Upscaling anaerobic digestion

 https://doi.org/10.58248/RR104 

Upscaling anaerobic digestion   

In 2022, 31% of total UK methane emissions (a greenhouse gas) arose from the waste sector (mostly from organic waste in landfill sites) and 48% from livestock and their manures. In anaerobic digestion (AD) plants, waste organic material is broken down by microbes into biogas and fertilisers, capturing this methane, which can then be used as a renewable fuel. Research evidence suggests AD has lower environmental impacts than disposal of organic wastes in landfill or by incineration.   

A range of feedstock materials are used in AD including sewage sludge, agricultural waste, purpose-grown crops (such as maize) and food waste, which remains high in the UK (10.2 million tonnes annually).  

The House of Lords Environment and Climate Change Committee has said the government should assess whether the number of anaerobic digestion plants is on track to keep pace with the amount of household-collected food waste, now weekly collections are required. It also noted concerns about fugitive methane emissions from anaerobic digestors and storage tanks and recommended reviewing regulatory oversight and considering a requirement for regular leak assessments for all sites.  

Charities and NGOs have concerns that upscaling and subsidies for anaerobic digestion are an incentive for generating food waste in the food supply chain and may disincentivise prevention and redistribution of edible surplus food. 

What is anaerobic digestion? 

Anaerobic digestion (AD) is the breakdown of organic material (feedstock) by microorganisms in the absence of air. This produces a mixture of: 

• methane (known as ‘biomethane’ when produced by AD) 
• carbon dioxide 
• digestate (fertiliser) 

The mixture of biomethane and carbon dioxide produced is called biogas and typically contains 50–70% biomethane and 25–45% carbon dioxide, as well as impurities. 

What can the products of anaerobic digestion be used for? 

Biogas 

Raw biogas can be burned in combined heat and power (CHP) units to generate renewable electricity and heat (Figure 1). However, if biogas is refined to increase the concentration of methane, it can be injected directly into the gas network, assisting decarbonisation of the national grid.  

As of March 2026, there were 750 municipal, commercial and smaller agricultural AD plants operating in the UK producing an estimated 21 TWh of biomethane annually, with around 7 TWh injected into the grid. 

One of the National Energy System Operator Future Energy Scenarios suggests 64 TWh of biomethane production will be required by 2050, a tenfold increase from current levels (in 2025, total UK methane demand was 689 TWh). 

The UK Government supports plants producing biomethane for the gas grid through the Green Gas Support Scheme, funded by the green gas levy applied to all licensed fossil fuel gas suppliers. Under the UK Emissions Trading Scheme, biomethane only qualifies as zero emissions if supplied directly via a dedicated pipeline, such as to an industrial use. Grid-injected biomethane does not qualify as there is no accounting mechanism in place. In contrast, under the EU Emissions Trading Scheme, grid-injected biomethane qualifies if certified by an approved scheme as meeting sustainability criteria. 

Biomethane can also be used as low-carbon fuel for heavy goods vehicles (HGVs) and shipping. Carbon dioxide can also be captured for industrial uses. 

Digestate 

AD digestate is a nutrient rich by-product that can be used as a renewable nitrogen-phosphate-potassium fertiliser. This is often cheaper than chemical fertilisers, costs of which can increase in response to geopolitical events. However, digestate fertiliser properties vary with the feedstock.  

What organic materials are used as feedstock?  

AD requires a continuous feedstock supply to remain profitable. Most AD operations use agricultural feedstock (manure and purpose grown crops), but most farms do not process their waste with AD; by 2025, 13% of farms used AD for multiple waste types. The percentage of farms processing each waste type using AD is listed in table 1. 

Table 1: Proportion of holdings processing by anaerobic digestion in 2025, separated by waste type. Source: DEFRA 

Different feedstocks have varying biogas production efficacies, which is reduced or slowed by:  

• Inhibitory chemical content interacting with digesting microbes.  

• Foaming and clumps that block impellers (a bladed rotor used to increase the pressure and flow of a fluid) and pipes.  

• Inaccessibility of organic feedstocks to microbes (due to structures and surface area).  

• Glass and plastic impurities in waste.  

Predigestion management of feedstocks uses technologies to prevent processing problems, increase biogas yield and accelerate AD, including mills and shredders, high heat, or chemical treatments. 

Why digest food waste?  

The Waste and Resources Action Programme (WRAP) reports that 58% of UK food waste comes from households, with between 80kg and 100 kg of food per person wasted in the UK annually causing 16 million tonnes of greenhouse gas emissions. The UK reporting guidelines do not distinguish between edible food and food that is no longer edible. For more information, and interventions to prevent edible food waste, see POSTbrief 60. 

AD disposal of food waste causes lower emissions than incineration and landfill, and using digestate as fertiliser could reduce the use of inorganic nitrogen fertilisers and the greenhouse gas emissions associated with them. Within the UK food and drink waste hierarchy (figure 2), AD is considered ‘recycling’: more preferable than incineration or landfill but less preferable than redistribution (for human or animal consumption). Estimates of household food waste processed by AD in the UK are uncertain, but WRAP reported an operational capacity of 3.2 million tonnes that could be used to process food waste in 2019.   

In 2024, 535,000 tonnes of separated food waste was collected by local authorities in England and the government announced £295 million in capital funding to fund specialist collection vehicles and food waste caddies for homes. The Environment Act 2021 made it mandatory for waste collection authorities in England to provide weekly household food waste collections from March 2026. However, some authorities agreed transitional arrangements with the government due to logistics, supply chain, and contractual issues.  

Researchers and NGOs raise concerns that edible food may be redirected from potential human or animal consumption, including on farms (often referred to as food loss). Companies and food redistribution charities report diversions of edible food to AD food waste due to government incentives for AD. The StopTheBiomethaneRush NGO coalition argue AD subsidies can undermine food waste prevention.  

Regulation of anaerobic digestion plants 

Some small AD plants using only on-farm waste fall under permitted development but most AD installations require planning permission. They require an environmental impact assessment if they are likely to affect the environment due to their nature, size, or location, such as those processing 50,000 tonnes or more of organic waste per year or those near protected areas. 

In England, regulations applied to AD plants include: 

• Environmental Permitting Regulations 2016. AD operators must obtain an environmental permit. To comply with the regulations, they must control, monitor, and report emissions to air, water, and land (including pollutant release to watercourses and groundwater). They must also maintain buffer distances from public receptors (public spaces such as residences), management of odour, noise and vibration and safe storage, treatment and disposal of waste materials.   

• Animal By-Products Regulations. These permit the treatment of low-risk animal by-products and catering waste which contains meat or which comes from premises handling meat. Manure and animal by-products containing digestive tract content are classified higher risk and must be treated to required standards, and the plants approved by the Animal and Plant Health Agency. 

• Duty of care for waste. The duty of care applies to anyone who imports, produces, carries, keeps, treats, or disposes of controlled waste, which includes AD digestate, and they must apply the code of practice. Digestate that meets the end of waste test and source-segregated biodegradable material standards can be spread on land as fertiliser in accordance with nutrient management plans.  

• Health and Safety at Work Regulations.  Employers must assess risks and minimise hazards to employees and others; risks associated with AD include flammable atmospheres, fire and explosion, toxic gases, confined spaces, asphyxiation, pressure systems and use of hazardous substances. 

Benefits of and challenges for upscaling AD  

Researchers state scaling up AD remains challenging as biomethane is more expensive to produce than natural gas, with uncertainty over whether the best policy approach is to adopt a producer-focused, state-driven, circular economy model or a market-driven model focused solely on biomethane.    

Some studies argue that reusing digestate as fertiliser could help mitigate the effects of climate change, recycle nutrients, and improve soil fertility, quality and health, but others suggest regulatory challenges can arise. For example, excess nutrients affecting soil, wildlife, air and water quality as they move through the environment  greenhouse gas emissions, and contamination with pollutants. A government-commissioned review from 2023 suggested that digestate applications that conformed to good practice (such as spring application and use of cover crops) had less environmental impact.  

There are also options to address other environmental impacts, such as reducing ammonia emissions from digestate during storage and use, methane emissions during storage, and contamination. Research suggests careful feedstock selection, post-treatment strategies and testing digestate could maintain compliance with regulatory pollution limits.  

If feedstocks that are not organic wastes are used, they may create trade-offs with food security and climate mitigation. The NGO Foodrise argues that purpose grown AD crops use land that could otherwise be used to produce food, and that other land uses provide more energy per hectare (such as such as solar panels) and greater climate mitigation per hectare (such as woodland).  

Acknowledgments  

• POST is grateful to Sophie Powell, whose POST fellowship was funded by NERC (UKRI), for researching and writing this briefing.  
• POST would also like to thank Dr Stephan Haefele, Soil Health and Management Team Leader, Rothamsted Research, for kindly giving up their time during the preparation of this article. 
• Questions about this briefing should be referred to Jonathan Wentworth, who acted as parliamentary lead for this work.