Information Sheet Anaerobic digestion by axu10828


									Anaerobic digestion
Anaerobic digestion is the             killing harmful pathogens. It is     Methanogenesis

                                                                                                                    Information Sheet
breakdown of organic material by       then pumped into the digester
micro-organisms in the absence         where bacteria break down the        Methane (60%) and carbon
of oxygen. Although this takes         material and form biogas, leaving    dioxide (40%) are produced
place naturally within a landfill,     a digestate. Figure 1 shows a        from the organic acids and their
the term normally describes an         typical schematic for an AD plant.   derivatives produced in the
artificially accelerated operation                                          acidogenic phase. The methane
in closed vessels, resulting in a      There are three main process         is a useful fuel source and
relatively stable solid residue.       stages in anaerobic digestion, as    methanogenic bacteria play a
                                       follows (as also in Figure 2):       further role in maintaining wider
Biogas is generated during                                                  breakdown processes.
anaerobic digestion (AD) - mostly      Hydrolysis
methane and carbon dioxide - this                                           Efficient mixing of the contents
gas can be used as a chemical          Insoluble organic polymers such      of the digester improves the
feedstock or as a fuel.                as carbohydrates, cellulose,         contact between the material and
                                       proteins and fats are broken         the resident bacteria. Mixing of
Anaerobic digestion can treat          down and liquefied by enzymes        the waste slurry in the digester is
many biodegradable wastes,             produced by hydrolytic bacteria.     important in maintaining a high
including wastes that are                                                   rate of anaerobic biodegradation
unsuitable for composting, such as     Carbohydrates, proteins and          and a high production level of
meat and cooked food.                  lipids are hydrolysed to sugars      gas. The mixing process disperses
                                       which then decompose further to      the incoming waste within the
History of AD                          form carbon dioxide, hydrogen,       digesting sludge, improving
                                       ammonia and organic acids.           contact with the micro-organisms.
Anecdotal evidence indicates           Proteins decompose to form
that biogas was used for heating       ammonia, carboxylic acids and        Monitoring the acidity within
bath water in Assyria 3,000 years      carbon dioxide. During this phase    the digester is necessary to
ago. The first digestion plant was     gas concentrations may rise          provide optimum conditions
said to have been built at a leper     to levels of 80 per cent carbon      for the balanced growth of
colony in Bombay, India in 1859.       dioxide and 20 per cent hydrogen.    bacteria. Monitoring takes place
AD reached England in 1895,                                                 in the reactor using probes. The
when biogas was recovered from         Acidogenesis                         concentration of volatile fatty
a sewage treatment facility to fuel                                         acids is an important parameter
street lamps in Exeter, Devon.         Organic acids formed in the          for monitoring as this can be the
                                       hydrolysis and fermentation          first indicator that digestion is not
In Germany in 1951 half the            stage are converted by acetogenic    progressing normally.
biogas from sewage sludge was          micro-organisms to acetic
being converted for use as fuel for    acid. At the end of this stage       Digesters
cars. AD has also been used to         carbon dioxide and hydrogen
treat agricultural waste for several   concentrations begin to decrease.    Digesters can be categorised
years and it now treats segregated                                          by dry or wet systems – below
municipal solid waste. Putrescibles                                         15 per cent dry solids is termed
and paper in household waste                                                wet. Also digesters can operate
are ideally suited to anaerobic                                             within two temperature ranges,
digestion, as are certain types of                                          either at 350C (mesophilic) or 550C
garden waste.                                                               (thermophilic) Some are loaded
                                                                            in batches while others have
The process                                                                 continuous feed. On completion
                                                                            of the process digesters are
The process begins with                                                     emptied leaving 10-15 per cent
separation of household waste                                               behind as a seed culture for the
into biodegradable and non-                                                 next batch.
biodegradable waste. The
biodegradable material is shredded,                                         Various AD processes have been
slurried and then screened and                                              developed, operating at different
pasteurised to start the process of                                         temperatures, moisture levels and

                                                                                                            reactor is connected to a fresh
Information Sheet

                                                                                  ELECTRICITY               batch of waste in a second
                        Manual                     Engine +                                                 reactor.
                        sorting                    generator                        HUMOTEX
                                                                                                            Dry continuous digestion

                                                                            Steam                           Waste is fed continuously into a
                       Shredding                                           generator                        digestion reactor with 20-40 per
                                                                                                            cent dry matter.
                                                 BIOREACTOR                         maturation              The process of anaerobic
                                                                                                            digestion in a digester takes
                                                                                                            about 35 days, which compares
                                                                                    Mixing unit
                                                                                                            favourably with a landfill site
                        Magnetic                                                                            which may remain active for 35
                                                              Mixer       POLYMER
                                                                                                            years producing methane and
                                                             + pump       + WATER                           leachates which can harm the
                            Dosing unit                                                Press
                        (add recycled water)        PRE-TREATED WATER
                                                                                                            Digestate use

                                                                                 WASTE WATER                Digestate is the residual fibrous
                     Figure 1                                                   TO TREATMENT
                                                                                                            material left at the end of
                     Schematic diagram for a typical Dry Continuous Single Stage Anaerobic Digestion
                     system - the DRANCO Process
                                                                                                            processing. End-use ranges from
                     Source: International Energy Agency technology review (1997)                           landfill cover, landspread for
                                                                                                            agriculture or the production of a
                                                                                                            high quality soil conditioner after
                                                                                                            an additional maturation process.
                                                                                                            The quality of the original input
                    speeds. The purity of material fed                digestion of biodegradable waste      biowaste determines the quality
                    into the AD process determines                    with sewage sludge.                   of the digestate at the end of
                    the quality of the end product.                                                         the process. The efficiency of
                    Some plants are designed to                       Multi-stage wet digestion             the source-separation systems is
                    remove as many other materials                    There are a range of multi-stage      important as the contamination
                    as possible (for example, ferrous                 wet digestion processes which take    of the biowaste with potentially
                    metals) before digestion. Others                  municipal solid waste and add to      toxic chemicals and too many
                    are designed to optimise gas                      recycled liquor. The mixture is       non-biodegradable inclusions will
                    collection for energy production                  fermented by micro-organisms to       be apparent in the final product.
                    and a soil conditioner may not be                 release volatile fatty acids. These   The presence of heavy metals
                    their main objective. Others might                are then converted to gas in a        severely limits its eventual use.
                    optimise the horticultural product,               specialised high-rate industrial
                    regarding energy of secondary                     digester.                             The digestate produced by most
                    importance. All of the processes                                                        operational plants is destined
                    share a common approach where                     Dry batch digestion                   for use as a soil conditioner
                    shredded materials and water are                  Waste is fed into the reactor with    and most have a useful level
                    held in a reactor for 6-25 days at                digested material from another        of nutrients resulting in less
                    a constant temperature between                    reactor and then the digester is      demand for inorganic fertilisers.
                    35-550C.                                          sealed. Leachate is collected from    There is also evidence that using
                                                                      the bottom and is re-circulated       digestate on land has the benefit
                    Wet continuous digestion                          to distribute nutrients and micro     of suppressing normal pathogen
                    Waste is slurried with a large                    organisms and maintain even           and parasite levels.
                    proportion of water to give a                     moisture levels.
                    feedstock of 10 per cent dry solids.                                                    Depending on the quality of
                    Glass and stones must be removed                  Leach-bed process                     the original feedstock, digester
                    to prevent them accumulating                      Similar to the dry batch method,      residue can be used for landfill
                    in the bottom of the reactor.                     but once the third stage of           cover or further matured into a
                    This method can be used for co-                   methanogenesis is reached the         compost product.
                                                       BIOWASTE                              Hydrolysis         hydrolytic

                                                                                                                             Information Sheet

During the process of anaerobic                     ORGANIC ACIDS                           Acidogenesis        acidogenic
                                                    ALCOHOLS ETC
digestion the organic wastes                                                                                     bacteria
produce biogas. This is composed
largely of methane (60%) and                          Acetogenesis                                          Acetic acid
carbon dioxide (40%). Methane                                                                               + acetate
is a greenhouse gas thirty                                                hydrogenating
                                                                        & dehydrogenating                   acetolclastic
times more damaging than the                             Hydrogen            bacteria                         bacteria
equivalent amount of carbon                               + CO2
dioxide. The quality of the biogas
produced from AD affects its final              REDUCTION
usefulness. The main concern                    hydrogenetrophic
in this context is the presence of                  bacteria
hydrogen sulphide which occurs as                                                                          METHANE
                                                                                                            + CO2
a metabolic bi-product of sulphur-
reducing bacteria in the digester.
Hydrogen sulphide can rapidly
corrode the gas-handling and
electricity generating equipment         Figure 2 Flowchart indicating the main stages in Anaerobic Digestion
in the plant.                            Source: Evans G, Biowaste and Biological waste treatment 2001

If one tonne of putrescible food
waste consists of 77 per cent water     useful bacterial populations.                  Energy use
and 23 per cent solids, the digester    This method can produce a
will convert approximately 75 per       faster reaction then the original              Whereas composting is an energy-
cent of the solids to biogas.           start-up. If all the liquor was                consuming process, requiring
                                        recycled in this way however, the              50-75 kilowatt-hours per tonne
The maximum possible yield of           concentration of contaminants                  (kWh/t) MSW input, AD is a net
biogas in is 400m3, but in practice     would become too high. Excess                  energy-producing process (75-150
is nearer to 100m3. This has an         liquor can be disposed of in                   kWh/t MSW).
energy value of around 21-28 MJ/        three ways: discharge, landfill or
m3. Between 20 - 50 per cent of         landspreading. Discharge is the                The amount of energy required to
the energy produced will be used        simplest but it may be expensive               run a digester is directly related
to run the plant.                       to achieve environmental                       to the moisture content of the
                                        standards required by regulators.              feedstock. High-moisture systems
Biogas may be used directly or                                                         use more heat but require less
as a replacement fuel for kilns,        Limits on certain chemical                     electricity to circulate the fluid
boilers and furnaces located close      components are likely to be                    digestate. Anaerobic digestion
to the AD site. If the gas is used in   in place. Discharge into the                   requires an additional 15kWh/
power generation gas clean-up is        sewerage system is more likely                 ton of energy in comparison to
required to remove corrosive trace      to be permitted than to a water                aerobic composting plants.
gases, moisture and vapours.            course. AD process liquors can be
                                        very polluting and treatments may              Odour
Process liquor                          include aeration de-nitrification
                                        and reverse osmosis techniques.                Several aerobic plants have been
There has been a focus on the                                                          closed or put under constant
digestate and biogas production         A major area of concern is the                 review due to odour complaints.
from AD but the process liquor          heavy metal content particularly               In anaerobic systems most volatile
is often overlooked. There              when considering application                   components are broken down by
are however environmental               to land, and also nitrogen and                 bacteria in the digester. A study
considerations and costs to be          phosphorous content.                           has shown that whereas 588g/t of
considered with the generation          Haulage charges for transporting               volatile organics were produced
of contaminated water. Some             contaminated water to disposal                 in aerobic composting, only 3g/t
process liquor is used to re-wet        may be a third of the total                    were produced in an anaerobic
incoming biowaste as it contains        transport and disposal fee.                    system.
                    Economics of AD                       be a problem for AD plants. In        Warmer Bulletin is published by
Information Sheet

                                                          summer households produce             Residua, a company formed to
                    The capital investment required       more organic kitchen wastes and       provide world-wide information on
                    for a modern AD plant are less        grass clippings, while in autumn      sustainable management of municipal
                    than those of an energy from          prunings and woody materials          solid waste.
                    waste plant but similar to those      predominate.                          Titles in this series include:
                    of a materials reclamation facility
                    (MRF). Experience in Europe           Anaerobic capacity                    advanced thermal processing
                    suggests that a plant which can                                             aluminium making & recycling
                    handle up to 15-20,000 tpa is         Anaerobic capacity currently          anaerobic digestion
                    the smallest scale which will be      stands at less than five per cent     batteries
                    financially viable. High costs are    of the total composting capacity      carpets
                    imposed by the superior technical     in Europe, with about 1.2 million     cartons
                    requirements to provide adequate      tonnes (Mt) of household and          compost
                    gas seals to prevent air ingress,     mixed waste being treated in          composting with worms
                    safe gas handling and internal        biogas plants.                        construction & demolition wastes
                    environmental controls and                                                  electrical & electronic wastes
                    monitoring techniques, such as        In Denmark 1.1 Mt biodegradable       energy from waste
                    the detection of very low levels of   wastes are treated annually each      glass re-use & recycling
                    concentrations of hydrogen (an        year. Plants can treat between 15     healthcare wastes
                    intermediate product).                - 100,000 tpa organic wastes.         household hazardous wastes
                                                                                                integrated waste management
                    When the digestion process is         Conclusion                            landfill
                    complete the digester is emptied                                            life cycle assessment
                    and 10 – 15 per cent may be left      The technology behind AD is           materials recovery facilities
                    behind as a starter for the next      able to process MSW biowaste          nappies
                    batch.                                but there are limitations to the      packaging
                                                          process. AD systems are best          paper making & recycling
                    Benefits                              designed for waste reduction or       plastics
                                                          methane production. If the aim is     re-use
                    Anaerobic digestion is a net          methane production, the quality       steel making & recycling
                    producer of energy – a renewable      of the feedstock is not critical.     textiles
                    energy source. It reduces leachate    However for most applications         tyres
                    and landfill gas production, while    the aim is to achieve maximum         vehicle recycling
                    generating a valuable soil            breakdown and have a usable           waste minimisation
                    conditioner (and less need for        product at the end of the process.    zero waste
                    artificial fertilisers).              The nature of the feedstock in this   Single copies of these information
                                                          case is of great importance.          sheets are also available free of charge
                    Enclosed AD systems enable all of                                           to Warmer Bulletin subscribers,
                    the gas produced to be used.          This, in conjunction with cost,       who can also collect copies from the
                                                          is the most common argument           internet at
                    Problems                              against AD.                           Additional copies may be purchased
                                                                                                at £1 each.
                    Anaerobic digestion is relatively     Cost is always an issue but where     Further details available from:
                    expensive and requires a major        the economics are favourable          Residua
                    capital investment. Waste water       the particular advantages of          1st Floor
                    from the process may contain          AD, especially the volumetric         The British School
                    a high concentration of metals,       reduction achievable over other       Otley Street
                    nitrogen and organic materials.       biological treatment methods,         Skipton
                                                          may make AD a desirable option.       North Yorkshire
                    Because of the complex                                                      BD23 1EP, UK
                    association of different types        Because of regulatory changes         Tel: (Int+44) (0) 1756 709 800
                    of bacteria, digesters have a         regarding biowaste management,        Fax: (Int+44) (0) 1756 709 801
                    higher risk of breakdown and          companies are now exploring AD        Email:
                    may be difficult to control. The      technology for future commercial      Int:
                    variable nature of the waste may      advantage.                            Printed on recycled paper

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