A floating-drum digester is a cylindrical pit with an inverted steel drum on top that rises as gas collects and falls as it is used, so the drum is both the gas holder and the pressure regulator: pressure equals the drum’s weight divided by its area, about 6.8 cm of water column for a 77 kg, 1.2 m drum. It needs less masonry skill than a fixed dome but the mild-steel drum corrodes and needs replacing in 10-15 years.
Part of Biogas Digesters
The floating-drum or Indian model is a digester pit with a floating inverted drum that stores the gas at constant pressure.
Weight Over Area Sets The Pressure
The parent describes the design (the “Khadi and Village Industries” type): a cylindrical underground pit, a drum above the slurry rising with the gas, simpler construction than a dome, and a drum life of 10-15 years in mild steel. The drum’s weight W presses on the gas under it, so the gas pressure is p = W ÷ A (weight force over area), steady at any fill level while the drum floats freely.
Worked example. A drum 1.2 m across and 1.0 m tall, made of 2 mm mild steel (7,850 kg/m³, an assumed gauge):
- Top area: π ÷ 4 × 1.2² = 1.131 m². Side: π × 1.2 × 1.0 = 3.77 m². Steel area 4.90 m².
- Mass: 4.90 × 0.002 × 7,850 = 77 kg.
- Pressure: 77 × 9.81 ÷ 1.131 = 668 Pa. One centimetre of water is 98.1 Pa, so 668 ÷ 98.1 = 6.8 cm of water column.
- To reach 10 cm of water column (981 Pa) the drum needs 981 × 1.131 ÷ 9.81 = 113 kg, so about 36 kg of added weight (stones on top, or a thicker plate).
The parent says a floating drum gives 5-15 cm of water column “naturally”, which fits. The gas volume held is drum area times rise: 1.131 × 0.5 m of rise is 0.57 m³, about the evening use of a household (0.4-0.6 m³ for a lamp, per the lighting page).
What You Need
- A masonry or concrete pit with a central partition or guide pipe (Waterproofing & Sealing)
- A steel drum, welded gas-tight, with a guide so it does not tilt
- Rust protection: paint or tar (Pitch and Waterproofing), renewed regularly
- A gas outlet from the drum through a flexible or telescoping joint
- Heavy stones or plates to adjust the pressure
Method
Step 1: Build the pit and make it watertight first, then the guide and the drum.
Step 2: Weigh or measure the drum and compute its pressure; add weights to reach the burner’s need.
Step 3: Seat the drum in the slurry and test that it rises and falls freely, not catching on the guide.
Step 4: Paint the drum and check it every year for rust; replace before it fails.
Common Mistakes
- Letting the drum tilt and jam with gas locked under one edge.
- Painting it once. The gas is wet and carries hydrogen sulfide, which corrodes steel.
- Overloading with weights so that the pressure exceeds what the pipe or lamp can take.
- A drum that cannot sink fully, so air stays in at start-up.
A drum of gas is a lid on a pit
The drum rises with the gas stored beneath it. Do not stand on it or lean over the slurry when it is low; the pit and drum space hold methane, carbon dioxide and hydrogen sulfide. Do not use a flame near the rim, and never enter the pit (Anaerobic Digestion Science). At first filling, the space under the drum holds air, so the first gas is an air-and-gas mixture that can burn explosively: vent it away from any flame and do not light it until the gas is established (Using Biogas).
What’s Next
- Learn next: Pressure Regulation
- Combine with: Gas Holder Design