Digestate is a good fertiliser with costs: it is still 94-97% of the mass fed in, so it must be stored and spread in nearly the volume that went in, and it keeps the nitrogen, phosphorus and potassium but loses 15-25 kg of carbon per tonne of manure as gas, so it builds less humus than compost. Its ammonia escapes if left open, and mesophilic digestion reduces pathogens without eliminating them, so digestate from human waste needs more treatment before it touches food crops.
Part of Biogas Digesters
Digestate is the slurry that leaves a digester after the microbes have taken their share, used as a liquid fertiliser and soil amendment.
The Parent’s Claims, Checked
The parent says digestate retains all the nitrogen, phosphorus and potassium of the manure, with nitrogen as plant-available ammonium, fewer pathogens (especially with 30+ days at mesophilic temperature), much less odour, and can be applied directly or composted. Each needs a qualifier.
Mass. A cubic metre of biogas at 60% methane weighs about 27.2 g/mol × 1,000 ÷ 24.47 L/mol = 1.11 kg. The parent’s 30-50 m³ per tonne of manure takes 33-56 kg out as gas, 3.3% to 5.6% of the tonne. So the digestate is 94-97% of the input mass. It is not less to move; it is a thin liquid, and fed with equal water it is about twice the manure’s volume.
Carbon. Each mole of gas carries one atom of carbon (CH₄ or CO₂), and a cubic metre holds 1,000 ÷ 24.47 = 40.9 mol, so 40.9 × 12.01 = 491 g of carbon. At 30-50 m³ per tonne, that is 14.7-24.6 kg, about 15-25 kg, of carbon per tonne of manure that does not come back to the soil. Digestate therefore returns less humus-forming carbon than the same manure composted, though it keeps the other nutrients (Composting, Pile Construction).
Nitrogen. It is retained, in the ammonium form that plants take up quickly. Ammonium turns to ammonia gas in warm, open, alkaline slurry and is lost to the air, so keep stores covered and incorporate the slurry into the soil soon after spreading. The fast form also means fresh digestate can scorch seedlings (recalled); test it on a small plot first, borrowing the germination test in Compost Maturity.
Pathogens. The parent says “significantly reduced”; that is not “gone”. For manure from healthy animals that is acceptable on field crops; for human waste, the corpus’s rule is measured heat plus a year of ageing (The Science of Humanure Composting). Do not put human-waste digestate on crops eaten raw, or on root crops, without further treatment such as that composting.
How Much Land
Animal Manure says one cow’s 10-15 tonnes a year fertilises about half a hectare, which is 20-30 tonnes of manure per hectare. Two cows making 20-30 kg of manure a day (the digester table’s rate) give 7.3-11 tonnes a year, which is 7.3 ÷ 30 = 0.24 to 11 ÷ 20 = 0.55 hectares. Mixed 1:1 with water, that is 14.6-21.9 tonnes of slurry a year, about 14-21 m³ of digestate. The corpus has no rate specific to digestate; start from its manure rates, lower because the nitrogen is faster, and adjust from the crop (Soil Testing, Nitrogen Sources).
What You Need
- A covered, vented store for digestate between spreading seasons, with a rail or lid
- A way to spread: buckets, a cart with a spout, or irrigation channels
- Ploughing or hoeing to work it in
- A small test plot
Method
Step 1: Draw digestate from the outlet tank; do not stand in it or lean over it.
Step 2: Store covered. Let thick solids settle; the liquid goes on fields, the solids can be composted.
Step 3: Apply in cool conditions to bare soil and work it in the same day.
Step 4: Keep it off crops that will be eaten raw for a margin of time; for human waste, compost first.
Step 5: Keep records: amount, date, crop, result.
Common Mistakes
- Leaving digestate open in the sun, and losing the nitrogen as ammonia.
- Spreading it uphill of a well or stream. Nutrients and pathogens run off (Manure Cycling).
- Treating it as sterile.
- Putting it on seedlings fresh.
A digestate tank is still a gas source
Stored digestate keeps giving off methane, carbon dioxide and hydrogen sulfide, and stirring or pumping it releases a burst; a closed tank can hold an explosive or deadly atmosphere. Keep it vented and covered with a rail, stir and pump in open air, never enter a tank or pit, and never go in after anyone: ventilate and haul out from outside (Manure Management, Safety Protocol). Wash hands after handling digestate.
What’s Next
- Learn next: Using Biogas, the other half of the output
- Combine with: N Ratio & Feedstock Selection