Biogas plants exist to produce methane — which means every anaerobic digestion (AD) site is, by design, producing a gas mixture that sits well above its own explosive limit. Gas detection isn’t a bolt-on safety measure here — it’s core infrastructure.
Why Gas Detection Is Core to AD Sites in the Biogas Industry
Gas detection is fundamental to anaerobic digestion operations because the breakdown of organic waste produces large volumes of methane. There are currently around 750 operating AD sites in the UK, producing an estimated 21 TWh of biomethane annually. Methane concentrations inside AD fermenters typically range from 50–75% by volume — a significant risk that must be actively managed. With methane’s explosive limit sitting at just 5–15% in air, the real danger comes from smaller leaks escaping confined spaces and sealed environments, not the fermenter itself.
Sludge and organic matter can also generate hydrogen sulphide (H2S), a gas that poses a serious threat to worker safety due to its toxicity, even at relatively low concentrations. On top of this, DSEAR regulations — which govern hazardous area classification and confined space safety — must be complied with, and in most cases, this is only achievable with an efficient, trusted fire and gas detection system in place.
How the UK AD Industry Is Changing
AD sites have shifted significantly over the last decade, moving away from on-site electricity generation (CHP) and toward purifying biogas into biomethane for injection into the national gas grid. This upgrade route allows biomethane to travel through existing gas pipelines, reducing heat loss compared to on-site power generation and helping sites align with UK Net Zero targets.
The 3 Most Dangerous Gases on an AD Site
Methane
Methane has a wide flammable and explosive range, from 5% to 15% in air — any ignition source within that range risks an explosion. It’s naturally colourless and odourless, though on biogas sites the presence of sulphur compounds often gives it a distinctive smell. With a relative density of around 0.55 (lighter than air), methane rises rapidly and can accumulate at the top of enclosed spaces, such as under warehouse roofs.
Hydrogen Sulphide (H2S)
H2S is tightly regulated due to its toxicity, and frequent exposure can cause serious health issues for both employees and the surrounding environment. In the UK, the Workplace Exposure Limit (WEL) is 5ppm as an 8-hour time-weighted average, with a 10ppm short-term (15-minute) limit. Exposure above 100ppm is classified as immediately dangerous to life. H2S is sometimes called an “invisible killer” because it deadens the sense of smell (olfactory fatigue) at higher concentrations — meaning workers can no longer rely on its “rotten egg” odour as a warning sign.
Carbon Dioxide
CO2 typically makes up around 40% of raw biogas. With a relative density of roughly 1.53 (heavier than air), it pools at floor level in confined or poorly ventilated spaces. Many operators underestimate the danger CO2 poses simply because it isn’t flammable — but as an asphyxiant, it’s just as capable of causing serious harm.
How Big Is the Gas Leak Problem on AD Sites?
Around 85% of AD plants surveyed across the UK and Germany showed some level of biogas leakage, based on a survey of nearly 1,000 sites over eight years. Separately, research led by Queen Mary University of London found that biogas plants across the UK, Poland and Germany lose an average of 5.4% of the methane they produce to fugitive emissions — with UK sites among the higher emitters in the study. Beyond the safety implications, this represents a direct and ongoing loss of revenue for operators.
DSEAR & Hazardous Area Classification
Under DSEAR, operators must assess, classify and identify where explosive atmospheres could occur in the workplace. This framework divides areas into hazardous zones (Zones 0, 1 and 2) based on the frequency and duration of explosive atmospheres. From this classification, Equipment Protection Levels (EPLs) are applied to ensure equipment — including fixed electrical installations — is suitably ATEX-rated for its environment.
If your AD site has changed — for example, retrofitting a biomethane upgrading unit, adding new gas compression stages, or altering grid injection pipework — your existing hazardous area classification is likely out of date and must be reviewed before operations continue.
- BS EN 60079-10-1:2021 — the British and EU standard used to calculate the physical size and shape of a hazardous zone based on gas release rates, ventilation and accumulation.
- Energy Institute Model Code of Safety Practice (EI IS) — provides industry-accepted, prescriptive methods for calculating hazardous areas.
High-Risk Areas on AD Sites
The highest-risk areas on any AD site are those handling contained or pressurised gases, or requiring work in confined spaces:
- Digester Tanks & Gas Holder Roofs — contain large volumes of flammable methane mixed with CO2. Membrane roofs and gas storage spaces are often classified as Zone 1 or Zone 2.
- CHP Units & Gas Pipework — gas lines, flanges, valves and engines carrying pressurised gas, which can leak from worn seals or vibrating pipework, creating potential ignition risks.
- Digestate Storage Tanks — leftover digestate can off-gas CO2 and ammonia, rapidly displacing oxygen directly above the tank.
Overall
At Protex Systems, we work with AD and biogas operators to make sure their fire and gas detection strategy matches the site they’re running today — not the one it was designed for. If your detection system hasn’t been reviewed since your site last changed, get in touch — we’d be glad to help you assess it.
