Anaerobic digestion (AD) is the breakdown of biodegradable organic material by microorganisms in the absence of air. It occurs extensively, for instance in landfills and the stomachs of cows. By controlling the process two useful products are obtained: biogas and residual digestate (a nutrient-rich fertiliser). AD technologies are well-proven and have been used in the UK for over 100 years to treat sewage sludge. However, there is growing interest in processing a wider range of materials including food waste, manure and crops.
The biogas produced by AD can be combusted directly to produce electricity and heat, or purified for injection into the gas network or for use as a transport fuel. In the UK, the main focus has been electricity generation. In August 2011:
147 plants were treating 18.3 million tonnes (Mt) a year of wet sewage sludge;
66 plants were treating approximately 1 Mt a year of wet food and agricultural waste.
Together these plants represent 151 MWe of generation capacity, which could provide the electricity for around 300,000 homes annually. AD is used more widely across Europe. Germany has about 6800 plants mainly producing electricity, while Sweden has 173 plants chiefly supplying heat and transport fuel.
The government has committed to an increase in energy from waste through AD, with the “AD Strategy and Action Plan” for England published in June 2011.5 AD can help to meet several UK targets by:
producing a flexible source of renewable energy. It could provide 0.2–0.7% of the energy delivered to UK consumers by 2020, as part of the 15% target.
recycling waste and avoiding the emission of methane, a powerful greenhouse gas. This is released when biodegradable waste breaks down in landfills and when manures and slurries are stored.
recycling nutrients, especially nitrogen and phosphorus, by using the digestate as a fertiliser, thus displacing manufactured or mined fertiliser.
This note examines some of the challenges related to maximising each of these benefits using AD.
Biogas is a mixture of methane (50-70%), carbon dioxide (25-45%) and minor impurities. The aims for energy production are to:
maximise the amount of methane produced, which partly depends on the raw material used.
efficiently convert this methane into electricity, heat or transport fuels.
The method of biogas use affects the level of carbon savings and the total amount of renewable energy produced. The Carbon Trust found that, if the electricity system is decarbonised, the greatest carbon savings from AD would be gained by upgrading the biogas to nearly pure methane (“biomethane”), equivalent to natural gas, and using this as a transport fuel or injecting it into the gas grid. Electricity-only generation gave the lowest carbon savings. Nevertheless, most AD plants in the UK currently burn the biogas to produce electricity as this has had the fewest technical and regulatory barriers and the most financial support. Incentives are now in place for all biogas uses, although barriers still remain.
Biomethane Injection into the Gas Grid
Biomethane can be injected into the gas grid, and is one of the few renewable alternatives to natural gas. The AD industry has welcomed the level of financial support within the Renewable Heat Incentive. However, biomethane injection is hindered by the gas specifications in the Gas Safety (Management) Regulations 1996. These are based on the composition and production volumes of North Sea gas, and require extremely low oxygen contents and expensive monitoring equipment. Although enforced by the Health & Safety Executive, any changes require the agreement of the Department of Energy and Climate Change (DECC). There are also issues around who should pay for and maintain gas connection equipment: biomethane producers or the gas grid owners. Regulators, gas grid networks and the AD industry are working to solve these issues. Even where there is a gas pipeline near an AD plant, it may not be able to receive the biomethane output all year round, due to lower demand for gas in the summer months. To overcome this, gas could be moved around the gas grid. Two AD plants in the UK are currently able to inject biomethane via exemptions from the gas regulations.
Biomethane can be used in vehicles designed to run on compressed natural gas. These are widely used throughout the world, although they are currently very limited in the UK. As well as decreasing carbon dioxide emissions, use of compressed biomethane can improve air quality by reducing particulate and nitrogen dioxide emissions. However, in the UK only 0.01% of renewable transport fuels are supplied by biomethane, partly due to the low price of the Renewable Transport Fuel Certificates in comparison with other financial incentives for biogas use. As an alternative, the European Natural Gas Vehicle Association advocates injection into the gas grid, to obtain the RHI, with removal at existing distribution depots. Currently no vehicles in the UK use biomethane from AD, although there are trials using landfill gas.
Combined Heat and Power (CHP)
By-product heat is produced when biogas is burnt to create electricity. If this heat is used, the overall energy efficiency and carbon savings of electricity production from biogas are greatly improved. However, there is a lack of infrastructure to distribute the heat, the value of heat is low and optimum use requires a site with constant heat demand throughout the year. Increasingly, some heat is used to maintain the temperature of the digester but most AD plants do not use all of their heat. The industry is waiting for information about RHI support from 2012 for larger CHP plants (>200 kWth).
Non-woody crops, such as grasses and maize, can be used in AD as the sole feedstock, or mixed with others. They have high methane yields and their production could be increased, unlike limited sources of waste. However, it remains uncertain how much can be produced in the UK for energy uses. The government recognises that crops for AD can be grown as part of a normal agricultural rotation or on land which is not suitable for food crops, and that some crops may need to be added to slurry-based AD to ensure efficient operation. However, it has stressed that it does not want to encourage the sole use of these materials, due to concerns about the displacement of food and feed or increased greenhouse gas emissions from land-use change. Operators compare this with support for other bioenergy technologies, with the planting of woody perennial crops for combustion assisted by grants worth ￡47 million.18 Defra and the industry are now working to study the “sustainability and role of purpose-grown crops in AD”. Knowledge of the optimum use of the UK’s biomass resource for energy purposes should also be improved by DECC’s Bioenergy Strategy and the Committee on Climate Change’s Bioenergy Review, both due in late 2011.
The main sources of waste feedstocks for AD are food and drink, sewage sludge, animal slurries and solid manures. Different environmental regulations and barriers apply to the supply and treatment of each material.
Food and Drink Waste
Several studies indicate that it is more environmentally beneficial to treat food waste by AD than by centralised composting or incineration. Although careful control of the process is required for all inputs, plants using food waste are the most complex due to the variable and protein-rich feedstock and the need for batch pasteurisation.
AD plants can currently charge a price (gate fee) to take food waste, providing an important source of revenue. However, over the past year, gate fees have decreased. In the light of potential future reductions, waste producers are unwilling to lock themselves into supply contracts, creating a lack of feedstock supply security and inhibiting access to finance. Local authorities offer longer term contracts, but these may be tied into other waste treatment technologies, and the food waste may not be “source-segregated”.
Separate collection and storage (“source-segregation”) of food waste is necessary to ensure that the digestate will meet “End of Waste” (EoW) criteria. If these are not met, environmental permits are required to use the digestate as a fertiliser. In England an estimated 13% of households receive a separate weekly food waste collection, rising to 82% in Wales. Evidence indicates that more food waste is captured when this is provided in conjunction with a fortnightly, rather than weekly, residual waste collection. It has been suggested that food waste disposers could reduce the need for separate kerbside collections.
The Welsh Government has introduced a statutory recycling target for local authorities (LAs) of 70% by 2025. Where LAs recycle source-segregated food waste using AD, it will fund up to 25% of the LAs’ food waste treatment costs under 15 year guaranteed contracts. Scotland is consulting on the Zero Waste Regulations 2011, which could require the source-segregation of all food waste from 2013, with a landfill ban from 2015. In England, some in the industry have argued for a similar landfill ban to increase feedstock supply and access to finance. However, the Local Government Group is adamant that LAs need to determine locally the best method for the collection and disposal of waste. The government will review the case for a landfill ban on biodegradable waste during the current Parliament.
The water industry is interested in co-digesting sewage sludge with other organic materials to use the small amount of spare capacity in their AD plants. However, because sewage sludge is excluded from the feedstocks allowed for a digestate to meet EoW criteria (Box 3), if it is co-treated with another material the digestate will always be a waste. The tests that apply to the use of treated sewage sludge (under the Sludge Use in Agriculture Regulations 1989) and EoW digestate as fertilisers are similar. However, there appears to be little desire in the EU or from retailers to include sewage sludge as an allowable material for co-digestion. If the water industry were able to accept a variety of organic materials, smaller AD companies are concerned about whether there will be a level playing field for competition over feedstocks. To address this, Ofwat, the independent regulator of the water industry, has asked the Office of Fair Trading to carry out a market study of organic waste. This should be published in September 2011.
Animal Slurries and Solid Manures
Solid manures and slurries are already spread on to land, but passing them through an AD plant first can produce a less odorous, more uniform fertiliser, as well as reducing methane emissions and pathogen and weed seed levels. Because manures are already partially digested, biogas yields and economic returns can be greatly improved when they are co-digested with other organic materials, such as purpose-grown crops or food waste. The amount required will vary during the year as manure and slurry will be collected only when animals are housed. As purpose-grown crops are a non-waste feedstock, their use for co-treatment adds fewer regulatory burdens than other materials. The Environment Agency is looking into whether low-risk agricultural wastes, such as vegetable peelings, could be controlled in the same way as manure, (i.e. the digestate is not a waste). An alternative model could involve the central pasteurisation of source-segregated food waste, with distribution to farms to combine with manures.
The digestate contains all of the non-degradable elements of the feedstock including any pollutants, water and a high proportion of the vital plant nutrients nitrogen, potassium and phosphate. The last are usually applied to fields via manufactured fertilisers. However, prices have increased dramatically and there are concerns about a future phosphate shortage and the impact of large-scale fertiliser manufacture on the nitrogen cycle. Initial data indicate that using one tonne of food waste digestate as a fertiliser could provide 4-6 kg of nitrogen, saving 20-40 kg of CO2 equivalent emissions.25 Digestate from sewage sludge has been used as a fertiliser for many years. However, several factors can affect the use from other sources including:
a lack of knowledge about the impacts on crop growth, soil quality and the environment. A government-funded study on “Digestate and Compost Use in Agriculture” should be completed by March 2014.
a lack of market understanding and acceptance in the farming, retail and farm assurance sectors. EoW criteria are expected to improve confidence.
the cost to AD operators of spreading digestate onto land, though future increases in fertiliser prices may improve the digestate value.
storage requirements during periods when digestate cannot be spread in Nitrate Vulnerable Zones.
odour levels, (can be reduced by controlling application)
uncertainty about whether digestate from non-household food sources can be used on certified organic land.
Several overarching issues may ultimately influence the scale of the AD industry and its environmental impacts:
efficiency of biogas use. The total amount of renewable energy supplied will depend on how the biogas is used. Electricity-only production is the least efficient use.
competition with other technologies for a continuous feedstock supply. High energy materials such as crops and fatty foods are useful for several technologies other than AD, including combustion and biofuel production.
food waste prevention versus use for AD. 64% of household food and drink waste is estimated to be avoidable.10 If prevented this could save 2.4% of the UK’s annual greenhouse gas emissions but would also reduce the potential feedstock for AD.
the impacts of digestate use on soil and water quality. The effects of heavy metals, persistent organic compounds, nitrates or phosphates, should be monitored.