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Anaerobic Digestion

 

The waste management process anaerobic digestion involves the decomposition by bacteria of complex organic materials in the absence of oxygen. The main product of anaerobic digestion is a biogas rich in methane (CH4) which can be harnessed as a source of energy.

Wastes buried in a landfill degrade under anaerobic conditions, since the layers of waste are compacted and covered, excluding air. A compost heap provides a similar bacterial break-down, but in an oxygen-rich (aerobic) atmosphere.

Although anaerobic breakdown takes place naturally within a landfill, the term anaerobic digestion (AD) is normally used to describe an artificially accelerated operation in closed vessels. This process has been used as a method of handling certain wastes - sewage sludge and animal slurry - for many years. In 1895, the English city of Exeter enjoyed street lighting using biogas from a sewage treatment plant. Indeed, one report points to anecdotal evidence that biogas was used to heat bath-water in Assyria three thousand years ago. As a treatment for municipal solid waste (MSW) it is relatively new. Perhaps the first large-scale investigation of AD for MSW was run by Waste Management International in Florida, USA between 1978-1985.

There are now more than 115 AD plants operating or under construction world wide, using MSW as their principal feadstock. The total installed capacity is almost five million tonnes per annum (Mtpa), rising to almost seven Mtpa when the 40 units now planned are built.

Waste disposal practices

Waste will cause environmental pollution if it is not carefully managed.

More waste is landfilled worldwide than is managed by any of the other options (eg recycling, composting, energy recovery). Standards vary, and while most of the developed world operate engineered, controlled landfills which reduce pollution, many countries rely on uncontrolled, insanitary dumps.

As organic wastes decompose anaerobically within a landfill, a gaseous mixture of methane and carbon dioxide in roughly equal parts is produced. This landfill gas can burn or even explode. To control the gas, many landfill sites have recovery systems to either burn the gas directly, or use it for electricity production or heat generation.

However, even when landfill sites are specifically engineered to recover landfill gas, collection is not very efficient, often capturing only 30-40 per cent of the gas. The remainder

can escape to the atmosphere. Since methane is a greenhouse gas with up to 60 times the impact of CO2, such emissions are not desirable.

Modern landfills are normally designed as sealed units, slowing the degradation processes. Wastes entombed in this way might continue to degrade and give off gases for a century or more.

In contrast, when wastes are digested anaerobically in a closed container, degradation is accelerated and all the gas generated can be collected for use as a fuel. Anaerobic digestion will produce around 100-200 cubic metres (m3) of biogas per tonne of the organic fraction of MSW, comprising typically 55-70 per cent methane and 30-45 per cent carbon dioxide. The process itself consumes 20-40 per cent of the energy produced, leaving around 100-200 kilowatts per tonne of organic fraction of MSW for use or sale elsewhere.

Only biodegradable wastes -those with organic or vegetable origins - can be processed in AD plants this way, but since such wastes can form 70 per cent of household waste (if paper is added to vegetable and kitchen wastes), there is considerable opportunity to divert wastes from landfill or to pre-treat them before landfilling. Waste paper is an ideal material to process in a digestion system when it is too contaminated for recycling, or where markets are temporarily poor. Anaerobic digestion schemes can provide a solution to a temporary waste paper gluts, balancing supply and demand. It is a far better approach than simply landfilling vast quantities of paper.

The diagram below shows the average components of a European household's waste. Even assuming paper is totally recycled, (an unlikely scenario), around one third of MSW is organic.

Vegetable and kitchen wastes can be readily digested. Garden waste - green wastes - can also be included, but woody material is less suitable because of its poor degradability (due to a high lignin content).

Anaerobic digestion does not compete with materials recycling schemes, as four of the most commonly recycled materials - steel, aluminum, glass and plastics - are unsuitable for digestion and can be removed for recycling before the remaining organic wastes are treated.

The process

The feedstock for an AD plant can either be organic wastes which have been separately collected and delivered for processing, or which have arisen from other mechanical or manual sorting plants. An alternative source of organic matter is green waste collected at centralized collection points.

The purity of the material fed into the AD process determines the quality of the end-product. Some plants are designed to remove as many other materials as possible, (for example ferrous metals), before digestion. Others are designed to optimize gas collection for energy production, and a soil conditioner may not be their main objective. Others might optimize the horticultural product, regarding energy as of secondary importance.

Having separated recyclables and unwanted materials from the waste, the organic material is shredded and fed into the digestor. If other very wet wastes (eg sewage sludge) are included, then addition of further water may not be necessary. Different AD systems call for different proportions of solid to liquid. While feedstocks often contain 25-40 per cent solids, certain technologies can cope with levels as high as 45 per cent.

ADVANTAGES OF ANAEROBIC DIGESTION

Modern AD processes can be designed to operate under different temperature conditions. Systems are described as mesophilic (20-40C, usually 35C) or thermophilic (45-60C, usually 55C). Which ever design is selected (governed by waste composition, treatment strategy and design preferences), the temperature is held more or less constant.

The wastes remain in the heated digestor for varying periods of time, depending on technologies, external temperature fluctuations, and other variables such as waste composition.

DISADVANTAGES OF ANAEROBIC DIGESTION

  • usually more expensive than composting
  • does not treat the whole waste stream
  • can experience materials handling difficulties
  • technology is rather novel

A standard single-stage mesophilic digestion plant will take between 12-25 days to fully process the waste. However, thermophilic systems can achieve the same results in six days. Increasing the number of stages linked in the AD plant will also reduce digestion times. Gases given off during the decomposition are continuously drawn off.

Mixing the reactor contents is not always necessary if the waste is held for short periods, or if the reactor has a recycle loop (through which more waste is fed). Mixing generally leads to a more efficient AD system by providing uniform conditions within the vessel, speeding up the biological reactions.

After biological degradation, the digestate is removed and usually cured aerobically, as well as being screened to remove oversized and unwanted items like glass shards or pieces of plastic. The degree of screening required varies according to the intended use of the final product.

Economies of AD

The capital costs of a modern AD plant are less than those of an energy from waste plant, but similar to those of a Material Reclamation Facility, which sorts recyclables diverted from household waste. A plant able to handle up to 50,000 tpa of organic and putrescible wastes might cost 8-lOm. Experience in Europe suggests that 15-20,000 tpa is the smallest scale which is financially viable.

As with many developing technologies, AD costs have fallen over time and one study cited a 10,000 tpa plant costing US$8.4 million in 1992, while a 20,000 tpa unit cost US$5.6 million only four years later. Overall, AD costs range between US$30-50/t for a 20,000 tpa facility, falling to less than US$30/t for a 100,000 tpa plant.

The income set against these costs could come from several sources: the gate fee (handling fee) for each tonne of waste delivered to the plant; the sale of recovered metals separated prior to digestion; the sale of biogas or energy, and; the sale of soil conditioner product. The latter is the least assured, as markets remain generally insecure. However, markets for compost are developing for parks and landfill cover, landscaping, agricultural and horticultural use.

Variable waste stream

One difficulty that AD plants may experience, which is less of a problem to other waste handling systems, is the variable nature and amount of organic waste, according to location and season. In summer, householders in suburban and rural areas produce grass clippings and other garden wastes, as well as more organic kitchen wastes. During autumn, gardens contain tree prunings and other woody material. In winter there are virtually no green garden materials in the incoming waste stream, only kitchen wastes.

The advantage of making full use of gases created by rotting wastes to substitute for other energy sources, with the added potential benefit of a soil conditioner product, make AD a valuable waste management technology. Perhaps more important is the fact that pre-treating organic wastes enables better management of landfill sites, minimizing uncontrolled emissions. Anaerobic digestion plants can provide a way to comply with requirements being introduced in many countries to limit the disposal of organic waste.

To landfill stabilized materials, having first captured the energy, appears to make far more environmental sense than landfilling untreated wastes which take many years to become stable.

Anaerobic digestion systems for municipal solid waste

Wet single-step

MSW is slurried with water, giving an input material with per cent solids, and is fed to a complete mix tank digester. This approach is amenable for co-digesting MSW with more dilute feedstoks.

Wet multi-step

Several ranges are involved, in which slurried MSW is first fermented by hydrolytic bacteria to release volatile fatty acids. These are then converted to biogas in a high-rate anaerobic digester. This technique is useful for biowaste and food processing wastes.

Dry continuous

This system uses relatively dry (20-40 per cent solids) material which is batch-loaded into a continuously fed digestion vessel. Minimal water addition makes this technique suitable for thermophilic (more heat-tolerant) bacteria.

Dry batch

Here the vessel is charged with feedstock, inoculated with digestate from another reactor and sealed. Leachate is re-circulated to maintain uniform conditions.

Sequencing batch

A variant of the dry batch process, leachate is exchanged between established and new batches. This helps start-up, inoculation and removal of volatile matter.

Bio-reactor landfills

With a landfill divided into cells with leachate re-circulation, the site can act as a large-scale high solids AD plant.

The World Resource Foundation provides information on international sustainable management of municipal solid waste. WRF is totally independent of commercial vested interests and is a registered British charity (Number 3265973).

The Foundation publishes Warmer Bulletin, and is advised by a council of eminent specialists, all of whom give their services free of charge.

Other titles in this series of Information sheets include:

Anaerobic digestion

Compost

Composting with worms

Construction & demolition wastes

Electrical & electronic wastes

Energy from waste

Glass re-use & recycling

Household hazardous waste

Landfill

Life cycle analysis & assessment

Materials recovery facilities

Packaging

Paper making & recycling

Plastics

Re-use

Steel making & recycling

Textiles reclamation

Vehicle recycling

The World Resource Foundation

Washable & disposable nappies

Waste minimization

Single copies of these information sheets are available free of charge. Additional copies may be purchased at 1 each. Further details from WRF.

The World Resource Foundation,

Heath House, 133 High Street, Tonbridge,

Kent TN9 1DH, UK

Tel: (Intl +44) (0)1732 368333

Fax: (Intl 44) (0)1732 368337

E-mall: library@wrf.org.uk

Internet:: http.//www.wrf.org.uk

WRF November 1998