Ducks overlooking outside scenery on bridge.

Building on renewable energy #3 – Anaerobic Digestion

24 September 2020

Anaerobic digestion is the process by which biodegradable materials are broken down in a controlled environment. Whilst being broken down, the materials emit gas (which can be used as a source of renewable energy), and produce heat (which can also be harnessed on or near-to site). The process also leaves a waste material (digestate), which can be used as fertiliser, contributing to the circular economy.

Anaerobic digestion plants have been in operation since the 1850s.[1] Thanks to encouragement from government, several significant public-private partnerships, and the divesting from traditional fossil fuels, anaerobic digestion plants have become far more prevalent in recent years.

Large anaerobic digestion plants are substantial and complicated pieces of engineering and will present various technical challenges for the project teams. We consider some of these challenges below.

Design and Construction

The selection of a site can present several initial difficulties. A comprehensive geotechnical survey will be required to assess ground conditions and ensure that site foundations are appropriately designed, as movement may affect the integrity of the digester (the sealed, oxygen-free tank in which the materials are broken down). Other site conditions, including water pressure, will need to be considered as part of the design process.

Professionals with design responsibility should pay careful attention to the feedstock intended to be fed into the digester. Some manures (poultry, for example), can contain a high percentage of ammonia, and the resulting sulphides could damage the digester tanks. Where the feedstock may contain contaminants, then appropriate pre-sorting will need to be specified to avoid blockages and ensure efficient operation of the plant.

AD plants require careful design. Safety measures and fail-safes to minimise the risk of explosion, and protect plant operatives such as maintenance contractors, will need to be incorporated; underwriters will pay careful attention to the experience and safety records of those involved with the design, construction, and operation of the digester.

Designers will need to ensure that appropriate filters are incorporated to avoid excessive odours (which may breach planning conditions). For the same reason, the design and specification of insulation, noise deflectors and soft landscaping to dampen and reduce operating noise, is also of import.

Other planning issues (including traffic impact, local development plans and many more), are outside the scope of this article.


Whilst not yet commonplace in the UK, larger AD plants are likely to become more prevalent over the next decade. These are substantial engineering projects, often with bespoke contractual arrangements.  Parties, and their insurers, need to be alive to contractual terms that are more onerous than a duty to exercise reasonable skill and care, including 'turnkey' EPC contracts with guaranteed performance specifications.

Consultants will invariably be required to enter collateral warranties with interested parties. It is vital to ensure that such collateral warranties comply with the terms of the consultant's professional indemnity insurance policy.  Warranting compliance with a higher duty than reasonable care and skill may leave a professional facing a large claim without the benefit of insurance cover.

Contractual disputes in construction projects are often referred to adjudication. The parties should ensure that the adjudication scheme set out in the contract complies with the terms of the professional's insurance policy.

Operation and Maintenance

Biogas contains hydrogen sulphide.  As well as having an unpleasant odour, hydrogen sulphide is corrosive and can damage the digester tanks (and will need to be extracted before the biogas can be used in engines, fuel cells or turbines).  Appropriate, periodic maintenance will be required to ensure the safe and efficient operation of the digester.

Maintenance can carry several risks. As above, hydrogen sulphide is a product of the digestive process.  Not only is the sulphide corrosive, but flammable and poisonous too. As well as this being a design risk, O&M contractors must undergo appropriate training and conduct proper risk assessments prior to undertaking maintenance work

The design/specification of an appropriate maintenance schedule will be of importance to the safe and efficient operation of the digester. Maintenance failings may impact upon the performance of the digester and result in the plant failing to meet its contractual performance requirements. Issues including sludge removal, general engine maintenance, and pipe and valve leaks, will need to be considered.

Insurance Coverage

In addition to the above issues, insureds and their brokers should ensure that the insurance policy wording is suitable to their needs. Engineering policies often contain exclusions which may limit the cover provided by the policy (an exclusion relating to concrete, for example).

Tank and pipe failures will invariably lead to pollution and environmental issues, and appropriate insurance will be necessary to cover the costs relating to the same.

Insurers will need to carefully consider the scope of their coverage, as the contents of a tank (both the feedstock and the cultures that are introduced to start the digestion process) can be valuable.  Indeed, replacement cultures may not be immediately available, which could lead to the digestor laying dormant for months and an associated business interruption claim.