ANAEROBIC DIGESTION IN CANADA
Anna M. Crolla, M.A.Sc.
Christopher B. Kinsley, M.Eng., P.Eng.
Collège d’Alfred - University of Guelph, Canada
Kevin Kennedy, Ph.D.
University of Ottawa, Canada
Around the world, pollution of air and water from municipal, industrial and agricultural
operations continues to grow. Governments and industries are constantly on the lookout
for technologies that will allow more efficient and cost-effective waste treatment. One
technology that can successfully treat the organic fraction of wastes and wastewaters is
anaerobic digestion (AD). When used in a fully-engineered system, AD not only
provides pollution prevention, but also allows for energy, compost and nutrient recovery.
AD is growing to become a key method for both waste reduction and recovery of a
renewable fuel and other valuable co-products.
This chapter will describe:
• Various feedstocks for anaerobic digestion
• Pre-treatment options for sewage sludge treatment
• Anaerobic digester options for agricultural wastes with Canadian examples
• Methane production from organic solid waste treatment using landfill and
designated anaerobic digestion systems
• Anaerobic digestion options for industrial wastewaters
ATAU Course Notes – Anaerobic Digestion in Canada 1
2.0 Feedstock for Anaerobic Digestion
2.1 Sewage Sludge
Digestion of sewage sludge provides significant benefits when recycling the sludge back
to land. The digestion process provides sanitisation and also reduces the odour potential
from the sludge. Typically between 30 and 70 % of sewage sludge is treated by AD
depending on national legislation and priorities (IEA, 2001). The energy generated from
the AD process is usually used to power the sewage treatment works and where
applicable (larger facilities) the excess biogas is exported from the plant.
2.2 Agricultural Wastes
Farm scale digestion plants treating principally animal wastes have seen widespread use
throughout the world, with plants in both developing and technically advanced countries.
In rural communities small scale units are typical; Nepal has some 47,000 digesters,
while China estimates they have 6 million digesters (IEA, 2001). These digesters are
generally used for providing gas for cooking and lighting for a single household. In more
developed countries, farm scale AD plants are generally larger and the biogas produced is
used to generate heat and electricity to run the farm and for off-farm export.
The small farm scale digestion plants are based on simple stirred tank designs that use
long retention times to provide the treatment required. Whereas modern developments in
agricultural waste digestion have developed the concept of centralised anaerobic
digestion (CAD) where many farms co-operate to feed a single larger digestion plant.
2.3 Municipal Solid Wastes
Organic wastes from households and municipalities provide potential feedstocks for
anaerobic digestion. Wastes can be treated to gain the biogas from the waste as well as
stabilising it to prevent further problems in the landfill.
2.4 Industrial Wastes
Organic solid wastes from industry are increasingly being controlled by environmental
legislation. Breweries, vegetable and meat processing industries are increasingly using
anaerobic digestion in their waste management strategies.
ATAU Course Notes – Anaerobic Digestion in Canada 2
3.0 Treatment of Sewage Sludge in Canada
Municipal wastewater treatment plants (MWWTP) produce primary and secondary
sludge streams that are high in organic content. Mesophilic anaerobic digestion of these
sludges is often employed to reduce the mass of solids for disposal, reduce their pathogen
content and to generate biogas for energy recovery. Presently in Canada there is a dual
focus pertaining to the treatment of MWWTP sludges. Reducing green house gas
emissions and utilization of alternative energy sources has created an interest in
evaluating options that are available for maximizing the generation of biogas from
anaerobic digestion for energy recovery. Additionally, pre-treatment options that
increase the potential of producing Class A biosolids have a tendency to have more
public support than those that only increase biogas production. The centre of Disease
Control (USA) has recommended “all sludges be treated to class A standard because of
the risk that disease could be transmitted through Class B sludge”. Recently, in Ottawa,
Canada a biosolids management program that included spreading of mesophilically
anaerobic digested municipal sludges on agricultural land was stopped as a result of
public health concerns.
The various emerging sludge treatment technologies are either reduction processes or
digestion processes (Kelly, 2003) that can be coupled with some type of pre-treatment.
Generally, sludge reduction technologies such as incineration, gasification, pyrolyis, wet
air oxidation, fuel from sludge and supercritical water oxidation tend to operate at high
temperatures, low pressures and short retention times. On the other hand sludge
digestion processes and associated pre-treatment processes such as Microsludge and high
pressure hydrothermal tend to operate at lower temperatures but higher pressures. On the
macro-scale ultrasound seems to operate at low pressure and low temperature. However
at the micro-scale the opposite is in fact true (discussed below).
This chapter focuses on high powered ultrasonication, chemical and pressure cell
disruption (Microsludge) and thermal and pressure (SUBBOR) pre-treatment that are
now available commercially. More experimental pre-treatment options such as
electropulse are not addressed. Additionally, these options do not include modifications
to operating practices or implementation of alternative digestion technologies that also
may lead to enhanced sludge stabilization and biogas production. Examples of other
options not described include enhanced primary settling, submerged combustion, sludge
ATAU Course Notes – Anaerobic Digestion in Canada 3
thickening, improved digester mixing (pancake versus eggshape design), increase solids
residence times incorporating sludge recycle, temperature phased anaerobic digestion
(TPAD) or aerobic thermophilic pre-treatment and dual digestion.
3.2 Sludge Production
Production of biogas at municipal wastewater treatment plants will be a function of many factors
including the sources of the sludge. Sources of sludge in most wastewater treatment plants
consist of the primary clarifier sludge and waste sludge from secondary biological treatment
processes. The generation and characteristics of primary sludges are influenced by the sources of
wastewater and other waters (i.e. infiltration) to the sewer system, by climatic conditions (i.e. dry
weather versus wet weather) and also operation of the primary settler (i.e. surface loadings, use of
Secondary sludges are influenced by the type of the secondary process employed (i.e. activated
sludge versus trickling filters or extended aeration activated sludge versus conventional activated
sludge) and by the operation of the secondary processes (i.e. solids residence time (SRT) in
activated sludge). These will impact on the quantity of sludges produced and their
biodegradability. For example, short SRTs generally produce more sludge that is more readily
It must therefore be recognized that the subsequently described pre-treatment options for
enhancing biogas production at municipal wastewater treatment plants will be impacted
by the sludge properties and direct comparison of options may be difficult. Hence, the
impact of implementing a pre-treatment option on biogas generation will vary from plant
to plant and may also vary temporally.
3.3 Pre-treatment Options
3.3.1 Microsludge Process
Microsludge (US Patent No. 6,013,183, international patents pending) is a chemical and
pressure pre-treatment process that significantly changes both the rate and the extent that
waste activated sludge (WAS) is degraded in a conventional mesophilic anaerobic
digester. The patented process uses alkaline pre-treatment and an industrial scale
homogenizer to provide an enormous and sudden pressure change to burst the cells. The
resulting liquefied WAS is readily degraded in an anaerobic digester to form methane and
ATAU Course Notes – Anaerobic Digestion in Canada 4
Conventional municipal wastewater treatment typically involves mesophilic anaerobic
digestion of both primary solids and secondary solids (WAS) from aerobic biological
treatment to produce methane and carbon dioxide. The rate limiting step is digestion of
the WAS. The rate-limiting step for anaerobic digestion of WAS is the destruction of the
cell membrane of each microbe (Parkin and Owen, 1986). Anaerobic digestion of WAS
is both slow and incomplete because the individual cell membranes are not significantly
degraded in conventional mesophilic (35 °C) anaerobic digesters that rely on enzymes to
promote cell lysis.
Anaerobic digestion of WAS without pre-treatment falls short of an ideal biosolids
management system for the following reasons:
1. Large quantities of undigested sludge still require disposal.
2. Partially digested sludge generates offensive odours and greenhouse gases.
3. Incomplete pathogen kill necessitates additional sludge processing before biosolids
are safe to use as a fertilizer.
4. Undigested sludge is a wasted resource since the methane generation potential is not
Description of Microsludge Process Operations
The Microsludge process (Stephenson and Dhaliwal, 2000) utilizes alkaline pre-treatment
to weaken cell membranes, mechanical shear to reduce particle size, a self-cleaning
screen to remove oversize debris, and an industrial scale homogenizer to provide an
enormous and sudden pressure change to burst or “lyse” the cells. Figure 1 illustrates
how Microsludge can be integrated into a WWTP.
The heart of the Microsludge process is an industrial scale homogenizer that provides a
large and abrupt pressure drop. At 12,000 psig (82,700 kPag), WAS in the cell disruption
homogenizing valve is accelerated up to 305 meters per second in about 2 microseconds.
This high velocity flow then impinges on an impact ring, disrupting the cell membranes
and producing a liquefied WAS homogenate.
ATAU Course Notes – Anaerobic Digestion in Canada 5
Figure 1: The Microsludge process (Stephenson and Dhaliwal, 2000)
Table 1: Optimum Microsludge Operating Conditions
PROCESSING STEP OPTIMAL SETTING
Chemical Pre-treatment 1. <300 mg/L Na (added as sodium hydroxide) in anaerobic
2. WAS pH ≥ 10 to promote cell lysis
3. Holding time of 1 hour or more to promote cell lysis
Homogenizer Pressure 12,000 to 14,000 psig (83,000 to 96,000 kPag) for maximum cell
Anaerobic Digester pH ≈ 7.0 to minimize ammonia toxicity
Table 2 compares no pre-treatment and Microsludge pre-treatment on digestion of a
40:60 primary:WAS sludge mixture at a municipal wastewater treatment plant in
Chilliwack (1 hr from the City of Vancouver), Canada. Comparison of soluble COD
(sCOD) remaining for conventional 15 day digestion vs Microsludge pretreated sludge
digested at 5,10 or 15 days indicated that the pre-treatment enhanced the anaerobic
digestibility of biosolids. Soluble COD was reduced by 90% after 5 days mesophilic
anaerobic digestion. Additional anaerobic digestion after 10 and 15 days resulted in 95%
and 97% sCOD destruction respectively. In contrast, conventional anaerobic digestion
resulted in just 17% destruction after 15 days.
ATAU Course Notes – Anaerobic Digestion in Canada 6
Table 2: Effect of Microsludge processing on 40:60 primary:secondary solids
Pre-Treatment Anaerobic sCOD VS Total BOD NH3
Digester (mg/L) (mg/L) VFAs (mg/L) (mg/L)
HRT (days) (mg/L)
40:60 - 9,490 22,640 2,723 4,230 527
Microsludge 5 920 4,940 195 415 1,160
Microsludge 10 498 4,780 74 135 1,210
Microsludge 15 305 6,460 159 230 1,180
Conventional Feed - 8,330 34,269 1,601 3,420 280
Conventional Digestion 15 7,870 20,330 2,415 3,200 1,180
Similar improvements in volatile solids (VS) reductions, 78% after 5 d digestion with
concomitant increased biogas production were reported for Microsludge pretreated
sludge versus 41% VS reduction after 15 d conventional mesophilic anaerobic digestion
(CMAD) with no pre-treatment. In terms of VS particulate remaining for disposal the
Microsludge pre-treatment resulted in less particulate sludge remaining for disposal.
Only about 12-14 % particulate sludge was remaining with Microsludge pre-treatment
followed by CMAD compared with about 44% with no pre-treatment followed by
Microsludge pre-treatment was also reported to produce US EPA Class A Biosolids
(Table 3). High pathogen kills and production of a Class A sludge is not commonly
attained with mesophilic anaerobic digesters.
Table 3: Pathogen destruction for heat treated primary solids and Microsludge
processed secondary solids (40:60 mix)
Pre-Treatment Anaerobic Faecal Coliform Salmonella sp.
Digester HRT (MPN/g) (Presence/absence)
Raw Feed - 1.0*108 Present
Microsludge Homogenized Feed - 2.5*104 Absent
Microsludge 5 7.1*103 Absent
Microsludge 10 3.5*10 Absent
Microsludge 15 2.4*10 Absent
Conventional Digestion 15 8.6*107 Present
ATAU Course Notes – Anaerobic Digestion in Canada 7
Advantages of Microsludge
1. Enhanced WAS biodegradability makes anaerobic digesters more effective within
WWTPs’ limited available footprint to destroy biosolids, thus avoiding large capital
expenditures for new sludge management infrastructure that is capacity constrained.
2. It reduces the nuisance aspect of biosolids because it destroys pathogens, reduces
odours, and yields lower quantities of stabilized biosolids for disposal.
3. It reduces net operating costs by significantly reducing the de-watered solids
requiring disposal, generating higher volumes of methane for heat/power generation,
and by operating the digesters more efficiently, thus lowering input costs for heating
3.3.2 Ultrasound Process
Ultrasound for sewage sludge treatment has started to become an established sludge pre-
treatment method for anaerobic digestion with several full-scale treatment plants using
this technology. As with the Microsludge process the most economical use of the process
is for treatment of WAS. Ultrasound plants consist of a single or series of high power
ultrasonic probes (Dirk European Holdings) or ultrasonic horns (Sonico Ltd) which emit
frequencies above the audible range (20 kHz or above) which come into intimate contact
with the sludge to be treated. Ultrasonic waves generated from the probes pass into the
sludge thus altering its structure and making it more biodegradable. Disintegration of
sewage sludge by means of ultrasound is based on the effects of acoustic cavitations in
the liquid sewage sludge. Cavitation is the formation, growth and implosive collapse of
bubbles in a liquid. Cavitational collapse produces intense local heating (5000K), high
pressures (1000 atm), large heating and cooling rates and very fast jet streams in the order
of 400km/h (Clark, 1998, Suslick, 1998). These intense localized “hotspots” result in the
destruction of cell walls and membranes in sewage sludge releasing cellular contents into
the mixed liquor as biodegradable COD and making cell components more susceptible to
biodegradation. Typical exposure times for ultrasound pre-treatment is in the order of
seconds and space requirements for the units is small and can be easily incorporated into
existing designs (Figure 2).
ATAU Course Notes – Anaerobic Digestion in Canada 8
Figure 2: Standard Sonix flow cell (left) designed to accommodate 5 horns and
installed on TWAS line on digester wall (Sonico Ltd)
The largest ultrasonic probes presently available can deliver 16 kW power each, and are
capable of treating a population equivalent of approximately 150 – 200,000 as secondary
sludge. Typical lifespan of an ultrasonic probe is about 2 years. Depending on sludge
thickness, 1 kW of ultrasound power can treat sludge with a population equivalent of
between 10 and 32 thousand.
Using ultrasound as a pre-treatment process increases hydrolysis of WAS. Hydrolysis
rates were improved by 25-50%. Ultrasound has also shown to improved biogas
production with increases from 25-50% (Table 4, Figure 3). Energy balances and organic
loading calculations indicated that application of Ultrasound pre-treatment could result in
a 25-30% decrease in digester retention time due to improved digestion. Additionally,
Ultrasound pre-treatment has resulted in a production of about 7 kW of power (after
losses) per kW of energy used for ultrasound. This would of course vary with operational
factors such as age of sludge and sludge concentration and so on.
It has also been reported that enhanced dewaterability of digested sludge resulted from
ultrasound pre-treatment (Barber, 2003). Benefits of ultrasound treatment to operation of
activated sludge operations were also reported including elimination of sludge bulking,
enhanced biological nutrient removal, reduction in polymer and other flocculants and
increased dry solids with dewatering (Barber, 2003)
ATAU Course Notes – Anaerobic Digestion in Canada 9
Table 4: Enhanced Performance of WWTP with Ultrasound Pre-treatment
Feature Guarantee Achieved Data
Biogas Production 20% 22% Increase of 90 litres/kg VS
Electricity 20% 22% Increase of 269000 kWh/year
VS Destruction 20% 22% From 50% VS destroyed to 62%
Dewatering 5 % points 5 – 7 % points From 16 – 23 % DS
Cake Production – 33% – 31% Reduction of 2218 tons cake per
Polymer – 30% – 31% Reduction of 40000 kg/yr
(m3 biogas/kg VS feed)
% increase in biogas
A B C D E F
Sewage treatment plant
Figure 3: Effect of applying ultrasound on gas production in full-scale digesters
Figure 4 shows a biogas production increase of about 50% in an anaerobic digester. The
figure illustrates that when the sonication was stopped and TWAS feeding returned to
about 20%, concomitantly biogas production in both the test and control reactors returned
to identical production levels (08/17/2002). Specific biogas production increases from
about 5.2-5.5 ft3/lbVS fed in the control to about 8.0-8.2 ft3/lbVS fed in the test digester
ATAU Course Notes – Anaerobic Digestion in Canada 10
indicating the positive effect of ultrasound on sludge stabilization and VS destruction
End of trial; TWAS feed reduced to ~20%
Power turned up
01/29/02 03/10/02 04/19/02 05/29/02 07/08/02 08/17/02
Control - New Gas Meter Test - New Gas Meter
Control - Old Gas Meter
Contol Test - Old Gas Meter
Figure 4: Mean daily gas production
Specific Gas Production (cuft/lbVS fed)
02/01/02 02/21/02 03/13/02 04/02/02 04/22/02 05/12/02 06/01/02 06/21/02
Figure 5: Specific gas production based on solids fed to the digester
ATAU Course Notes – Anaerobic Digestion in Canada 11
3.3.3 Super Blue Box Recycling (SUBBOR) Process
The Super Blue Box Recycling (SUBBOR) process is a patented (Vogt. et al. 1998)
enhanced, multi-stage anaerobic digestion process for mixed municipal solid waste
(MSW) and other biomass feedstock materials. The technology centers on enhanced high
solids, thermophilic digestion after steam-pressure disruption of the ligno-cellulosic fibre
components that are recalcitrant to conventional anaerobic digestion. Mixed MSW, rich
in organic components but also containing inorganic materials, such as glass, aluminum
and steel, as well as non-digestible plastic materials, has been pilot tested with a fully
integrated process train designed to treat and recycle all of the MSW components.
Methane yields from the MSW were 0.36 m3 CH4/kg VS representing a 40% increase
over the yield obtained from conventional single stage digestion. The secondary digestion
step after steam pressure disruption also shows a 40% improvement in total solids and
volatile solids reduction. The residual organic fraction following two-stage digestion is
fine in texture and is recovered as a clean Class A peat fraction with reduced contents of
heavy metal and other fugitive non-digested contaminants. Mass and energy balance
determinations indicate a high degree of MSW diversion from landfill disposal (greater
than 80%) can be achievable by the SUBBOR process as well as substantial net electrical
and thermal energy production. Continuous long-term trials of the SUBBOR process at
25,000 tonnes/year are underway.
The SUBBOR process has been primarily used for MSW in the past, but current
applications of the process to sludge treatment are underway. The following sections will
describe the process for MSW conditions.
The heart of the SUBBOR process is the sequential two stage anaerobic digestion (Unit
II, Figure 6) that employs steam pressure disruption of the primary digestate prior to its
re-digestion in the secondary stage. Anaerobic digestion is carried out at medium to high
solids (15-30% (w/w)) content and under thermophilic conditions (55oC).
The primary digestate after approximately 25 days digestion is removed from the primary
digester and processed through a steam pressure disruption circuit. This step causes a
“steam-explosion” of the internal water of the remaining non-digested fibers, causing
fiber disruption (Liu et al., 2002). The extent of disruption is affected both by cooking
time and cooking temperature which range from 5-20 minutes and 190- 270 oC
respectively. The disrupted material is subsequently re-inoculated and re-digested in the
secondary digestion stage to provide additional digestion and biogas production from the
ATAU Course Notes – Anaerobic Digestion in Canada 12
substrate made accessible from pressure disruption. Digestate from the secondary
digester is then further processed to provide secondary recovery of non-digested
materials (metals, plastic, glass) and heavy metal removal, providing a recovered cleaned
organic peat by-product fraction. Biogas produced during the primary and secondary
digestion stages is routed to an energy recovery circuit where biogas is the energy source
to produce electrical power and steam/heat co-generated energy products. A portion of
the recovered energy is utilized for internal process needs while the balance is exported
as product energy.
Primary Products Recovery/
UNIT I Milling/ Aluminum
Steam Pressure Disruption (Energy)
Secondary Products Recovery
Figure 6: SUBBOR two stage enhanced digestion process for mixed MSW
ATAU Course Notes – Anaerobic Digestion in Canada 13
The 35-day primary digestion biogas yield is summarized in Table 5. The 60% CH4 in
the biogas and overall CH4 yield of 0.25 m3/kg VS (provided for digestion) are average
yields when using MSW as a feedstock (Owens and Chynoweth, 1993, Oleszkiewicz and
Poggi-Varaldo, 1997, Mata-Alvarez et al., 1993).
Table 5: Methane yields and mass reductions from enhanced two-stage digestion of
Digestion Time of Biogas Mass Reduction %
Stage Digestion CH4 content Yield TS VS
(days) % m3CH4/kgVS
Primary 35 60 0.25 40 48
Secondary 15 65 0.11 16 19
Total 50 60-65 0.36 56 67
Typically, digestion in commercial anaerobic facilities is terminated after 15-20 days and
processing is then completed by aerobic composting (DeBaere, 1999), presumably
because biogas production beyond 20 days of digestion is not justified economically.
SUBBOR has also reported that further enhanced secondary digestion yields are
achievable by shortening the course of primary digestion, followed by steam pressure
disruption of the digestate and its secondary digestion. Thus, a correspondingly larger
portion of the overall biogas yield can be obtained in the secondary digestion stage
through an earlier termination of the primary stage followed by steam pressure disruption
and re-digestion. This aspect is being further investigated as it holds potential for
achieving similar overall yields with further reduced overall digestion times. The
substantial boost to biogas production kinetics provided by the secondary digestion stage
following disruption therefore provides process flexibility for adjusting the length of the
overall digestion time needed for optimum gas yields.
The SUBBOR process has successfully been shown to enhance digestion of MSW,
however the application to municipal wastewater treatment plant sludges is just
beginning. It is believed that as steam explosion technology can disrupt lingo-cellulose
bonds in MSW it should be able to disrupt cell walls and membranes of municipal
sludges. Studies using the SUBBOR process to enhance digestion of 6% thickened waste
activated sludge (TWAS) and anaerobic biosolids cake after steam pressure disruption
has shown an average of 75% increase in biogas production (Hamzawi, 1998 a&b).
ATAU Course Notes – Anaerobic Digestion in Canada 14
4.0 Use of Anaerobic Manure Digestion in Canada
4.1 Background of Anaerobic Manure Digestion in Canada
The adoption of manure digesters is much more advanced in Europe than in North
America, and especially more so than in Canada. This practical long-term European
experience indicates that manure digesters do operate well and are of economic value in
cooler climates, similar to that of Canada. There are an estimated 760 manure based
biogas plants in Europe (IEA, 2001; Bo Holm-Nielson et al., 1997). Table 6 identifies
the approximate number of biogas plants in various European and North American
countries distributed among different digester feedstocks. Bo Holm-Nielson et al. (1997)
identifies Germany as having the largest number of plants in Europe, dominated by small
scale farm plants, while Denmark has the largest biogas production of 1.05 Peta Joules
(PJ), originating mainly from co-digesting slurry with an average of 25 percent agro-
industrial wastes. Sweden has also been identified as having a large manure based biogas
production of about 0.4 PJ. Canada and the U.S.A. are beginning to grow their numbers
of manure based biogas plants and currently stand at approximately 10 and 28 plants,
A study conducted by Agriculture and Agri-food Canada from 1973 to 1986 showed that
sufficient biogas could be produced to provide supplemental heat to maintain the
digestion process in the winter and for a farm to be almost self sufficient in the
production of electricity. However, it was also concluded that for small to medium sized
Canadian farms anaerobic digestion was not economically feasible because it was too
labour and capital demanding. The results indicated that even when the technology was
fully integrated into a farming operation involving energy production and recovery of
protein, the energy and feed cost savings could not justify in economic terms the large
capital investment, operating costs and management time that the farmer had to supply
(Van Die, 1987).
Increasing volatile energy costs and, increasing concerns about greenhouse gas
emissions, odour and pathogens in manure has lead to increased interest in anaerobic
digestion for the agricultural industry in Canada. Improved digester designs with lower
capital and operating costs, and the successful implementation of manure digesters in
Europe has also justified this second look into digesters for the Canadian agricultural
industry (ManureNet, 2004).
ATAU Course Notes – Anaerobic Digestion in Canada 15
Table 6: Number of Anaerobic Digester Plants in
Various European and North American Countries (IEA, 2001)
Country Sewage Landfill Gas Biowaste or Agricultural Industrial
Sludge Production Industrial Wastewater1
Austria 100 31 3 100 25
Canada 50 33 1 10 2 13
Czech Republic N/A 3 N/A N/A 10 4
Denmark 64 10 N/A 20 5
Finland N/A N/A 1 N/A 3
Germany N/A 170 MW of 49 380 91
Greece 2 1 N/A 1 2
Italy N/A N/A 4 50 38
Netherlands N/A N/A 2 35 84
Norway 17 40 N/A 2 5
Portugal N/A N/A N/A 94 3
Spain N/A N/A 1 6 27
Sweden 134 73 4 3 8
Switzerland 70 15 11 69 20
United Kingdom 200 160 1 25 26
U.S.A. 1600 270 N/A 28 92
Note: Data presented in table is not complete. Often data was not available because the information was
not centrally located.
1 AD used for industrial wastewater pre-treatment
2 (ManureNet, 2004)
3 N/A: Not Available
4 Germany has a large landfill gas programme producing 170 MW of electricity, but the number of plants
is not available
5 (Bo Holm-Nielson et al., 1997)
4.2 Benefits of Anaerobic Manure Digestion for Canada
The renewed interest in anaerobic manure digestion in Canada has been driven by the
technology’s potential to accomplish the following:
1. Greatly reduce odour levels during manure processing, creating a relatively odour-
free end product.
2. Reduce pathogen levels in the final products - Additional post-digester technologies
can ensure pathogen-free end products.
3. Conserve nutrients - more than 90% of nutrients entering anaerobic digesters are
conserved through the digestion process. By conserving nitrogen during digestion, the
N:P ratio of the treated manure is more favourable for plant growth.
ATAU Course Notes – Anaerobic Digestion in Canada 16
4. Reduce greenhouse gas (GHG) emissions - Since anaerobic digestion operates in a
closed system, substantial reductions in greenhouse gas emissions (methane, nitrous
oxide) are achieved.
5. Co-generation and Energy Independence - Anaerobic digesters produce methane
which can be captured for supplying energy (heat, electricity) for the operation.
6. The final products of anaerobic digestion are quite homogenous and are more
predictable as sources of plant nutrients since they are in a more mineral form (50%
of carbon is converted to methane).
In Canada, the digester option is likely to be more attractive to larger operations, or to
well-established existing operations wanting to expand and modernize their manure
management systems. When considering manure digesters as an option, it may enhance
the chances of success if local municipalities or other agricultural industries (vegetable or
fruit processing, slaughter houses) or commercial industries (distillers, bio-fuel
production) are considered as potential partners. This appears to be the trend of several
European digesters. The addition of off-farm fatty wastes, animal rendering wastes or
vegetable/cooking oils can act as an accelerant for methane production, increasing
outputs by up to 4-fold.
4.3 Characteristics of Manure
Organic waste management in livestock operations has long been an area of concern.
Tables 7 and 8 illustrate typical annual manure production quantities and characteristics
from various animal types in Canada, respectively.
Table 7: Typical Annual Manure Production per Animal (Semmler, 2002; OMAF, 2000)
Livestock Manure Production Animal Units
Dairy Cow 18.25 1
Beef Cattle 9.13 0.5
Calf 3.65 0.2
Swine 2.92 0.16
Poultry 0.073 0.004
Note: Amounts of manure produced may vary depending on farming practices
ATAU Course Notes – Anaerobic Digestion in Canada 17
Table 8: Typical Manure Characteristics As Excreted
(CH2MHill et al., 1997; Fulhage et al., 1993)
Total Nitrogen Total Phosphorus BOD5 Volatile
Manure Type (kg/day) (kg/day) (kg/day) Solids
(Animal Weight) (kg/day)
Dairy Cow (545 kg) 0.20 0.032 0.73 4.31
Beef Cattle (454 kg) 0.15 0.054 0.54 2.27
Swine (68 kg) 0.19 0.07 0.94 0.32
Poultry (1.8 kg) 0.38 0.14 1.68 0.02
Note: Manure concentrations will vary depending on feed composition
Anaerobic digestion will treat manure by converting organic materials to carbon dioxide
and methane gas (biogas). The conversion of solids to biogas results in a much smaller
quantity of solids that must be disposed of after digestion. During the anaerobic
treatment process, organic nitrogen compounds are converted to ammonia, sulphur
compounds are converted to hydrogen sulphide, phosphorus is converted to
orthophosphates, and calcium, magnesium, and sodium are converted to a variety of salts
All waste constituents are not equally degraded or converted to gas through anaerobic
digestion. Anaerobic bacteria do not degrade lignin and some other hydrocarbons. The
digestion of manure containing high nitrogen and sulphur concentrations, like swine
manure, can produce toxic concentrations of ammonia and hydrogen sulphide. Waste
constituents that are not particularly water soluble will breakdown more slowly. For
example, dairy manure has been reported to degrade slower than swine or poultry
manure. Approximately 10 % of volatile solids in dairy manure is lignin, thereby
reducing the percentage of volatile solids available to be converted to biogas.
4.4 Overview of Manure Digester Types and Reactor Design for Manure
Treatment in Canada
An array of anaerobic digesters have been developed and placed in operation in North
America over the past fifty years. The main objective associated with an anaerobic
manure digester is to convert solids to biogas while meeting the goals of anaerobic
digestion. The goals of manure anaerobic digestion are as follows:
1. Reduce the mass of solids
2. Reduce the odours associated with the waste products
3. Produce clean effluent for recycle and irrigation
ATAU Course Notes – Anaerobic Digestion in Canada 18
4. Concentrate the nutrients in a solid product for storage or export
5. Generate biogas for energy
6. Reduce pathogens
The processes, either pilot or full scale, that have been used in Canada and the U.S.A. for
the digestion of manure can be subdivided into high rate and low rate processes. Low
rate processes consist of covered anaerobic lagoons, plug flow digesters, and mesophilic
completely mixed digesters. High rate reactors include the thermophilic completely
mixed digesters, anaerobic contact digesters and hybrid contact/fixed film reactors.
4.4.1 Factors controlling Anaerobic Digestion and Reactor Design
The Solids Retention Time (SRT) is the most important factor controlling the
conversion of solids to gas. It is also the most important factor in maintaining digester
stability. The solids retention time is defined as:
(V )(C d )
SRT =  (Burke, 2001)
(Qw )(C w )
where V = digester volume, m3
Cd = solids concentration in digester, kg/m3
Qw = volume wasted each day, m3/day
Cw = solids concentration of waste, kg/m3
In a conventional completely mixed, or plug flow digester, the hydraulic retention time
(HRT) equals the SRT. However, in a variety of retained biomass reactors the SRT
exceeds the HRT. As a result, the retained biomass digesters can be much smaller while
achieving the same solids conversion to biogas. Figure 7 illustrates the percentage
volatile solids destruction of manure as a function of SRT. The goal of process engineers
over the last twenty years has been to develop anaerobic processes that retain biomass in
a variety of forms such that the SRT can be increased while the HRT decreased.
Effective retention systems will have SRT/HRT rations exceeding 3 (Burke, 2001). At
an SRT/HRT ratio of 3 the digester will be 1/3 the size of a conventional digester
ATAU Course Notes – Anaerobic Digestion in Canada 19
Figure 7: Manure Volatile Solids Destruction with increasing SRT (Burke, 2001)
The digester loading rate (kg/m3/day) is designed to maintain the necessary bacterial
balance and prevent ammonia toxicity from occurring. The loading rate is the most
appropriate measure of the waste on the digester’s size and performance. The loading
rate is often reported as the mass of waste per digester volume. The digester loading can
be calculated if the HRT and the influent waste concentration is known. The loading rate
can simply be defined as:
L= (C I )  (Burke, 2001)
where HRT = volume of tank (V)/daily flow (Q), day
CI = influent waste concentration, kg/m3
Digester loading rates (based on volatile solids) and hydraulic retention times for various
manure types are presented in Table 9.
ATAU Course Notes – Anaerobic Digestion in Canada 20
Table 9: Typical Loading Rates, Detention Times and Digester Volumes for Dairy,
Beef, Swine and Poultry Manure (Fulhage et al., 1993)
Dairy Beef Swine Poultry
Loading Rates *, 0.37 0.37 0.14 0.12
lbs solids/ft3/day (5.9) (5.9) (2.2) (1.9)
Hydraulic Retention 17.5 12.5 12.5 10.0
Digester Volume, 26.0 13.5 5.0 0.37
ft3/animal (m3/animal) (0.74) (0.38) (0.14) (0.01)
Digester Volume for a 75 cows: 300 cows: 500 hogs: 15000 birds:
Typical Livestock 1950 4050 2500 5550
Operation, ft3 (m3) (55) (115) (71) (150)
* Loading Rates are based on mass of volatile solids per digester volume per day
The Food to Microorganism (F/M) ratio is the key factor controlling anaerobic
digestion. At a given temperature, the bacterial population can only consume a limited
amount of food each day. The F/M ratio is the mass of waste supplied to the mass of
bacteria available to consume the waste. This ratio is the controlling factor in all
biological treatment processes.
F M =  (Burke, 2001)
VS D − VSUP
where LVS = volatile solids loading rate, kg/m3/day
VSD = concentration of volatile solids in digester, kg/m3
VSUP = concentration of unprocessed volatile solids, kg/m3
For any given loading, efficiency can be improved by lowering the F/M ratio by
increasing the concentration of biomass in the digester. Also, for any given biomass
concentration within the digester, the efficiency can be improved by decreasing the
4.4.2 Anaerobic Lagoons (Very Low Rate)
Anaerobic lagoons are covered ponds. Manure enters at one end and the effluent is
removed at the other. The lagoons operate at psychrophilic (< 20 oC), or ground
temperatures. Consequently, the reaction rate is affected by seasonal variations. Since
the reaction temperature is quite low, the rate of conversion of solids to gas is also low.
ATAU Course Notes – Anaerobic Digestion in Canada 21
In addition, solids tend to settle to the bottom where decomposition occurs in a sludge
bed. Little contact of bacteria with the bulk liquid occurs.
Figure 8: Covered Anaerobic Lagoon System (Burke, 2001)
The biomass concentration is low, resulting in very low solids conversion to gas. In
anaerobic lagoons there is a high F/M ratio with poor growth rates at low temperatures.
There is little or no mixing thereby lagoon utilization is poor. Periodically the covered
lagoons must be cleaned out due to solids accumulation. Solids may be screened and
removed prior to manure entering the lagoon to minimize solids accumulation. However,
a considerable amount of energy potential is lost with the removal of particulate solids.
The advantage of anaerobic lagoons is the low cost. The low cost is offset by the lower
energy production and poor effluent quality.
A review by Burke conducted by (2001) determined that on average lagoon systems
accepting a screened manure feed would achieve a 35% conversion of volatile solids to
biogas at a loading of 0.04 kg/m3/day.
4.4.3 Completely Mixed Digesters (Low Rate)
The most common type of anaerobic digester is the completely mixed reactor. Most
sewage treatment plants and many industrial treatment plants use completely mixed
reactors to convert solids to biogas. The completely mixed digester is heated and mixed.
Most completely mixed digesters operate in the mesophilic range. Most of the initial
anaerobic digesters installed to treat manure in Canada and the U.S.A. were completely
mixed mesophilic digesters (Burke, 2001). The cost of mixing is high, especially if sand,
silt and floating materials present in the waste stream must be suspended throughout the
digestion period. Some completely mixed reactors operate in a thermophilic range where
sufficient energy is available to heat the reactor.
ATAU Course Notes – Anaerobic Digestion in Canada 22
Figure 9: Completely Mixed Manure Digester (Burke, 2001)
Most completely mixed reactors are heated with spiral flow heat exchangers. These heat
exchangers apply hot water to one side of the spiral and the anaerobic slurry to the other.
The spiral heat exchangers have proven to be a successful method of efficiently
transferring heat to the digesters.
Completely mixed reactors can have fixed covers, floating covers, or gas holding covers.
Floating covers are more expensive than fixed covers.
Mixing can be accomplished with a variety of gas mixers, mechanical mixers, and draft
tubes with mechanical mixers or simply recirculation pumps. The most efficient mixing
device in terms of power consumed per gallon mixed is the mechanical mixer (Burke,
2001; Fulhage et al., 1993).
The advantage of the completely mixed digester is that it is a proven technology that
achieves reasonable conversion of solids to gas. It can be applied to the treatment of
slurry wastes such as manure. The disadvantage of the completely mixed reactor is the
high cost of installation and the energy costs associated with mixing. The completely
mixed conventional anaerobic digester is a biomass growth based system. The process
requires a constant conversion of a portion of the feed solids to anaerobic bacteria rather
than biogas. Since anaerobic bacteria are constantly wasted from the process, new
bacteria must be produced to replace the lost bacteria.
Completely mixed thermophilic anaerobic digesters have a rapid conversion of solids to
gas and biomass. It has been shown that the rate of conversion can be three times greater
with thermophilic reactors (Burke, 2001).
ATAU Course Notes – Anaerobic Digestion in Canada 23
Generally mesophilic completely stirred digesters will achieve a 40% conversion of
volatile solids to biogas at a loading of 5.7 kg/m3/day of dairy and swine manure (Burke,
2001). Better conversions could be achieved at lower loadings. Thermophilic reactors
appear to achieve greater conversions at higher loadings while mesophilic reactors appear
to achieve greater conversions at lower loadings (Fulhage et al., 1993).
4.4.4 Plug Flow Digesters (Low Rate)
The plug flow anaerobic digester is the simplest and least expensive form of digester.
The plug flow digester can be a horizontal or vertical reactor. The horizontal reactor is
the most common configuration. The waste enters on one side of the reactor and exits on
the other. Since bacteria are not conserved, a portion of the waste must be converted to
new bacteria, which are subsequently wasted with the effluent. Since the plug flow
digester is a growth based system, it is less efficient than a retained biomass system and
converts less waste to biogas.
Figure 10: Plug Flow Reactor (Burke, 2001)
Plug flow system are subject to stratification wherein the sands and silts settle to the
bottom and the organic fibers migrate to the surface. The stratification can be partially
inhibited by maintaining a relatively high solids concentration in the digester.
Periodically, solids must be removed from the plug flow digester. Sincere there is not
easy way of removing the solids, the reactor must be shut down during the cleaning
period. Cleaning costs can be considerable.
Plug flow reactors are normally heated by a hot water piping system within the reactor.
The hot water piping system can complicate the periodic cleaning of the reactor.
ATAU Course Notes – Anaerobic Digestion in Canada 24
The plug flow reactor is a simple and economical system. Applications are limited to
concentrated liquid dairy manure containing minor amounts of sand and silt. If
stratification occurs because of a dilute waste or excess sand, significant operating costs
will be incurred. Generally, a mesophilic plug flow reactor will achieve a 32%
conversion of volatile solids to biogas at a loading rate of 2.5 kg/m3/day of dairy and
swine manure (Burke, 2001).
4.4.5 Contact Digesters (High Rate)
The contact digester is a high rate process that retains bacterial biomass by separating a
concentrating the solids in a separate reactor and returning the solids to the influent.
More of the degradable waste can be converted to biogas since a substantial portion of
the bacterial mass is conserved. The contact digester can be either completely mixed or
plug flow, thermophilic or mesophilic. The contact digester can treat both dilute and
Figure 11: Contact Anaerobic Digester System (Burke, 2001)
Gravity separators (settling tanks) and solids thickeners have been used in the past for the
solids separation. It was discovered that the solids could not be sufficiently concentrated
in a gravity separator without degassing to remove the gas bubbles attached to the solids.
Gravity separation techniques are only effective with dilute waste following a completely
mixed reactor. Separation requires several days of detention and the digester solids
concentration should be less than 2.5 % for gravity separation (Burke, 2001).
Mechanical separation devices have been tested to reduce the detention time required by
gravity separation. Centrifuges, gravity belts, membranes, and other mechanical
separators have been used with limited success. The mechanical separators have been
shown to inhibit the recirculated bacteria population and thus limiting the effectiveness of
the contact process.
ATAU Course Notes – Anaerobic Digestion in Canada 25
During the contact process, refractory organic and inorganic solids accumulate within the
system. The accumulated sands, silts and non-degradable organic fibers dictate the rate
of solids wasting. Wasting the non biodegradable solids causes the loss of bacterial mass
and reduced process efficiency.
4.4.6 Sequencing Batch Digesters (High Rate)
A sequencing batch reactor is a contact digester, which utilizes the same tank for
digestion as well as separation. Generally, two or more tanks are used. The tanks are
operated in a fill and draw mode. The separation is accomplished by gravity.
Consequently, a more dilute, screened waste is treated.
4.4.7 Contact Stabilization Digesters (High Rate)
The anaerobic contact stabilization process is a more efficient contact process. The
process has the advantage of efficiently converting slowly degradable materials as
cellulose in a highly concentrated reactor (Burke, 2001). Organic materials, which can be
degraded rapidly, are digested in the contact reactor. The bacteria and slowly degradable
organics are removed and degraded in a highly concentrated reactor.
A contact mesophilic stabilization anaerobic digester can achieve a 78% conversion of
volatile solids to biogas at a solids loading of 3.7 kg/m3/day of dairy manure (Burke,
Figure 12: Contact Stabilization Anaerobic Digester (Burke, 2001)
ATAU Course Notes – Anaerobic Digestion in Canada 26
4.4.8 Phased Digesters
Both acid phased and temperature phased digestion have been used to convert municipal
sludge to biogas. Acid phased digestion takes advantage of the fact that the acid forming
bacteria have a much higher growth rate than the methanogens.
Consequently, the initial reactor can be much smaller than the subsequent methane
producing digester. Acid phased digestion offers greater efficiency in the size of the
Figure 13: Acid Phased Anaerobic Digester (Burke, 2001)
Temperature phased digestion has been used to digest dairy manure in the U.S.A. (Burke,
2001). The temperature phased digestion takes advantage of the pre-thermophilic
digester to reduce pathogens from the manure.
Figure 14: Temperature Phased Digester (Burke, 2001)
A temperature phased sequencing batch reactor when treating dairy manure can achieve a
30 to 41% conversion of volatile solids to biogas, while producing a biogas with 62 to 66
% methane (Burke, 2001). These values are based on a system with a SRT/HRT ratio of
3 to 4 and a three-day HRT.
ATAU Course Notes – Anaerobic Digestion in Canada 27
Table 10 summarizes the expected performance of each the various types of anaerobic
digesters. Table 11 presents a summary of parameters of the anaerobic processes that can
be used to convert all or a fraction of manure to biogas.
Table 10: Expected Percentage of Volatile Solids Conversion to Biogas in Manure
Process Load Conversion of Volatile Solids
Entire Waste Stream
Completely Mixed Mesophilic High 35 to 45 %
Completely Mixed Thermophilic High 45 to 65 %
Contact High 50 to 65 %
Partial Waste Stream
Plug Flow Mesophilic High 35 to 45 %
Fixed Film High 55 to 65 %
Lagoon Low 35 to 45 %
Table 11: Summary of Anaerobic Digestion Parameters to Convert All or Farction
of Manure to Biogas (Burke, 2001)
Complete Mix -
Complete Mix -
Parameter Plug Flow –
Not limited by Solids Concentration X X X
Not limited by Foreign Material X X X
Digest Entire Manure Waste Stream X X X
Sand & Floating Solids Processing X X X
Highly Effective at Odour Control X X
Concentrate Nutrients in Solids X X
Treat Additional Substrate X X X
Stability X X X X
Simplicity X X
Net Energy Production X X
ATAU Course Notes – Anaerobic Digestion in Canada 28
4.5 Qualitative Analysis of Anaerobic Digestion of Manure
Professionally designed and implemented anaerobic manure digestion plants typically
have the following results:
1. Odour reduction is usually in the order of 80% (Semmler, 2002)
2. Production of virtually odourless high grade organic liquid or marketable solid
fertilizer (depending on digester configuration), which can even be applied to
growing crops without damage.
3. Production of biogas, with a methane content of approximately 60% (Semmler
2002), which can be stored and used on demand
4. Reduction of pathogens of up to 100% depending on configuration (Semmler,
5. Reduction of greenhouse gas emissions.
6. Reduces land base requirements for manure applications.
7. Provides for the possibility of reclaiming water.
8. Permits the addition of various substrates to increase biogas production, known as
4.5.1 Nutrient Concentration and Retention
The process of anaerobic digestion will convert nutrients from an organic form to an
inorganic form. In plug flow, completely mixed and thermophilic digesters the quantity
of nutrients entering the reactor equals the quantity of nutrients exiting the digester.
However, retained biomass digesters such as the contact process, sequencing batch
reactors and fixed film reactors, nutrients may be concentrated in a separate waste solids
stream. In manure anaerobic digesters 90% of the phosphorus and 43% of the total
nitrogen can be concentrated in the waste solids (Burke, 2001). Often the waste solids
volume is only 1/5 of the influent volume. The ability to concentrate nutrients is an
important characteristic of the anaerobic process as it proves the livestock producer with
the control necessary to manage nutrient application to land.
4.5.2 Energy Production
The quantity of energy produced from each cubic meter of manure processed is strictly a
function of the percentage conversion of volatile solids to gas. Each pound (454 g) of
volatile solids destroyed will produce 5.62 ft3 (0.16 m3) of methane (Burke, 2001). Each
cubic foot (0.028 m3) of methane will contain 1000 Btu’s of energy. Therefore each
pound (454g) of volatile solids converted will produce 5620 Btu’s of energy. At a 35
percent typical conversion efficiency, each pound (454 g) of volatile solids destroyed will
produce 0.58 kWh of energy (Burke, 2001).
ATAU Course Notes – Anaerobic Digestion in Canada 29
The conversion of volatile solids to gas is a function of the organic loading to the
digester. Higher percentage conversions to gas are achieved at lower organic loadings.
Low loadings however, translate into larger digestion facilities. However, it is possible to
achieve a higher volatile solids conversion to gas by increasing the digester loading while
maintaining a higher biomass concentration in the digester. In other words, the F/M
ration remains low resulting in a higher rate of conversion (Burke, 2001; Fulhage et al.,
1993). Conventional completely mixed and plug flow digesters, which do not retain
biomass, will have comparable volatile solids destructions, while high rate retained
biomass reactors will have higher rates of solids conversion to gas.
Table 12 describes the potential gas production of swine, dairy, beef and poultry manure.
Table 12: Potential Gas Production of Swine, Dairy, Beef and Poultry Manure
(Fulhage et al., 1993)
Dairy Beef Swine Poultry
(545 kg cow) (454 kg cow) (68 kg hog) (1.8 kg bird)
Gas Yield, 7.7 15 12 8.6
ft3/lb (m3/kg) (0.48) (0.93) (0.75) (0.54)
Volatile Solids Produced, 9.5 5 0.7 0.044
lb/day (kg/day) (4.32) (2.27) (0.32) (0.02)
% Reduction of 31 41 49 56
Potential Gas Production, 22.7 31 4.1 0.21
ft3/animal/day(m3/animal/day) (0.64) (0.87) (0.12) (0.006)
Energy Production Rate,
Btu/hr/animal 568 775 103 5.25
Net Available Energy (after
heating digester), 380 520 70 3.5
4.6 Examples of Manure Digesters in North America
4.6.1 BioGem Power Systems, Inc., Alberta, Canada (Centralized digester)
BioGem Power Systems Inc. is a company from Alberta, Canada. The company has
developed a biogas technology that utilizes organic waste from intensive livestock
operations, and through anaerobic digestion, generates: electricity and thermal energy,
reusable water and a dry nutrient rich organic material.
ATAU Course Notes – Anaerobic Digestion in Canada 30
BioGem built the first commercial biogas facility in Alberta at the Hutterite colony that
sells power to the Alberta Power Pool. The facility has been successful at providing
power to the digester facility and selling excess power to the grid. Hutterite colony
consists of a 1500 farrow to finish sow operation.
The BioGem process consists of taking organic wastes (manure, sewage, dead stock, and
plant matter) that has been made into a slurry and feeding it to the digester. Under
controlled anaerobic conditions at 32oC, methane gas is produced. The methane is used
to fuel an engine which drives a generator; producing electricity. Heat from the engine is
captured through a hydronic heating system and returned to the facility for heating
purposes. The digestion process takes approximately 35 days. At the completion of the
digestion, the “spent liquor” is drawn out of the digester into a holding a tank. The liquor
is similar in nutrient content to the raw feedstock, with the exception that 95% of the
odours have been reduced and the material is in a non-settling form (Henteleff, 2004).
The spent liquor can then be used directly as fertilizer or further treated in a wastewater
treatment plant. This Hutterite facility has chosen to install a wastewater treatment plant
to recover and reuse the water as washwater and livestock water. The sludge from the
anaerobic digestion process is further dried through composting, where the end product is
similar to peatmoss. This dried material represents 4% volume of the original raw
volume of waste slurry (Henteleff, 2004).
The Hutterite digester produces 350 kW/h of electricity. The expected payback period
for this facility is 6 years. Figure 15 illustrates the Hutterite digester system.
Figure 15: Hutterite Organic Waste
(swine manure, sewage, plant material) Digester
ATAU Course Notes – Anaerobic Digestion in Canada 31
4.6.2 Lethbridge Bioreactor, Alberta, Canada (Centralized digester)
The Lethbridge system is currently under construction and is scheduled to be in operation
in the fall of 2004. The Lethbridge system is an Integrated Waste Management System
(IWMS) that involves a combination of solutions: anaerobic digestion (biogas), aerobic
treatment (composting) and wastewater treatment. The IWMS concept is quite similar to
that of the BioGem system.
The Lethbridge system is strategically situated in an area known as “feedlot alley”, which
produces between 60 to 65% of the slaughter beef in Canada. The ECB Enviro Berlin
AG digester will accept a 60:40 split of beef manure and other organic waste (sewage,
food processing waste, slaughter house waste, etc.). The digester has been designed to
treat 100 million kg (100,000 tonnes) of manure per year. The system promises to
generate 15 Gigawatt-hour of electricity while reducing equivalent CO2 emissions by 15
kilotonnes (Loh, 2003).
This IWMS will produce methane that will be used to fuel a combined heat and power
plant and/or a fuel cell to produce thermal and electrical energy. Treated solid effluent
will be recycled into an aerobic digestion (composting). The liquid effluents will be
purified through a biofiltration process and recycled. A portion of these effluents will be
used as wash waters for barns, farm machinery and vehicles.
The construction costs of this IWMS system is estimated at $5.5 million (CAN$).
Figure 16: Lethbridge Bioreactor, Alberta, Canada
ATAU Course Notes – Anaerobic Digestion in Canada 32
4.6.3 Klaesi Digester, Ontario, Canada (On-farm digester)
The Klaesi farm is a 200 cow dairy operation in Ontario, Canada. The digester is built
into an existing concrete manure pit. The digester is an off-the-shelf Bohni design from
Switzerland. A rubber membrane on top expands or contracts depending on the amount
of gas that is collected. The manure mixture is heated to 40 oC and, when the gas is
drawn off, the liquid left is moved to a second holding tank and from there is spread onto
the farm’s 500 acres of farmland. The biogas runs a piston generator that produces power
which is "net metered" to Hydro One (Ontario's major generating company). Net
metering measures the energy used against the energy generated, resulting in a "net"
energy total from which your bill is calculated. The Bohni digester cost between
$170,000 (CAN$), with a payback of 10 years with electricity savings. The digester
produces about 450 kW per day metered into the grid.
Excess heat collected from the generator does two things: keeps the digester running at an
optimal temperature and heats water which is connected to an outdoor furnace and heats
two farmhouses as well. Maintenance on the system takes about 10 minutes a day,
mostly on the generator.
Figure 17: Klaesi Dairy Manure Digester (Paul Klaesi forefront), Ontario, Canada
ATAU Course Notes – Anaerobic Digestion in Canada 33
5.0 Treatment of Solid Waste in Canada
5.1 Composition of Organic Solid Waste
Table 13 identifies the average composition of various components typically found in
municipal solid waste in Canada; average was taken from 8 different landfills.
Table 13: Average Composition of Municipal Solid Waste from 8 Canadian
Landfills (Georgia Basin, 2002)
Components % of Total Standard Deviation
Organics 37.41 11.11
Paper 32.29 10.58
Plastic 13.31 5.37
Household Hygiene 3.80 3.27
Metals 3.36 1.50
Glass 3.11 2.30
Inorganic 2.92 3.81
Household Hazardous 2.15 2.07
Fines 1.19 1.70
Small Appliances 0.45 1.41
5.2 Organic Solid Waste Management Options
The two common options for the disposal of organic solid waste in Canada is in a landfill
or through decomposition in a dedicated reactor vessel.
5.3 Energy Recovery from Landfill Gas
5.3.1 The Production of Landfill Gas
Landfill gas (LFG) is produced by the decomposition of organic materials in municipal
solid waste (MSW). Typically, methane (CH4) and carbon dioxide (CO2) comprise 99 per
cent of LFG, with trace gases including carbon monoxide, hydrogen, nitrogen and
oxygen. The precise composition of LFG depends on the age of filled waste, and its
exposure to water.
ATAU Course Notes – Anaerobic Digestion in Canada 34
Table 14: Formation of landfill gas occurs in five stages (AGO, 1997):
1. Initial adjustment: The biodegradable portion of MSW is decomposed by microbial activity,
under mostly aerobic conditions. Aerobic decomposition is sustained by air trapped within
the landfill. Daily soil cover on the MSW supplies the necessary microorganisms for this
2. Transition stage: The trapped air is depleted, and anaerobic conditions begin to prevail.
Nitrate and sulphate become active in the biological degradation process. As a consequence,
some nitrogen gas and hydrogen sulfide gases are emitted during this stage.
3. Acid stage: A different set of micro-organisms becomes active during this stage Acidogens,
or acid formers, are now the principal microorganisms responsible for biodegradation. Their
activity is characterised by a three-step process: hydrolysis, acidogenesis and carbon dioxide
4. Methane fermentation stage: During this stage, methanogens or methane formers, become
active. These organisms convert the acetic acid and hydrogen gas formed by the acid formers
into methane and carbon dioxide. Most LFG is generated during this stage.
5. Maturation stage: Maturation occurs after the readily available biodegradable material has
been converted into methane and carbon dioxide in the previous stage. The biodegradation
process is now slow, because the nutrients available and easily biodegradable organic
material was exhausted in earlier phases. LFGs are generated at a very slow rate.
Figure 18: Concepts of a Bioreactor Landfill (AGO, 1997)
5.3.2 Potential Yield and Release of LFG Over Time
The volume of LFG generated from a landfill depends on the proportion of materials in
the MSW deposits with some organic content, which is the food for microorganisms in
the anaerobic digestion process. Organic materials are not decomposed in equal
ATAU Course Notes – Anaerobic Digestion in Canada 35
proportions, because the degradable fraction varies from product to product. For example,
a larger proportion of food wastes degrade slower than yard wastes. The amount of
degradable materials in MSW is determined by the composition of waste, and its
exposure to moisture in the landfill. Exposure to moisture is particularly important, since
it can be managed as part of the landfill operations.
The total yield of LFG is not released as soon as decomposition commences. LFG is
generated over time, and the degradation rate varies across types of waste. Table 15
shows the length of time over which half of the degradable fraction is transformed in
LFG. In all cases, decomposition of the remaining degradable matter extends over long
periods of time.
Table 15: Time taken for decomposition of
half of the degradable content (AGO, 1997)
Type of waste Years
The basic pattern is that LFG generation from a particular quantity of MSW is highest in
the two years after waste has been filled. During this time, anaerobic digestion of most of
the degradable content of food wastes occurs. LFG generation continues after this time
but at slowly decreasing rates. While gas generation can extend for periods of up to fifty
years, in most cases LFG release occurs within five years, because food and garden waste
typically comprise a large proportion of all organic materials in MSW (AGO, 1997).
The annual rate of methane generation is the proportion yielded in one year of the total
amount of LFG that the landfill has the potential to yield over its lifetime. The annual rate
of methane generation is higher if more of the MSW is food waste or exposed to optimal
amounts of water. If a landfill comprises a relatively high proportion of paperboard, or is
located in a dry climate, then the annual rate of methane generation will tend to be lower.
The total yield of LFG and the annual rate of generation are key factors in the annual
flow of LFG from a landfill. Table 16 lists five factors that determine the flow of landfill
ATAU Course Notes – Anaerobic Digestion in Canada 36
Table 16: Five factors that determine the annual flow of LFG (AGO, 1997)
Factor Units Label
1 Total LFG yield per kg of MSW m3/kg Lo
2 Filling rate kg/yr R
3 Time since landfill opened (years) number of years t
4 Time since landfill closure (years) number of years c*
5 Annual rate of LFG generation 1/years k
The US EPA has set 0.05 as the default value for k in their model of LFG generation
(AGO, 1997). The US EPA model (first-order decay model) is used to estimate LFG:
LFGt = LoR[e-k(c) - e-k(t)]
The model can be used to get an idea of the LFG generated at a landfill with particular
characteristics. It does not produce estimates of LFG flow rates. Model assumptions
about climate, composition of waste and landfill management, will not apply.
Table 17: Typical Methane and Energy Production from Landfill Gas
(Environment Canada, 2001)
CH4 captured in LFG 48 m3/tonne waste processed
Percent CH4 in LFG 50-53 %
Annual Energy generation 143 kWh/tonne of waste processed
Greenhouse gas emission
128,000 tonnes eCO2/yr
reductions (LFG combustion)
Leachate management and filling practices comprise a set of important decisions that
influence LFG generation and collection. Moisture content fundamentally affects
anaerobic digestion in landfills. Filling practices affect the anaerobic process. The more
frequently waste is covered, the more quickly anaerobic decomposition commences.
Some landfill characteristics are beyond the direct influence of the landfill owner. For
example, the site’s area and depth is typically set by the local government, and may not
be able to be increased. Proximity to groundwater, and exposure to rainfall, are fixed.
Thorough assessments of the landfill’s area, depth and proximity to groundwater are vital
to the examination of potential LFG recovery.
ATAU Course Notes – Anaerobic Digestion in Canada 37
5.4 Dedicated Anaerobic Digestion of Organic Solid Waste
Unlike energy recovery from landfill gas which can be unreliable, anaerobic digestion of
organic solid waste in a dedicated facility provides the opportunity to obtain energy from
the biogas and compost from the sludge residue in a more controlled environment.
Anaerobic digestion may have a number of advantages over modern sanitary landfills in
that anaerobic digestion:
• Makes landfills easier to manage by removing potentially problematic organic wastes,
although the microbial activity in the landfill can assist in the breakdown of other wastes.
• Avoids the generation of gas in landfills.
• Contributes to recycling targets (or enables recyclable material to be reclaimed).
• Provides an enclosed system that enables all of the gas produced to be collected for use.
• Provides a soil conditioner as one of the end products (energy from methane being the
other end product).
Modern sanitary landfills sites that are designed for methane recovery yield only 30-40
per cent of the amount of gas actually generated. They are designed as tightly sealed
units slowing the degradation process, with some biodegradable materials that might
continue to degrade for 50-100 years (AGO, 1997). By comparison a closed vessel
bioreactor can enable 100 per cent of gas generated to be recovered and over a much
shorter time period.
5.4.1 Anaerobic Digester Technologies MSW
The leading anaerobic digestion concepts for MSW are dry continuous digestion, dry
batch digestion, leach bed processes, wet continuous digestion, and multi-stage wet
Dry continuous digestion involves a continuously-fed digestion vessel with a digestate
dry matter content of 20-40 per cent. Both completely-mixed and plug-flow systems are
available. Plug flow systems rely on external recycling of a proportion of the outgoing
digestate to inoculate the incoming raw feedstock. In both cases, the requirement for only
minimal water additions makes the overall heat balance favourable for operation at
thermophilic digestion temperatures (50-55oC).
ATAU Course Notes – Anaerobic Digestion in Canada 38
Dry batch digestion involves loading a vessel with MSW and digestate from another
reactor. The vessel is sealed and left to digest naturally. Leachate is tapped from the base
of the vessel and recirculated to maintain a uniform moisture content and redistribute
soluble substrates and methane-producing bacteria.
The leach-bed process is similar to the dry continuous process except that leachate from
the base of the vessel is exchanged between established and new batches to facilitate start
up, inoculation and removal of volatile acids in the active reactor. The concept has also
been described as "sequential batch anaerobic composting" (SEBAC).
Wet continuous digestion involves the MSW feedstock being slurried to about 10 per
cent dry solids to provide a feedstock that can be fed to a conventional completely-mixed
digester similar to those commonly used for sewage sludges or farm slurries. Effective
removal of glass and stones is required in the feed preparation stages to prevent their
rapid accumulation in the bottom of the main digester tank.
Multi-stage wet digestion involves making a slurry of the MSW with water or recycled
liquor. It is then fermented with hydrolytic and fermentative bacteria to release volatile
fatty acids which are then converted to biogas in a specialist high-rate industrial
anaerobic digester, usually an anaerobic filter or an upflow anaerobic sludge blanket
5.4.2 Methane Production from Designated Anaerobic Digestion of MSW
The main determinant of the amount of biogas is the amount of carbon in the organic
waste. When the waste degrades some of the carbon becomes part of the cellular material
of the microbes (assimilated carbon) and the rest of the carbon forms methane and carbon
dioxide (dissimilated carbon). The more anaerobic the process, the more of this carbon is
converted to methane. The amount of carbon is expressed in terms of the percentage of
fresh weight (AGO, 1997):
Table 18: Typical Methane and Energy Production from Anaerobic Digestion of
MSW (Environment Canada, 2001)
Biogas generation 110 m3/tonne waste processed
Percent CH4 in biogas 52-54%
Annual Compost produced 665 kg/tonne of waste processed
Annual Electricity generation 320 kWh/tonne of waste processed
Greenhouse Gas Emission Reduction
ATAU Course Notes – Anaerobic Digestion in Canada 39
(Biogas combustion) 161,500 (tonnes eCO2/yr)
6.0 Use of AD in the Treatment of Industrial Wastewaters in Canada
6.1 Anaerobic digestion of industrial wastewater
Factors which should be considered when assessing the suitability of anaerobic treatment
of industrial wastewater include:
• The nature of the wastewater
• The concentration of organic pollutants
• The concentration of suspended solids
• Presence of toxic compounds
• Nutrient requirements
Most of the larger industrial anaerobic treatment plants are in the food, drinks and
fermentation sectors, and the pulp & paper industry. The high organic content of food
industry waste means that in principle they should be easily treated with anaerobic
digesters. In practice, several factors impede treatment. High strength and fluctuations
that occur in the type and quantity of wastes to be treated along with cleaning aids and
sanitisers create problems.
Dairy product effluents are warm, strong and ideal for anaerobic digestion if the process
is well controlled. Starch effluents are high strength but with a high proportion of
colloidal solids which reduces biodegradability. The high strength means that anaerobic
digestion using a solids tolerant process is an efficient treatment process. Sugar industry
effluent is suitable for anaerobic digestion although over a period of time the effluent
becomes highly acidic. Lime is used to offset acidity but this accumulates reducing the
space for active biomass. Effluent from the manufacture of confectionery and soft drinks
can be high strength although they are periodic and usually hot. The strength of the
effluents means that discharge costs are high and can be reduced through anaerobic
6.2 Digestion technologies
The choice of digestion process is driven by the type of wastewater. The more rapid the
treatment process the more likely it is to be viable. Provided that the process is properly
controlled the determinant of the rate of digestion is the concentration of bacteria.
ATAU Course Notes – Anaerobic Digestion in Canada 40
Industrial processes have been designed to retain the bacteria or to recycle the bacteria.
The three main processes are the contact stirred tank reactor, the upflow anaerobic sludge
blanket, and fluidised or expanded bed reactors.
6.2.1 The contact stirred tank reactor (CSTR)
In the CSTR bacteria are physically separated from the effluent by settlement or filtration
and recycled back into the reactor. The warmth of the effluent (relative to ambient
temperatures) and the methane gas being given off make the solids float. Settlement has
to be assisted by degassing, cooling, filtration or inclined plates.
6.2.2 Upflow anaerobic sludge blanket reactor (UASB)
Key elements in the feed substrate for successful formation of the sludge blanket are
calcium, phosphorus, aluminium and silicon, along with a large population of filamentous
bacteria and the generation of bacterial polymers. To reduce the time required for
acclimatisation and adaptation it is now usual to start with a large inoculum of an already
granulated sludge. The reactor baffles are used to promote gas/solid separation to retain
the granules. The degree of mixing in the sludge blanket is a function of the gas
production and the upward flow velocity of the influent.
6.2.3 The fluidised bed
The fluidised bed is designed to overcome the difficulties of biomass separation in
completely mixed reactors and the loss of granulation or blockage in plug flow reactors.
This is a relatively new technology and has acquired the reputation of being difficult to
operate. Significant further development appears to be required before it is adopted more
6.3 Industries Utilizing Anaerobic Digestion
6.3.1 Brewing Industry
Brewing produces a cold relatively weak effluent from which several by-products have
already been recovered such as, malt residues, spent hops, yeast, and carbon dioxide.
Reductions in water-use leading to a more highly concentrated effluent increases the
opportunities for anaerobic digestion. Water charges and effluent charges are decisive in
forcing changes to waste management practices. Distillery and fermentation industry
effluents are high strength even where some by-products have already been recovered.
ATAU Course Notes – Anaerobic Digestion in Canada 41
Anaerobic digestion is a sound solution to effluent problems in an industry that has few
alternatives. Main process - Contact Stirred Tank Reactors
6.3.2 Vegetable Processing Industry
Most vegetable processing produces a weak and cold effluent that favours aerobic
treatment. Exceptions are pea processing and snack foods such as potato chips. These
processes produce a higher strength waste with soluble starches that are suitable for
anaerobic processes. A high degree of seasonality in pea processing militates against the
high capital costs of anaerobic treatment. Main processes - Upflow Anaerobic Sludge
Blanket and Contact Stirred Tank Reactors
6.3.3 Meat Processing Industry
Meat processing wastes are difficult to handle irrespective of the treatment process used.
They contain blood, faecal matter, grease, bone fragments, and hair along with biocides
and disinfectants. Long retention time processes are considered to have the greatest
potential. Chemical contamination from organic and inorganic sources poses unique
problems as does the slow degradation of cellulosic material. The effluent is high strength
and warm which favours anaerobic treatment despite the difficulties.
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ATAU Course Notes – Anaerobic Digestion in Canada 44