There are three common household digester designs: the fixed-dome (Chinese) plant, a masonry pit with a gas-tight dome that lasts over 20 years but needs skilled bricklaying; the floating-drum (Indian) plant, a pit with a steel drum that rises on the gas, simpler to build but with a drum that corrodes in 10-15 years; and the plastic bag or tube digester, the cheapest and fastest, lasting 3-5 years. Pick by the skills and materials you have.
Part of Biogas Digesters
Digester types are the standard designs for a small biogas plant, differing in how they hold the slurry and store the gas.
Comparison
| Fixed dome | Floating drum | Bag / tube | |
|---|---|---|---|
| Structure | brick or concrete pit with a dome | pit with an inverted steel drum on top | plastic tube in a trench |
| Gas stored in | the dome | the drum | the tube’s upper part and a separate bag |
| Pressure | varies with the amount of gas | steady, set by the drum’s weight | low, set by the bag and any weights |
| Life | 20+ years (parent) | drum 10-15 years in mild steel (parent) | 3-5 years (parent) |
| Skill needed | skilled masonry | less | little |
| Page | Fixed-Dome | Floating-Drum | Tube Digester |
Whatever the shape, the feed arrangement is either continuous (daily) or batch (once, then sealed); see Batch vs Continuous Feed. The parent’s typical household fixed-dome size is 6-10 m³ and that figure is for a family with a few cattle; sizing is worked in Sizing the Digester.
Choosing
- Masons and cement or lime available, long term, flat budget: fixed dome. The parent says over 50 million are installed in China alone (a figure from the parent that this page has not checked).
- A metalworker, little masonry skill, and rust protection available: floating drum.
- Urgent, temporary, or no masons: bag or tube.
Whatever the choice, a settlement needs somewhere to put the digestate (Digestate as Fertilizer), animals close by to feed it, and a site downhill and at least 30 m from drinking water (the corpus’s manure guide gives that distance for manure, Manure Management).
Common Mistakes
- Choosing the fixed dome without a mason. Poor masonry means a dome that leaks gas.
- Building a plastic digester where animals or children can reach it.
- Under-sizing it and expecting a family’s cooking from a few kilograms of manure.
Every digester is a gas vessel and a confined space
Any pit, dome, tank or bag holds methane, carbon dioxide and hydrogen sulfide. Never enter one, never stand in the inlet or outlet pit, keep naked flames away, and give every pit a lid or rail. Hydrogen sulfide dulls the smell at the concentrations that matter (Anaerobic Digestion Science); the corpus warns never to enter a manure pit (Manure Management) and never to go down a well to rescue anyone (Safety Protocol).
What’s Next
- Learn next: Construction, to build one
- Combine with: Gas Collection & Storage