Do Biogas and Anaerobic Digestion Plants Need ATEX Zoning?

Yes. Biogas is typically 45 to 75 % methane and forms explosive mixtures with air, so every anaerobic digestion plant needs an explosion risk assessment and, in most cases, hazardous zones around the points where gas can escape: over/underpressure relief devices, membrane gas holders, condensate traps, gas treatment, blowers and the CHP engine room. The inside of the gas system is normally too rich to burn, but air ingress, start-up and maintenance change that. The operator sets the zones in the explosion protection document (the DSEAR assessment in Great Britain). In Germany, TRGS 529 and the DGUV example collection give detailed guidance; elsewhere, IEC 60079-10-1 is the method.

The hazard: methane, hydrogen sulfide and sometimes hydrogen

TRGS 529 gives a typical dry composition of 45 to 75 % methane, 25 to 55 % carbon dioxide, up to 5 % nitrogen, up to 2 % oxygen and up to 0.4 % hydrogen sulfide, with traces of ammonia, hydrogen and higher hydrocarbons. For an example biogas of 60 % methane it gives an explosive range of about 6 to 22 vol % and a density ratio to air of 0.9.

Methane and hydrogen sulfide values are from the IFA GESTIS database; older references often quote 5 vol % for the methane LEL, and the SVLFG safety rules also use 4.4 vol %. Three points follow for classification:

• Density is not fixed. TRGS 529 notes that biogas can be lighter or heavier than air depending on composition, moisture, temperature and pressure, and that it does not separate out by gravity. Do not assume it always rises when placing gas detectors or judging ventilation.
• Hydrogen sulfide is mainly a toxic hazard: TRGS 529 gives typical concentrations of 100 to 4,000 ppm, occasionally up to 2 vol %, and notes that the sense of smell is deadened from about 100 ppm. It is also group IIB and T3 in its own right, so the classification should state the group and T-class it assumes for the actual gas rather than defaulting to methane values.
• Hydrolysis gas. Where a separate hydrolysis stage is used, TRGS 529 warns that hydrogen can form in higher concentrations and needs further explosion protection because of its wider explosive range, lower ignition energy, smaller safe gap and higher explosion group. See hydrogen explosion protection.

Where explosive atmospheres form

The zone drawing follows the release sources. The main ones on an agricultural or waste AD plant:

• Digester and gas holder gas space. In normal operation it contains biogas well above the upper explosion limit. It becomes a problem when air gets in: underpressure, air dosing for biological desulfurization (TRGS 529 limits the air flow to at most 6 % of the biogas flow, even if the flow control fails), and emptying or refilling. The SVLFG safety rules treat start-up and shutdown as a special operating state, not normal operation, requiring its own assessment and operating instructions.
• Overpressure and underpressure safety devices. TRGS 529 requires a second gas consumer (typically a flare) with enough capacity to start before the relief device opens, and relief discharge at least 3 m above ground or the operating level, 1 m above the roof or tank rim, and 5 m horizontally from buildings and public traffic routes. In one example in the DGUV explosion protection example collection (point 4.8.9, reproduced in SVLFG Technical Information 4), an outdoor relief device arranged this way, with a discharge rate of up to 250 m³/h, a flare sized for the plant's maximum gas production and daily function checks, is assigned no zone because it should only open very rarely; the release case is handled separately.
• Membrane roofs and gas holders. The gas membrane, its clamping at the tank rim, and the support-air exhaust of double-membrane holders are potential release points. How they are zoned depends on the design, the tightness classification and the inspection regime; TRGS 529 lists membrane covers among the dynamically loaded parts that tightness checks must cover, and gives monitoring of the support air of air-supported roofs as an example of fixed atmosphere monitoring.
• Condensate traps and shafts. TRGS 529 requires condensate separators to be checked and maintained without entering shafts or pits, and designed so gas cannot escape in any operating state. Pits collect gas, which makes them both a zone and a confined-space hazard.
• Pipework, valves, blowers and gas analysis. Under the German approach (TRBS 2152 Part 2, summarized in SVLFG TI 4), parts that are "permanently technically tight" (welded joints, tongue-and-groove flanges, double mechanical seals, magnetically coupled pumps) create no hazardous area around them when closed; parts that are only "technically tight" (plain flanges, single mechanical seals) can have rare releases, the usual basis for a Zone 2.
• Substrate reception and digestate drying. TRGS 529 requires fire and dust explosion hazards of digestate drying to be covered in the risk assessment; dried digestate is a combustible dust.

CHP engine rooms and gas treatment rooms

TRGS 529 requires technical measures that ensure no hazardous explosive atmosphere can form in CHP rooms. The SVLFG safety rules describe the usual package:

• Non-closable supply and exhaust openings for cross-ventilation, with exhaust taken from the ceiling area; forced ventilation sized to dilute the maximum possible gas release to no more than 20 % LEL.
• Fixed gas detection, for example a first alarm at 20 % LEL (0.9 vol % methane) with warning and ventilation to full output, and a second at 40 % LEL (1.8 vol %) that also closes the gas supply automatically outside the room. The detection keeps running after the second alarm.
• Two automatic shut-off valves in the gas line before each engine, closing when it stops, with the space between them checked for tightness.
• A manual shut-off for the gas supply outdoors, close to the CHP room.

TRGS 529 also requires flame arresters in the gas lines to consumers (CHP, boilers, gas analyzers, flares), placed as close to the consumer as possible and supplied as autonomous protective systems. The SVLFG rules add that where rooms with gas-carrying parts cannot be shown to be permanently technically tight, ignition sources must be avoided and zones assigned, or ventilation with airflow monitoring or gas detection with coupled ventilation used to limit them.

Ventilation and IEC 60079-10-1

Outside Germany, zones are normally derived with IEC 60079-10-1:2020: grade of release, release rate, degree of dilution and availability of ventilation. Most AD plant is outdoors, where dilution is usually good, but look for sheltered pockets (between tanks, under roof overhangs, in pits and valve chambers) and low-velocity releases, which dilute poorly. TRGS 529 requires gas storage installation rooms to have effective ventilation, cross-ventilation where possible, and rooms with substrate-handling or not permanently tight gas-handling parts to have at least natural cross-ventilation.

Equipment selection

• Zone 1: EPL Gb / ATEX category 2G. Zone 2: EPL Gc / category 3G or better. Zone 0 (only where the classification finds it, for example inside a gas space subject to air ingress): EPL Ga / category 1G. See EPL and equipment categories.
• Gas group and T-class: methane alone is IIA and T1. Hydrogen sulfide is IIB and T3; hydrolysis gas with hydrogen needs a higher group. State the basis in the classification and select equipment to match. See gas groups and temperature classes.
• Non-electrical equipment in zones (blowers, compressors, agitators, pumps) needs ATEX conformity under Directive 2014/34/EU as well.
• Lightning and static: TRGS 529 calls for lightning protection measures and for TRGS 727 to be applied against electrostatic charging.

Regional notes

• Germany: TRGS 529 "Tätigkeiten bei der Herstellung von Biogas" (edition July 2024, amended 2026) applies to biogas plants but not to sludge digesters that are part of wastewater treatment, nor to plants that upgrade and inject biogas into gas grids, which are covered by DVGW G 265-1. Zone examples: DGUV Regel 113-001 (EX-RL) example collection, point 4.8. The SVLFG publishes the safety rules for agricultural biogas plants (Technical Information 4).
• Great Britain: DSEAR 2002 and ACOP L138 apply; there is no AD-specific approved code. HSE ran an inspection initiative at 50 farm-based AD sites from early 2023, focused on DSEAR, process control, safe maintenance and managing contractors. The industry AD Certification Scheme, promoted by ADBA, includes DSEAR compliance.
• US: digesters at wastewater treatment plants fall under NFPA 820 (current edition 2024); electrical classification follows NEC Articles 500/501 or 505. See NEC 500 vs ATEX/IEC.

Common mistakes

• Assuming "the digester is too rich to explode" without assessing air ingress, desulfurization air dosing, start-up and emptying.
• Relief devices discharging next to walkways, roof access points or buildings, or no flare capacity to keep them closed.
• Gas detectors mounted high on the assumption that biogas always rises.
• Non-rated equipment added to digester roofs later: cameras, lights, agitator drives, level sensors.
• Hot work on or near membranes and gas lines without a gas-free check; opening membrane covers (for example to change an agitator) without treating it as a planned release.
• Entering condensate shafts or pits, which combines a flammable atmosphere with H₂S and oxygen depletion.
• Forgetting the dust hazard of digestate drying and pelleting.

What else applies

Explosion protection on AD plants is mostly about preventing releases and ignition: technically tight construction, flares and relief devices, flame arresters, gas detection linked to ventilation and shut-off, and permit-controlled maintenance. Toxic and asphyxiation hazards (H₂S, CO₂, NH₃) need the same attention; where TRGS 529 relies on gas-free measurement during work, portable detectors must at least measure methane, CO₂, H₂S and oxygen, plus ammonia where its exposure limit may be exceeded. This page is general guidance; the zone drawing for a specific plant has to come from its own assessment by a competent person.

Frequently asked questions

What is the LEL of methane and biogas?

Methane has a lower explosion limit of 4.4 vol % (GESTIS; older sources quote 5 vol %). Because biogas also contains CO2, its explosive range is narrower: TRGS 529 gives about 6 to 22 vol % for an example biogas with 60 % methane.

Is the inside of a biogas digester Zone 0?

Not by default. In normal operation the gas space is far above the upper explosion limit. Air ingress, desulfurization air dosing, start-up, shutdown and emptying can create an explosive atmosphere, so the classification must assess those states and may assign a zone inside.

What zone is around a biogas pressure relief valve?

It depends on the discharge arrangement and how often it opens. TRGS 529 requires a flare to start before the relief opens and sets minimum discharge heights and distances; in one DGUV example with those measures in place, the outdoor relief device is assigned no zone.

Does a biogas CHP engine room need to be a hazardous area?

The aim is that it is not. TRGS 529 requires technical measures so no hazardous explosive atmosphere forms in CHP rooms, typically cross-ventilation, gas detection that raises ventilation and shuts off the gas supply, and double shut-off valves before each engine.

Which gas group and temperature class for biogas equipment?

Methane alone is IIA and T1, but hydrogen sulfide is IIB and T3 and hydrolysis gas can contain hydrogen. The area classification should state which group and T-class it assumes for the actual gas, and equipment is selected to match.

Sources

1. BAuA: TRGS 529 Tätigkeiten bei der Herstellung von Biogas
2. SVLFG: Technische Information 4, Sicherheitsregeln für Biogasanlagen
3. IFA GESTIS Substance Database: Methane
4. IFA GESTIS Substance Database: Hydrogen sulfide
5. IEC: IEC 60079-10-1:2020 Classification of areas - Explosive gas atmospheres
6. ADBA: HSE announce inspections of 50 farm-based AD plants
7. NFPA: NFPA 820 Standard for Fire Protection in Wastewater Treatment and Collection Facilities

Related Topics

• Zone Classification
• Hazardous Area Classification
• Gas Groups
• Hydrogen Explosion Protection
• DSEAR Regulations (UK)
• ATEX Directive
• ATEX by Application