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Anaerobic Digestion of food waste in the UK

Anaerobic Digestion (AD) plants prevent millions of tonnes of organic waste from being dumped in landfills every year.

They achieve this by converting your employees’ apple cores, orange peels, and organic residues your business generates into biogas and natural solid fertiliser.

AD plants can process organic waste on an industrial scale, and the resulting biogas is becoming an increasingly important renewable energy source, a win-win for both businesses and the environment.

This article explains Anaerobic Digestion in simple terms and explores its pros and cons for UK businesses. Here’s what we cover:

• What is Anaerobic Digestion?
• Benefits and drawbacks of Anaerobic Digestion for waste management
• How does Anaerobic Digestion work?
• Types of Anaerobic Digestion plants
• Anaerobic Digestion facilities in the UK

What is Anaerobic Digestion?

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.

Benefits and drawbacks of Anaerobic Digestion for waste management

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

Here, we present the four main advantages of anaerobic digestion for waste management in the UK.

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.

Understand how this compares to other waste disposal options using our waste carbon emissions calculator.

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.

Disadvantages of anaerobic digestion

Here, we present the four main disadvantages of anaerobic digestion for waste management in the UK.

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 simpler composting, anaerobic digesters require higher capital investment, especially for large-scale facilities that include the reprocessing of biogas into biomethane. This has increased commercial waste collection costs for both businesses and local councils.

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.

How does Anaerobic Digestion work?

All anaerobic digesters (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 (a fossil fuel) and can serve as a direct renewable replacement, capable of being injected into the national gas grid. After undergoing a refinement process to remove CO₂ and other impurities, energy suppliers can sell biogas as a green business energy product.

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 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:

💡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.

Anaerobic Digestion facilities in the UK

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 rural waste 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:

Growth of Anaerobic Digestion

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.

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