Feed a digester through a 15-20 cm pipe that ends below the slurry surface, so slurry seals the gas in, and let the digestate leave through a pipe of the same diameter on the opposite side; the height of the outlet lip sets how high the slurry, and so the gas pressure, can rise. Both pipes slope down, both pits need a lid or rail, and the mixing pit is a place where hydrogen sulfide escapes as slurry is stirred.
Part of Biogas Digesters
The inlet and outlet are the pipes and pits through which feed enters and digestate leaves, and which hold the slurry level that seals the gas.
The Parent’s Layout
- Inlet: a 15-20 cm pipe entering below the slurry level; mix manure with equal parts water before feeding.
- Outlet or overflow: the same diameter, on the opposite side, positioned so the slurry level determines gas pressure.
- Both pipes slope downward into and out of the digester so gas cannot escape.
An inlet pipe that opens above the slurry would let gas out as it fills; one that ends below it is a liquid seal. Feeding from a mixing pit above the digester lets gravity do the work.
Pressure From The Outlet Lip
Slurry is about as dense as water (1,000 kg/m³ assumed), so a height difference h of slurry makes a pressure of ρ g h. For h = 15 cm: 1,000 × 9.81 × 0.15 = 1,472 Pa, about 15 cm of water column. In a floating-drum plant the outlet lip fixes the slurry level and the drum sets the pressure (Floating-Drum). In a fixed dome the gas pushes slurry up the outlet, so every metre of rise is 9.81 kPa (Fixed-Dome), so the outlet tank’s height is a pressure limit as well as a size.
Pipe Size And Flow
A 15 cm pipe has an area of π ÷ 4 × 0.15² = 0.0177 m²; a 20 cm pipe, 0.0314 m². A household feeds 40-150 litres of slurry a day (0.04-0.15 m³), a few buckets, so flow is trivial; the big diameter is for straw and fibre, which block narrow pipes.
What You Need
- Pipe: clay, concrete or salvaged plastic, 15-20 cm (Clay Pipe, Salvaged Modern Pipe, the latter warning against any pipe that carried fuel or chemicals)
- A mixing pit (inlet) and an outlet pit (digestate)
- Lids or rails on both
- A stirring stick, long enough that no one leans into the pit
Method
Step 1: Set the inlet pipe at a steep slope, entering low, well below the working slurry level.
Step 2: Set the outlet on the opposite side at the planned overflow height.
Step 3: Seal each pipe into the wall (Waterproofing & Sealing).
Step 4: Cover or rail the pits so that no child or animal can fall in.
Step 5: Feed by mixing manure and water in the inlet pit and letting it flow in; clear a blocked pipe with a long rod, never by looking into it.
Common Mistakes
- An inlet that ends above the slurry, letting gas out and air in.
- Narrow pipes, which clog with straw.
- No cover on the outlet pit, a drowning and gas hazard.
- An outlet too high for the dome’s strength.
The mixing pit gives off gas and drowns
Stirring fresh and fermenting slurry releases hydrogen sulfide and carbon dioxide, which collect low in a pit and numb the sense of smell; a person who leans in can collapse into the slurry. Stir from above with a long stick in open air, never enter the pit, and never go after someone who has fallen: ventilate and haul from outside (Manure Management: never enter a manure pit). Cover or rail every pit.
What’s Next
- Learn next: Mixing & Agitation
- Combine with: Pressure Regulation