1. What is digester gas?
Digester gas (biogas) forms when organic matter is decomposed without air by bacteria. It consists of ca. 50–70 % methane (the burnable part) and 30–50 % carbon dioxide. Suitable feedstocks: slurry and manure, kitchen scraps (not much meat!), grass clippings, leaves, digestible waste. The residue after digestion ("digestate") is a high-quality fertilizer — nothing is lost.
2. The DIY digester
- Vessel: 200-liter drum, IBC container or drum in a pit — with a lid (but NOT pressure-tight! vent the gas via a hose)
- Inoculum: slurry or mature compost — the bacteria must be there from the start
- Feeding: small daily amounts of chopped organic waste + water (slurry consistency)
- Temperature: 20–37 °C is the sweet spot; below 15 °C production collapses (winter: bury it, surround with compost, place in sun, insulate)
- Gas off the top, digestate released at the bottom — feed continuously, never build pressure
Small biodigester layout (photo: Miguel, CC0, via Wikimedia Commons)
3. Realistic gas yield
| Substrate | Biogas | Notes |
|---|---|---|
| Kitchen scraps (cooked is better) | ca. 100–150 L per kg | Chop it! Bones/shells slowly |
| Cattle slurry | ca. 20–30 L per kg | Perfect inoculum/carrier |
| Grass clippings | ca. 100 L per kg | Fast to get; avoid acidification |
For comparison: A 200-liter drum delivers only a few hundred liters per week depending on feeding. 1 m³ of biogas equals ca. 2 kWh of electricity or ca. 6 kWh of heat. For 1–2 hours of cooking on a gas burner you need ca. 0.5–1 m³ per day — this is a serious project, not a side effect.
4. Use: cooking, light, power
- Cooking: gas burner with a larger jet (biogas has less heating value than bottled gas) and adjusted air mix — ready-made biogas burners exist.
- Lighting: gas mantle lamp (like historic gas lanterns) — without electricity!
- Power: small generator with gas engine (CHP) — requires engine conversion/tuning, expertise.
- Heat: combustion chamber/heating coil in the greenhouse — heat is always a by-product.
5. Storage: low pressure is enough
Biogas is stored not compressed but at minimal overpressure: a gas bag (UV-resistant), a floating bell over water, or an inverted drum in a water-filled bucket. As much as needed, as low as possible — if it leaks, nothing flies into the air, it just escapes slowly.
6. Safety: the three fatal mistakes
- Ignoring H₂S: hydrogen sulfide smells of rotten eggs — but at high concentration the smell fades and it is lethal! Open digesters only from a distance, never lean over, never indoors.
- Sealing pressure-tight: a digester is not a pressure vessel — it must always be able to vent (water seal/safety valve), otherwise it bursts.
- Flashback: water wash or backfire valve between storage and burner — otherwise the gas ignites inside the digester.
7. Winter operation
- Production drops sharply below 15 °C — bury the digester, surround it with compost, paint it black or place it in a greenhouse
- Insulation (straw, foil) and small solar absorbers on the south side help
- Plan bottled-gas reserves for winter
8. Checklist: biogas micro-setup
- ☐ Digester 200 L–1,000 L (drum/IBC), ventable
- ☐ Gas storage (bag/hose storage) + water seal
- ☐ Biogas burner or converted burner (larger jet)
- ☐ Shredder (scissors/blender) for kitchen waste
- ☐ H₂S warning + distance to buildings
- ☐ Digestate basin (fertilizer storage)
9. Did you know?
- In China millions of simple family digesters were counted — biogas from slurry has been the everyday solution there for cooking and light for decades.
- Bacteria work around the clock — also at night and under clouds. That makes biogas the perfect complement to the sun.
- The digestate is a full-value fertilizer — the cycle closes.