Estimate methane production potential from your organic wastewater streams. Calculate biogas yield based on COD load, wastewater type, and process efficiency.
From chemistry to engineering reality: why theoretical yield is never achieved.
At standard temperature and pressure, 1 kg of COD destroyed anaerobically produces 0.35 Nm3 of CH4 (Buswell equation). The complete stoichiometry for a carbohydrate (C6H12O6) is:
C6H12O6 → 3 CH4 + 3 CO2
ΔG° = −404 kJ/mol (thermodynamically favourable)
However, real digesters never achieve 100% COD-to-methane conversion. Some COD is diverted to biomass synthesis (yield Y ≈ 0.05–0.10 g VSS/g COD), some remains as residual VFA, and some is consumed by sulfate reduction or other competing pathways.
QCH4 = Qww × CODinf × ηrem × YCH4 × (1 − Y)
Typical practical methane yields range from 0.25 to 0.32 Nm3 CH4 per kg COD removed, depending on wastewater composition, temperature, and reactor configuration.
| Wastewater Type | COD (mg/L) | Reactor | HRT (d) | OLR (kg COD/m3·d) | COD Removal | CH4 Yield (Nm3/kg CODrem) |
|---|---|---|---|---|---|---|
| Brewery | 2,000–6,000 | UASB | 0.5–2.0 | 5–15 | 75–85% | 0.28–0.32 |
| Dairy | 2,000–10,000 | EGSB / IC | 0.3–1.5 | 10–25 | 80–90% | 0.27–0.31 |
| Pulp & Paper | 1,500–4,000 | CSTR | 10–20 | 2–5 | 40–60% | 0.22–0.28 |
| Food Processing | 3,000–15,000 | UASB / EGSB | 0.5–3.0 | 5–20 | 80–95% | 0.28–0.33 |
| Municipal Sludge | 30,000–60,000 | CSTR | 15–30 | 2–4 | 35–50% | 0.20–0.25 |
| Landfill Leachate | 5,000–30,000 | UASB + CSTR | 2–15 | 3–8 | 60–80% | 0.24–0.30 |
Temperature matters: mesophilic (35°C) operation typically yields 10–15% more methane than psychrophilic (<25°C) for the same COD load. Thermophilic (55°C) can increase rates further but requires tighter VFA control.
Gas engines (32–42% electrical efficiency) or micro-turbines (25–35%) convert biogas to electricity and recoverable heat (50–60% thermal). A 500 kW engine running on 0.30 Nm3/kg COD biogas can offset –400k/year in energy requirements for a 5,000 PE plant.
Direct-fired boilers achieve 80–90% thermal efficiency. Biogas with 60% CH4 has a lower heating value of ~21 MJ/Nm3. Typical project benefits for boiler retrofit is 2–4 years against natural gas.
Membrane or PSA upgrading strips CO2 and H2S to produce >97% CH4 biomethane for grid injection or vehicle fuel. Energy consumption 0.15–0.25 kWh/Nm3 raw biogas.
10–30% of produced biogas is typically consumed heating the digester to 35°C or 55–60°C. Insulation (100–150 mm PU foam) and sludge-to-sludge heat exchangers minimise this parasitic load.
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