Double-membrane gas holders (biogas storage tank)

A gas holder is a buffer storage vessel that decouples biogas production from downstream consumption — letting the digester produce gas at its natural rhythm while upgrading, CHP or compression equipment runs at steadier rates. Without a gas holder, every short-term fluctuation in digester output would either over-pressure the system or starve downstream equipment.

Two construction families dominate Indian CBG practice. Double-membrane gas holders consist of an outer weatherproof EPDM or PVC membrane and an inner gas-tight membrane that inflates and deflates with biogas volume. A small fan maintains air pressure in the annular space between the two, holding the outer shape and providing constant biogas pressure at the outlet (typically 3-5 mbar gauge). They are usually mounted directly on top of the digester or as a separate ground-level unit. Rigid steel or fibreglass gas holders use either a floating-roof design (a gas-tight inverted cup that rises and falls in a water seal) or a fixed dome with pressure control. Rigid holders are more durable but heavier and more capex-intensive.

Sizing follows production patterns. Plants with steady continuous-flow digesters and continuous downstream consumption need 2-4 hours of typical production as holder volume. Plants with batch or semi-batch operation, or with intermittent CHP, may need 8-24 hours of buffer. A 5 TPD CBG plant producing 750 Nm³/hour of raw biogas needs roughly 1,500-3,000 m³ of holder volume for 2-4 hour buffering.

The trade-offs are condition-monitoring and safety. Double-membrane holders are cheaper and easier to install (₹50-120 lakh for a 1,500 m³ unit) but the outer membrane is exposed to UV, dust and bird damage; typical service life is 15-20 years with annual inspection. Inner-membrane leaks are hard to detect without specialised methane sensors, so plants should monitor the inflation fan's power consumption — rising power use indicates increased air leakage past the inner skin and possible methane migration into the annulus. Rigid holders cost 50-100% more but have well-defined inspection regimes under Indian Boiler Regulations equivalents. Both designs require flame-arrestors at all gas-line connections, pressure-relief vents sized for full digester production runaway, and emergency-flare capacity to burn off gas safely during downstream equipment outages. Skimping on these safety items is the single biggest source of catastrophic incidents at biogas plants in India.