Run biogas through 12-20 mm plastic pipe, sloped to a low point with a water trap and drain because the gas is saturated with moisture, with a ball valve at each appliance and a master valve at the holder. Remove hydrogen sulfide with a scrubber of iron filings or steel wool and replace it on a fixed schedule, not when the smell changes, because the gas is least detectable when it is most dangerous, and keep copper and brass out of the line.
Part of Biogas Digesters
Biogas piping is the plastic or steel line, traps, scrubber and valves that carry gas from the holder to appliances.
The Parent’s Design
- Plastic pipe (PVC or polyethylene), 12-20 mm.
- A water trap at the low point of the run, with a drain valve, because the gas is saturated with moisture that condenses in the pipe.
- A simple ball valve at each appliance to control flow.
- H₂S removal: pass the gas through a container of iron filings or steel wool, which absorbs hydrogen sulfide (it corrodes equipment and smells of rotten eggs).
Size: The Flow Is Small
A burner using 0.4 m³ an hour (an assumed top rate, above the parent’s 0.3-0.4 m³ per meal) in a 15 mm bore (7.5 mm radius, area 1.77 × 10⁻⁴ m²) has a gas speed of (0.4 ÷ 3,600) ÷ 1.77 × 10⁻⁴ = 0.63 m/s. That is a gentle flow, so the pressure lost along a short pipe is negligible; diameter matters for long runs and for the wider tube that does not block with condensate. Keep the run short and the line sloped so that water runs to the trap.
The Trap: Its Depth Must Beat The Pressure
A U-shaped water seal has a depth in centimetres of water; if the gas pressure exceeds that depth, the water blows out and gas escapes through the drain. For a line that could see the 20 cm relief pressure (Pressure Regulation), the seal must be deeper than 20 cm, say 30 cm. Locate the trap and drain outdoors, in the open, where escaping gas disperses, never in a pit or a kitchen, and empty the condensate regularly.
The Scrubber: Replace On A Calendar
Iron removes hydrogen sulfide by forming iron sulfide: iron oxide in rust reacts Fe₂O₃ + 3 H₂S → Fe₂S₃ + 3 H₂O. On a molar basis, 159.7 g of Fe₂O₃ takes up 3 × 34.08 = 102.2 g of H₂S, so at best 0.64 g of H₂S per gram of rust. How much gas does that treat? Raw biogas at 0.1% hydrogen sulfide holds 1 litre of H₂S per cubic metre, 1.39 g (34.08 ÷ 24.47 g/L), so a cubic metre at 0.1% needs at least 1.39 ÷ 0.64 = 2.2 g of rust, and at 0.5% about 11 g. A family using 2 m³ a day at 0.1% needs about 4.4 g a day, 1.6 kg a year, at the theoretical best. Real scrubbers use their material far less completely, so allow many times that and replace it on a schedule you set from the quantity of gas, not “when the smell comes back”.
Why not by smell: hydrogen sulfide numbs the sense of smell at the levels that matter, and scrubbed gas has no smell at all, which also removes the warning of a methane leak. Methane itself is odourless (Anaerobic Digestion Science).
Spent iron sulfide can heat up and may ignite when exposed to air (recalled, not from the corpus): keep spent material wet, and bury or dispose of it outdoors away from anything that burns.
What You Need
- Pipe and fittings of PVC or polyethylene; steel as an alternative. Avoid copper and brass, which hydrogen sulfide corrodes (recalled)
- A water trap with a drain, of a depth greater than the maximum pressure
- A ball valve at the holder and at each appliance
- A scrubber container with a gas inlet at the bottom, packed loosely with moist rusty iron or steel wool
- Soapy water for leak checks
Method
Step 1: Lay the pipe sloped to the trap, supported, shaded and buried or protected where it crosses paths.
Step 2: Fit the master valve at the holder, the trap at the low point, then the scrubber, regulator and flashback seal, then the appliance valves.
Step 3: Test every joint with soapy water, never a flame.
Step 4: Replace scrubber material on a calendar.
Step 5: Drain condensate every week.
Common Mistakes
- A trap shallower than the pressure.
- A long, sagging pipe that holds water in the middle.
- Relying on smell for the scrubber or for leaks.
- Copper pipe or brass fittings.
Leaks, numbness and fire
A leak indoors collects methane near the ceiling (0.55 times the density of air) and hydrogen sulfide near the floor (1.18 times), and raw biogas, at 0.84-0.99 times the density of air, can drift either way. Shut the valve at every appliance when not in use, ventilate the room, and keep the line short. Spent scrubber material can overheat. No flame to find leaks.
What’s Next
- Learn next: Using Biogas
- Combine with: Cooking Burners