Anaerobic digestion plants in the UK

[edit] Introducing anaerobic digestion

Anaerobic (or anoxic) means oxygen deficient (or oxygen depleted), so anaerobic digestion (AD) means the microbial degradation of organic material known as ‘feedstock’ (such as farm waste, food waste and energy crops) in anaerobic digestion plant or facility to produce biogas

The process sometimes referred to as biomethanation typically takes place in sealed, insulated tanks (digesters) in the absence of oxygen. It requires a heat source, either a mesophilic process at temperatures of 35 to 40°C or a thermophilic process at 50 to 60°C. The mesophilic process tends to be used for low-solid wastes such as animal slurry, while the thermophilic process tends to be used for high-solid materials such as a garden waste.

[edit] Defined as a renewable technology

NB BG 34/2021, The Illustrated Guide to Renewable Technologies, 2nd edition, written by Michelle Agha-Hossein and David Bleicher and published by BSRIA in 2021, states: ‘Anaerobic digestion is a process that produces gas from organic material with a high methane content, such as agricultural, household and industrial residues and sewage sludge. In addition to methane gas, a solid residue similar to compost and a liquor are also produced. The former can be used as a soil conditioner while the latter can be used for fertiliser (depending on the level of contamination). The solid residue can also be burnt as a fuel or gasified. The methane gas can be used to power a gas engine or turbine to generate electricity and heat. This process is used in very large applications.’

[edit] What does the AD process produce ?

The products of anaerobic digestion are; biogas (sometimes called biomethane or renewable natural gas (RNG)), water and fermented organic material (digestate). The digestate can be used as a high-quality fertiliser. After a relatively striaght-forward clean-up process, biogas can be used as a fuel. Biogas is mainly composed of methane and carbon dioxide, but may also contain small amounts of nitrogen, hydrogen and carbon monoxide, as well water vapour and contaminants such as hydrogen sulphide and siloxanes.

Biogas is often used to fuel combined heat and power (CHP) plant that produces both electricity and heat. The heat can be used for the anaerobic digestion process and can also be used to pasteurise animal-derived waste so that it can be used as fertilisers. Combined heat and power plant can feed electricity into the national grid, and so biogas installations may qualify for payments under the renewable heat incentive (RHI) scheme (other than biogas from landfill).

Anaerobic digestion is recognised by the UK government and by the United Nations as having significant potential to help achieve sustainability targets as it produces local energy and fertiliser and makes use of local waste products that might otherwise be difficult to dispose of. Biogas can be fed into the national gas network if it is cleaned of carbon dioxide.

[edit] The AD market in the UK

BSRIA World Market Intelligence had the chance to explore the evolving landscape of waste management and sustainable energy production. Through many conversations with industry experts, it discovered the pivotal role AD plants play in transforming bio-waste from commercial establishments into valuable resources.

AD technology is on the rise in Europe, and particularly prominent in the UK. These plants specialise in on-site treatment solutions for organic waste, with a focus on sectors like hotels and restaurants. The dual approach of on-site treatment using bacteria and off-site treatment through anaerobic digestion, form the backbone of this sustainable solution addressing the urgent need for eco-friendly waste management.

The AD process creates many valuable by-products, including biofertiliser, biogas, biomethane, and bioplastics. These by-products open doors to future green fuels, underscoring the potential for carbon capture technologies. Notable among these green fuels are green hydrogen, renewable DME (rDME), syngas, and sustainable aviation fuel (SAF).

BSRIA had the chance to understand the versatility of biomethane, a significant output of AD, with applications ranging from replacing fossil fuel gas on the grid, to serving heating needs and generating combined heat and power (CHP). The low carbon footprint of the electricity generated aligns with government targets, shaping a more sustainable energy landscape.

Diving deeper into the intricacies of AD plants, it discovered the two main types present in the UK: commercial and farming. Commercial plants handle diverse organic waste sources, while farming-based plants focus on utilising on-farm waste to generate power. In 2024 there are 532 operational AD plants in the UK, a testament to the growing acceptance of this sustainable technology.

Conversations brought the team face-to-face with the operational processes of AD plants, where converters facilitate the digestion process using organic waste, bacteria, dry matter, and heat. The adaptability of AD systems is clear, such as the addition of wood pellets for generating heat in specific scenarios.

Entities acting as intermediaries in this space emerged as crucial players, either locally treating waste at businesses or collecting and delivering it to AD plants. The exploration concluded with a profound realisation – the rise of AD plants in the UK symbolises a significant stride towards a sustainable and eco-friendly approach to waste management and energy production. With the potential to generate clean energy, reduce carbon emissions, and produce valuable by-products, AD plants offer a promising solution for a greener and more sustainable future in the UK's energy landscape.

The above section appears on the BSRIA news and blog site as "Exploring the Inner Workings of Anaerobic Digestion Plants: A First-hand Introduction to Sustainable Energy Production" dated January 2024 and was written by Frantisek Hejl as part of the BSRIA World Market Intelligence group.

--BSRIA

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• Liquefied petroleum gas (LPG).
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