A silo generates gas (2) from a charge of fermenting organic waste under a gas membrane (11). The membrane is secured to the silo wall from which it can be detached by operation of a mechanism outside the silo wall. A silo generates gas (2) from a charge of fermenting organic waste under a gas membrane (11). The membrane is secured to the silo wall from which it can be detached by operation of a mechanism outside the silo wall. The membrane inner face is exposed to the gas, while the outer face is attached to a thermally insulated (3) silo wall by loops tied or hooked to the wall around the circumference. The membrane is impervious to methane and is strengthened by woven fibres. The membrane rim hangs into a water-filled gutter (20) on the silo inner face forming a gas-tight seal The membrane rises and falls in accordance with the gas volume and ambient pressure, and is protected by a removable roof (6). The membrane is positioned and sealed or removed only after first removing the roof. The silo wall has a cap (24) preventing the ingress of rain water.
