Build a biogas digester in this order: choose a site close to the animals, downhill and at least 30 m from drinking water; size the plant from the daily feed and retention time; dig and build the body; waterproof and seal it; fit the inlet, outlet and gas outlet; then test it with water and with air pressure before loading. Every pit must have a lid or rail, and nobody enters a digester once it has held slurry.
Part of Biogas Digesters
Digester construction is the sequence of siting, sizing, building, sealing, fitting and testing a household biogas plant.
The Order Of Work
| Stage | What | Page |
|---|---|---|
| 1 | Size from the feed and retention time | Sizing the Digester |
| 2 | Build the body (dome, drum pit or tube) | Digester Types |
| 3 | Make it watertight and gas-tight | Waterproofing & Sealing |
| 4 | Inlet and outlet | Inlet & Outlet Design |
| 5 | Stirring access | Mixing & Agitation |
| 6 | Gas holder, regulator and piping | Gas Collection & Storage |
Choosing The Site
- Near the animals, so manure reaches the inlet without long hauling.
- At least 30 m from drinking water and downhill of it, the distance the corpus gives for manure (Manure Management); digestate and leaks carry nutrients and pathogens.
- Warm and sunny, because a warmer digester gives more gas (Temperature & Retention Time).
- Away from the house and from any fire, with the gas pipe as short as practical.
- Dry, stable ground with the water table below the pit floor, since a high water table floats and cracks an empty pit.
What You Need
- Labour and a digging plan, with shoring for pits that are deeper than a person’s waist, and a second person above ground
- Masonry, concrete or plastic, per the chosen type
- Plaster and waterproofing materials
- A means of testing: water, an air pump and a water-column gauge
Method: The Tests
Step 1: Water test. Fill the digester with water, mark the level on the wall, and recheck after 24 hours. A fall means a leak; find and repair it (Pond Lining and Waterproofing uses the same 24-hour check).
Step 2: Gas-tightness test. With the gas outlet connected to a water-column gauge, put air in to the working pressure, close the valve and watch the gauge for a day. Any fall is a leak. A structure that holds water can still leak gas (Waterproofing & Sealing).
Step 3: Seed and load with active culture.
Common Mistakes
- Building before sizing.
- No lid on the mixing pit or outlet tank.
- Skipping the gas test because the water test passed.
- Putting the digester above a well or spring.
Pits and confined spaces
A fresh or empty pit can hold carbon dioxide from decay or from nearby rotting matter, and a loaded one holds hydrogen sulfide and methane. Never go down alone, never rely on smell, and never go in to rescue someone; ventilate and use a line from outside (Safety Protocol, Manure Management). Shore every excavation against collapse.
What’s Next
- Learn next: Sizing the Digester
- Combine with: Gas Collection & Storage