A biogas digester must be both watertight and gas-tight, and the second is harder: any crack that passes water will pass gas, but a wall that holds water can still leak gas through hairline cracks. Seal masonry with several coats of cement or lime plaster, line plastic pits with welded sheet, and test first with a marked water level over 24 hours, then with air pressure, then with soapy water on every joint.
Part of Biogas Digesters
Waterproofing and sealing make a digester’s walls and joints hold both slurry and gas, and the tests show whether they do.
The Parent’s Methods
- Brick or concrete: several coats of cement plaster, finished with a waterproofing agent: bitumen, lime wash or acrylic sealant if available.
- Plastic-lined pit: excavate, line with heavy HDPE or PVC sheeting, seal joints by heat welding or an appropriate adhesive.
- Test before loading: fill with water and check for leaks; any leak that passes water will leak gas.
The corpus’s cement plaster guide gives a standard for a cistern: two coats of 1:2 cement:sand, each 10-15 mm thick, cured damp at least 7 days (Cement Plaster). For a gas dome use more coats and cure thoroughly. Pitch and bitumen finishes are described in Pitch and Waterproofing; plastic membranes and other options for earth roofs are compared in Waterproof Membranes.
Watertight Is Not Gas-Tight
Gas can pass through finer pores and cracks than liquid water will; a surface with hairline cracks can pass biogas while water stays put. A dome plastered wet and cured slowly has fewer cracks than one plastered fast and left to dry. The parent says any leak that passes water will leak gas, which is true; the converse is not, so the water test alone is not enough.
What You Need
- Plaster materials and curing time, with sacking or sand kept wet
- Waterproofing finish: bitumen or lime wash, or heat-welded plastic
- Water, a marker for the level, and time
- An air pump and a water-column gauge (a transparent U-tube half full of water; each centimetre of difference is about 98 Pa)
- Soapy water in a bottle and brush
Method
Step 1: Plaster in two or more coats, damp-curing each for at least 7 days.
Step 2: Water test. Fill, mark the level on the wall, wait 24 hours, remeasure. Repair any fall and repeat.
Step 3: Air test. With every opening closed except the gas outlet, pump air to the highest pressure the digester will see (the working pressure, not more) and watch the gauge for a day. A steady reading is a pass.
Step 4: Soap test the joints. After gas has appeared, brush soapy water on every fitting, valve and weld. Bubbles show a leak (the parent gives the same test). Never use a flame to find a leak.
Step 5: Repeat after the first season; ground movement opens cracks.
Common Mistakes
- Declaring it sealed because it holds water.
- Pumping air to over-pressure, which cracks the dome.
- Skipping the joints. Most leaks are at pipes, valves and plastic welds, not walls.
- Using a flame to find the leak.
A gas leak is a fire and a poison
Leaking biogas carries methane that explodes at 5-15% in air and hydrogen sulfide that is lethal and numbs the nose. Test with soapy water, never a flame, ventilate any enclosed space where gas piping runs, and fix leaks before use. The corpus’s mortar and plaster work (lime, cement) burns skin and eyes; wear gloves and goggles (Protective Gear).
What’s Next
- Learn next: Inlet & Outlet Design
- Combine with: Piping & Valves, where most leaks are