cattle, sheep and horses. However, in order to ensure economic viability, it was also
assumed that the quantities, availability and transportation requirements of food-
processing and other organic wastes were critical factors in choosing feasible site(s)
for CAD plant location.

The main tasks carried out were as follows:-

2.1     State of technology review
The current implementation of CAD technology was reviewed (internationally and
nationally). The biological and engineering aspects of the technology were examined
and the environmental and cost implications of the use of centralised anaerobic
digestion were documented.

2.2    Legislation overview
A review of the current National/EU legislation/regulations and guidance documents
which may impinge on the application of centralised anaerobic digestion was

2.3     Quantification of organic waste arisings in Ireland
This aspect of the study involved (i) evaluation of published reports on organic waste
arisings in Ireland; (ii) a county-by-county analysis of sources and locations of
organic waste arisings, and (iii) identification of the most appropriate potential CAD

2.4     Specific evaluation of the feasibility of installation of a CAD plant in a
        selected catchment area
Based on the data from 2.3, a site was selected for installation of a potential CAD
plant. A comprehensive technical assessment was carried out; plant design
specifications were developed, and an economic assessment was carried out in order
to determine the cost benefits of installation of a CAD plant at the chosen site.


Organic wastes, such as animal manures, sewage sludge etc. present two main
environmental risks to receiving waterbodies:- (i) an oxygen demand, due to
degradable organics, that can result in oxygen depletion and ultimately fish-kills, and
(ii) introduction of inorganic nutrients (nitrate, phosphate, etc.) that result in
eutrophication of the receiving waterbody. While anaerobic digestion reduces the
oxygen demand of organic wastes, it does not reduce the eutrophication potential of
the wastes undergoing treatment. In fact, anaerobic digestion renders the nitrogen and
phosphorus content of animal manures and sewage sludge more amenable to uptake
by growing plants or algae. Consequently, AD treated animal wastes, in particular,
represent, on the one hand, a valuable nutritional resource for plant uptake and, on the
other hand, a significant eutrophication potential, if leached or run off to receiving
water bodies.

Subsequent to anaerobic treatment, the digestate from animal manures and sewage
sludge is generally landspread on grassland or ploughed-in in tillage crop production
areas. Alternatively, the digestate can be further processed by solids/liquid separation
to yield (i) a high solids, fibrous fraction that can be composted to produce a peat-
moss equivalent of value as a soil conditioner, an horticultural resource or a soil
reclamation agent, and (ii) a liquid fraction that can be irrigated to farmland or used as
a liquid fertiliser for parkland, golf courses or other amenity areas.

3.1      Development of CAD technology
The concept of centralised anaerobic digestion plants developed from the growing
need to manage, re-use and recycle animal manure and slurry arisings in an
environmentally sustainable manner. As indicated in Section 1.2.2, on-farm AD
plants in Europe dated mainly from the energy crisis of the 1970s when many
European countries provided financial incentives for renewable energy production
and on-farm energy recycling. A 1982 EU-funded survey of AD application
identified a total of 546 on-farm and agro-industrial biogas plants in the EU and
Switzerland (Demuynck et al., 1984). The total working digester volume involved
was 296,000 m3, with a net biogas production of 33,000 tonnes of oil equivalent
(TOE). About one third of the on-farm plants were “do-it-yourself” units and the
operational problems identified included pipe blockages, inefficient digester mixing,
feedstock pump failure, gas leakage and poor general plant maintenance (Demuynck
et al., 1984). Inefficient gas usage also resulted in poor plant economics and was a
major factor resulting in the shut-down of many of the early on-farm plants.

Although the number of on-farm EU plants constructed in the late 1980s and early
1990s were relatively few, they were generally of better design, with consequently
improved operational performance and reduced maintenance costs. These more
efficient on-farm digesters were typically mono-substrate (treating only the animal
manure/slurry arisings generated on site); were operated at mesophilic temperatures
and were of CSTR or plug-flow design, with hydraulic retention times of 20-30 days.
The majority of these on-farm plants used the biogas produced for space-heating
(domestic, piggeries, greenhouse) or for combined heat and power (CHP) production,
with sale of the electricity produced to the national or local grid. In virtually all cases,
the treated slurry was disposed of by landspreading (Warburton, 1997; AD-Nett
Conference, 1999).

Centralised AD (CAD) plants for animal waste treatment were initiated in Denmark
in the late 1980s/early 1990s. The primary objective was to ensure sustainable
management and recycle of animal waste arisings on a centralised basis in defined
geographical areas. CAD plants are generally operated by a local farmers’ co-
operative or by a private energy utility. The CAD plants accept animal manures from
a defined geographical area, together with other waste arisings from local food-
processing plants, abattoirs, breweries, distilleries etc. Sewage sludge and the source
separated organic fraction of municipal solid waste (OFMSW) is increasingly also
used as an additional organic substrate for CAD plants.

CAD plants are operated at either mesophilic or thermophilic temperatures and at
solids/liquid retention times of 12-20 days. Multiple digesters of circa 2-4,000m3 size
are generally utilised. A pre- or post-pasteurisation/hygienisation stage is usually
included to prevent faecal pathogen carryover in the treated waste. The biogas
produced is used to provide district heating with, or without, electricity generation in
CHP plants, or to generate steam or electricity for local industry. The treated waste is
either landspread directly or separated into a liquid and solid fraction. The solid
fraction is usually composted prior to sale as a soil conditioner or horticultural
product (Tipping, 1996; AD-Nett Conference, 1999).

Farmers subsequently became aware of the economic potential of co-digestion at
individual farm level. The majority of on-farm digesters constructed in EU countries
over the past ten years resemble CAD plants in treating animal manures/slurries as the
primary organic substrate, while accepting other agricultural/industrial/municipal
organic wastes as co-digestion substrates. These new on-farm plants differ from CAD
plants in that they are owned and operated by an individual farmer, rather than by a
cooperative or public utility, and are typically of much smaller size than CAD plants.
The treated waste is disposed of by landspreading on the farmer’s land or on adjacent
farmlands, by agreement. In some of these on-farm co-digestion plants, the digestate
is separated into a solids and liquid fraction. The latter is recycled to farmland using
irrigation techniques, with the former composted for sale for soil-conditioning and
horticultural purposes. The biogas is used in CHP plants to provide electricity, which
is sold to the national or local grid, and heat which is used for on-farm applications
(AD-Nett Conference, 1999).

3.2    Benefits and potential problems of CAD

3.2.1 Benefits
Better manure management:
• Farmers who are members of CAD co-operatives benefit from improved storage
    facilities for raw animal wastes and CAD digestate, have better management
    regimes, and can optimise landspreading of the digestate.

Improved utilisation of the fertiliser value of organic wastes:
• The mixing of different animal wastes in CAD plants provides a digestate with a
   more balanced NPK content and with improved fertiliser potential (i.e. mixtures
   of cattle manures rich in potassium; pig slurries with high phosphorus content,
• The fertiliser value of CAD digestate is more nutritionally defined than raw
   manures or slurries. Consequently, livestock and tillage farmers can maximise
   landspreading of the digestate, with substantial reduction in synthetic fertiliser
Reduced greenhouse gas emissions:
• Methane is a major contributor to the greenhouse effect when it escapes to the
   atmosphere. Current disposal practices for slurry and food residues cause methane
   to be released through natural processes. Anaerobic digestion exploits this process
   so that the gas can be used as a fuel.

Reduction of fossil fuel usage:
• Energy generated from anaerobic digestion can reduce the demand for fossil fuels.
   For example, the digestion of one tonne of cattle manure can generate sufficient
   methane to produce 1.7 kW of electricity (Warburton, 1997).
• The use of the digestate as a contribution to fertiliser regimes can in turn reduce
   fossil fuel consumption and emissions associated with the production of synthetic

Cost effective and environmentally-sustainable waste recycling:
• When co-digesting organic wastes with animal manures, it is possible to achieve
   environmentally attractive recycling of a number of suitable wastes.
• The environmental benefits include the sanitising effect of pretreatment and
   digestion, which achieve significant pathogen reduction at 55°C and pathogen kill
   at 70°C, weed seed destruction, as well as efficient fertiliser utilisation of the
   effluent. In doing so, the CAD plants can provide farms, industries and municipal
   authorities with a lasting and relatively cheap solution to their organic waste
   disposal problems.

Reduction of odour nuisances:
• Landspreading of raw animal manures and slurries is associated with significant
   odour nuisance. Anaerobic digestion can reduce odour nuisance during
   landspreading by up to 80%.

Public and animal health benefits:
• Anaerobic digestion (mesophilic or thermophilic) results in significant die-off of
   the bacterial, viral and protozoan pathogens present in animal manures. The
   inclusion of a hygienisation step, as is commonly practiced in CAD plants, further
   reduces the pathogen load in the digestate.

Generation of electricity from renewable sources (biomass/waste):
• AD plants can provide an on-site energy source, displacing existing bought-in
• Due to Government policies promoting energy from renewable sources and the
   de-regulation of the electricity sector, the market for electricity from renewable
   sources, such as AD, is likely to grow significantly, and opportunities for AD
   operators to sell their energy will, therefore, be increased.

Tried and tested technology:
• The CAD concept has an established track record in several European countries,
    such as Denmark, Germany, Austria and Sweden.

3.2.2 Potential problems
Economic viability:
• CAD plants have high capital and operating costs. Their installation, for
    renewable energy generation alone, is not financially viable. Consequently, the
    other benefits of CAD technology must be maximised and must ensure either
    cost-savings or additional income:- i.e. improved organic waste management;
    optimal inorganic nutrient recycle; savings on synthetic fertiliser use; sale of
    liquid fertiliser and compost.

Potential emissions:
• During anaerobic digestion and subsequent landspreading of the digestate, various
   emissions to air, soil, ground and surface waters may occur that are potentially
   damaging to human health and to the environment. For example, the production of
   low levels of hydrogen sulphide (H2S) during digestion may give rise to potential
   odour problems and also result in the generation of sulphur dioxide (acid rain
   constituent) during subsequent burning of the biogas. The increase in ammonia
   concentration during digestion of organic-N-containing wastes (e.g. animal
   manures, sewage sludge) may result in increased volatilisation of ammonia during
   landspreading of the digestate. Ammonia is a very significant contributor to acid
   rain in some countries (e.g. the Netherlands). However, it should be emphasised
   that landspreading of raw animal manures and slurries results in more damaging
   emissions to air, soil, ground and surface waters.

Traffic movements:
• All waste management systems create traffic movements but, although overall
   quantities of vehicle movements may not be a major issue, the traffic may be
   concentrated in a small area, especially where a CAD plant is established. All
   CAD developments should aim to minimise traffic movements in order to reduce
   inconvenience to other road users and to minimise pollution problems. This
   should also serve to reduce transport costs and improve plant viability.

• CAD plants can generate noise. Consideration needs to be given to minimisation
   of potential noise from deliveries, pumps, compressors, the power plant, etc.

Visual Impact:
• Large CAD plants may have visual impact because of the scale of the overall
   development and the size of the individual digester tanks. The latter may be
   ameliorated to some extent by partial sinking below ground. This has the added
   advantage of making the digesters easier to load. Screening with trees also
   reduces the visual impact of CAD plants.

3.3     Organic wastes (feedstocks) suitable for AD
Feedstocks for CAD plants are derived from agriculture, industry and municipal
wastes and wastewaters. The primary ingredients of these wastes/wastewaters are
carbohydrates, proteins, fats and long-chain organic acids (Table 3.1). Although the
majority of these organic compounds are readily degradable by anaerobic bacteria, the
presence of sand, grit, inorganic additives, heavy metals and non-biodegradable
plastics may pose problems during the anaerobic degradation process (Table 3.1).

Table 3.1: Sources, composition and biodegradability of anaerobic feedstocks (Steffen et al., 1998).

    Compounds                    Sources                       Examples                    Anaerobic           Disturbing effects        Inhibitory effects
Carbohydrates                                         breweries, distilleries,
  Sugars                beet, corn                    sugar beet processing,           excellent                                       pH decrease2
  Starch                potatoes, maize, etc.         milk processing,                 excellent             foaming
  Cellulose             straw, grass, wood            chip & starch processing,
                                                      farmyard manure,
                                                      harvest remains                  poor - good           lignin encrustation
Proteins                animals & animal              milk processing,                 excellent             foaming               pH decrease2
                        products                      pharmaceutical industry                                                      ammonia increase3
Fats                    animals & animal              slaughterhouses,                 excellent1            scum layers, poor     VFA increase3
                        products                      rendering plants                                       water solubility      pH decrease2
Long chain fatty        fats, grease, oils,           rendering plants, oil mills      excellent1            poor water solubility specific inhibition
acids                   evaporation                                                                          of fats and oils      of different
                        condensates                                                                                                bacterial groups
Trace organic           pesticides, antibiotics,      pharmaceutical industry, poor                          foaming               antibiotic reactions
compounds               detergents                    manure
Inorganic material      salts, food additives,        slaughterhouses, manure, non-biodgradable              precipitation4
                        silica gel (filtration)       food & pharm. industry                                 sludge formation
Sand, Grit              stable walls and floors       manure                           “                     precipitation
Metals                  packaging material,       OFMSW, industry                               “            blocking,
                        process remains                                                                      precipitation
Plastic                 packaging material        OFMSW, industry                               “            flotation
Heavy metals            metal refining, batteries OFMSW, industry                               “                                      toxic reactions

1: necessity for long retention times; 2: depending on bulk capacity; 3: inhibition depending on pH value; 4: can have a positive effect by elimination of sulphide

Lignocellulosic components of animal manures and municipal wastes require long
retention times and digestion is usually incomplete. However, the stabilised,
incompletely-digested fraction is of value as a soil conditioner and as a horticultural

Although animal manures/slurries continue to represent the primary substrates for
CAD plants, the range of other organic waste substrates is increasingly varied. Table
3.2 summarises the chemical characteristics, potential biogas yields, and operational
problems associated with co-digestion of some common CAD substrates. The
agricultural and food-processing wastes commonly used in AD plants include: (i)
cattle and pig manures/slurries, (ii) poultry manure (with or without litter); (iii)
abattoir wastes; (iv) vegetable processing residues (e.g. from potatoes, sugar beet
etc.); (v) silage effluent; (vi) dairy processing residues (e.g. cheese and yoghurt
processing); (vii) brewery residues; (viii) fish oil and fish processing wastes and (ix)
canning wastes and wastewaters. Feedstocks for CAD plants may also include sewage
sludge and the organic fraction of municipal solid waste (OFMSW).

Toxic substances that may inhibit the AD process or cause die-off of the participant
microbes should be precluded, in as far as possible, from AD feedstocks. With respect
to animal manures and agricultural and food processing residues, these substances
• Toxic materials that inhibit digestion (e.g. high ammonia levels, pesticide
    residues, sheep dip, heavy metals, oil).
• Bioagents (aflatoxins and antibiotics).
• Disinfectants (e.g. cresol, phenol, arsenic, etc.).

Long straw and non-biodegradable materials should be avoided as they cause
blockages in the system (Warburton, 1997). Pre-treatment may be required, prior to
digestion, for some agricultural wastes. Examples of pre-treatment include:-
• Screening for foreign matter (e.g. sand, grit and long straw).
• Addition or removal of H2O. In general, it is not advisable to add water as the
    more water there is in the feedstock, the more energy is needed to heat the
    digester influent. However, for some wastes (i.e. poultry litter), addition of water
    may be necessary to ensure the correct consistency. Reduction of the water
    content of some animal slurries (i.e. pig slurry) is best achieved by changes in
    farm practices - e.g. diversion of run-off from roofs, yards etc.
• Maceration: For animal manures containing bedding, maceration is a necessary
    pre-treatment. This may also be required for other agricultural residues, such as
    fruit and vegetable wastes (Steffen et al., 1998).

Since anaerobic digestion proceeds satisfactorily only within a narrow pH range (6.8-
8.0) (Steffen et al., 1998), feedstocks should ideally be within this range. Because
acidification of carbohydrate-rich wastes can proceed rapidly at ambient
temperatures, feedstocks should be fed to the digester as soon as possible. This will
prevent souring and also maximise the biogas yield. The risk of souring is greatest
with feedstocks of low alkalinity (i.e. silage effluent). Animal slurries have a high
buffering capacity which reduces the risk of souring when different materials are co-

   Table 3.2: Characteristics and operational parameters of agricultural waste digesters (Steffen et al., 1998)

  Feedstock        Total solids         Volatile         C : N ratio         Biogas              Retention        CH4 content      Unwanted         Inhibiting       Potential
                     TS (%)              solids                               yield3               time              (%)           substances       substances       problems
                                       (% of TS)                           (m3/kg1VS)               (d)
Pig slurry              3-84             70-80               3-10           0.25-0.50             20-40             70-80       Wood shavings,     Antibiotics,    Scum layers,
                                                                                                                                bristles, H2O,     disinfectants    sediments
                                                                                                                                sand, cord,
Cow slurry             5-124             75-85              6-201            0.20-0.30            20-30             55-75       Straw, soil,       Antibiotics,    Scum layers,
                                                                                                                                H2O, NH4+          disinfectants    poor biogas
Chicken               10-304             70-80               3-10            0.35-0.60             >30              60-80       NH4+, grit,        Antibiotics,        NH4+
slurry                                                                                                                          feathers           disinfectants     inhibition
                                                                                                                                                                    scum layers
Whey                    1-5              80-95                n.a.           0.80-0.95             3-10             60-80       Transporation,                     pH reduction
Fermentative            1-5              80-95               4-10            0.35-0.55             3-10             55-75       Undegradable                         High scid
slops                                                                                                                           fruit remains                      concentration
Leaves                   80                 90              30-80           0.10-0.302             8-20              n.a.       Soil               Pesticides
Wood                     80                 95               511               n.a.                n.a.              n.a.       Unwanted                           Mechanical
shavings                                                                                                                        material                            problems
Straw                    70                 90                90            0.35-0.455            10-505             n.a.       Sand, grit                             Poor
Garden waste           60-70                90             100-150           0.20-0.50             8-30              n.a.       Soil, cellulosic   Pesticides          Poor
                                                                                                                                components                         degradation
Food remains             10                 80                n.a.           0.50-0.60            10-20             70-80       Bones, plastic     Disinfectants    Sediments
   Depending on: 1straw addition; 2dry weight; 3retention time; 4dilution; 5particle size; n.a. = not available

There are several advantages of co-digestion of food-processing and other organic
wastes with animal manures and slurries:

•   The primary advantage is the enhancement of the biogas yield per m3 of reactor,
    with consequent financial benefit to the plant operator.
•   Co-digestion results in more efficient digestion of some biomass materials. This
    may be due to co-metabolic or other synergistic effects of the mixed digestion
•   Solid wastes are converted into pumpable slurries when mixed with liquid
    manure. This can result in easier handling, both in the digestion process and
•   When organic wastes are accepted for co-digestion in CAD plants, the co-
    operative that owns the plant takes responsibility for end-use of the digestate.
    Controlled landspreading, irrigation of the liquid fraction or composting of the
    solid fraction ensures relatively inexpensive and environmentally sustainable
    recycling of these organic wastes (Kuhn, 1995).

The main disadvantage is the dependency of large CAD plants on the availability of
organic wastes fom the food-processing and other sectors for financial viability (i.e.
income from gate charges and from enhanced biogas generation). The growth in the
number of CAD plants in Denmark, for example, means that individual plants are
now competing with each other for access to high biogas-potential, off-farm organic
wastes. However, the growing restriction, within the EU, of landfill disposal of
organic wastes means that the availability of off-farm organic wastes for co-digestion
is likely to increase, rather than decrease.

3.4   Reactor design
The main requirements for optimal design of CAD plants include:

•   minimisation of mechanical and electrical equipment requirements;
•   effective insulation and use of corrosion resistant materials;
•   simple design and automatic operation;
•   equipment fail safe devices throughout;
•   environmental controls (Warburton, 1997).

There are also some essential functions that must be realised in the digester. These
include: (i) the continuous provision of nutrients to the bacteria and the removal of
metabolic products from the viable biomass, (ii) adequate retention times for the
feedstock organic matter and (iii) prevention of uncontrolled accumulation of solids in
the digester and of blockages in the material flow through the digester (Baader, 1981).

Traditionally, both plug-flow and completely mixed systems, such as the continuously
stirred tank reactor (CSTR), have been used for anaerobic treatment of animal
manures and have been adapted for the co-digestion of animal manures and food-
processing wastes (Metcalf and Eddy, 1991). The principle of the plug-flow design is
a one-step, once-through biomethanation system with a predominantly horizontal
flow, which may be mechanically assisted.

While plug-flow digesters are commonly located at smaller scale on-farm plants,
completely mixed systems (CSTR) are the design of choice for larger CAD plants.
Other CAD digester designs include: (i) serial/contact (ii) two-phase and (iii) upflow
anaerobic sludge blanket (UASB) digesters. The CSTR is the preferred option as it is
simple in design and, as such, is one of the least costly digester designs on the market
(Prisum and Norgaard, 1992).

Anaerobic digestion tanks are either cylindrical, rectangular or egg-shaped. The most
common design is a low, vertical cylinder (Metcalf and Eddy, 1991). Cylindrical
digesters have no “dead” corners, where biomass can accumulate, provided an
appropriate mixing system is installed (Prisum and Norgaard, 1992). In general,
anaerobic digestion plants are provided as prefabricated, modular turnkey packages.
At both CAD and on-farm level, prefabricated section tanks are increasingly the
norm. An insulation layer is provided to prevent heat loss. On-farm plug-flow
systems are usually of concrete construction but concrete CSTRs are now rarely used
in CAD and on-farm plants due to their more expensive construction costs.

Although the main process stage is the anaerobic digestion tank, there are a range of
preliminary and post-digester units and a range of structures which are necessary for
CAD plants. These include: (i) appropriate waste reception, loading facilities and
influent holding tanks, (ii) hygienisation tanks, (iii) mixing devices, (iv) gas holder,
(v) gas handling equipment, including pipework, valves, flare stack/heat dump, (vi)
electricity generating equipment, powered by ignition engines converted to run on
methane, gas turbines and electricity generators, (vii) boilers and heat exchangers to
provide heat for the digester, (viii) appropriate storage for digestate and fibre, (ix)
control and monitoring equipment and (x) odour control equipment (Warburton,
1997). Increasing legislative requirements and the increasing use of CHP plants at
farm level means that this range of ancillary unit processes is also required at on-farm

The main characteristics of CSTR systems are that they are operated on a continuous
basis and are completely mixed. Several methods of mixing are commonly used,

  (i) gas mixing where biogas is collected, compressed and injected back into the
      digester, usually through lances, promoting mixing from the gas bubbles. Gas
      mixing can also be used in plug flow reactors;
 (ii) internal mechanical mixing using propeller mixers within the digester. Such
      mixers were widely used in early completely mixed reactors, but their
      maintenance requires emptying the reactor and they are now rarely installed in
      new plants;
(iii) external mechanical slow speed mixers using an external motor and gearbox.
      The mixing device may be a paddle arrangement or a single large blade
      propeller. Such systems are frequently used in completely mixed reactors;
(iv) recycling of the sludge or slurries.

Efficient mixing is important to ensure complete digestion; to prevent short circuiting;
to maximise pathogen removal; to ensure uniform heat transfer and to prevent
sedimentation of silt and other nodular formations in the reactor (Kiely, 1997).

In CAD designs, the primary aim is to treat the combined wastes as efficiently and as
cost-effectively as possible. Therefore, a digester design which incorporates an
internal gas circulation system for mixing is more favourable than the more expensive
option of a mechanical stirrer (van Handel and Lettinga, 1994). In gas circulation
systems, the pipework should be suitably designed to prevent leakage and include
appropriate safety measures, such as water traps, flame arrestors and burner
interlocks. If mechanical mixing is used, slow speed mixers are the favoured option as
they use less energy and the external motor and gearbox are accessible for
maintenance. Single large mixing blades, or top and bottom paddle designs are less
susceptible to fouling by fibrous materials (Warburton, 1997).

The biogas production rate remains more or less constant for CSTR digesters
operating under steady state conditions. A mean average production rate of 1m3 of
biogas per m3 working volume of digester per day is usually obtained with mesophilic
digesters treating animal manure (Demuynck et al., 1984). However, the biogas
production can be significantly higher under thermophilic conditions or where
mesophilic co-digestion with food processing wastes is practised. The collection of
biogas is carried out by using either floating or fixed covers (Prisum and Norgaard,
1992). The floating covers fit on the surface of the digester contents and allow the
volume of the digester to change without allowing air to enter the digester (if gas and
air are mixed, an explosive mixture can result). Fixed covers provide a free space
between the roof of the digester and the liquid surface. Gas storage must be provided
so that (i) when the liquid volume is changed, gas, not air, will be drawn into the
digester, and (ii) gas will not be lost by displacement (Metcalf and Eddy, 1991).

CAD plants are operated at either mesophilic (30°-40°C) or thermophilic (50°-60°C)
temperatures. For hygienisation purposes (pathogen reduction), a pre- or post-
sanitisation treatment (60°-70°C for one or more hours) may be required for
mesophilic CAD plants. Pre- or post-sanitisation is not usually required for
thermophilic plants because of the operational temperature and the long retention time
of the feedstock within the digester. Pre- or post-sanitisation is capital expensive
because of the requirement for additional tanks, pumps and other ancillary equipment.
However, thermophilic plants are more difficult to operate and are more sensitive to
abrupt changes in operating temperature and influent supply than mesophilic plants
(Prisum and Norgaard, 1992).

The heat requirements of digesters consist of the amount needed (i) to raise the
incoming wastewater to digestion tank temperatures, (ii) to compensate for heat losses
through the walls, floor and roof of the digester, and (iii) to make up for losses that
might occur in the piping between the source of heat and the tank. Such heat may be
provided by external or internal plate exchangers (Metcalf and Eddy, 1991). When
external heat exchangers are installed, preheating of the wastewater occurs before it
enters the digester. The wastewater is pumped at high velocity through connecting
tubes, while water circulates at high velocity around the outside of the tubes. The
circulation promotes high turbulence on both sides of the heat transfer surface and
results in higher heat transfer coefficients and better heat transfer (Metcalf and Eddy,
1991). An advantage of external heat exchangers is that the untreated wastewater on
its way to the digester can also be warmed by heat exchange with the digested
material. Heat recovery from the digested material is beneficial to the energy balance
of the plant although the efficiency of the liquid/liquid heat exchange is generally low
due to problems of ragging and blockage of tubes or deposition of struvite
(magnesium ammonium phosphate) (Tipping, 1996). A sufficient heat transfer area
should be provided to minimise high temperature differences and prevent local high
hot water temperatures leading to fouling of exchange surfaces and more frequent
maintenance (Metcalf and Eddy, 1991). Alternatively, steam heating (the injection of
steam directly into the incoming feed line) is used in some Danish digesters (e.g.
Ribe) eliminating the requirement for digester heat exchangers (Christensen, 1995).
In digesters with internal heating systems, the cold wastewater is pumped directly into
the digester tank and the temperature is regulated by either wall-embedded piping or
mixing tubes equipped with hot water jackets. Because of the inherent operational and
maintenance problems associated with this type of heating system, internal heating is
not frequently used (Metcalf and Eddy, 1991).

For all anaerobic systems, digester capacity is directly related to the design retention
time, the operating temperature and the quantity of material processed. Longer
retention times within the reactor releases more biogas and cuts down post digestion
methane release. However, excessive retention times are not economic. Therefore,
installed capacity becomes an economic balance between the capacity cost and the
biogas production. In general, thermophilic reactors are smaller than mesophilic
reactors for a given quantity of influent material. A suitable monitoring system, both
manual and instrumental, is essential to ensure stable reactor operation (especially for
thermophilic plants) and to minimise operational difficulties, such as foaming, which
may lead to odour and aesthetic problems. Additionally, excess foam in the sludge
makes dewatering very difficult.

The number of reactors employed for a given capacity is also important in aesthetic,
economic and operational terms. In most cases, completely mixed reactors are built as
above ground structures and therefore have the maximum aesthetic impact. Plug-flow
reactors may be totally or partially buried in the ground resulting in less aesthetic
interference. Also, the ground provides added insulation for the reactor which in turn
minimises overall energy consumption (Tipping, 1996). In large scale CAD plants,
reactors should be multiple units to (i) ensure continuity of treatment if a reactor
malfunction occurs or maintenance is required, and (ii) reduce the visual impact of the
plant by the use of smaller reactors. The number of reactors for a given capacity will
depend on the optimum financial solution, local planning restrictions and operational
factors (Warburton, 1997).

3.5    Products of anaerobic digestion

3.5.1 Biogas
The biogas produced during AD of agricultural and food processing wastes consists
primarily of a mixture of methane and carbon dioxide with some trace contaminants,
including sulphur compounds, nitrogen, hydrogen, volatile organic compounds and
ammonia. The typical composition of biogas is shown below:-

               Methane                         60-80%
               Carbon dioxide                  20-40%
               Nitrogen                        0-5%
               Hydrogen Sulphide               0-3%
               Volatile Compounds              traces

One m3 of biogas with a methane content of 70% (20 MJ/m3) is equivalent to:

               0.61 litres of petrol
               0.58 litres of alcohol
               0.90 kg of charcoal
               1.70 kWh of electricity (assuming a conversion efficiency of 30%)
                2.50 kWh of heat only (assuming a conversion efficiency of 70%)
                1.70 kWh of electricity and 2.5 kWh heat in a CHP system.

Table 3.3 summarises the biogas production and energy output potential from various
types of animal manures/slurries. Many food processing wastes (e.g. fish oils, fats,
blood, distillery slops etc.) yield considerably greater quantities of biogas per unit
volatile solids.

Table 3.3:    Biogas production and energy output potential from 1 tonne of various
fresh animal waste feedstocks1

  Feedstock         No. of animals to       Dry matter          Biogas yield          Energy value
                        produce             content (%)          (m3/tonne              (MJ/m3
                      1 tonne/day                                feedstock)             biogas)
  Cattle slurry2              20-40               12                   25                23-25

  Pig slurry                 250-300                  9                  26               21-25

  Laying hen            8,000-9,000               30               90-150                 23-27

  Broiler            10,000-15,000                60               50-100                 21-23
  1: Figures should be regarded as indicative values only;
  2: Cattle slurry refers to both dairy and beef cattle;
  3: Because of the susceptibility of poultry manure to acidification, fresh manure should be digested
  as soon as possible.

Up to one third of the biogas energy may be needed to heat the influent and maintain
the digester temperature, although the average value is closer to 10% (Warburton,
1997). The surplus biogas can be burned to generate heat, either on site or piped
elsewhere. Depending on the local heat requirement, it may be possible to export
excess heat to industrial processes or to provide district heating to the local
community. If a combined heat and power (CHP) plant is used, all of the biogas
produced is generally consumed to provide both heat and electricity. High quality
biogas can be exported to the distribution network to replace natural gas. However, a
gas cleaning process is first required to remove carbon dioxide and trace
contaminants. Purified biogas can achieve a methane content of 95% which compares
favourably to the typical natural gas methane content of around 85%. It is also
possible to use the purified methane to run motor vehicle engines which need prior
conversion to gas use. This is currently carried out successfully in Sweden for city bus

The co-digestion of manure with different kinds of organic wastes containing easily
digestible organic matter results in an increase in the volume of gas produced. The
yield of gas per tonne of biomass feedstock is a significant economic indicator, since
costs of operation are closely related to the amount of biomass materials processed,
while income is primarily tied to gas production.

The Danish Energy Agency issued a report on CAD plants in 1995 (Christensen,
1995). As part of the study, gas production improvement, regulation, handling and
purification were examined in 10 CAD plants. Part of the improvement in gas
production was shown to originate from continued biogas production in the digestate
storage tanks. All plants originally used one step continuous digestion in fully mixed
digesters at either mesophilic or thermophilic temperatures. Experience showed,
however, that a considerable amount of biogas was subsequently produced in the
digestate storage tanks. Therefore, for economic and environmental reasons, all
Danish CAD plants have now been equipped with systems for collecting this gas.
This means that the original one step has turned into a two step process, the second
step being lower temperature post digestion with one or a few weeks retention time.
Additional gas production of 10-15% or more has been gained from this second gas
collection stage.

Biogas production can be deliberately controlled to a considerable extent by
regulating the organic loading rate. The dry matter content of the feedstock can be
increased by adding solid wastes. The loading can also be changed to some extent by
changing the hydraulic retention time. In some cases, industrial wastes with very high
contents of digestible organic matter, such as sludge from fish oil production, are used
to regulate gas production through a separate loading system. Some of the plants use
these regulating possibilities to produce more gas during the winter than in summer.
The plant in Hashøj (Denmark) attempts to optimise gas production in the daytime
(from approx. 6am to 9pm) by loading the digester only during these hours. There is
an incentive to do so since the gas can be burned in a CHP plant during this daytime
period. In Denmark, sales prices for electricity are considerably higher during these
peak usage hours (Christensen, 1995).    Biogas handling and purification

The H2S content of biogas used in internal combustion engines for combined heat and
power production should preferably not exceed 700-1500 ppm. Danish CAD plants
constructed in the late 1980s were generally able to meet these conditions without
special purification due to the presence of iron in the feedstock, e.g. from sludge
flocculated with ferric chloride. This led, mistakenly, to the establishment of new
plants without any H2S purification included. Since many of the newer plants used a
more diverse feedstock range and a greater percentage of non-manure feedstocks,
biogas H2S levels were found, in many instances, to exceed these levels. Initially,
addition of ferric chloride to the feedstock was used to minimise H2S levels. This
method of biogas “purification” was, however, quite expensive. Consequently,
aerobic biological removal of H2S from the produced gas was tested at full-scale in
the Fangel plant in 1993. The procedure involves addition of approximately 5% air to
the gas as it enters a separate purification vessel. The filter vessel is filled with plastic
carriers and a liquid made up from the gas condensate and the liquid fraction of
digestate separation is continuously recirculated over the filter. The temperature is
maintained at c. 35°C. In this way, the H2S is biologically converted to sulphur or
sulphate which is retained in the liquid in the filter. Surplus liquid is returned to the
digestate storage tanks, resulting in the eventual return of the sulphur or sulphate to
the fields as fertiliser. H2S purification with air addition is now being implemented at
all new CAD plants in Denmark and in many modern on-farm plants in Germany.

3.5.2 Digestate use
The digestate from manure-based CAD plants must be considered to be only partially
treated and should be stored appropriately until ready for application to farm land. In
many countries, long term storage capacity (up to 9 months) may be required if the
digestate cannot be applied to the land outside the growing season (Tipping, 1996).
Although there is no specific EU or National legislation covering spreading of treated
or untreated animal manures and slurries on land, most countries have developed
agricultural codes of practice. For example, in the UK slurry spreading is carried out
according to the MAFF Code of Good Agricultural Practice for the Protection of
Water. More specific guidelines apply to nitrate sensitive areas or nitrate vulnerable
zones. Where sewage sludge is used as a co-digestion substrate, tighter regulations
apply and the digestate must be landspread in conformity with the EU Sewage Sludge
Directive (86/278 EEC).

The recycling of livestock waste to land is environmentally appropriate and
constitutes a valuable source of macro- and micro-nutrients. Since livestock wastes
constitute the primary feedstock in CAD plants, and the only or main feedstock in on-
farm plants, the macro and micro nutrient content of the digestate largely resembles
that of untreated manures/slurries. Although AD treatment does not significantly
reduce the NPK content of livestock wastes, it does affect the chemical form of
nitrogen, in particular. The content of ammonium-N (NH4+-N) is generally 20%
higher in digested than in untreated cattle slurry (Holm-Nielson et al., 1997). By
contrast, the ammonium-N content of AD-treated and untreated pig slurry is almost
the same. Anaerobic digestion may also result in some conversion of organic to
inorganic phosphate. However, digestion has a minimal impact on either the
availability or the overall content of P and K in manure-based digestates.

Table 3.4 illustrates the results of nutrient analysis of digestate from the Ribe Biogas
plant, in Denmark, over the 1991-1996 period of operation. The feedstock to the plant
consisted of 84% manure and 16% industrial organic waste. For comparative
purposes, the nutrient content of untreated cattle manure and pig slurry are also
included in Table 3.4. Since the Ribe plant treats a range of different livestock and
food-processing wastes, the NPK ratio in the digestate is quite different from that of
the individual animal waste components. Mixing of different livestock wastes during
digestion results in the production of a digestate that has a better NPK ratio and, as a
result, is a more valid alternative to artificial fertilisers (Table 3.4). Digested slurry
has a lower dry matter content than untreated slurry since approximately 50% of the
dry matter content is converted to CH4 and CO2 during digestion (Nielson et al.,
1997). This facilitates more efficient landspreading and better uptake of nutrients.

The advantages of CAD operation with respect to subsequent digestate nutrient use
were clearly demonstrated by this detailed study of the Ribe CAD plant (Holm-
Nielson et al., 1997). In 1992, feedstock to the digester was contributed by 71 farms
and 6 industrial plants (Table 3.5). The quantity of feedstock delivered to the plant in
1992 was approximately 140,000 tonnes. Approximately 14% of the nitrogen content
was contributed by food-processing waste intake (primarily abattoir wastes). The
livestock wastes were contributed by 56 dairy farms, 7 pig farms, 3 mixed dairy/pig
farms and 5 mink or poultry farms. The digestate was returned to 68 of the 71
manure-producing farms (3 of the mink farms had no land availability for spreading)
and to 41 tillage farms that had not contributed livestock wastes to the CAD plant. In
nitrogen terms, the 41 tillage farms accepted 19% of the CAD plant digestate N
content (Table 3.5). Payment for the digestate was calculated on the basis of kg
nutrient (NPK) delivered from the CAD plant to private or centralised storage tanks.
The analysis illustrated the potential role of CAD plants in ensuring redistribution of
digestate NPK from livestock farms with surplus manure production to tillage farms
requiring high levels of nutrient application. The co-operative system utilised in
Denmark for CAD plant operation clearly facilitates optimal use of the NPK content
of CAD digestates. Co-digestion of livestock and food-processing wastes also ensures
that the NPK content of food-processing wastes/wastewaters is also returned to the
land, thereby maximising nutrient recycle and obviating the need for nutrient removal
in food-processing wastewater treatment plants.

Table 3.4: Average nutrient and metal content of digestate and influent manures at
the Ribe Biogas Plant between 1991-1996 (adapted from Holm-Nielson et al., 1997)

                                  N-total     NH4-N        P-total    K-total        Mg-     Cu- total             Ca- total
       Year1         %DM2          kg/t        kg/t         kg/t       kg/t       total kg/t    g/t                  kg/t
       1991           5.6          4.7         3.3          0.9        3.7           0.5       9.7                  1.9
        1992            6.4         4.6         3.1         0.9         3.5           0.5         12.8               1.5
        1993            6.2         5.2         3.4         1.2         4.1           0.8              2.7           2.5
        1994            5.8         5.0         3.2         1.1         3.3           0.6         11.6               2.0
        1995            5.8         4.9         3.2         1.1         2.9           0.5         11.8               1.4
        1996            5.8         4.8         3.2         1.1         3.2           0.5         10.8               1.5
    Cattle slurry3      8.5         4.7         2.7         0.6         4.4
     Pig slurry3        6.0         5.3         3.7         1.5         2.3
 Average yearly values for the plant digestate; 2: % dry matter; 3: influent cattle and pig slurries

Table 3.5: Nutrient circulation during operation of the Ribe Biogas Plant in 1992
(adapted from Holm-Nielson et al., 1997)

                                Slurry (T)         N (kg)            P (kg)            K (kg)            Number
     To the plant:
         From farms              117,530           549,082           104,869          470,912                71
         From industries          21,916            92,082            20,683           17,345                 6

     Total                       139,446           641,709           125,552          488,257                 77

     From the plant:
        To suppliers             113,498           522,091           102,148          397,243                68
        To buyers                 26,004           119,618            23,404           91,014                41

     Total                       139,502           641,709           125,552          488,257                109

3.5.3 Solid/liquid separation of digestates
In CAD plants, the digestate is often separated into solid (fibre) and liquid fractions.
A variety of fibre separator systems are used: (i) rotary screens, (ii) flat belt
separators, (iii) roller presses, (iv) vibrating screens, (v) centrifuges and (vi) screw or
ram presses. Separation performance depends on the characteristics of the feedstocks
being processed and the separator type/screen size. Generally, increasing the feed dry
solids content produces a higher separated fibre dry solids content. For 10% dry solids
content materials (not digested), vibrating screens are not suitable. Roller presses and
rotary screens produce up to 20% dry solids fibre and belt presses can achieve 25%
dry solids fibre (Pain et al., 1978). Rotating sieves produce between 10.4 and 20.6%
dry solids content from a variety of input slurries. A discostrainer can produce up to
16.6% dry solids fibre and a continuous screw sieve up to 38.4% dry solids fibre
(Chiumenti et al., 1991). Energy use is between 0.01 and 0.6 kWh/m3. Screw and ram
presses may be able to produce up to 60% dry solids material which is suitable for
combustion (i.e. in the wood chip CHP units used in Denmark).

In the majority of CAD plants practising digestate separation, the fibre fraction is
aerobically composted to provide a stable and marketable peat moss substitute. Open
air composting in windrows or aerated heaps results in uncontrolled ammonia losses
to the environment, odour release and aerosol formation. Composting under
controlled conditions (static piles) with application of forced aeration in ventilated
sheds enables emissions to air to be controlled and treated using biofilters prior to
release. When composting is carried out under enclosed conditions, negative pressure
is maintained by ventilation fans which extract the air and either recirculate it through
the compost piles or pass it to biofilters for treatment. An electrical consumption of c.
30 kWh/tonne is typical for composting systems (Novem, Holland - personal
communication). After composting, the treated fibrous material can be stored with
minimal air emissions, although cover is required to minimise rainfall uptake. The
fibre can be utilised locally or packaged and sold as a peat moss substitute.

Alternatively, the separated fibrous fraction can be returned, without composting, to
land in the biogas plant catchment area. Spreading fibre requires much less power
than conventional digestate land-spreading and can be carried out using a small
tractor rather than specialised equipment. It is also less offensive to neighbours,
having significantly less odour after digestion. Return of the fibrous fraction can play
an important soil-conditioning role, particularly in regions where the soil organic
content is low. The Filskov Biogas Plant in Denmark is located in a sandy soil region
specialising in potato poduction. The potato producers willingly pay for the fibre
fraction of the CAD plant digestate in order to maintain the organic content of the
potato production lands.

In CAD plants that do not utilise composting techniques, storage facilities for the
separated fibrous fraction are required. Since subsequent slow aerobic and anaerobic
decomposition may occur, resulting in emissions of ammonia and other volatile
compounds, storage under cover is required. Covered storage is also essential in
order to prevent rainwater dilution.

In some Danish CAD plants, the separated fibrous fraction (without composting) is
used as a fuel, either alone or in combination with wood chips, MSW or sewage
sludge, in CHP plants dedicated to the provision of electricity and district heating.

The liquid fraction of digestate, after solid/liquid separation in CAD plants, is either
locally landspread or marketed as a liquid fertiliser for amenity areas, such as golf
courses. The dry solids content of separated liquor increases with the dry solids
content of the feedstock, irrespective of the separator type used (not including screw
presses or centrifuges). For a 10% dry solids feed, a separated liquid dry solids
content of 4 to 5% would be expected (Pain et al., 1978). The liquor from AD
processes has a low, but diverse range of nutrients. It can be used as a liquid fertiliser
in a planned fertiliser regime. As it has a high water content, the liquid also has
irrigation benefits, so it can be used for 'fertigation' on agricultural land. Other
advantages of the separated liquor are reduced odour and material homogeneity
improving ease and accuracy of distribution. However, as it contains particles, it
should not be used for fertigation in greenhouses because it can block feeder pipes if
not separated effectively. It is extremely important that the liquor be used or disposed
of in a way which prevents run-off to surface waters.

As indicated above, the use of fibre and liquor from centralised biogas plants has led
to improved fertiliser utilisation and therefore less chemical fertiliser consumption.
This is an aspect of increasing environmental importance as can be seen from the
increasingly stringent regulations which are laid down in order to protect surface and
groundwaters from nutrient enrichment. However, a risk of increased loss of ammonia
as a consequence of digestion must be taken into account. Digested livestock manures
have a higher ammonia content and a higher pH value than fresh manure. Therefore,
ammonia release from digested material during open storage or landspreading is
almost inevitable unless the digestate pH is lowered. The surface area available for
volatilisation of stored digestate is reduced by covering or introducing a flotation
layer or if return of the digestate to land is by irrigation rather than by landspreading,
thereby reducing atmospheric emissions (Nielson, 1997).

Since the ratio of organic nitrogen to inorganic nitrogen (mainly ammonium-N) in
digested livestock waste is lower than in raw manures/slurries, plant nitrogen uptake
from digestate (complete digestate or separated liquor) is greater than from raw
manure (Klinger, 1998a). Consequently, the replacement of synthetic fertiliser by
landspreading of digestate or separated liquor maximises plant N uptake and
minimises the risk of nitrate leaching to ground or surface waters.

The P/K content of digestate or separated liquor may not be in balance with
simultaneous nitrogen requirements or may be excessive to plant requirements (i.e. on
pastureland). Consequently, the amount of digestate that is landspread or injected
should be tailored to the P/K needs and additional N can be added via synthetic
fertiliser application.    Safeguards against pathogens

Manure from domestic animals may contain a wide variety of pathogenic bacteria,
animal viruses and parasites (Bendixen, 1994). These pathogens may be shed from
sick animals and also from apparently healthy carriers. Thus, handling of manure
involves the risk of spreading infections. In centralised biogas plants, where manure is
collected from a large number of herds and returned for spreading on different farms
after digestion, this risk must be counteracted by the use of hygienic measures for the
collection, transportation and handling of untreated and treated manure (Bendixen,
1994). Industrial wastes from animal and fish-processing plants (i.e. abattoir wastes)
may also contribute pathogens to co-digestion feedstocks. Vegetable wastes may
contribute plant pathogens.

In general, the long retention times and elevated temperatures used in anaerobic
digesters result in considerable pathogen destruction. For on-farm digesters treating
manures generated on the farm and spread on the farmland after treatment, further
hygienisation steps are not considered necessary. However, for centralised biogas
plants receiving manures from a variety of sources and where the treated manure is
landspread on a number of farms, it is recommended that the manures be subjected to

thermophilic temperatures at some point during the overall treatment process. At least
50°C for a minimum of several hours is recommended (Bendixen, 1994).

Sewage sludge or the organic fraction of MSW may contain a wide range of human as
well as animal pathogens. In the UK, the pathogenic risk of spreading sewage sludge
on land has been recognised and a code of practice has been introduced which
requires sewage sludge to be stabilised, prior to surface spreading, by approved
methods, including mesophilic digestion and storage. Approved stabilisation
processes reduce infection risks to acceptable limits for most of the recognised
pathogens present. In addition, a time period prohibiting grazing after spreading is

In Denmark, CAD plants that co-digest sewage sludge or household waste must
incorporate a pre- or post-hygienisation treatment at 70°C for at least 1 hour. It is
furthermore recommended that the hygienic standard of the digested product should
be controlled by official supervision of the plants and by checking the sanitation
effect at regular intervals. A microbiological indicator test (the faecal streptococcal
(FS) test) has been developed for this purpose (APHA, 1985). Since not all of the
feedstocks used in CAD plants require such elevated temperature treatment, different
feedstocks are subjected to different hygienisation regimes to reduce the energy costs
involved (see Section 4.1.11).

3.6    Storage of feedstocks and products
For large CAD plants, storage facilities for incoming raw feedstocks are required.
Feedstocks from different sources are usually segregated allowing blending of
materials prior to digestion to achieve requisite influent mixtures (Tipping, 1996). The
feedstocks may be stored near the digester or elsewhere, although the need to
minimise transport movements will affect the decision on siting storage.

Feedstock storage facilities must be planned in accordance with environmental, health
and safety issues and regulations. It may be necessary to balance the planning
requirements (such as for feedstock to be stored in a totally enclosed space with tank
covers to reduce the escape of odours), with health and safety regulations (to ensure
that there is no exposure to substances hazardous to health) (Warburton, 1997).

After the specified retention time within the reactor, digested material is usually
displaced into a holding/storage tank. The digested material at this stage is warm and
actively producing biogas. Accelerated ammonia release can also be expected under
open, warm storage conditions. Heat exchangers may be used to recover energy from
the digested material to heat the incoming feed steam. Removing heat from newly
displaced reactor contents can reduce subsequent, residual biogas production. The
importance of collecting biogas from storage tanks has recently been widely
recognised. Short-term impoundment of the digestate in enclosed storage tanks and
recovery of evolved gases is now a common feature of Danish plants. Some 10 to
15% of the total biogas production may be recovered from the post reactor storage
tanks (Christensen, 1995).

In some of the new German on-farm digesters, treated slurry from plug-flow digesters
is passed to a large covered circular tank which serves both as a second ambient
temperature digester and a gas storage tank. Additional biogas produced during 10-12
days storage is obtained in this way and the covered gas storage system allows
simultaneous biological H2S oxidation by oxygen injection.
At the Ribe Biogas plant in Denmark, local farmers interested in being connected to
the plant experienced difficulties re the location of storage facilities, since their land
holdings were split into two sections, some close to the farm and the remainder
situated further away. In order to reduce the transportation of digestate as much as
possible, a network of 26 local storage tanks for digested slurry was built by the
biogas plant. It is now common practice for new CAD plants to provide a network of
strategically located digestate storage tanks within their catchment regions (Holm-
Nielson et al., 1997).

3.7     Transportation
In any centralised operation, waste materials must be collected from source and
products delivered to their destination (Tipping, 1996). The primary means of
transport are heavy goods vehicles, usually diesel powered, and sized to meet the
compromise between economies of scale in using large articulated vehicles, and the
practicality of accessing farms served by narrow rural roads. The impact of transport
movements needs to be minimised through logistics and use of alternative methods of
transport (such as rail), as well as careful design of the location of storage tanks so
that distances travelled between the site of feedstock production, the storage tanks and
the digester are minimised. The possibility of piping pig slurry and other liquid wastes
has been investigated in some CAD plants. Optimum siting of the digester between
adjacent piggeries has facilitated piping of pig slurry in a number of plants in
Denmark and Italy, with consequent decreased transport costs (AD-Nett Conference,

As mentioned in Section 3.6, construction of a network of digestate storage tanks for
CAD plants facilitates landspreading by participant livestock and tillage farmers. In
addition to benefiting those farmers who produce slurry in excess of their
landspreading needs, the provision of a network of large storage tanks can decrease
individual farm storage requirements, reduce overall transport costs and appropriate
siting can facilitate delivery and turning facilities for large tanker vehicles (Holm-
Nielson et al., 1997).

Reduction of the H2O content of animal slurries may also reduce transport costs. The
higher the water content of the waste, the greater the volume that has to be transported
to sustain the digester loadings at efficient levels. Moving excessively dilute waste
materials around the countryside increases environmental impacts and decreases

3.7.1 Vehicle emissions
Organic wastes, including industrial waste, animal slurries and manures, typically
contain odorous substances and pathogenic microorganisms. Transport of liquid and
semi-solid materials in enclosed tankers of appropriate design should ensure little or
no odour or pathogen release to the environment, although road accidents and human
error cannot be discounted. Solid wastes, such as cattle manure, chicken litter or
industrial waste are generally transported in tipper vehicles where there may be a need
for covering to minimise odour release and fugitive dust (Tipping, 1996).

A further practical transport difficulty is contamination of digested material with
pathogenic microorganisms from vehicles which have carried unprocessed materials.
Vehicles transporting unprocessed, raw wastes need to be thoroughly washed (and
even disinfected) prior to carrying digested material. If separate vehicles are used to
transport raw and digested materials, this doubles the number of journeys necessary,
and therefore doubles the transport costs. The whole project can become uneconomic
unless transport movements are minimised (Warburton, 1997).

3.8     Odour
Odour release from organic material processing is a key issue for planning authorities
and local residents. Good operational practice and the inclusion of appropriate odour
removal technologies in the plant design are essential requirements for successful
siting of industrial scale digestion plants in urban or semi-urban locations (Warburton,

Anaerobic or septic conditions at waste treatment plants may result in the presence of
hydrogen sulphide and a range of other malodorous compounds which singularly, or
in combination, contribute to odour nuisance. Malodorous compounds tend to be
volatile, sparingly soluble materials with a low odour threshold value to the human
nose. The main components of odour nuisance from digestion plants are typically
sulphides, mercaptans, ammonia and volatile fatty acids.

There are several odour point sources in a typical anaerobic digestion plant and the
degree of nuisance involved varies according to the type of storage, waste type and
age, and efficiency of abatement measures installed. In addition, there may be diffuse
area sources emitting odour at a low rate, such as quiescent storage tanks or from
occasional ground spillages (Demuynck et al., 1984).

Odour release is most intense from putrescible organic materials prior to anaerobic
digestion, especially when the untreated material is agitated, for example during
unloading, tank filling or mixing operations. Odour release is also significant in off-
gases from heat treatment and is commonly prevented by collection and treatment of
the odorous air before release to the environment.

Historically, unloading of tanker vehicles has been a major odour releasing activity at
sewage treatment works. Air displaced from the storage tank or reception pit during
filling typically contains organic malodours and reduced sulphur compounds, causing
severe odour nuisance if uncontrolled. Unloading of solid materials such as chicken
litter, manure or industrial waste is known to be a highly odorous activity. Tipping
material into hoppers or bunkers inevitably releases odour, dust or aerosols, and may
constitute a health hazard to workers. In addition, ammonia may be released to air
(Tipping, 1996).

Poultry litter combustion plants and recently constructed CAD plants receive waste
consignments in enclosed buildings maintained under negative air pressure to prevent
dust or odour release during unloading or handling operations. The buildings are
designed to accommodate the maximum size of vehicle such that the access doors can
be closed prior to unloading. Odorous air is extracted and used as excess combustion
air (Tipping, 1996).

Unloading of tanker vehicles may be acceptable in open air situations if waste
reception tanks are air tight and ingress and egress of air is through filters capable of
removing odorous substances to acceptable levels. In most cases, the best available
technique (BAT) for reception of organic materials is a dedicated building (or discrete
area within a building) which is maintained under negative pressure to prevent

external release of odour, dust, aerosol or ammonia. The ventilated air must be treated
to remove odorous compounds and to reduce potential pathogen transfer.

Liquid and semi-solid materials may be stored in above or below-ground tanks. Solid
materials may be stored in bunkers or silos and moved by mechanical handling.
Storage tanks should be enclosed and should either be gas tight with air venting
through suitable scrubbers or filters, or maintained under negative pressure by air
extraction. Typical odour releases from quiescent open slurry storage tanks are
compared to background odour concentrations in Table 3.6 (De Bode, 1991).

Table 3.6: Average Odour Release during Slurry Storage (adapted from De Bode,

                                           Odour Concentration (o.u./m3)1

                                       Summer                           Winter

  Pig Slurry                               200                           120
  Cattle Slurry                            110                            60
  Background                                20                            10-20
   o.u./m3 refers to odour units per m3, as defined in De Bode (1991)

Post digestion activities may also release odour, but to a lesser extent than during
unprocessed material handling. The major concerns with digestate material handling
and storage are biogas and ammonia release. Digested material has a recognisable
odour associated with reduced sulphur compounds and other volatile species. The
odour intensity is, however, much reduced compared to untreated materials since
volatile acids are removed during digestion (Tipping, 1996).

3.8.1 Odour removal
Good process design can minimise or eliminate odour generation in some cases, but
odorous air from point sources needs to be collected and treated. A variety of methods
can be applied depending on the digestion plant design, e.g. combustion,
physicochemical or biological treatment.

In treatment plants involving combustion processes, odorous air is often collected and
used as excess combustion air for boilers or engines. This results in total removal of
odorous compounds, such as sulphide and ammonia, and of methane, but sulphur
dioxide levels in flue gas emissions from the boilers may consequently be elevated.
Collection and combustion probably represents BAT at most sites, although the
volume of malodorous air which can be burnt is related to the combustion plant type
and size (Warburton, 1997).

Biofilters, such as peat bed filters, have proved successful in removing many odorous
compounds from contaminated air, but removal may not be complete for some
insoluble malodorous species, resulting in perceptible outlet odour. Off-gases may be
particularly difficult to treat biologically due to the inlet gas temperature.
Physicochemical treatments including activated carbon adsorption, ultraviolet light
and chemical scrubbing are successful for the removal of specific categories of
odorous compounds. A condensation stage may be necessary to remove water from
saturated air before chemical or activated carbon treatment (Warburton, 1997).

Hydrogen sulphide may be removed from biogas by wet or dry scrubbing procedures.
Water scrubbing involves the use of alkaline reagents and a packed tower. Wet
scrubbers or packed bed biofilters continually recirculate water which needs
intermittent blowdown to maintain acceptable ionic concentration. Dry chemical
scrubbers are of various designs and require frequent reagent replacement, with the
result that chemical costs can be considerable. As indicated earlier, iron salts are often
added to sewage sludge digesters in the UK and Denmark in order to minimise the
level of H2S in the produced biogas. This practice is expensive and is not advised for
centralised co-digestion plants. It is now generally accepted that the least expensive
method of removing H2S from biogas is by biological conversion to sulphur or
sulphate which is removed in the circulating liquor or condensate in separate
biological treatment units. Alternatively, storage of gas above the digestate may
provide an opportunity for biological H2S oxidation with removal of the produced
sulphur/sulphate in the digestate.

3.9      Nutrient removal
As indicated earlier, the digestates from on-farm or CAD plants treating animal
manures as the primary feedstock have a significant fertiliser value. For
environmental and economic considerations, the logical use of this material is to
landspread it or inject it into grassland or soils used for grazing, silage production or
for tillage purposes. In Denmark, one of the declared objectives of the Centralised
Biogas Programme is to maximise the utilisation of the fertiliser value of animal
manures and other food processing wastes in order to reduce the usage of synthetic
fertiliser and to decrease farmers’ costs. A further bonus of digestate rather than
artificial fertiliser use is the soil-conditioning achieved by return of the digested
fibrous fraction of the treated waste.

In areas where manure production is in excess of the quantities required by
surrounding farmers or where groundwater nitrate levels are already high (e.g. in the
Po Valley in Italy), some form of post-treatment may be required. Reverse osmosis is
a process by which water is separated from dissolved solutes by filtration through a
semi-permeable membrane at a pressure greater than the osmotic pressure caused by
the solutes in the wastewater. An applied pressure gradient ensures flow of water
from the more concentrated to the less concentrated solution. Reverse osmosis trials
were carried out in Denmark at the Lintrup Biogas Plant with the objective of
reducing the water content of the liquid fraction of separated digestate by 75%. The
intention was to produce a concentrate with high fertiliser value which could be
marketed or transported to a wider spreading area with minimised transport costs.
Although the trials at the Lintrup plant were not successful due to blocking and
clogging of the membrane, the Danish company, Bioscan A/S, is now marketing a
number of post-treatment options, including reverse osmosis. To date, there is no
published information on the type of membranes used, nor on the efficiency or
operational cost of the process (S. Tafdrup, Danish Energy Agency, personal

Removal of nutrients from the treated digestate can be achieved using the same
chemical or biological methods used for industrial wastewaters. In Italy, one
centralised plant has been equipped with full post-treatment because of the already
serious nitrate contamination of groundwater in the region. The very high costs
involved are justified in order to maintain the pig-farming population in this region
and to reduce potable water production costs (AD-Nett Conference, 1999).

3.9.1 Chemical nutrient removal methods
Nitrogen removal:
 (i) Breakpoint chlorination is a process which involves the addition of chlorine to
     treated wastewater to oxidise the ammonium nitrogen in solution to nitrogen gas
     and other stable compounds.
(ii) Ion exchange is a unit process in which ions (in this case ammonium, NH4+) are
     removed from the waste stream and exchanged with ions from an ion exchange

Phosphorus removal:
The addition of certain chemicals to wastewater produces insoluble or low-solubility
salts when combined with phosphate. The principal chemicals used for this purpose
are aluminium sulphate, ferric chloride and lime. The chemical sludge produced
requires safe disposal under licenced conditions.

3.9.2 Biological nutrient removal methods
Nitrification of ammonium is a two-step process involving two genera of micro-
organisms, Nitrosomonas and Nitrobacter, where ammonium is first converted to
nitrite and subsequently to nitrate, in the presence of oxygen.

Nitrate formed from the nitrification process is converted to nitrogen gas by a variety
of denitrifying bacteria under anoxic conditions. For this to occur, sufficient organic
carbon is needed to provide the energy source for the conversion of nitrate by the
bacteria. This may be provided by residual wastewater organics or by the introduction
of precise amounts of a readily utilisable carbon source (e.g. methanol).

Combined nitrification-denitrification systems:
In these systems, low molecular weight organics (e.g. volatile fatty acids) present in
the wastewater are used as the organic carbon source for denitrification. Ammonium
N is oxidised by nitrifying bacteria in the presence of oxygen to nitrate N during the
nitrification step, while denitrifying bacteria convert nitrate N to nitrogen gas (N2) in
the absence of oxygen. Processes used include (1) the Bardenpho process and (2) the
oxidation ditch (Metcalf and Eddy, 1991).

Biological phosphorus removal:
Biological phosphorus removal processes are based on sequential use of anaerobic
and aerobic conditions in a single reactor or cycling of the wastewater through
separate anarobic and aerobic tanks, followed by sludge settlement and return (the
A/O process). Under anaerobic conditions, volatile fatty acids and other low
molecular weight organics in the wastewater are taken up and stored by the microbial
biomass. The ATP energy needed for organic compound uptake is provided by the
hydrolysis of intracellular polyphosphate. Consequently, operation of the anaerobic
stage is associated with phosphate release to the wastewater. Subsequently, under
aerobic conditions, the stored organic compounds are metabolised fully to CO2 and
H2O, releasing a large amount of energy that is used for bacterial growth but also for
uptake of phosphate and re-synthesis of intracellular polyphosphate. The amount of
phosphate taken up in the aerobic zone/tank greatly exceeds that released under
anaerobic conditions, with the bacteria involved (e.g. Acinetobacter sp.) accumulating
phosphate in amounts that are excessive to their growth requirements. This
phenomenon has been referred to as "luxury phosphate uptake". An alternative
biological P-removal system is referred to as the PhoStrip process. This results in
removal of the P, not in sludge form, but as a phosphorus-rich supernatant which can
be later chemically precipitated with lime or another coagulant (Metcalf and Eddy,

Combined biological nitrogen and phosphorus removal:
A number of biological processes have been developed for the combined removal of
nitrogen and phosphorus. Many of these use a type of activated sludge process that
employs combinations of anaerobic, anoxic and aerobic zones or compartments to
accomplish nitrogen and phosphorus removal. The most commonly used processes
are (1) the A2/O process, (2) the five-stage Bardenpho process, (3) the UCT process
and (4) the VIP process (Metcalf and Eddy, 1991).

3.10   Economics

3.10.1 Financing
Centralised biogas plants are characterised by investments of long duration and a
relatively low current income compared to the invested capital. This calls for types of
financing that provide low average mortgage payments and long repayment periods.
Traditional investors do not recognise the environmental benefits and sustainability of
CAD and view it in the same way as other high risk commercial projects, demanding
high security and high returns on invested capital, leaving less for other investors and
shareholders. Ethical or 'green' bank funding is, however, beginning to be provided in
some countries. These lending institutions take a more sympathetic view of renewable
energy in general and seem willing to invest on less onerous terms (Warburton, 1997).

In Denmark, all of the centralised plants received investment grants from the Danish
Energy Agency for the "energy" part of the plant (Table 3.7). In the 1980s, these
grants amounted to 30-40% of this part of the investment. The grants given in the
1990s were approximately 20% and further reductions are expected (Christensen,
1995). For all projects involving district heating, a special financing scheme with
indexed loans was established in 1987. This type of loan, where remaining debts and
mortgage repayments are significantly lower, result in average yearly payments that
are less than traditional mortgage loans. Most of the newer district heating projects
and almost all of the centralised biogas plants are primarily financed by indexed loans
(Christensen, 1995).

3.10.2 Costs
The main financial costs of establishing a CAD project include capital, project
development, operational and training costs.

Capital costs:
The equipment has to be manufactured to a high standard to prevent corrosion, and
the resulting high capital costs may only be justified if the equipment has a long
active life. Construction of the plant and associated site works, including any
landscaping required under planning permission, will also incur costs. A CAD plant
of 1MW capacity (requiring a digester with a capacity of 10,000 m3) is likely to
require an initial capital investment of £3 to £4 million sterling (Warburton, 1997).
Table 3.7: Invested capital and financing costs (IR£ x 1000) for two Danish biogas
plants (adapted from Christensen, 1995)

                                                     Ribe        Lemvig

               Start-up year                         1989         1991
               Invested Capital
               Biogas plant                           3063         4586
               Equipment for slurry transport          392          377
               Slurry storage                         1333          878
               Effluent separation                       -            -
               Total                                  4788         5841
               Public investment grant               1873          1502
               Grant, % of total investment          39%           26%
               Indexed mortgage loan                 2619          3704
               Bank loan                                -           635
               Own capital                            296             -
               Total                                 4788          5841

Project development costs:
These can be very significant and include (i) technical, legal and planning consultants'
fees, (ii) costs of arranging finance, (iii) electrical connection costs and (iv) licence

Operational costs:
These vary enormously depending on size, technical construction, plant age and plant
management and include staff, insurance, consumable and transport costs. Biomass
transportation accounts for 35% to 50% of the total operating costs at centralised
biogas plants in Denmark. Transportation costs for solid manure are approximately
double those of slurries. Reduction of transport costs can only be achieved by siting
new CAD plants in such a way as to minimise transport costs, by reduction in the
water content of slurries and optimised siting of digestate storage tanks (Christensen,

Training costs:
CAD plant operators need to be fully trained in the safety, financial and
environmental implications of the project. These skills need to be updated as
technology and safety considerations develop.

3.10.3 Income
The largest revenue streams from CAD plants are likely to be from the sale of energy,
in the form of electricity (primarily) and heat (Table 3.8). Other income streams are
likely to include treated fibre and liquor sales and gate fees (Table 3.8). Gate fees
come from the receipt of organic wastes, primarily from food processing industries
and municipal authorities (sewage sludge and OFMSW). These fees are increasing as
charges for alternative waste management methods, such as landfill taxes, also
increase (Warburton, 1997). In Denmark, biogas energy sales are favoured as (1) the
electricity produced can be sold directly to the national grid at competitive prices (2)
there is a tax on fossil fuels used for heat production for domestic purposes, (3)
electricity for domestic use is also taxed and (4) to promote renewable energy, a part
of the tax is refunded to producers of renewable-energy-based electricity
(Christensen, 1995). In Ireland, markets for all of the products will need to be
developed and realistically priced in order for a CAD project to be economically
viable. The operation of centralised biogas plants also results in several socio-
economic advantages, such as improved fertilisation efficiency, lower greenhouse gas
emissions, cheap and environmentally sound waste recycling, and reduced nuisances
from odours and flies. The economic implications of these are beyond the scope of
this report.

In Table 3.8, the economic results are expressed per m3 of feedstock treated. The
analysis is based on the realised sale revenues and the current running costs according
to the accounts of the plants. The capital costs are calculated on the basis of initial
investments and the expected lifetime of various plant components. Initially, the
plants received a grant amounting to 39% of the investments at Ribe and 26% at
Lemvig (Table 3.8).

Table 3.8: Sales and costs analysis for the Ribe and Lemvig CAD plants between
1994 to 19961

                                                     Ribe                    Lemvig
                                        1994         1995    1996    1994     1995        1996

Feedstock, m3 per day                    401   391   403   413                  369        394
Biogas production, m3 per day          11784 11759 11811 14797                14003      13512
m3 of biogas/m3 of feedstock              29    39    29    36                   38         34

Sales (IR£/m3 of feedstock):
    Gas                                  3.72        4.02    4.02    5.94        6.33      6.03
    Gate fees                            0.70        1.00    1.00    0.91        1.31      1.00
    Storage rental fee                   0.50        0.50    0.50    0.00        0.10      0.10
                       Total             4.92        5.52    5.52    6.85        7.74      7.13

Costs (IR£/m3 of feedstock):
   Production of biogas                  3.12        3.42    3.52    4.83        4.93      5.23
   Transport of biomass                  2.01        2.11    1.91    2.11        2.31      2.11
   Storage of biomass                    0.81        0.70    0.70    0.10        0.10      0.10
                     Total               5.94        6.23    6.13    7.04        7.34      7.44

Costs breakdown:
(IR£/m3 of feedstock)
    Operating costs                      2.92        3.21    3.32    4.23        4.23      4.43
    Capital costs2                       3.02        3.02    2.82    2.82        3.12      3.02

Profit/loss                             -1.02        -0.71   -0.61   -0.19       0.40     -0.31
(IR£/m3 of feedstock)
    Adapted from the follow-up progamme for biogas plants carried out by the Danish Institute of
    Agricultural and Fisheries Economics.
    Capital costs do not include investment subsidies

It appears that the operating costs for both plants exceeded sales revenue during the 3-
year study period, except in 1995 when the Lemvig plant showed a profit (Table 3.8).
In practice, the losses are covered by the initial grant which, as previously mentioned,
is not included in the analysis.

The general conclusion from the assessment of the results obtained for these two
Danish centralised plants is that it is possible, under favourable conditions, to achieve
a balance in the company economy without grants for initial investments. However,
the risk is still too big to justify the removal of the investment grants completely
(Christensen, 1997).

It must be emphasised, when comparing Ireland with our European neighbours, that
biogas plants in countries such as Denmark, Germany and Austria, for example, are
favoured with respect to energy prices, energy sales options, financing possibilities
and investment grants.

3.11 Renewable source of energy
The biogas produced from large CAD plants is a significant source of renewable
energy. In Denmark, renewable energy sources are exempt from Danish state taxes. It
is clearly a pre-condition for the viability of CAD biogas plants that this exemption is
maintained. The selling of electricity produced from biogas to the National Grid is
commonplace in Denmark and, even though variations in electricity prices can be
considerable (in the range of 10-20%), the tax exemption secures a net energy
production value corresponding to 0.94 - 1.00 DK per kWh (1DK equals £0.10)
(Christensen, 1995). In Germany, electricity prices range between 15-18
Pfennig/KWh (6-7.2p/kWh).

Western and Southern Europe have lagged behind their northern European
counterparts in development and implementation of renewable energy sources.
However, in the past decade, initiatives have been put in place to improve this
situation. In the United Kingdom, the Electricity Act of 1989 made provisions for the
Secretary of State to place an obligation on the Regional Electricity Companies
(RECs) to purchase at premium prices power produced from non-fossil fuel sources.
This obligation is known as the Non-Fossil Fuel Obligation (NFFO) and has given
rise to a process whereby developers can bid for NFFO contracts to supply non-fossil
or renewable electricity. The premium price paid for non-fossil power is a form of
subsidy and the extra money required to pay for this is raised by a "Non-Fossil Fuel
Levy" placed on all the electricity suppliers and passed through to customers
(Tipping, 1996). Under the NFFO4 scheme, 195 renewable schemes were contracted,
and of these, 6 schemes come under the technology band of 'Anaerobic Digestion of
Agricultural Wastes'. The highest price contracted from this source was 5.2p/kWh.
However, under the NFFO5 scheme, anaerobic digestion of agricultural wastes is not
a specific technology band and instead comes under the bands 'Energy from Waste'
and 'Energy from Waste using CHP' where the highest prices contracted are
2.49p/kWh and 2.9p/kWh, respectively (I. Higham, ETSU, personal communication,

In Ireland, the first Alternative Energy Requirement (AER 1) programme was
initiated in 1994 in order to fulfil Ireland’s obligation under the EU ALTENER
programme which aims to triple electricity production from renewable sources in the
EU as a whole, during the period 1991-2005. The Minister for Transport, Energy and
Communications initiated a competition aimed at securing additional electricity
generation capacity from wind energy, hydro energy, biomass and/or waste to energy
systems. The Electricity Supply Board's Power Procurer agreed to offer successful
bidders a 15 year Power Purchase Agreement (PPA) for the purchase of their net
electricity output. The AER I programme resulted in PPAs being awarded to a range
of alternative energy sources with 12 MW awarded to landfill gas and waste to energy
projects. To encourage this area of renewable energy, a second AER competition
(AER II - 1995) was initiated which resulted in the offer of a contract for the
construction and operation of a 30 MW waste to electricity generating facility.
Unfortunately, the most recent AER competition (AER III - 1997) has only set a
target of 7 MW for Biomass/Waste to Energy for the period 2000-2010. Although the
price cap for bidders was 3.9p/kWh, the unit price is expected to be lower.

3.12   Status of anaerobic digestion of agricultural wastes in the UK and Ireland

3.12.1 UK
About 45 farm-scale digesters have been installed in the UK since 1975. Many of
these digesters were installed with the aid of a capital grant which is no longer
available. Typically, the digesters were sized between 50 and 1000 m3 and generated
gas for on-farm heating only, utilising mainly cattle, pig and poultry manure as
feedstocks, either alone or in combination. A few digesters were fitted with small
combined heat and power (CHP) engines. Many of the farmers also sold some of the
digestate to local householders for use as a fertiliser and soil conditioner (AD-Nett
Conference, 1999).

Of the 45 units installed, only about 25 are currently operating. The primary reasons
for closure of on-farm plants were poor initial design and lack of operator training.
Operational problems included pipe blockages, equipment failure, inability to
maintain mesophilic temperatures during winter months, digester pH instability and
difficulties in maintaining constant organic and hydraulic loading rates. Although the
majority of these operational problems are easy to rectify, given improved on-farm
digester designs, the UK history of poor performance has left the technology with a
bad reputation among the farming community. It should be noted that most of those
farmers who continue to operate digesters now possess a good knowledge of digester
operation, resulting in efficient operation. Very few farm-scale digesters have been
installed in the last few years since the removal of grant funding and unless there is
compelling legislation the situation will remain so (Warburton, 1997).

Recent interest in the UK has focused on larger CAD schemes due to the financial
support available under the Non-Fossil Fuel Obligation (NFFO) programme. To date,
seven centralised anaerobic digesters have received NFFO contracts, one under
NFFO3 in 1995 and six under NFFO4 in 1997, as shown in Table 3.9. These NFFO
projects are mainly based on chicken litter but there is some use of pig slurry, cow
manure and turkey litter. The NFFO rules allow intake of up to 20% (on a dry weight
basis) of food processing waste and it is expected that most projects will take
advantage of this. Although the NFFO contracts are for electricity only, it is likely
that the projects will utilise CHP plants where an appropriate heat use can be

Table 3.9: NFFO contracts for anaerobic digestion1

                         Developer              Capacity (MWe)

               Attwell Farms Ltd                        0.30
               LRZ Ltd                                  1.05
               Agtec Ltd                                1.00
               Agtec Ltd                                2.00
               Agtec Ltd                                0.50
               Agtec Ltd                                0.60
               North Tamar Business LTD                 1.43
               Adapted from AD-NETT Conference (1999)

3.12.2 Ireland
Application of anaerobic digestion at single farm or centralised level in Ireland lags
far behind that of our European neighbours. One of the early on-farm plants in
Ireland was commissioned in the late 1980s at Bethlehem Abbey, Portglenone, Co.
Antrim. The plug-flow digester installed treats a mixture of cattle manure, poultry
litter and silage effluent. The produced biogas is used to heat the monastery and for
grain-drying in season. The digestate is separated to yield a solids fraction which is
composted and marketed as a peat-moss substitute. The liquid fraction of the
digestate is used to provide nutrients for the farm tillage fields and is also sold for
golf-course and municipal parkland application.

On-farm interest in AD application in Ireland is currently increasing. Three on-farm
plants have been recently installed (Patrick Berridge, Co. Wexford; Vicky Heslop,
Co. Waterford and Camphill Community, Co. Kilkenny). Off-farm organic wastes
are accepted by these plants, with or without gate charges, and the biogas produced is
utilised in CHP plants for electricity and heat generation.

3.13    Status of anaerobic digestion of agricultural wastes in other European
Localised production of significant point sources of animal manures from intensive
pig, cattle and poultry production units has led to the development of centralised
(CAD) and commercial on-farm AD-based treatment plants in a number of European
countries. Table 3.10 summarises the current application of AD at centralised and on-
farm level in Denmark, Austria, Sweden, Italy, Germany, the U.K. and Ireland.

3.13.1 CAD plants
The concept of centralised biogas plants has been developed in Denmark since 1987.
Currently, there are >20 operational plants with capacities ranging from 50 to 500
tonnes biomass feedstock per day. Approximately 80% manure, mainly as slurry, is
co-digested with 20% of organic wastes of primarily plant residue and agro-industrial
origin. A few plants co-digest small quantities of sewage sludge or the source
separated organic fraction of MSW. The resulting biogas is mainly used for combined
heat and electricity generation, with the heat generated being used locally for district
heating. The digested biomass is redistributed to a wide range of livestock and tillage
farms as nutritionally defined fertiliser. All of the plants received investment grants,
ranging from 30-40% in the late 1980s to 20% today. The focus is now on further
economic improvements that will enable new plants to be built without public

Table 3.10: Distribution of centralised and commercial on-farm biogas plants in
selected European countries1

            Country       No. of CAD biogas plants2           No. of on-farm biogas

          Denmark                       20                               19
          Austria                       0                               >25
          Sweden                        8                                5
          Italy                          5                               50
          Ireland                       0                                3
          U.K.                          7                                25
          Germany                        3                             >1,600
          Switzerland                    -                             C. 100
        Adapted from AD-NETT Conference, 1999
        These plants do not include the large number of centralised plants that have
      been installed to treat the organic fraction of MSW only or MSW plus sewage
        Data from German Biogas Association Website (www.biogas.org).

Plates 4 and 5 illustrate two of the Danish CAD plants. The Ribe plant (Plate 4) was
commissioned in 1990 and consists of three thermophilic digesters, each of 1,750 m3
volume, with associated influent and effluent storage tanks, reception area, etc. The
Filskov plant (Plate 5) was started up in mid-1995 and consists of a power plant
dedicated to the generation of electricity and district heating for the village of Filskov.
The plant consists of two 500 m3 thermophilic digesters and associated storage
facilities for influent wastes/wastewaters, digestate and biogas. It is of interest to note
that the power plant (which can utilise wood chips as well as biogas) and the district
heating grid were installed first (1992/1993) with the biogas plant being
commissioned subsequently in August 1994, only after a guaranteed use of the
produced biogas had been put in place. The power station and biogas plant are owned
and operated by Filskov Energy Company (a non-profit cooperative consisting of the
farmers who supply the manure and the consumers who are connected to the district
heating grid and/or who utilise the produced electricity).

Sweden (Table 3.10) has also opted for agricultural waste-based CAD plants. Four of
the existing eight CAD plants purify the produced biogas and use it as a vehicle fuel.
There is also growing interest in using the residue from anaerobic digestion for soil
improvement. This requires that digested material must fulfill strict environmental
demands, such as sanitation, and must be free of hazardous residues (Nordberg, AD-
Nett Conference, 1999).

The installation of manure-based CAD plants is also growing in importance in Italy
(Table 3.10). The CAD plant at Perugia is unique in that manure/slurry is pumped to
the plant through a collection pipe network some 56 kilometres in total length. The
Italian plants generally practice digestate solids/liquid separation with compost
production from the separated solids fraction. Because of the current high levels of
nitrate pollution of groundwater in the Po Valley, further treatment
(nitrification/denitrification) of the liquid digestate fraction is required in some of the
Italian CAD plants prior to its irrigation to land or its discharge to receiving

               Plate 4. Aerial view of the Ribe CAD plant in Denmark

       Plate 5. The Filskov CAD plant and associated power plant in Denmark

3.13.2 On-farm commercial AD plants
As illustrated in Table 3.10, Germany is the leading European nation with respect to
construction of on-farm biogas plants. Figure 3.11 illustrates the growth of on-farm
AD plants in Germany between 1992 and 2001. By the end of 2001, there were more
than 1,600 on-farm plants in operation in Germany and the German Farm Biogas
Association suggests that current agricultural waste arisings could potentially support
in the region of 220,000 on-farm plants (AD-Nett Conference, 1999). Plates 6 and 7
illustrate two examples of on-farm biogas plants in Germany. The Pellmeyer digester
(Plate 6) is located on a dairy farm and co-digests food-processing wastes generated
locally. The Beer plant (Plate 7) co-digests pig slurry, grass clippings, vegetable
wastes and other local food waste arisings.

                            1992 1993 19941995 1996 19971998 1999 20002001

Figure 3.11. On-farm biogas plants in Germany (1992-2001).

Plate 6. The biogas plant on the Pellmayer dairy farm in Bavaria, Germany
Plate 7. The biogas plant on the Beer pig fattening farm in Bavaria, Germany

Two of the main factors leading to the installation of on-farm digesters in Germany
have been the development of proven, low-cost digester designs and the availability
of technical advice from the German Farm Biogas Association. Installation of on-
farm plants is actively promoted by the Association and case-histories and site visits
are made available to interested farmers. In the majority of cases, on-farm digesters
accept a variety of non-manure wastes, including distillery slops, food production and
processing residues, grass clippings and, in some instances, sewage sludge and the
organic fraction of MSW. Sale of the electricity generated by CHP plants is widely
practised and the electricity produced is sold at peak times in order to maximise the
financial return. The heat generated is used to heat the digester and for other on-farm
uses. The digestate is rarely separated into fibrous and liquid fractions and is
landspread on the farm or, by arrangement, on adjoining farms. Gate fees for off-farm
feedstocks supplement the income to the farmer. However, recently published
regulations, stringent approval conditions, decreased subsidies and the constant
uncertainty with the law of supplying renewable energy to the grid
(Stromeinspeisegesetz) makes further sustained development of new on-farm plants
uncertain and endangers the current viable operation of existing German plants (AD-
Nett Conference, 1999). Information on the German on-farm plants can be accessed
from the German Farm Biogas Association website (www.biogas.org).

In Austria, more than 25 farm-scale biogas plants are operating, most of them having
CHP-production but with none of them connected to the gas grid. The biogas sector is
growing since 10 of these farm-scale biogas were built in 1997. At present, co-
digestion of manure with other wastes does not take place (AD-Nett Conference,

There are 19 operational on-farm digesters in Denmark, with the majority co-
digesting animal manures and small amounts of off-farm organic waste (AD-Nett
Conference, 1999). Interest in farm-scale plants in Denmark has been growing from
1995, with particular emphasis on large pig farms which have a high consumption of
heat and power. The anaerobic digestion of animal manures has flourished in
Denmark primarily due to investment grants and subsidies and also because biogas, as
a renewable energy source is exempt from Danish state taxes, whereas taxes must be
paid for fossil-derived energy (AD-Nett Conference, 1999).

In Italy, installation of commercial on-farm digesters has also developed in recent
years (Table 3.10), with the majority of plants being sited in pig and poultry
production facilities. In some plants, advantage is taken of the generally high ambient
temperatures and heating of the influent is not practiced. Novel plug-flow digester
designs, with gas storage on top, have been developed in order to minimise initial
construction costs.

There are approximately 100 on-farm digesters in Switzerland treating liquid manure.
Sixty of these are equipped with CHP plants for electricity and heat generation
(Wellinger, AD-Nett Conference, 1999).

The three on-farm plants in Ireland (Table 3.10) are located on beef cattle or dairy
cattle farms and all either currently accept or will, in the future, accept off-farm food-
processing wastes for co-digestion. The current restrictions on landfill disposal of
organic wastes in Ireland is ensuring that co-digestion substrates are available and that
gate charges will enhance the financial viability of these on-farm plants. Plate 8
depicts the on-farm co-digestion plant located at the Camphill Community farm in
Co. Tipperary.

Plate 8. The on-farm digester at the Camphill Community farm in Co. Tipperary.


The current legislative framework within the EU clearly favours the re-use and
recycle of organic waste, while restricting landfill disposal and prohibiting marine
dumping. At the same time, the requirement to limit greenhouse gas emissions under
the Kyoto Protocol is focussing attention on the development of renewable energies,
resulting, in many countries, in the provision of financial incentives for energy from
biomass projects. The development of renewable energy resources in Ireland, over
the past five years, has focussed almost exclusively on wind-to-energy projects. It is
evident that biomass/waste-to-energy projects must be encouraged and supported to a
greater extent if Ireland is to meet the renewable energy targets set out in the Green
Paper on Sustainable Energy and fulfil the legally binding commitments of the Kyoto
Protocol. In other EU countries, the need to develop biomass/waste-to-energy
technologies has resulted in a renewed interest in anaerobic digestion and in the
installation of an increasing number of large CAD plants by individual member states.
Given our very sizeable quantities of animal waste arisings, CAD presents a proven
technology that could significantly contribute to the required increase in energy
generation from renewable resources in Ireland.

Given the mixture of organic feedstocks likely to undergo digestion in CAD plants
and the need to return the digestate to land in order to maximise inorganic nutrient
recycle and enhance soil conditioning, there is an evident need to develop agreed
standards that can be used in the licencing of CAD plants. These standards should
include limits for heavy metals in the digestate; sanitation requirements for individual
feedstocks and for the final digestate; recommended digestate application rates in
order to optimise NPK uptake and prevent over-spreading; hygiene requirements for
compost generated from the solids fraction of the digestate; appropriate safety
standards for CAD plant operation, etc. Both national and international standards
should take into account the balance required to ensure sustainable management and
environmentally-acceptable recycling of organic wastes, without entailing excessive

4.1    National and EU Legislation and Policies that may impact on CAD plant
       installation and operation

4.1.1 Planning control
Given the potential size and scale of CAD plants, planning permission for their
location and construction will be required under the Local Government (Planning
Development) Acts of 1963 to 1993 (and subsequent amendments thereof).

4.1.2 Environmental Impact Assessment
Because of their potential environmental impact, it is likely that CAD plants may
come under the remit of the EU Environmental Impact Directives (85/337/EEC and
93/99/EEC) which require assessment of the effects of public and private projects on
the environment. Waste management facilities dealing with wastes of >1000
Population Equivalents (P.E.) are likely to be subjected to Environmental Impact

4.1.3 Waste Licensing
Licensing of waste activities by the EPA under the Waste Management Act, 1996,
commenced in May 1997. As with IPC licences, waste licences are granted by the
EPA on an integrated basis, with each licence dealing with all environmental
emissions, in addition to regulating the overall environmental management of the
facility. By the end of December, 2001, 187 applications for waste licences had been
received by the EPA and 101 licences had been granted (Personal Communication;

CAD plants in Ireland are likely to require a Waste Licence rather than an IPC licence
from the EPA. Waste licences, in addition to requiring compliance with strict
emission limits, are required to establish and implement an Environmental
Management System (EMS) and to prepare an Annual Environmental Report (AER).
Waste licences for CAD plants are likely to specify acreage requirements for
landspreading of the digestate and set limits for the amounts landspread in line with
the following Codes of Practice:- (i) Control of Farm Pollution (Dept. of Agriculture,
Food and Forestry, 1992) and (ii) Code of Good Agricultural Practice to Protect
Waters from Pollution by Nitrates (Dept. of the Environment and Dept. of
Agriculture, Food and Forestry, 1996); and in accordance with national regulations,
such as the “Use of Sewage Sludge in Agriculture” (S.I. No. 148 of 1998). Small on-
farm or off-farm AD or composting plants are unlikely to require an EPA Waste
Licence but will be subject to Local Authority permitting.

4.1.4 The Nitrate from Agricultural Sources Directive (91/676/EEC)
Nitrate is a highly toxic substance, whose presence in surface and groundwater
potable water sources presents a public health risk, requiring costly treatment of the
source water to meet EU Drinking Water Standards. Landspreading, irrigation and
injection of animal manures may result in increased nitrate levels in surface and
groundwaters. The Nitrate Directive requires member states to specify Codes of
Good Agricultural Practice for management and landspreading of animal manures
and slurries. These codes are required to be implemented by farmers on a voluntary
basis and must specify restrictions on ways and amounts of manures and chemical
fertilisers to be landspread (Scannell, 1995). Member States are also required to
designate zones vulnerable to water pollution and to draw up action plans to reduce
nitrogen leaching. Limits must be placed on the amounts of livestock manures that
may be spread in vulnerable zones (Scannell, 1995).

In order to assist the statutory authorities to meet their responsibility to protect
groundwater, a methodology for the preparation of groundwater protection schemes
has been jointly published by the EPA, the Dept. of the Environment and Local
Government and the Geological Survey of Ireland (Groundwater Protection Schemes,
1999). These schemes will require land surface zoning and groundwater protection
responses. The conditions set for CAD plant siting and operation will be determined
by these guidelines.

Where groundwater is sourced for potable water production, it may also be necessary
to take Directive 80/68/EEC into account (Protection of Groundwaters against Certain
Dangerous Substances). In addition to potentially toxic or environmentally polluting
chemicals (phosphorus, copper, etc.), animal manures and other organic wastes (e.g.
wastes from slaughtering and rendering plants) are likely to contain a wide variety of
animal and human pathogens - bacteria, viruses, protozoa and helminthic parasites
(see section 2.12). Bacterial pathogens and parasites that are infective at very low
dose levels are of particular concern. These include E. coli 0157 (infective dose of
10-50 viable cells) and Cryptosporidium which may cause gastroenteritis at levels as
low as <10 viable oocytes (Ball, 1997). Viral pathogens are also of particular concern
since they may survive for longer periods in soil and groundwater than bacteria and
their presence in potable water sources may not be detected by standard faecal
indicator bacterial test methods (Berg & Metcalf, 1978; Macler, 1995).

4.1.5 EU Directive on Use of Sewage Sludge in Agriculture (86/278/EEC)
The first sewage sludge directive (86/278/EEC) regulated the spreading or injection of
sewage sludge on pasture or tillage lands and was incorporated into Irish Legislation
by the European Communities (Use of Sewage Sludge in Agriculture) Regulation,
1991 (S.I. No. 183 of 1991). The most recent relevant Irish Regulation is the “Use of
Sewage Sludge in Agriculture”, (S.I. No. 148 of 1998). Since sewage sludge is a co-
substrate in a number of existing CAD plants, the provisions of this Directive are of
relevance to the landspreading of CAD plant digestate.

4.1.6   EU Directives governing bathing water quality and freshwater, brackish or
        coastal waters supporting freshwater salmonid and cyprinid species and
        marine shellfish species
Licensing of landspreading of CAD digestate will have to take the above Directives
into account, particularly in areas where a risk to bathing waters or finfish/shellfish
habitats may exist.

The EU Bathing Waters Directive (76/160/EEC) has been implemented in Ireland by
the European Communities (Quality of Bathing Waters) Regulations 1992-1994. This
directive defines imperative and guide values for microbiological, physicochemical
and other substances in bathing waters.

The Freshwater Fish Directive (78/659/EEC) was implemented in Ireland via the
Local Government (Water Pollution) Acts 1977/1990 and the European Communities
(Quality of Salmonid Waters) Regulations, 1988. The purpose of this directive is to
ensure maintenance of the freshwater quality necessary to support either salmonid or
cyprinid fish. More stringent imperative and guide values are defined for 14 physical
and chemical parameters for salmonid than for cyprinid waterbodies. Orthophosphate
is not included in the chemical parameters specified by Directive 78/659/EEC.

The Shellfish Directive (79/923/EEC) was implemented in Ireland via the Local
Government (Water Pollution) Acts 1977/1990; the Fisheries Acts 1979/1990, and the
Quality of Shellfish Waters Regulations 1994. Imperative and guide values for 12
physicochemical and microbiological parameters are defined in the Directive.

4.1.7   EU and national legislation on orthophosphate levels in discharged
        wastewaters and in surface freshwater bodies
As indicated in Section 4.1.6, the Freshwater Fish Directive (78/659/EEC) did not set
guideline or imperative values for orthophosphate levels in salmonid or cyprinid
waters. Guideline levels for phosphorus were, however, set by the Surface Water
Directive (75/440/EEC) for surface waters intended for abstraction for potable water
supply purposes. The Surface Water Directive defined three categories of source
water (A1, A2, A3) based on raw water quality using 46 physical and chemical
parameters. The Directive guideline values for orthophosphate for water categories
A1, A2, A3 were, respectively, 0.4, 0.7 and 0.7 mg phosphate (as P2O5) per litre.

The Drinking Water Directive (80/778/EEC) set guideline (G) and maximum
admissable concentrations (MAC) for orthophosphate in potable water - i.e. 400 µg/l
and 5000 µg/l (as P2O5), respectively. However, the new EU Drinking Water
Directive (98/83/EEC) does not specify either G or MAC values for orthophosphate
in potable water.

The EU Urban Wastewater Directive (91/271/EEC) specifies maximum total
phosphorus concentrations in treated domestic wastewaters discharged to receiving
waterbodies that are considered to be sensitive (i.e. at risk from eutrophication). The
maximum discharge concentrations specified in the Directive are 1 and 2 mgP/litre,
respectively, from sewage treatment works serving population equivalents of 10,000 -
100,000 and > 100,000. The Directive was implemented in Ireland by S.I. No. 254 of
2001 (Urban Wastewater Treatment Regulations).

Stringent standards for phosphorus concentrations in surface waters in Ireland were
set by S.I. No. 258 of 1998 (Local Government (Water Pollution) Act, 1997 (Water
Quality Standards for Phosphorus) Regulations, 1998). S.I. 258/1998 requires that
existing surface water quality be either maintained or improved over a ten-year period
until December 31st, 2007. The existing biological quality rating of rivers and the
trophic status of lakes are defined in S.I. 258/1998 and are based on monitoring data
collected between the 1st of January, 1995 and the 31st of December, 1997. Target
values range from ≤ 5 µgP/litre for ultra-oligotrophic to >20 - ≤50 µgP/litre for
eutrophic lakes. For rivers, the target values range from 15 to 70 µgP/litre based on
the quality class and quality rating (Q index) determined over the 1995-1997
monitoring period. The number and frequency of sampling are also specified in S.I.

Implementation of S.I. 258/1998 can only be achieved by the imposition of strict
maximum phosphorus discharge levels for industrial and domestic wastewater
treatment plants; by control of landspreading of animal manures and sewage sludge in
order to prevent run-off; by reduction in fertiliser application where soil P levels are
already high, and by enforcement of conditions relating to operation of septic tanks
and other small-scale domestic or commercial wastewater treatment systems. S.I.
258/1998 will impact on the operation of proposed CAD plants since it will be
required to demonstrate that landspreading of the digestate (with or without solids
separation) does not result in phosphorus run-off or leaching to surface waters.

4.1.8  EU Directives and national legislation governing quality standards in the air
Atmospheric pollution legislation may be of relevance to the operation of CAD plants
because of (i) potential odour release during transport and holding of organic wastes
(manures, sewage sludge, etc.); (ii) the possible presence of H2S (hydrogen sulphide)
in the biogas produced during digestion; (iii) the release of sulphur dioxide (a
causative agent of acid rain) during burning of the biogas for electricity generation or
space heating, and (iv) potential release to the atmosphere of H2S, ammonia or other
odorous compounds (e.g. butyric acid) from digestate holding tanks or during
subsequent landspreading. Legislation relating to air pollution in Ireland has been
reformed and updated by the Air Pollution Act of 1987 (Scannell, 1995).

Throughout the European Union, national ambient air and emission standards
generally rely on the German TA Luft, USEPA and WHO Air Quality

Guidelines/Standards (Kiely, 1997). A variety of EU Directives (the most recent
being 99/30/EEC) have set limit values for ambient air and emissions standards .

4.1.9 EU Directives relating to the maintenance of biodiversity and ecosystems
Both the location of CAD plants, and the landspreading of CAD digestate within
protected areas, will require environmental assessment in the context of a number of
EU Directives governing biodiversity, protection of endangered species and habitat

EU Directives 79/409/EEC, 85/441/EEC and 91/224/EEC on the conservation of wild
birds have resulted in the designation of twenty Irish Special Protection Areas (SPAs)
under the European Communities Act 1972-1992 (Scannell, 1995). At least 10
further SPAs are currently being designated, with possibly more to follow. Many of
these SPAs had previously been afforded protection as Nature Reserves or refuges
under the 1976 Wildlife Act. Should CAD plants be in receipt of EU grants,
appropriate steps must be taken in order to protect the birdlife in SPAs against
potentially damaging effects of the proposed development.

The Habitats Directive (92/43/EEC) provides for the protection of natural and semi-
natural habitats and of wild flora and fauna (Scannell, 1995; Scannell et al., 1999).
The purpose of this directive is to establish a comprehensive network of Special Areas
of Conservation (SACs) of European significance for rare, endangered and vulnerable
species and habitats throughout the EU. The network will be known as Natura 2000
and will consist of sites of international importance. The process of designating Irish
SACs is currently in train. CAD plants located in, or impacting on, SACs will be
subjected to the EIA process.

The Wildlife (Amendment) Act (2000) sets conditions for development control in
National Heritage Areas (NHAs). NHAs were previously defined as Areas of
Scientific Interest by an Foras Forbartha in 1981 and include outstanding landscape
areas (Inventory of Outstanding Landscapes: an Foras Forbartha, 1977) and
exceptional peatland sites (Peatland Sites of Scientific Interest in Ireland; Wildlife
Advisory Council, 1980). The location of CAD plants, or the landspreading of CAD
digestate, within NHAs is likely to require special evaluation and control under the
amended Wildlife Act.

4.1.10 The Rural Environment Protection Scheme
The preference of the European Community to ensure protection of natural habitats
by agreement with landowners, rather than by resorting to legal regulation, resulted in
the adoption of EU Regulation 2078/92/EEC (Agricultural Production Methods
compatible with requirements of the protection of the environment and the
maintenance of the countryside). Implementation of Regulation 2078/92/EEC in
Ireland is via the Rural Environment Protection Scheme (REPS), administered by the
Dept. of Agriculture, Food and Forestry. The primary aim of this EU Regulation is to
provide an aid scheme for farmers in order to encourage the introduction of farming
practices and production methods which reflect increasing concern for the
conservation of wildlife habitats, conservation of endangered species of flora and
fauna, and protection of the landscape (Hickie, 1997; Scannell et al., 1999).

The agri-environment plans drawn up by approved REPS planners must take a
number of compulsory measures into account, including retention of wildlife habitats,
protection and maintenance of waterbodies, development of grassland management
plans, adoption of waste management, liming and fertilisation plans (Dept. of
Agriculture, Food and Forestry handbook "Rural Environment Protection Scheme:
Agri-Environmental Specifications).

The REPS plans may impose limits on the spreading of CAD plant digestate on
farmlands owned by farmers who are participating in the scheme. These restrictions
will have to be taken into account when granting planning permission for CAD plants
in environmentally sensitive areas.

4.1.11    National legislation on sanitation requirements for CAD plant operation in
          different EU countries
The mixing of animal and human wastes as feedstocks for CAD plants, together with
the potential return of the digestate for landspreading on pasture and tillage lands,
raises the question of risk to public and animal health. Animal manures, sewage
sludge and the organic fraction of MSW may contain a wide variety of pathogenic
bacteria, viruses and parasites. The transport of these feedstocks to CAD plants, their
mixing during digestion, and the return of the mixed CAD digestate for landspreading
on farmland creates new potential pathways for pathogen dissemination by direct
contamination or through the animal and human foodchains.

In countries where CAD plants are already operational, this risk has been addressed
by recent national legislation or codes of practice. Indicator organisms, as in public
health legislation for potable water supplies, have been used to monitor the hygiene
and sanitation standards of CAD plants. E. coli is the faecal indicator of choice in
public health monitoring of potential contamination of water supplies, bathing water,
shellfish production waters etc. However, faecal streptococci (FS) are present in
higher numbers in animal than in human faeces and they also display longer survival
times in holding tanks and greater thermostability during anaerobic digestion than the
majority of other bacteria, viruses and parasites tested to date (Bendixen, 1994, 1999).

Standardised test procedures have been developed in Denmark, using FS as the
indicator organism, in order to monitor the Pathogen Reducing Effect (PRE) achieved
during CAD plant operation. The PRE effect is defined as the reduction in log10 units
of FS numbers/g of waste during feedstock holding, exposure of influent to pre-
sanitation temperatures, anaerobic digestion, post-sanitation procedures etc.
(Bendixen, 1999). The data obtained in these studies led to a Danish Ministerial
Order (1989) requiring pre-sanitation of high-risk feedstocks prior to treatment in a
CAD plant. Pre-sanitation was preferred to post-sanitation, with respect to cost and
energy usage considerations, since only the high-risk feedstocks were deemed to
require special sanitation processing.

The Danish Order specifies a controlled pre-sanitation period of one hour at 70°C for
sewage sludge and the organic fraction of MSW. For other feedstocks (animal
manures, food processing wastes, etc.) a range of alternative operational conditions
(Table 4.1) which would achieve the required sanitation, are defined by the
Ministerial Order (Bendixen, 1999). These conditions include minimal guaranteed
retention times for the feedstocks within mesophilic and thermophilic digesters or in
pre-sanitation tanks at temperatures ranging from 52°C to 65°C (Table 4.1). Regular
analysis of FS levels in incoming feedstocks, holding tanks, sanitation tanks and in
digesters is required in order to monitor hygienic and sanitation standards and to
detect operational problems, such as short-circuiting of raw wastes during plant
Table 4.1: Sanitation equivalent to one hour at 70°C as required by Danish
Notification No. 823 (Ministry for Energy and Environmental Protection)1

    Temperature     MGRT2 in a thermophilic            MGRT in a separate sanitation tank
                       digestion tank3

                                                       Before or after        Before or after
                                                        digestion in a         digestion in a
                                                        thermophilic            mesophilic
                                                       digestion tank3        digestion tank4

      52.0°C                    10 h                           -                      -
      53.5°C                    8h                             -                      -
      55.0°C                     6h                          5.5 h                  7.5 h
      60.0°C                      -                          2.5 h                  3.5 h
      65.0°C                      -                          1.0 h                  1.5 h

  Adapted from Bendixen (1999)
  MGRT is the minimum guaranteed retention time of feedstock in the digestion tank.
  Thermophilic digestion is defined as 52°C or greater. The HRT must be at least 7 days
  Mesophilic digestion between 20°C and 52°C. The hydraulic retention time (HRT) must be at least 14

The Danish Order, in addition to requiring an FS log10 reduction of 4 units for biogas
plants co-digesting high risk wastes, specifies that the final digestate must contain less
than 100 FS per gram and that Salmonella species must be absent from a specified
number of routinely tested 25 g samples (Bendixen, 1999).

In Germany and Austria, a pre-sanitation treatment at 70°C for one hour for
mesophilic AD plants or for 0.5 h for thermophilic plants is recommended (Table
4.2). In Germany, validation procedures, developed initially for composted organic
wastes, have been adopted in the recent "Biowastes Ordinance". These procedures
involve the use of Salmonella senftenberg W775 (H2S negative) as test organism in
specially-designed test carrier systems. The standards required for AD plants treating
high risk feedstocks are:- (i) absence of Salmonella sp. in 50 g samples, and (ii) less
than two germinative tomato seeds and reproducible plant parts in one litre of
digestate (Bohm et al., 1999; Amon & Boxberger, 1999).

Although other countries within the EU have not, so far, set hygiene or sanitation
requirements for CAD plant digestates, the increasing application of CAD for organic
waste management, re-use and recycle will undoubtedly lead to further national and
EU legislative control with respect to animal and human health.

Table 4.2: Sanitation requirements for risk substances in biogas plants in Austria1

                                  Heating before anaerobic     Sanitation during
                                         digestion            anaerobic digestion

Digester Type                      Temperature       Time    Temperature     Time
                                      (°C)            (h)       (°C)          (h)

Mesophilic biogas plants                   70         1           -            -
20 - 40°C

Thermophilic biogas plants                 70        0.5        55°C          24
52°C or greater
    Adapted from Amon & Boxberger (1999)

4.1.12 Proposed EU Legislation on biological treatment of “Biowaste”.
The EC working document on Biological Treatment of Biowaste (2nd draft; Feb.
2001; DG ENV.A.2/LM/biowaste) defines biowaste as “any waste that is capable of
undergoing anaerobic or aerobic decomposition, such as food and garden waste,
paper and paperboard.” Proposed standards are indicated in the working document
for home, community and collective composting and for anerobic digestion of
separated biowaste. These standards include sanitation requirements (chosen
indicator organism is Salmonella senftenberg W775 (H2S negative)) and also specify
heavy metal limits. With respect to anaerobic digestion, it is recommended that a
minimum temperature of 55°C is maintained in thermophilic digestors over a period
of 24 hours and that the hydraulic retention time (HRT) of the waste in the reactor is
at least 20 days. For digesters operated under mesophilic conditions or at shorter
HRTs, it is recommended that the influent biowaste be pre-treated, or the digestate
post-treated, at 70°C for one hour.

The working document proposes that the compost and anaerobic digestate are deemed
to be adequately sanitised only if they comply with the following suggested

*Salmonella spp. – absent in 50g of compost/digestate
*Clostridium perfringens – absent in 1 g of compost/digestate

[* - Under Review]

4.1.13 The EU Directive on the Landfill of Waste (99/31/EEC)
The EU directive on the landfill of waste sets targets for reduction of the
biodegradable fraction of MSW going to landfill (i.e. a reduction, within 15 years, to
35% of the total amount, by weight, of biodegradable MSW produced in 1995).
Member States are obliged to prepare national strategies to implement this reduction,
including measures to achieve the specified targets by means of “recycling,
composting, biogas production or materials/energy recovery”.

A number of Member States have set limits on the use of landfills for organic waste
disposal. In Sweden, the disposal of nutrient-rich, wet organic waste in landfills is
subject to a special tax from 2000 onwards and a total ban on landfilling of organic
waste will be introduced in 2005. Austria has recently prohibited the landfilling of
organic waste above 5% of the total waste landfilled (Amon & Boxberger, 1999). In
Ireland, the Department of the Environment and Local Government policy statement
of 1998 – “Waste Management: Changing our Ways” set targets for reduction on
reliance on landfill. While emphasising the preferred options of waste minimisation
and re-use, the Policy Statement specifies (i) a diversion of 50% of overall household
wastes from landfill; (ii) a minimum of 65% reduction in biodegradable waste
(biowaste) consigned to landfill. The proposed targets were given a fifteen year
timescale from 1998.

4.1.14 Proposed EU Legislation on Animal By-products
In the area of Food Safety and Health, one particular European Community initiative
has especial relevance to the biological treatment of biodegradable waste. This is the
Proposed EU Regulation of the European Parliament and of the Council laying down
health rules concerning animal by-products not intended for human consumption. The
Proposed Regulation is presently under negotiation and the current text seeks to
include stringent controls on the management of animal by-products that is destined
for beneficial re-use within its scope. The scope of animal by-products currently
includes “catering waste”, the definition of which effectively includes all food waste
originating from household and commercial kitchens. The full text of the Common
Position on the Proposed Animal By-products Regulation as adopted by the Council
on 20th November 2001 can be downloaded from website address:

4.2     Reduction of Greenhouse Gas Emissions and Promotion of Renewable
The requirement to reduce greenhouse gas emissions is one of the primary factors
underlying the recent upsurge of interest in anaerobic digestion technology in EU
Member States. Since combustion of fossil fuels represents the greatest source of
CO2 emissions, strategies to counteract global warming must promote development of
renewable energies (biomass, wind, wave, solar, hydro, etc.). Anaerobic digestion of
single or combined organic waste feedstocks provides a mechanism for the production
of methane from biomass (organic waste or purpose-grown biomass), thereby
generating a renewable form of energy and resulting in a net decrease in CO2

Ireland is a signatory to a variety of international agreements designed to offset
climate change and promote sustainability. These include the Montreal Protocol
(1987, revised in 1990), the Rio Declaration (1992) and the associated Agenda 21,
and the U.N. Framework Convention on Climate Change (1992). Ireland's
committment to the development of renewable energy resources and the reduction of
greenhouse gas emissions has been highlighted in national policy documents, such as
"Sustainable Development: A Strategy for Ireland", published in April, 1997; the
"Statement of Strategy" published by the Department of Public Enterprise in April
1998, and the "Green Paper on Sustainable Energy" published by the Department of
Public Enterprise (September, 1999).

The U.N. Framework Convention on Climate Change (UNFCCC) set a primary
objective of reducing global greenhouse gas emissions to 1990 levels by the year
2000. This Convention took into account the requirements of under-developed
countries by demanding greater reductions in CO2 emissions from developed
countries. Ireland, as a cohesion country, was expected to contribute less to CO2
emission reductions than the majority of the then EU Member States. The target set
for Ireland was to limit its CO2 emissions in the year 2000 to 20% above the emission
rate of 1990.

The Gothenburg Protocol sets emission ceilings for 2010 for four pollutants including
ammonia. The ceilings were negotiated on the basis of scientific assessments of
pollution effects and abatement options and once the Protocol is fully implemented,
Europe’s ammonia emissions should be cut by 17% compared to 1990 levels. Farmers
will have to take specific measures to control ammonia emissions. It has been
estimated that once the Protocol is implemented, the area in Europe with excessive
levels of acidification will shrink from 93 million hectares in 1990 to 15 million
hectares in 2010 while those with excessive levels of eutrophication will fall from 165
million hectares in 1990 to 108 million hectares in 2010.

The 1997 Kyoto Protocol to the UNFCCC set legally binding targets for developed
countries for the period (2008-2012). The Kyoto Protocol includes commitments to a
reduction in hydrofluorocarbon (HFC), perfluorocarbon (PFC) and sulphur
hexafluoride (SF6) gases, in addition to the three gases covered by the 1992 UNFCCC
agreement (i.e. CO2, CH4 and N2O). Under the Kyoto Protocol, the EU agreed to
reduce its emissions of greenhouse gases by 8% below 1990 levels by the period
2008-2012 (for the three new gases, HFCs, PFCs and SF6, the reference year is 1995).
As part of the internal EU burden-sharing arrangements under the joint fulfillment
provisions of the Protocol, Ireland agreed to limit its increase in emissions of the six
greenhouse gases to 13% above 1990 (CO2, CH4, N2O) and 1995 (HFCs, PFCs, SF6)
levels by the same period (2008-2012).

Within the EU as a whole, energy use and production is by far the most important
source of greenhouse gas emissions, representing 80% of total 1990 emissions (Green
Paper on Sustainable Energy, 1999). Table 4.3 summarises the actual and projected
Irish greenhouse gas emissions for selected years between 1990 and 2010. The limit
emissions set for Ireland for the period 2008 - 2012 has, in fact, been reached in the
year 2000. Unless significant interventions to reduce greenhouse gas emissions are
put in train (i.e. if we continue on a business as usual basis), Ireland will exceed its
1990 emission levels by 25% by the year 2010 (compared to an agreed increase limit
of 13% under the EU burden-sharing agreement). The projected emissions of CFCs,
PFCs and SF6 in the year 2010 also show a more than 3-fold increase over 1995
emission levels (Table 4.3).

Table 4.3: Actual and Projected Irish Greenhouse Gas Emissions (1990-2010)

                      Emissions (x Million Tonnes of CO2 Equivalents)

              CO2          CH4         N2O            HFCs, PFCs              Total3
                                                        & SF6

    19901   30.719       17.038        9.105             0.046                56.907

    19951   34.116       17.099        8.110             0.256                58.511

    19981   39.107       16.398        7.981             0.685                62.519

    20002   41.439       17.425        8.227             0.971                65.642

    20052   45.581       17.516        7.728             1.125                68.481

    20102   49.350       17.594        7.638             1.279                71.331
 Actual emissions; 2projected emissions; 3total values with forestry CO2 sinks taken into account
(Adapted from the 1999 Green Paper on Sustainable Energy).

Energy-related CO2 emissions account for the bulk of the projected increase in
greenhouse gas emissions. By the year 2010, it is projected that the production,
transmission, supply and end-use of energy will contribute 18.2 million tonnes more
of CO2 to annual greenhouse gas emissions by comparison with the 1990 CO2
emission levels (i.e. a 62.7% increase in energy-related CO2 emissions). The
projected data also indicate that energy-related CO2 emissions will account for 66%
of the 2010 emissions compared to 51% in 1990.

It is evident, therefore, that the Kyoto Protocol imposes extremely challenging targets
for greenhouse gas emission abatement on Ireland by the target period of 2008-2012,
particularly in the context of a period of extremely rapid economic growth rate.
Based on advice presented in reports commissioned by the Government (i.e. the ERM
Report (1998), the London School of Economics Report (1999) and the 1992 ESRI
Report), the Green Paper on Sustainable Energy (1999) sets out a framework of
policies and measures designed to reduce energy-related CO2 emissions in the context
of an overall National Abatement Strategy.

Overall abatement strategies include energy consumption reduction; switching fuels
for electricity generation, especially from coal and peat to natural gas; implementation
of new standards of insulation, heating, ventilation and lighting systems in new
buildings and encouragement of improved systems in existing building stock;
promotion of renewable energies; imposition of carbon taxes; implementation of
vehicle efficiency standards and promotion of cheap public transport, etc. - in addition
to proposed EU burden-sharing agreements. It has been projected that Ireland can
expect a massive bill of up to 0.5 billion dollars by the year 2012 for the purchase of
carbon dioxide credits (Kiely, 1997).

Positive relevant measures taken by Ireland to reduce greenhouse gas emissions and
to promote renewable energies include the following:-

 (i) The establishment of the Irish Energy Centre (IEC), a joint initiative of the Dept.
     of Public Enterprise and Enterprise Ireland. The priorities of the IEC include (a)
     support for energy auditing; (b) support for investment in energy efficient
     technology and systems; (c) technical advice in relation to energy supply and
     use, and (d) information campaigns and back-up measures.

(ii) The launching of the Alternative Energy Requirement (AER) scheme in 1995
     and its subsequent renewal.

(iii) Irish participation in EU energy programmes, such as JOULE, THERMIE,
      SAVE and ALTENER.

In 1995, renewable energy represented 5.7% of total electricity generation capacity in
Ireland (representing only 1.7% of Total Primary Energy Requirement (TPER) in that
year). Assuming a "business as usual" scenario, renewable energy would only
account for 2% of TPER in Ireland by the year 2010. This contrasts with the target of
12% of TPER and 23.5% of electricity generation from renewable sources proposed
in the European Commission's White Paper, "Energy for the future: Renewable
sources of energy". Within the European Union as a whole, electricity generation
from large hydro-power plants is projected to provide greater than 50% of the
electricity generated from renewable sources by the year 2010. However, since the
potential for new large hydro-power plants in Ireland is limited, no significant
increase in electricity generation from this source can be expected.

Consequently, increased use of renewable energy in Ireland must focus on wind,
wave and biomass resources. This is reflected in the targets set in the AER
competitions, as illustrated in Table 4.4. The target set by the four AER competitions
was an additional 190 MW by the end of 1999, of which 63% and 27%, respectively,
were to be sourced from wind and biomass/waste. To date, twenty-two AER projects
have been completed with a total installed capacity of 73.4 MW and the winning
projects in AER III and IV are at various stages of completion.

The Green Paper on Sustainable Energy (1999) proposes a target of installed
electricity generating capacity of 500 MWe from renewable sources in the period
2000-2005.It is proposed to achieve this target by a combination of AER support,
direct sales and successful EU Fifth Framework projects. However, it is evident that
a greater focus on biomass/waste will be required in order to meet the targets set for
the contribution of renewable energy resources to Ireland's energy requirements in

Throughout the EU, renewable energy development is being promoted by a variety of
positive measures, including ecotaxes on fossil fuel usage; subsidies and tax
exemptions for "green" electricity and combined heat and power (CHP) projects, and
decentralisation and privatisation of national electricity and gas supply systems. A
number of Member States have given a particular focus to anaerobic digestion. An
Energy Action Plan, "Energy 21" was developed in Denmark in 1996, with medium
and long-term scenarios to 2005, 2020 and 2030 (Tafdrup, 1997). This plan
anticipates a doubling of biogas production between 1996 and the year 2000 and a
further doubling again by the year 2005. It is anticipated that the bulk of the biogas
produced will be used for electricity generation in CHP plants because of the very
favourable subsidy paid for electricity generated from renewable energy (0.26

Table 4.4: Targets set for renewable energies and CHP installations in AER
Competitions in Ireland (1995-1999)1

                                         Target MWe (Installed Capacity)

                             AER I       AER II       AER III     AER IV     Total

   Biomass/Waste                15          30                7      0          52
   Hydro                        10            0               3      0          13
   Wind                         30            0          90          0        120
   Wave                          0            0               5      0           5
   Total R.E.                   55          30          105          0        190
   CHP                          20            0               0     35          55
   Total                        75          30          105         35        245
   Adapted from the 1999 Green Paper on Sustainable Energy.

In Sweden, the use of biogas as a vehicle fuel is being promoted in order to reduce
transport emissions of carbon dioxide and sulphur- and nitrogen-oxides. The focus on
utilising biogas as a vehicle fuel entails the construction of large CAD plants since the
cost of upgrading biogas to vehicle standards would be too expensive for small on-
farm or sewage treatment works.

In the UK, the Non Fossil Fuel Obligation (NFFO) programme provides the main
impetus for renewable energy projects. NFFO is a competitive process with
successful projects being awarded a contract to supply electricity on favourable terms
for a fixed period of 15 years. The most recent NFFO contracts for biogas plants
guarantee a price of 5p/kWh for 15 years (increasing with inflation). This compares
very favourably with an annual average price of 2.4p/kWh for electricity sold to the
UK national grid.


5.1     Quantities of organic waste arisings annually in Ireland.
Total annual organic waste arisings suitable for anerobic digestion were derived from
a variety of national reports, such as the 1998 National Waste Database Report (EPA,
2000); the State of the Environment Report (EPA, 1996); Ireland's Environment: a
Millennium Report (EPA, 2000); the Fehily, Timoney Inventory of Non-Hazardous
Sludges in Ireland (1998), and the Strategy Study on Options for the Treatment and
Disposal of Sewage Sludge in Ireland (Weston-FTA Ltd., 1993).

5.1.1 Agricultural wastes
Animal manures and slurries (Table 5.1) present major point sources of organic waste
in many countries due to changes in animal husbandry and intensification of cattle,
pig and poultry production (Colleran, 1992). The total amount of animal manure and
slurry arisings from intensive livestock production units and from over-wintering of
cattle, sheep and horses in Ireland in 1998 was estimated to be 43.28 million tonnes
wet weight (1998 National Waste Database Report, EPA 2000). The estimate of total
annual arisings was based on 1998 livestock numbers (CSO, 1998) and on average
winter housing periods of 20, 6 and 26 weeks, respectively, for cattle, sheep and
horses. For pigs and poultry, it was assumed that all of the slurry and litter produced
is from intensive units, thereby requiring appropriate management. Manure from the
overwintering of cattle accounted for 86% of the manure and slurry arisings. As
indicated in Table 5.1, silage effluent and “dirty water” from dairy farms contribute
significantly to the total annual agricultural waste arisings. Silage effluent presents a
potential seasonal feedstock for CAD plants. “Dirty water” is not regarded as a viable
substrate for CAD plants because of its low BOD/COD content, its large volume,
problems of collection and prohibitive transport costs.

5.1.2 Urban wastewater sludges
The total urban wastewater sludge arisings in 1998 from agglomerations with
population equivalents of greater than, or equal to, 1000 were estimated to be
approximately 493,011 tonnes wet weight, corresponding to approximately 37,577
tonnes dry solids (National Waste Database: Report 1998). Significant amounts of
sewage sludge are also generated from small-scale sewage treatment plants and from
septic tanks. The annual contribution of sewage sludge from the small-scale sector
was estimated by Fehily, Timony (1998) to be approximately 12,675 tonnes wet
weight (507 tonnes dry solids). Consequently, the total estimated sewage sludge
generation in Ireland in 1998 was approximately 505,686 tonnes wet weight. This is
equivalent to only 1.2% (on a wet weight basis) of the estimated animal waste arisings
in 1998 from intensive livestock production and over-wintering.

5.1.3   Biological sludge arisings from food and other industrial wastewater treatment
Industries that biologically treat their wastewater on-site generate sizeable quantities
of waste sludge. Table 5.2 summarises the quantity of sludge arisings (tonnes dry
solids) reported in the Inventory of Non-Hazardous Sludges in Ireland (Fehily,
Timony Report, 1998). The 1998 National Waste Database Report quantified these
sludge arisings in wet weight terms (Table 5.1). The percentage dry solids of these
sludges is extremely variable due to different degrees of dewatering on-site.
Consequently, it is not possible to make valid comparisons between the Fehily,
Timony and EPA data (Tables 5.2 and 5.3).

Table 5.1. Total estimated agricultural waste arisings in Ireland in 19981

                    Waste category                              Quantities arising
                                                          tonnes/annum2             %
                    Cattle manure and slurry                37,098,470             57.4
                    Sheep manure                               338,063              0.5
                    Horse manure                               365,310              0.6
                    Pig manure and slurry                    2,623,350              4.1
                    Poultry manure                           1,847,531              2.9
                    Silage effluent                          2,684,500              4.2
                    Dirty water (dairy only)                19,578,724             30.4
                    Total                                   64,578,724           100.0
                 Adapted from 1998 National Waste Database Report (EPA, 2000)
                 Tonnes wet weight

Table 5.2 Summary of reported annual biological sludge arisings from industrial
sources in Ireland1

                                       Sludge type                          Tonnes (Dry Weight)
                    Industrial biological wastewater sludges                        57,446
                    Food industry sludges                                           88,851
                    Abattoir sludges (excluding offal)                              19,369
                    Total                                                          165,666
                 Inventory of Non-Hazardous Sludges in Ireland (Fehily, Timony, 1998)

According to the 1998 National Waste Database, the food and beverage sector
contributed 94% of the total industrial biological treatment sludge arisings (wet
weight basis) in Ireland in 1998 (Table 5.3). The chemical/pharmaceutical sector
contributed 3%, with other industrial treatment plants making up the balance of 3%.
Table 5.3 also summarises the current disposal/recycle routes for industrial biological
treatment sludges.

Table 5.3 EPA estimate of biological sludge arisings from industrial wastewater
treatment (tonnes wet weight) and sludge disposal/recovery routes1

        Industry               Total     Landfill disposal      Landspreading             Other/
                             668,485             7,948              614,334                 46,013
    Food and
                              20,036           13,441                  5,056                 1,539
                              19,549           10,699                  8,772                    78
    Other industries
                             708,070           32,088               628,162                 47,630
 1998 National Waste Database Report (EPA, 2000)
5.1.4    High strength organic wastes and wastewaters from the food processing,
         beverage and other industrial sectors
Section 5.1.3 summarises the organic sludge arisings from on-site treatment of
industrial wastewaters. This section does not include data on raw organic waste and
wastewater arisings in the industrial and food-processing sector. Currently, there is
little published information on the quantities and geographical location of these waste
and wastewater arisings in Ireland. In countries practising CAD technology, food-
processing wastes and wastewaters from abattoirs, canneries, breweries, fish-
processing and edible oil production plants are regarded as valuable co-digestion
substrates because of their high biogas production potential. The availability of these
raw wastes/wastewaters can greatly boost the daily CAD plant biogas productivity,
resulting in increased heat/electricity generation, with consequent improvement in
economic viability.

5.1.5 The organic fraction of MSW
The source-separated organic fraction of MSW (OFMSW) is currently co-digested
with animal manures in a number of EU CAD plants. Although Ireland lags far
behind many of its partner EU countries with respect to source separation of MSW,
the potential for co-digestion of source-separated OFMSW in CAD plants is
significant for the future. The 1998 National Waste Database Report (EPA, 2000)
estimated that 1,220,856 tonnes of household waste, 754,797 tonnes of commercial
waste and 80,999 tonnes of street cleaning waste were generated in Ireland in 1998.
Analysis of the composition of household and commercial waste arisings by local
authorities indicated an organic content of 32.9% and 15.1%, respectively.
Consequently, the organic fraction of household and commercial waste in Ireland in
1998 amounted, respectively, to approximately 0.4 million and 114,005 tonnes in
1998. The 1998 National Waste Database report also highlighted the fact that some
commercial sectors were not represented in the local authority surveys. Most notable
of these omissions is the hotel and catering sector, which tends to produce wastes with
a high content of digestible organic matter. Despite the deficiencies in data
collection, it is evident that source separation of household and commercial wastes in
Ireland could generate considerably more than 0.4 million tonnes of putrescible
organic waste per annum in Ireland. The local availability of this highly-digestible
waste would provide a valuable co-substrate for CAD plants, resulting in their
improved commercial viability in defined urban and rural geographical areas.

5.2     County by county analysis of organic waste arisings and identification of
        potentially suitable catchment areas for CAD plant location
Databases of national organic waste arisings quantify the total potential substrates
suitable for anaerobic digestion. However, such databases give little information on
the geographical location of waste arisings with respect to catchment areas that might
be suitable for CAD plant location. Consequently, a county by county survey was
carried out in order to pinpoint suitable catchment areas. This was primarily based on
written enquiries to local authorities, followed by visits by the project team.
Information was also obtained from a variety of other sources:-

•     EPA Integrated Pollution Control (IPC) applications and reports from food-
      processing industries, large dairy plants, slaughterhouse and meat-rendering
      companies, breweries, etc.
•     Department of Agriculture information on farm type, size, density and location.

•   Teagasc reports, such as the Teagasc Manure Guidelines with Special Reference
    to Intensive Agricultural Enterprises
•   Ordinance Survey of Ireland
•   Geological Survey of Ireland reports, including Groundwater Protection in
    Ireland: A Scheme for the Future, 1995 and Landspreading of Organic Wastes
    and Groundwater Protection, 1998.
•   Central Statistics Office reports including Farm Structures Survey, June 1995 and
    Crops and Livestock Survey, June 1997.
•   The Irish Farmers Association

The purpose in carrying out the county by county survey was to identify:-
1. The type and quantity of waste arisings within each county.
2. The presence of large point sources of suitable wastes, such as the arisings from
   pig and poultry production plants.
3. The location of sensitive catchments.
4. The average county farm size and density.
5. The availability of spreadlands for treated wastes.
6. The interest of local authorities in centralised anaerobic digestion as determined
   by response and feedback from questionnaires and visits.

5.3     Weighting system for catchment selection
The criteria and weighting system applied in the survey was adapted from “The New
Rational Manager” (Kepner and Tregoe, 1981), which details a system known as
“Decision Analysis”. The purpose of Decision Analysis is to identify what needs to be
done, develop the specific criteria for its accomplishment, evaluate the available
alternatives relative to those criteria, and identify the risks involved.

The objectives required from the first phase of the project were defined and weighted
according to their importance for the successful selection of suitable sites for CAD.
The level of interest and feedback gained from individual local authorities was
deemed to be a significant factor, as full cooperation was required for the successful
completion of phase 2 of the project. The quantities of biosolids available for CAD
were calculated from the total arisings on a county by county basis (Fehily, Timony
Report, 1998). The location of sensitive catchments and the availability of
spreadlands (taking into account REPS uptake, NHA, SAC designations etc.) within
each county were also estimated. Other factors which were regarded as significant
were the farm size distribution and the track record of the particular local authority in
the application of anaerobic digestion technology.

Weighting Criteria

Interest/Feedback: The level of interest or feedback shown by the local authorities
was ranked on the basis of information received on sludge arisings and expressed
interest in anaerobic digestion technology.

Available Biosolids: The quantities of agricultural wastes and industrial (primarily
food processing and slaughterhouse) wastes within each county were obtained from
the Fehily, Timony Report (1998) and ranked according to total amounts.

Number of potential sites: This refers to the number of prime sites within each county,
(20km X 20km area) where there are substantial quantities of both agricultural and
industrial wastes.
Sensitive catchments: The sensitivity of a catchment site was based on EPA reports
on the quality of rivers and lakes in Ireland and also on GSI reports on the level of
pollution of groundwater. The presence of NHAs and SACs in certain areas was also
used as an index of the sensitivity of a catchment area.

Availability of spreadlands: The availability of land for the spreading of digestate
from the CAD plant was analysed. The presence of tillage farms within the locality
was an added bonus, as crop production requires larger quantities of manure by
comparison with the dairying and beef farming sectors. The level of uptake of the
REPS scheme within each county was seen as a negative factor for available
spreadlands due to restrictions on nitrogen and phosphorus application levels.

Farm Sizes: The density of larger farms to smaller farms within a county was
investigated and the presence of large-scale pig and poultry farms was favoured.

Track Record: If a particular county or local authority already had experience with
anaerobic digestion technology, whether it be for sewage sludge treatment or for any
other area of waste management, this was taken as being a positive factor and given a
higher score.

5.4    Results of National Survey
Table 5.4 summarises the data obtained from the survey and illustrates the weighting
grade scores of individual local authority areas. The grades illustrated in Table 5.4
and in Figure 5.2 are as follows:- A = >700; B = 600-700; C = 500-600; D = 400-
500, and E = <400. As indicated earlier, the weightings were based on the “Decision
Analysis” system developed by Kepner and Tregoe (1981).

A more detailed breakdown of the quantity and type of organic waste arisings on a
county by county basis is presented in Table 5.5. Figures 5.1 and 5.2 geographically
illustrate the total quantities of biosolids and the weighting scores for each county,

5.5     Catchment selection - analysis of potential CAD plant locations in six
        selected counties
On the basis of data obtained from the county by county survey and from other
sources, six counties were selected for further study. The survey weightings for the
selected counties were as follows:- 785 (Cork); 725 (Limerick); 700 (Monaghan); 685
(Kilkenny); 640 (Meath) and 630 (Cavan).

The six counties selected were further analysed using the following criteria:

1.    Nature and location of waste arisings
2.    Infrastructure
3.    Sensitive catchments
4.    Interest/alternatives

Table 5.4: Summary of evaluation of potential sites for CAD on a national basis.

County             Interest/         Available        Potential      Sensitive    Availability of   Track    Farm size Weighting
                   Feedback       Biosolids (tdsa)*      sites         areas       spreadlands      record              grade**
Carlow                 none           103119             low            low           good           poor     v. large     E
Cavan                 v. good         147995           medium          high            low           poor      small       B
Clare                   fair          159919             low           high            low            fair    medium       C
Cork                 excellent        629440             high        medium           good           good      large       A
Dublin                  fair           48452             low            low            low           poor      large       E
Donegal                good           100062           medium        medium          medium          good      small       C
Galway                  fair          228531             low           high            low            fair     small       C
Kerry                  good           200689           medium          high            low           good     medium       B
Kildare                none            86109             low            low            high           fair    v. large     E
Kilkenny             excellent        184986             low            low            high          good     v. large     B
Laois                  none           116197             low            low          medium          poor      large       E
Leitrim                none            45368             low           high            low           poor      small       E
Limerick             excellent        223567           medium          high          medium           fair    medium       A
Longford               good            64902             low            low          medium          poor     medium       C
Louth                  good            54083             low            low            high          good     medium       C
Mayo                    fair          161944             low           high            low           poor     v. small     D
Meath                  good           177814           medium        medium            high          poor      large       B
Monaghan             excellent        167315           medium          high            low           poor     v.small      A
Offaly                 none           113697             low            low          medium          good      large       E
Roscommon              good           125499           medium           low            low           poor      small       C
Sligo                   fair           66507             low           high          medium          poor      small       D
Tipperary (N)          none           165932           medium        medium          medium          good      large       E
Tipperary (S)          none           181407           medium        medium          medium          good      large       E
Waterford               fair          142231           medium           low            high           fair    v. large     C
Westmeath              good           108122             low           high            low           poor     medium       C
Wexford                none           148565           medium           low            high          good     v. large     E
Wicklow                none            68192             low            low            high          good     v. large     E
*tdsa - tonnes dry solids per annum; ** See section 5.3 for explanation of Weighting System.

Table 5.5: National biosolids arisings summarised on a county by county basis (Tonnes Dry Solids per annum- tdsa).

County            Biol. Ind.   Sewage         Food      Slaughtering         Cattle     Pig       Poultry            Poultry    Total
                               sludge       industry       waste            manure    slurry      (Litter)           (Slurry)
Carlow                 0          897        46080          5804             49027      735         147                 429     103119
Cavan                 13          859         1032           155            114488    24549         6899               1119     147995
Clare                380         1874           0             0             156299     1190         138                 183     160064
Cork                53432        2010        15903         18898            507093    23932        3184               6192      630644
Donegal               0          2189          26           3802             90478     2832         162                 658     100147
Dublin               27         15561           0           7528             23850      939         331                 243      48452
Galway                 0          801           50            0             224733     1193         1237               1119     229133
Kerry                200          368         1750          1581            190012     6199          350                482     200942
Kildare               10         1931          312          7396             74186     3309          54                1100      88298
Kilkenny              84          849        14125          8656            154683     6185         122                 282     184986
Laois                 0          715            0           2822            106122     5980          57                 501     116197
Leitrim             1823         217            0             0              41528     1446         294                  60      45368
Limerick               0          879         1336          3824            202016     2217        11765               1879     223916
Longford             240          309           0             0              58011     5787         478                  77      64902
Louth                 0          341          4566            0              44998     2010         427                2088      54430
Mayo                  0           840           0             0             155054     2499        3110                 441     161944
Meath                  0         1199            0          8139            159056     5607         2793               1020     177814
Monaghan             162          603          788          3416             91629     4526        51321              15094     167539
Offaly                 0          814            0           752            102813     7544           86               1905     113914
Roscommon             0          335            0          16601            106793     1500          43                227      125499
Sligo                 0          184            2             0             65263      913           32                113       66507
Tipperary (N)         80          779        25123         19387            138437     4641           55                189     166081
Tipperary (S)        459          398           90          5176            164042    10987         190                 132     181474
Waterford              0          121            1          4977            119791     8538         8348                455     142231
Westmeath             0           966           0             0              98633     8277          59                 157     108122
Wexford               20         1652          277          4253            134503     6211         411                1388     148715
Wicklow              514          599           0            180             62838     1682        1163                1320      68296

Figure 5.1 :   Total Quantities of Biosolids Arisings in Ireland in Tonnes Dry
               Solids per Annum (Based on “Inventory of Non-hazardous Sludges
               in Ireland”, Fehily, Timoney Report, 1998).

Figure 5.2 : GIS Representation of the weighting of counties for potential sites
            for CAD.

The aim of this stage of the study was to attempt, on a preliminary basis, to locate
potential sites within these counties (of about 20km2 or 12.5 miles2) where a CAD
plant could be located. The primary parameter of importance at this stage was the
location of large point sources of waste arisings, such as pig or poultry production,
food processing plants, slaughterhouses etc. within the six counties selected. The
proximity of these sources to each other was of particular importance due to the
requirement to minimise influent and effluent transportation costs in order to ensure
the energy and financial sustainability of any proposed CAD plant. The likelihood of
local large waste production units to function as end users of the generated biogas for
heat, steam or heat/power generation was also a factor taken into consideration.

Data on the intensity and type of farm activity (CSO, 1995, 1997; Dept. of
Agriculture, Food and Forestry, 1998) within the counties was collected, as was
information on the infrastructure in terms of road networks and the potential
availability of spreadlands (Dept. of Arts, Culture and the Gaeltacht, 1997; Dept. of
Agriculture, Food and Forestry, 1998 and Nugent, 1999). All selected counties have
varying levels of cattle slurry arisings which are potentially available as feedstock for
proposed CAD plants. Data from surveys on national water quality (EPA, 1996a, b;
1997, 1999b) and sewage outflow locations were used to identify sensitive
catchments in each of the six counties.

The locations chosen for more detailed study in each of the six counties are presented

1. Pallasgreen/Dromkeen/Doon (East Limerick):
The location has three poultry producers and one food processing plant in Dromkeen
in very close proximity to each other. The road network is good.

2. Patrickswell/New Kildimo (West Limerick):
There are two poultry producers in close proximity. The area has a good road
network. There is an AIBP beef slaughtering plant in Rathkeale.

1. Monaghan/Emyvale (North Monaghan):
There is a large scale duck production facility (Silverhill Foods Ltd.) and a
slaughtering plant in the area. This is a sensitive catchment area.

2. Clones/Smithboro: (West Monaghan):
There are several small poultry production units in the Smithboro area and an AIBP
plant for beef slaughtering in Clones. This is a sensitive catchment area.

3. Lough Egish/Castleblayney/Carrickmacross (South Monaghan):
There is a large dairy plant (Lakeland Dairy) as well as a number of food processing
industries located close to Lough Egish. The area has a large number of dairy farms.
Lough Egish is extremely sensitive due to its hypertrophic nature (EPA, Water
Quality Report, 1995-1997).

1. Ballineen/Bandon/Clonakility (South Cork):
The area has some small pig and poultry production units. There are poultry
slaughtering plants in Ballineen and Clonakilty and an AIBP beef slaughtering plant
in Bandon. The area has significant levels of tillage farming (Carton and Magette,

2. Kildorrey/Mitchelstown/Charleville (North Cork):
There are large scale food processing plants in Mitchelstown and an animal
slaughtering plant in Charleville. The area has significant levels of tillage farming
(Carton and Magette, 1998).

1. Kells/Summerhill (West Meath):
The area has some poultry production units and a good infrastructure. However, point
sources of suitable wastes are scattered, which has implications for transportation

1. Ballyragget (North Kilkenny):
There is a large scale dairy/food processing industry (Glanbia) in this location. Other
significant waste arisings derive mainly from large local dairy farms. The
dependence on a single food-processing industry for additional feedstocks could
affect the long-term viability of a CAD plant in this area.

2. Granagh (South Kilkenny):
There are two animal slaughtering plants and a number of large pig production units
in this area. There is a significant level of tillage farming (Carton & Magette, 1998).
However, the road network is poor.

1. Shercock/Cootehill/Ballyjamesduff/Baillieborough (East Cavan):
There are a number of large-scale poultry and pig production units in east Cavan.
Other potentially available organic wastes include abbatoir wastes (offal, paunch
contents, etc.) and dairy processing wastes from Baillieborough.

5.6    Choice of location for more detailed study
Based on the data obtained in the more detailed analysis of potential CAD plants in
the selected counties (Section 5.5), six locations were identified on the basis of
quantity and proximity of waste arisings, potential end-users of the produced biogas,
and infrastructure. These were North, West and South Monaghan, East Cavan, East
Limerick and South Cork.

In the context of the assured co-operation of Monaghan Local Authority, the potential
for at least three CAD plants in the county, and the sensitivity of the catchment area
(EPA, 1996a, 1996b), it was decided to choose Co. Monaghan for detailed study in
Phase 2 of the project.


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