Anyone operating a biogas plant in Germany is responsible for building and running it safely, even when it is a small manure plant. The main building blocks are a risk assessment, an explosion protection document with hazardous zones, effective protection against overpressure and vacuum, consistent protection against hydrogen sulphide, regular inspections and complete records. The Technical Rule for Installation Safety TRAS 120 serves as the benchmark for the state of the art in safety.
Legal framework and TRAS 120
Several sets of rules interlock for biogas plants:
- Occupational Safety and Health Act (Arbeitsschutzgesetz): duty to carry out a risk assessment and instruct staff as soon as employees or family members work at the plant.
- Hazardous Substances Ordinance (Gefahrstoffverordnung, GefStoffV): protection against hazardous substances such as biogas, hydrogen sulphide and ammonia, including explosion protection.
- Industrial Safety Regulation (Betriebssicherheitsverordnung, BetrSichV): inspection duties for work equipment and installations in potentially explosive atmospheres.
- Federal Immission Control Act and Major Accidents Ordinance: for larger plants and plants holding large quantities of gas.
- Ordinance on Installations for Handling Substances Hazardous to Water (AwSV): requirements for tanks and pipes to protect soil and water.
TRAS 120 “Safety requirements for biogas plants”, issued by the German Commission on Process Safety (Kommission für Anlagensicherheit), brings together requirements for construction and operation. It is binding mainly for plants covered by the Major Accidents Ordinance or requiring an immission control permit. For smaller plants it describes the state of the art, and authorities often refer to it when granting building permits. In addition, the social insurance for agriculture, forestry and horticulture (SVLFG) publishes practical safety rules for biogas plants.
Risk assessment and instruction
The risk assessment is the starting point for all protective measures. It covers every task at the plant, from the daily inspection round and solids feeding to maintenance, repairs and entering tanks. For each hazard you define measures and check that they work.
Typical hazards include:
- explosion and fire from escaping biogas
- poisoning by hydrogen sulphide, suffocation from carbon dioxide or lack of oxygen
- falls from tanks and roofs, dangers from agitators and pumps
- electrical hazards and noise at the combined heat and power (CHP) unit
- drowning in open slurry or digestate tanks
Tasks outside routine operation are particularly dangerous: entering tanks and shafts, repairing agitators, opening gas-carrying pipes or working on tank roofs. A written permit-to-work system has proven useful for such tasks. It sets out who authorises the work, which measurements are required beforehand, how drives are isolated and secured against restarting, and who acts as standby. Contractors must be briefed on the plant’s hazards before they start.
The risk assessment leads to written operating instructions. Everyone working at the plant must be instructed before starting and regularly afterwards. Specialist training for operators and staff is state of the art, acquired through courses and refreshed through further training.
Explosion protection and ATEX zones
Methane forms an explosive mixture with air at roughly 4.4 to 17 per cent by volume. Wherever biogas can escape, you therefore have to define hazardous areas and describe them in the explosion protection document under section 6 GefStoffV. Areas are classified into three zones:
| Zone | Meaning | Examples at a biogas plant |
|---|---|---|
| Zone 0 | explosive atmosphere present continuously, for long periods or frequently | gas space inside tanks, depending on operation |
| Zone 1 | likely to occur occasionally in normal operation | around overpressure and vacuum relief devices, gas pipe openings |
| Zone 2 | not likely in normal operation, or only briefly | around flanges, gas storage membranes, the CHP gas train |
Only equipment suitable for the relevant zone (ATEX marking) may be used in these areas. Ignition sources such as open flames, sparks, hot surfaces and static electricity must be avoided. Work involving ignition risks, such as welding, requires a written permit. Lightning protection and equipotential bonding are also part of explosion protection.
Mark hazardous areas with warning signs, prohibit smoking and open flames, and restrict access to the plant. Fencing not only keeps out unauthorised persons but also protects children and visitors on the farm. Vehicles and machinery with hot surfaces should be kept away from pressure relief devices and gas storage.
Hydrogen sulphide and other gases
Hydrogen sulphide (H2S) forms during the digestion of sulphur-containing material and is regularly present in slurry and biogas. It is heavier than air and collects in reception pits, pump shafts, cellars and tanks. The German occupational exposure limit is 5 ml/m³. Even at higher concentrations that are not yet fatal, the sense of smell fails, so the typical rotten egg odour disappears. High concentrations can cause unconsciousness and death within a short time.
Proven protective rules:
- Before entering shafts, pits and tanks, measure with a suitable multi-gas detector (methane, hydrogen sulphide, oxygen, carbon dioxide).
- Force-ventilate tanks and shafts before entry.
- Never work alone; always have a standby person and rescue equipment.
- Install fixed gas detection in rooms with gas pipes, such as the CHP room, and maintain it regularly.
Overpressure and vacuum relief, flare, gas storage
Digesters, secondary digesters and gas-tight digestate stores need protection against excessive overpressure and vacuum. These are usually liquid seals that release gas to the atmosphere under overpressure and let air in under vacuum. They must be frost-proof, checked regularly and correctly filled.
So that biogas does not escape through the pressure relief when the CHP unit fails, the state of the art generally requires an additional gas consumer, usually an emergency flare. Gas storage with membrane roofs needs fill level monitoring and protection against wind load and mechanical damage. Gas pipes should be labelled and drain condensate safely.
Preparing for emergencies is also part of the protection concept. Agree an alarm and emergency response plan with the local fire brigade, provide a site plan showing gas pipes, shut-off valves and tank contents, and invite the fire brigade for a site visit. Emergency stop switches and gas shut-offs must be easy to reach and clearly labelled. Keep rescue equipment, fire extinguishers and an up-to-date list of emergency numbers at the plant. On family farms in particular, where often only one person knows the plant, other family members should know how to shut off the gas supply and call for help in an emergency.
Inspection duties and operating log
Under the BetrSichV, installations in potentially explosive atmospheres must be inspected before first commissioning, after modifications requiring inspection and periodically. Added to this are inspections under water law, checks of electrical installations and maintenance of gas detection systems. An overview helps:
| Item inspected | Basis | Note |
|---|---|---|
| Explosion protection of the whole plant | BetrSichV | before commissioning and periodically, at least every six years |
| Equipment, protective systems, safety devices | BetrSichV | periodically, at least every three years |
| Gas detection systems and ventilation | BetrSichV | annually |
| Tanks and pipes for digestion substrates | AwSV | expert inspection before commissioning and periodically |
| Electrical installations | DGUV accident prevention rules | regularly, based on the risk assessment |
The specific intervals follow from the regulations, the risk assessment and your permit. Record all inspections, maintenance, faults and gas readings in an operating log. For the EEG payment you also have to document feedstocks; for small manure plants this means proving the minimum manure share of 80 per cent by mass.
Independent experts for inspections under the BetrSichV and AwSV are listed in the directory under experts and surveyors. How to organise maintenance and inspections over the year is covered in the guide to biogas plant maintenance, and the permitting basics in the guide to biogas plant permits. The regulation texts are available in German at gesetze-im-internet.de, for example the Industrial Safety Regulation.
Frequently asked questions
Does TRAS 120 apply to small manure plants?
TRAS 120 is binding mainly for biogas plants covered by the Major Accidents Ordinance (Störfall-Verordnung) or requiring an immission control permit. For smaller plants it serves as a reference for the state of the art in safety. Permitting authorities often rely on it and write individual requirements into permits.
Who prepares the explosion protection document?
The employer or operator is responsible. In practice, qualified planners or experts draw up the document, often during the planning stage. It must be in place before work begins and updated whenever the plant changes.
How dangerous is hydrogen sulphide?
Hydrogen sulphide is heavier than air, collects in pits, shafts and reception tanks and is toxic even at low concentrations. At higher concentrations the sense of smell fails, so the gas can no longer be noticed. Measure before every entry and never work there alone.
Do I have to keep an operating log?
For biogas plants, continuous records of operation, maintenance, faults and inspections are state of the art and are regularly required in permits. For the EEG payment you must also document the feedstocks used, for example in a feedstock log (Einsatzstoff-Tagebuch).
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