Biogas Yield of Cow Dung per kg: Rates, Calculation, and Realistic Figures
Fresh cow dung yields about 0.02–0.03 m³ of biogas per kg (20–30 L/kg) as excreted at 12–16% total solids, or 0.20–0.30 m³ per kg of volatile solids (VS) digested, at 55–65% methane. Per kg of fresh dung that is roughly 10–18 L of CH₄. Yield per kg of material falls as moisture rises, so dried dung reads higher per kg but identical per kg of VS.
The Two Ways to Quote Yield
Per kg of fresh dung is intuitive for farm planning: 0.02–0.03 m³/kg. Per kg of volatile solids is the engineering basis: 0.20–0.30 m³/kg VS. They are the same gas expressed differently—fresh dung is only about 12–16% solids, and roughly 75% of those solids are volatile, so one kg of fresh dung carries only ~0.09–0.12 kg of VS. Always check which basis a quoted figure uses before comparing.
Comparative Data Table: Cow Dung Yield by Basis
| Basis | Biogas Yield | CH₄ Content | Notes |
| Per kg fresh dung | 0.02–0.03 m³ | 55–65% | 12–16% TS as excreted |
| Per kg dry solids | 0.15–0.22 m³ | 55–65% | After drying |
| Per kg volatile solids | 0.20–0.30 m³ | 55–65% | Engineering basis |
| Per cow per day (30 kg) | 0.6–0.9 m³ | 60–65% | Matches measured 0.5–0.8 |
How to Calculate Your Own Output
Use: Biogas = Dung (kg/day) × TS × VS-fraction × yield-per-kg-VS. For 30 kg/day at 14% TS and 75% VS, VS mass is 30 × 0.14 × 0.75 = 3.15 kg VS/day. At 0.20–0.30 m³/kg VS that is 0.6–0.95 m³/day, consistent with the 0.5–0.8 m³/day typically measured once collection and mixing losses are counted. Scale by herd size for a plant figure.
Frequently Asked Questions (FAQ)
Q1: How much gas per kg of volatile solids?
A: Cow dung yields about 0.20–0.30 m³ of biogas per kg of volatile solids digested at 55–65% methane. This is the engineering basis and is lower than food waste or energy crops, which can exceed 0.40 m³/kg VS.
Q2: Does fresh or dry dung yield more?
A: Per kg of material, dried dung yields more simply because water has been removed—about 0.15–0.22 m³/kg dry solids versus 0.02–0.03 m³/kg fresh. Per kg of volatile solids the yield is the same; drying changes the basis, not the gas potential.
Q3: How do you calculate biogas from cow dung?
A: Multiply dung mass by total solids, by the volatile fraction, then by yield per kg VS. For 30 kg/day at 14% TS and 75% VS: 3.15 kg VS × 0.20–0.30 = 0.6–0.95 m³/day, before collection losses.
Q4: Why do quoted figures differ so much?
A: Because sources quote different bases—per kg fresh, per kg dry solids, or per kg volatile solids—and different solids contents. A figure of 0.02 m³/kg fresh and 0.25 m³/kg VS can describe the same manure. Always check the basis.