Double-Membrane Gas Holder: How Biogas Is Stored
How a double-membrane gas holder stores biogas: outer and inner membrane, air blower, working pressure, capacity and safety for biogas plants.
A double-membrane gas holder stores the biogas produced by a digester in the space between two PVC-coated polyester membranes: the inner membrane contains the gas and the outer membrane protects it and gives it shape. An air blower keeps a pressurized air cushion between the two, so the outer membrane holds its dome shape and applies a constant working pressure on the gas, typically 2 to 6 mbar. This delivers biogas to the consumers (CHP genset, boiler, flare) at stable pressure, without relying on water bells or low-pressure flexible bags.
How a double-membrane gas holder works
The principle is simple and robust. Biogas enters near the bottom and collects under the inner membrane, which inflates like a balloon inside the dome. As the gas level rises it displaces air from the intermediate chamber; when consumption exceeds production, the blower tops up the air to keep the pressure steady.
The two membranes
- Outer membrane (dome): the visible skin of the dome. It withstands wind, snow, UV radiation and hail. It is anchored to the tank wall or to a circular concrete base with a clamped fixing channel.
- Inner membrane: holds the biogas and expands or contracts with the balance between production and use. It sets the usable storage volume.
- Bottom membrane (optional): on ground-mounted holders or holders sitting on the digester itself, a third membrane seals the base and isolates the gas from the ground.
The blower and pressure control
A low-pressure air blower runs almost continuously to pressurize the outer chamber. A position sensor (ultrasonic or a counterweight cable) measures how far the inner membrane has inflated, i.e. how much biogas is stored. With that reading the control system decides when to start the genset, when to modulate the boiler, and, if the holder fills completely, when to open the safety flare to burn the surplus and prevent overpressure.
Capacity, pressure and typical dimensions
Gas-holder capacity is sized to provide a few hours of buffer between production and consumption, not to store days of gas. In practice it is designed for 2 to 8 hours of average production, depending on the genset or boiler consumption profile.
| Parameter | Typical range | Note |
|---|---|---|
| Gas capacity | 50 – 5,000 m³ | Depends on plant size and desired buffer |
| Working pressure | 2 – 6 mbar | Set by the blower and counterweights |
| Membrane material | PVC-coated polyester | High tenacity, UV- and H₂S-resistant |
| Membrane service life | 10 – 15 years | Depends on UV, H₂S and maintenance |
| Dome geometry | Height ≈ 0.5 – 0.6 of diameter | Hemispherical dome |
| Storage buffer | 2 – 8 h | Buffer, not long-term reserve |
Raw biogas contains methane (CH₄, 50–65%), CO₂ (35–45%) and traces of hydrogen sulfide (H₂S), a corrosive gas. That is why the inner membrane uses H₂S-resistant coatings and, in many plants, biological desulfurization is added by dosing a small amount of air inside the holder.
Safety: what a well-designed gas holder protects against
Biogas is flammable and H₂S is toxic, so safety is not optional. A well-integrated double-membrane holder includes:
- Hydraulic safety valve (water seal): relieves overpressure and vacuum without letting air into the gas circuit.
- Emergency flare: burns the surplus when storage fills or consumption stops.
- Level/position sensor: prevents the membrane from over-inflating or collapsing.
- Redundant or backup blowers: if the blower fails, the outer membrane must not collapse onto the inner one.
- Gas detection and ATEX zoning: the area around the holder is classified as an explosion-risk zone.
Why it beats old gas bags and water bells
Before double membranes, plants used low-pressure flexible gas bags and water bells (wet gas holders). Both have drawbacks compared with the double-membrane system.
- Stable pressure: a water bell gives variable pressure with the water level and a flexible bag has almost none; the double membrane delivers constant, adjustable pressure.
- No seal water to maintain or freeze: water bells need seal maintenance and frost protection; the double membrane is dry.
- Smaller footprint and fast installation: it mounts on the digester itself or on a concrete base, with no water pit.
- Better weather resistance: the outer membrane shields against UV, wind and hail, extending service life.
- Automation-ready: the gas level is a direct control input for the genset and flare.
The gas holder does not work in isolation: it is one component of the biogas system. To understand the whole —reactor, mixing, heating, valorization— see what an industrial biodigester is and how it works. And if you are planning the scale of the system, review how to size a biodigester for your farm, because holder volume depends on the projected biogas flow.
At Tech Tank we integrate the gas holder into the complete biogas and biodigester solution, using bolted steel tanks as the reactor body and domes and covers for gas containment. A reference case is the Marks Foods biogas plant, where agro-industrial effluent is turned into energy. If you want to evaluate biogas storage for your plant, talk to our engineering team to size capacity, pressure and safety for your consumption profile.
Frequently Asked Questions
How much pressure does a double-membrane gas holder handle?
Typical working pressure is 2 to 6 mbar, set by the air blower and, in some designs, by counterweights. It is a low but stable pressure, enough to feed gensets, boilers and flares without additional compression in most cases.
How much biogas can it store?
Commercial gas holders range from around 50 m³ up to several thousand m³. Capacity is chosen to provide 2 to 8 hours of buffer against the digester's average production; it is not a multi-day reserve but a cushion between production and consumption.
How long does the membrane last?
A good-quality PVC-coated polyester membrane lasts 10 to 15 years. Its life depends on UV exposure, the H₂S concentration in the biogas and maintenance. Desulfurization and periodic cleaning help reach the upper end of the range.
Is it safe to store biogas in a membrane?
Yes, provided the system includes a hydraulic safety valve, an emergency flare, level sensors, backup blowers and explosion-risk area classification. Biogas is flammable and contains toxic H₂S, so safety engineering and trained operation are essential.
Do I need an environmental permit to install a gas holder in Paraguay?
A biogas plant normally requires an Environmental Impact Assessment under Law 294/93 and a license from MADES. We recommend verifying the current regulations and the scope of the environmental license with MADES before building, since requirements depend on the scale and type of effluent.
Does the double membrane replace the water bell?
In new projects, yes. The double membrane delivers more stable pressure, needs no seal water or frost protection, takes less space and integrates better with automation. Water bells remain a legacy technology in older installations.