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Anaerobic digesters
Anaerobic digesters (ADs) prevent millions of tonnes of organic waste from being dumped into landfills every year.
They do this by turning your employees’ apple cores, orange peels and any organic residues generated by your business into biogas and a natural solid fertiliser.
ADs can process organics at industrial scales, and the resultant biogas is becoming a growing source of renewable energy, a win-win for business and the environment.
This article explains anaerobic digestors in simple terms and explains the pros and cons for UK businesses.
Contents:
- What is an anaerobic digester?
- How does an anaerobic digester work?
- Types of anaerobic digesters
- UK anaerobic digesters
- Benefits and drawbacks of anaerobic digesters
What is an anaerobic digester?
An anaerobic digester (AD) is a system designed to break down organic waste, such as commercial food waste, within a closed container devoid of oxygen.
The breakdown is carried out by naturally occurring microorganisms within the container and has two main by-products:
- Biogas, which can be harnessed as a renewable replacement of natural gas, the fossil fuel.
- Solid ‘digestate’ that accumulates at the bottom of the container and is used as a potent soil fertiliser, similar to compost.
Anaerobic digesters can be built at any scale, from small businesses to industrial scales, without risks of pest infestations or odours when managed properly.
They are extremely versatile, with many designs available to optimally process a large range of organic waste streams, from sewage to solid food residues and commercial garden waste.
💡 In terms of the waste hierarchy, anaerobic digestion falls into the “recycling” tier, as waste is turned into useful resources through the process.
How does an anaerobic digester work?
All anaerobic digestors (ADs) require the following:
- Feedstock: An organic waste feed to provide food for the microorganisms.
- No oxygen: An anaerobic environment is necessary to ensure biogas production.
- Optimal conditions: The right mix of microorganisms will efficiently break down the target feedstock.
- By-products: Equipment designed to store and collect biogas and the ‘digestate’ to ensure profitability.
Here are more details on each and how they fit into turning organic waste from UK businesses into something useful:
Feedstock
Digester feedstocks can come from various sources, including general business waste, farms, water treatment, etc.
In 2019-20, British Anaerobic Digesters received their feedstock from the following sources:
- 30% Crops – Grown for biogas like maize.
- 29% Food waste – Segregated waste collected directly from supermarkets, restaurants, homes, etc.
- 20% Other waste – Includes garden waste, biodegradable municipal waste and even commercial cardboard waste,
- 17% Manure/slurry – Sourced from farms and sewage treatment.
- 4% Crop waste – Residues after harvesting like cereal straw and husks.
Waste from these sources goes through pre-processing, in which non-organic contaminants are removed, and organic material is broken down to reduce particle size and increase the surface area available for digestion.
Anaerobic digestion
Pre-processed organic waste fills the digester’s container to be broken down by tailored colonies of microorganisms. The container is air-tight to ensure it remains oxygen-free and usually has sensors to maintain optimal conditions, such as humidity and temperature.
The time it takes for an anaerobic digester to process the entire feedstock, known as retention time, typically ranges from 15 to 60 days, depending on the feedstock type and the season (summer digesting is faster than winter digesting due to higher temperatures).
The detailed chemistry of the reactions is outside the scope of this article, but you should know that there are four marked processes undertaken by microorganisms: hydrolysis. acidogenesis, acetogenesis and methanogenesis.
Biogas collection
Biogas, primarily methane (CH₄) and carbon dioxide (CO₂), is produced during digestion. Since biogas is less dense than humid air, it rises to the top of the digester chamber and is directed into storage for accumulation.
97% of the biogas produced in the UK is burned there and then for electricity and heat generation in CHP (Combined Heat and Power systems).
This allows businesses running the digesters to be energy self-sufficient, save on transmission costs and reduce greenhouse gas emissions by using this unavoidable methane and turning it into less harmful CO₂ (i.e. as decomposing organics in landfills simply lose this methane to the atmosphere).
Some digesters, especially the larger ones, have the size to accumulate significant amounts of biogas, making it suitable for refinement into biomethane.
This product has properties similar to natural gas (the fossil fuel) and can act as a direct “renewable” replacement and be injected into the national gas grid. The refinement process requires removing CO₂ and other impurities from this by-product to ensure it is at least 95% methane.
Digestate Output
After the digestion process, the remaining solid digestate is extracted. This nutrient-rich by-product can be used as a soil conditioner or fertiliser.
In the UK, many anaerobic digestion facilities supply digestate to agricultural businesses, helping to recycle organic waste by returning valuable nutrients to the soil and supporting a circular economy.
Types of anaerobic digesters
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? The Dagenham biogas plant is one of the largest in the UK. It produces biogas from food and liquid collected by London commercial waste firms. Its capacity is 1.5 MW of electricity, comparable to the output of one medium-sized wind turbine.
UK anaerobic digesters
Around 700 anaerobic digesters are distributed across the UK (see location map here). They process a whopping 1.6 million tonnes of food waste (and more from other sources) every year, adding a combined capacity of 500 MW to the UK electricity and gas grids.
From these:
- Around 600 digesters are dedicated to combined heat and power (CHP) generation, with small farm-fed systems producing as little as 250kW and larger installations exceeding 300MW.
- Around 100 are biomethane-to-grid plants producing between 5MW and 6MW.
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 digesters
The last few years have seen significant growth in the number of anaerobic digesters in the UK, driven by their ability to reduce landfill waste and emissions, manage food waste, and extract value from organic by-products.
In 2021, the Green Gas Support Scheme was introduced to encourage anaerobic digesters to inject biomethane (refined biogas) into the national gas grid. It provides financial incentives to cover the higher costs of producing green gas than natural gas.
Commercial food and garden waste as feedstock
Municipal organic waste is a key feedstock to the anaerobic digestion industry. Commercial waste collection services are key to accumulating them from any business site.
ADs benefit from businesses signing commercial waste collection contracts with waste firms as this allows digesters to secure a reliable stream of organics for decomposition.
Anaerobic Digesters thrive from businesses like restaurants, cafes, farms, and food and beverage producers because of their predictable stream of raw organics.
Composting vs anaerobic digestion
The alternative process for managing certain types of organic waste is composting for businesses, where raw materials are decomposed aerobically (i.e. in the presence of oxygen) to produce CO₂ and a soil amendment called ‘compost’ as by-products.
While this process requires very little infrastructure and effort, it is more restrictive in its choice of feedstock, requires carefully controlled conditions and cannot generate power or heat for human use.
In contrast, anaerobic digestion is more effective for industrial-scale operations or mixed, liquid-heavy waste, making it better suited for large businesses and renewable energy generation.
Benefits and drawbacks of anaerobic digesters
As useful as anaerobic digestion is for waste management, its has a few drawbacks and limitations. Here’s a summary of advantages and disadvantages:
Advantages of anaerobic digestion
Renewable energy production: Biogas and biomethane produced from the anaerobic digestion of organic wastes are renewable energy sources because they can be perpetually produced from organic wastes, which cannot be exhausted. Societies worldwide go to the toilet, have gardens, and consume food, providing a reliable feedstock to anaerobic digesters.
Landfill waste reduction: Digesting organic waste prevents it from being lost to landfills, which have significantly worse environmental impacts, such as methane emissions, soil and water contamination, and increased demand for North Sea natural gas. This also helps businesses save on their landfill tax obligations.
Greenhouse gas reductions: Anaerobic digesters reduce greenhouse gas emissions by trapping useful biogas instead of directly releasing it into the atmosphere. This way, we can extract power and heat from the biogas’s methane (CH₄) by combusting it, which becomes carbon dioxide (CO₂), a less powerful greenhouse gas. This also reduces the amount of fossil fuels that need to be extracted to produce this energy!
Nutrient-rich digestate: The process’s by-product, digestate, can be used as a fertiliser, closing the nutrient loop and reducing the need for chemical fertilisers. This is similar to compost but with slightly different properties.
Green economy: Anaerobic digesters provide additional jobs and add value to the local economy where they are built.
Disadvantages of anaerobic digestion
Carbon footprint: Anaerobic digesters may be the most effective way of reducing the unavoidable emissions resulting from the natural decomposition of organic waste. However, some argue that the continuous production of biogas is hindering efforts to reduce our reliance on fossil fuels, as these depend on the same combustion and distribution (pipeline) infrastructure.
Requires capital: Compared to much simpler composting, anaerobic digesters require higher capital costs, especially if they are large-scale and include the re-processing of biogas into biomethane.
Complexity: While anaerobic digesters don’t require rocket or nuclear scientists to run, they still require more technical expertise and care than simple composting done at a smaller scale.
Odours: Poorly managed or failing anaerobic digesters risk having odour issues due to their process being akin to “rotting”, which can affect the welfare of employees and neighbours.
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