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Anaerobic Digestion (AD)

Anaerobic Digestion is a complex biochemical process which takes place in a vessel in the absence of oxygen and results mainly in the formation of a carbon dioxide and methane gas mixture known as "Biogas" (similar to Landfill Gas in composition) and a solid and liquid residue (the liquid may be reused in some processes). Mechanical processing and separation of MSW is essential for most AD systems and the non-biodegradable materials should be removed prior to processing. There may be an element of post digestion extraction of contrary materials for recycling. Greenfinch AD (on-farm digestor)A typical feedstock comprises biodegradable municipal solid wastes (BMW) plus other enriched organic waste streams (sewage sludge, agricultural, food processing wastes).

The organic waste stream is passed to the anaerobic digester where the material remains for a suitable retention time under controlled conditions. The "Biogas" produced during digestion is transferred to a gasholder and used to provide internal electrical power generation and heating requirements. In the UK surplus electrical power may be sold as power which is eligible for a subsidy  known as Renewables Obligation Certificates (ROCs), which the UK government awards.The fossil fuel UK Power Industry are then required to pay the biogas producer for the power produced under the RO Certificates, to achieve their own annual  non-fossil fuel derived power quota. Since 2009, there have also been a UK system of enhanced Feed-in-Tarrifs for renewable power, meaning that the biogas power producer also gains from a higher price paid to him per unit of electricity fed out inot the national energy grid.

Schematic of Inputs and Outputs of a typical Anaerobic Digestion process

Anaerobic Digestion process schematic

Typical capacity: 5,000 tpa - 60,000 tpa (modular facilities, larger capacity can be achieved by a number of digesters on one site).

Land requirements: Less than for aerobic composting and waste-to-energy plants, depends on the process adopted. Estimated requirement is 1m2 per tonne, depending on technology employed.

Capital costs: For a 50,000 -60,000 tpa plant - costs in the range of (See full report).

Operating costs: Estimates of (UK) gate fees are likely to be in the region of (See full report) .feedstock. Estimated processing costs are in the region of (See full report).

Staffing requirements: Staffing levels, including technical competence, management and administrative resources will vary depending on the size of facility.

Strengths & Weaknesses of Anaerobic Digestion

Strengths

Weaknesses

Relatively low capital costs compared to most thermal processes

Uncertainties over the economics and practical applications of AD to treat MSW. AD technology for various homogenous waste streams is widely proven in Europe, but there are no full scale plants in operation in the UK on municipal derived wastes. There are however proposals to develop facilities at present.

State-of-the-art technology in global use including pollution control technology

AD of MSW will need to rely on comprehensive pre-processing of the waste or source separation; plastics for example, can cause operational difficulties. Some systems however are designed to operate with mixed municipal type wastes.

Energy recovery potential (methane generation) and possible sale of surplus

Odour emissions during material handling

Contributes to national recycling /recovery objectives

Does not treat the whole MSW stream, only the organic fraction, however may be used on residual municipal waste stream with contaminants rejected as part of the process

Reduces organic wastes from landfill which reduces the production of landfill gas and leachates; a key aim of the landfill Directive

AD is more capital intensive than composting

Totally enclosed system, reduces environmental impacts

Materials handling problems with front end processing, can be costly

Complies with Animal By-Products legislation

Contamination of final product often difficult to avoid; marketing problems

Eligible for Renewables Obligation Certificates on electricity generated

Gas handling, storage and clean up facilities are required, which can be costly

Reduces the mass of organic waste input

Digestate produced, if landfilled may still count as BMW and be subject to active Landfill Tax

It is anticipated that AD will count as composting under the BVPIs in future

 

Other issues

Odour is a major concern at any plant processing the biodegradable elements of municipal waste. The bio-reactor and associated waste handling facilities (tipping floor, waste storage halls/bunkers, processing areas, conveyors etc.) can be designed to mitigate against the escape of odour/leachate.

The digestate products of AD will depend on the nature of the process, the feedstock and any post treatment maturation. Good quality products derived from source segregated wastes may be of a sufficient quality for agricultural / horticultural applications and could even produce a minor income stream. It is more likely however to assume a zero income for digestate when considering this aspect. It is likely that the product will count towards the composting / recycling targets under the BVPIs in the near future (at present it does not). There is also uncertainty over whether the product counts as biodegradable municipal waste under the Landfill Diversion targets (at present it does) and the 'active' waste residue will be subject to Landfill Tax if disposed to Landfill.

Utilisation of waste heat as well as electricity can be an additional source of revenue, however this is dependent on having an accessible customer/s for the heat. The sale of electricity generated from the Biogas falls under the Renewables Obligation and receives preferential market value.

The first commercial scale AD plant for municipally derived wastes is due to be commissioned by June 2004.

Other AD sites

Anaerobic Digestion information source web site

Biogas Digester web site

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