Our use of cookies 
Anaerobic digestion: Process, feedstocks, advantages and disadvantages
Anaerobic digestion (AD) turns organic waste like food scraps, slurry and sewage into biogas and nutrient rich fertiliser. The UK has approximately 750 commercial AD plants processing 36 million tonnes of organic waste a year.
This guide to anaerobic digestion covers:
What is anaerobic digestion?
An anaerobic digester is a sealed, oxygen-free container that breaks down organic waste such as commercial food waste.
Naturally occurring microorganisms inside the container digest the waste and produce two by-products:
- Biogas, a renewable replacement for gas.
- Solid ‘digestate’ that settles at the bottom of the container and works as a potent soil fertiliser, similar to compost.
Anaerobic digesters work at any scale, from small businesses to industrial sites, and when managed properly, they avoid pests and odours.
They also handle a wide range of organic waste streams, from sewage to solid food residues and commercial garden waste.
💡 In the waste hierarchy, anaerobic digestion sits in the “recovery” tier because the process turns waste into useful resources.
How the anaerobic digestion process works
Anaerobic digestion turns organic waste into biogas and digestate through 5 core stages inside a sealed tank.
1. Feedstock enters the digester
Organic waste enters the digester after pre-processing, where staff remove contaminants like packaging and shred the material to a smaller particle size. Smaller pieces give microorganisms more surface area to work on, which speeds up digestion.
Common feedstocks include food waste, crop residues, manure, sewage sludge and garden waste.
2. The air-tight tank keeps out oxygen
The digester is air-tight. Without oxygen, the microorganisms inside switch to anaerobic respiration, the chemical process that produces methane as a by-product.
Sensors track temperature, humidity and pH to keep conditions stable. Most digesters run warm, between 35°C and 55°C, to encourage bacterial activity.
3. Bacteria break the waste down in four stages
Different groups of microorganisms work on the waste in sequence:
- Hydrolysis: Bacteria break large molecules like fats, proteins and carbohydrates into smaller, soluble compounds.
- Acidogenesis: A second group converts those compounds into volatile fatty acids, alcohols and gases.
- Acetogenesis: The acids and alcohols turn into acetic acid, hydrogen and carbon dioxide.
- Methanogenesis: Methane-producing bacteria (methanogens) convert the acetic acid and hydrogen into biogas.
The full cycle takes between 15 and 60 days, depending on the feedstock and the temperature.
4. Biogas rises and is captured
The biogas produced is roughly 60% methane and 40% carbon dioxide, with small traces of other gases. Because it is lighter than air, it rises to the top of the tank and feeds into a storage system.
From there, it either powers a Combined Heat and Power (CHP) unit on-site or goes through further refinement into biomethane for injection into the gas grid.
5. Digestate is separated and removed
Once the bacteria have finished, what is left at the bottom of the tank is a nutrient-rich slurry called digestate. Operators separate it into:
- Liquid digestate, used as a fast-acting liquid fertiliser.
- Solid digestate, used as a soil conditioner with slower nutrient release.
Both go to farms and growers as a replacement for chemical fertilisers, closing the nutrient loop.
What feedstocks can be used for anaerobic digestion?
UK digesters run on a mix of organic materials. The best ones are high in energy, low in contamination, and steady in supply. Most plants blend two or more to keep the bacterial population stable and biogas output predictable.
Not all organic waste is suitable. Here’s what works and what doesn’t.
| Material | Suitable? | Notes |
|---|---|---|
| Food waste | Yes | Best source for business waste streams |
| Agricultural slurry and manure | Yes | Common on farm digesters |
| Energy crops | Yes | Maize, grass silage, fodder beet |
| Crop residues | Yes | Straw, husks, spoiled grain |
| Bakery, brewery and dairy processing waste | Yes | Consistent, high energy |
| Sewage sludge | Yes | Two thirds of UK sewage sludge processed this way |
| Garden and green waste | Yes | Soft plant material only |
| Plastics and bioplastics | No | Do not break down reliably |
| Glass, metal and grit | No | Cause blockages and equipment wear |
| Wood and lignin rich material | No | Too tough for the bacteria |
| Large volumes of fat or cooking oil | No | Floats and blocks the system |
| Chemicals and pharmaceutical waste | No | Kill the bacterial population |
| Garden waste with persistent herbicides | No | Damages crops when digestate is spread |
Why feedstock quality matters
The bacteria inside the digester are sensitive. They need a consistent organic content, a balanced carbon to nitrogen ratio (ideally 20 to 30 parts carbon to 1 part nitrogen), and the right moisture level for the digester design. Get any of these wrong and biogas yield drops.
Contamination issues
Most contamination comes from packaging. Plastic, glass and metal in food waste causes blockages, wears down equipment, and ends up as microplastics in the digestate spread on farmland. UK plants invest in screening and depackaging to remove it, but cleanly separated business food waste always produces better results.
Benefits and drawbacks of Anaerobic Digestion for waste management
As useful as anaerobic digestion is for waste management, it has a few drawbacks and limitations. Here are the main pros and cons.
| Benefits | Drawbacks |
|---|---|
| Produces renewable energy as biogas and biomethane | High capital cost to build, which feeds into commercial waste collection costs |
| Diverts organic waste away from landfills and reduces wider environmental impacts | More complex to run than basic composting |
| Cuts greenhouse gas emissions by capturing methane, as shown by our waste carbon emissions calculator | Risk of odour issues if poorly managed |
| Produces nutrient rich digestate as a fertiliser | Some argue ongoing biogas use slows the shift away from gas infrastructure |
| Reduces landfill tax costs for businesses | Feedstock contamination from packaging adds processing costs |
| Closes the nutrient loop and supports a circular economy | Yield depends heavily on feedstock quality and consistency |
Types of anaerobic digestion plants
There are hundreds of anaerobic digester variants to fill in different industry niches (e.g. a milk industry digester will be different to sewage).
However, we can group them into five different types, all of which are commonly used by UK waste management facilities and in specific industries like food, beverage, farms and supermarkets:
| Type of Anaerobic Digester | Description & Advantages | Applications & Examples |
|---|---|---|
| Continuous Stirred Tank Reactor (CSTR) | A sealed tank where organic waste is continuously added and stirred for even digestion. It’s versatile and efficient for handling a wide range of business and food wastes. | Widely used for mixed organic streams like food waste in UK waste management facilities. |
| Plug Flow Digester | Organic waste moves in a linear flow through the digester, efficient for solid waste with minimal mechanical mixing. It’s simple and ideal for food scraps. | Often used by food manufacturers and supermarkets. |
| Upflow Anaerobic Sludge Blanket (UASB) | Liquid-rich waste flows upward through sludge for digestion. Compact and effective for wastewater from food processing industries. | Used by breweries and food processors. |
| Anaerobic Filter | Waste flows through a bacteria-packed filter, highly efficient for low-strength liquid waste like food processing effluents. | Common in beverage industries. |
| Batch Digester | Waste is added in batches, allowing for simple operation, suitable for small-scale or intermittent waste inputs. | Used by smaller waste producers. |
💡Did you know? Dagenham hosts two of the UK’s most notable AD plants side by side. East London Biogas processes 70,000 tonnes of food waste a year and powers around 10,000 homes, while the nearby ReFood Dagenham plant handles over 160,000 tonnes a year and produces enough biogas to power 12,600 homes.
Both turn food waste collected by London commercial waste collection firms and households into renewable energy.
Anaerobic Digestion facilities in the UK
The UK has around 750 commercial anaerobic digestion plants in operation, or over 1,000 once sewage sludge digesters at wastewater treatment sites are included.
Together they process roughly 36 million tonnes of organic waste a year and have a combined capacity of more than 2,800 MW, supplying renewable heat, electricity and biomethane.
From these:
- Most plants are dedicated to combined heat and power (CHP) generation, ranging from small farm-fed rural waste systems producing as little as 250kW up to large industrial installations of several megawatts.
- A growing number are biomethane to grid plants, refining biogas to inject directly into the national gas grid.
Here are five notable anaerobic digesters in the UK, each with a different feedstock and usage for its by-products:
| AD Plant (Location) | Typical feedstock | Processing & Energy Capacity | Biogas or Biomethane | Notable because... |
|---|---|---|---|---|
| Poplars (near Birmingham) | Cafe waste, expired packaged foods, food scraps from retailers like Sainsbury's. | 120,000 tonnes/year, 6 MW (electricity) | Biogas | One of the largest UK AD plants, powering 6,000 homes. |
| Gonerby Moor (Lincolnshire) | Crop residues, vegetable trimmings, spoiled grains. | 75,000 tonnes/year, 1.3 TWh (planned gas) | Biomethane | A large-scale project aimed at producing green gas. |
| Deerdykes Bioresources (Scotland) | Kitchen scraps, waste cooking oil, discarded fruits and vegetables, dairy waste. | 30,000 tonnes/year, 2 MW (electricity) | Biogas | Scotland's first large-scale AD plant for food waste. |
| Dagenham (London) | Restaurant waste, catering waste, supermarket food waste. | 50,000 tonnes/year, 1.5 MW (electricity) | Biogas | First commercial AD plant in London, aiding urban waste recycling. |
| Walsh Mushrooms (Worcestershire) | Spent mushroom compost, straw, corn husks, manure. | 20,000 tonnes/year, 500 kW (electricity) | Biogas | Specialised in recycling agricultural waste into mushroom compost. |
Growth of anaerobic digestion
The UK AD sector has grown significantly over the past decade, driven by the need to reduce landfill waste, manage food waste, and extract value from organic by-products.
Two recent policy changes are accelerating that growth:
- The Green Gas Support Scheme, launched in 2021, provides tariff payments to new biomethane plants injecting green gas into the national grid. The scheme has been extended, with new applications accepted until 31 March 2028 and commissioning deadlines running to March 2030.
- Mandatory separate food waste collections for businesses in England, introduced in 2025, are significantly boosting the supply of organic feedstock for AD plants.
Together, these mean AD capacity in the UK is expected to keep expanding through the late 2020s.
Back to blog- Share:
