Anaerobic digestion (AD) plants are facilities designed to break down organic matter—such as agricultural waste, food waste, sewage sludge, or manure—in the absence of oxygen. The process produces biogas (a mixture of methane and carbon dioxide), which can be used as a renewable energy source, and digestate, a nutrient-rich substance that can be used as fertiliser.
There are four key stages in an anaerobic digestion plant which are described below and seen in Figure 1:
Feedstock Collection – the organic materials such as sewage sludge, food waste, etc is collected from multiple sources.
Pre-treatment – the organic materials are pre-treated to remove contaminants and diluted with water. It is also heated to approx. 70 oC to kill pathogens in the food.
Anaerobic digestion – the waste materials are fed into the anaerobic digester where the bacteria breakdown the waste which converts the matter into biogas and digestate (left over waste).
Biogas Capture and Digestate processing – biogas is captured and used for electricity & heat generation and the digestate goes through post treatment and can be used as a fertiliser or soil conditioner.
AD Plants can be classified into different types depending on the type of feedstock, operating conditions, and the way the process is carried out. For example, if the plants are based on feedstock type, then there are four main types of AD plants which are listed below:
Agricultural AD plants – use of manure, crop waste as the feedstock.
Industrial AD plants – use waste from food processing, breweries and other industries with organic waste.
Municipal AD plants – use sewage waste and are operated by wastewater treatment facilities.
Co-digestion AD Plants – use a mixture of wastes i.e. manure, food waste to balance nutrients which helps optimise the biogas production.
Other factors which affect the type of AD plant used is process configuration which can mean the use of either wet digestion plants, dry digestion plants, batch or continuous AD plants. Typically, these plants will use different digester designs depending on the application i.e. plug flow digester for continuous AD plants. The choice of plant is dependent on factors including the waste to be treated, energy output, available space and budgetary considerations.
Why is Process Safety so important for AD plants?
Process Safety is very important with AD plants since they have typically the following risks:
Methane is a highly flammable gas where if there is a leak or uncontrolled release then a fire or explosion can take place. Therefore, proper containment, venting and monitoring of flammable gases is critical.
Toxic gases such as hydrogen sulphide & ammonia can be produced within the biogas which is toxic and corrosive. Improper use can cause major exposure issues to personnel and the environment.
Pressure and temperatures within the digester itself can be critical since if the pressure vessel or temperature control systems fail then can lead to catastrophic equipment failure leading to downtime and potential for loss of containment to the environment and risk to personnel safety.
Incompatible materials with different types of feedstocks may produce variable biogas yields and different gas compositions which can cause process instability.
Human factors/operator errors during operation and maintenance such as improper gas venting can lead to serious safety incidents.
Corrosion and equipment failure due to age or corrosion from gases such as hydrogen sulphide can increase loss of containments which can eventually find an ignition source.
Examples of Process Safety events at AD plants in the UK?
Tank Explosion at AD Plant, Harper Adams University (2014)
Location: Harper Adams University, Shropshire
Date: May 2014
Incident: A digester tank exploded at a biogas plant. The explosion occurred when the gas produced in the digester tank ignited, causing structural damage to the plant. Thousands of gallons of slurry spilled into the nearby farm area contaminating the land around the plant.
Cause: The exact causes were attributed to a mechanical failure in the plants biogas system. A probable gas leak from the equipment allowed methane to find an ignition source resulting in an explosion.
Impact: No one was injured in the incident, but the explosion caused significant damage to the plant, and the operation was disrupted.
Lessons Learned: This incident emphasised the critical need for preventative maintenance and effective gas detection systems and suitable ATEX-certified equipment. Suitable grounding should also be used to prevent static discharge especially where biogas is stored or processed.
Figure 2: Picture showing the tank after explosion at Harper Adams University
Incident: Lightning struck the biogas tanks and caused a large explosion and the orange glow from the explosion could be seen from a great distance. It took over 40 fire and rescue personnel to put the fire out and luckily no personnel were injured during the explosion.
Cause: The main cause was of the lightning strike hitting the biogas tanks which is a type of ignition source and the flammable atmosphere within the biogas tank must have been within the flammable limits to ignite and cause the explosion.
Impact: Although there were no injuries there was significant damage which meant the plant was temporarily shut down which would have had substantial rebuild costs and operational costs.
Lessons Learned: This incident highlights the importance of lightning protection systems. Always ensure there are suitable lightning protection systems in place when there are potential explosive atmospheres present.
Figure 3: Photo showing the Severn Trent Plant after the incident.
Overall, maintaining rigorous process safety standards in anaerobic digestion plants protects workers, communities, and the environment, while ensuring reliable and efficient production of renewable energy.
At Finch we have experience in not only assisting companies with Process Safety elements such as HAZOPs & DSEAR on AD plants, but also with accident investigation, and expert witness work following AD plant fires and explosions. If you want further information or advice on AD plants or other processes then please contact Tristan Pulford: [email protected] : 01530412777, Sohail Khan: [email protected], Nick Freer: [email protected] .
We can run HAZOP workshops/DSEAR studies/accident investigation at your workplace and assist in identifying where there are gaps and support you with finalising solutions and closing the gaps.
Tristan is a chartered mechanical engineer and process safety engineer and is Finch Consulting’s Capability Director. Sohail is a chemical engineer experienced in process safety management. Nick Freer is a Chartered chemical engineer experienced in Process Safety and Asset management.
Finch is a leading risk management consultancy, working worldwide with blue-chip clients in multiple sectors.
As a “critical partner” to our clients, we provide confidence to be a better business by helping identify, manage and mitigate risks associated with engineering, health and safety, and regulatory compliance. This is delivered through three core areas: Asset Management, Health & Safety Management and Process Safety Management.
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