biogas yield (gas yield)

Biogas yield is the volume of biogas generated per unit mass of feedstock processed in an anaerobic digester, expressed most rigorously as cubic metres of biogas per kilogram of volatile solids destroyed (m3/kg VS) and at standard temperature and pressure (Nm3). It is the single most important performance metric for evaluating feedstock economics in a CBG plant, since both revenue (CBG sales) and the FOM by-product flow directly from it.

Typical Indian biogas yields by feedstock include:

• Pressmud (sugar mill waste): 200-280 Nm3 per tonne VS.
• Cattle dung: 180-250 Nm3 per tonne VS.
• Poultry litter: 280-400 Nm3 per tonne VS.
• Napier grass and paddy straw: 250-380 Nm3 per tonne VS.
• Food waste: 380-550 Nm3 per tonne VS.
• Municipal sewage sludge: 200-350 Nm3 per tonne VS.

Yield is governed by feedstock biodegradability (lignin content suppresses it strongly), the carbon-to-nitrogen ratio (optimum 20-30:1), digester temperature (mesophilic 35-37 degC vs thermophilic 50-55 degC), hydraulic retention time (15-40 days typical), organic loading rate (1-4 kg VS/m3/day for wet digestion), and process stability (pH, VFA, ammonia inhibition).

Operators distinguish theoretical (BMP, biochemical methane potential) yield from actual achieved yield. The theoretical BMP is measured in 30-60 day lab batch tests under ideal conditions and represents the upper bound. Real plants typically achieve 70-90% of BMP, with the gap explained by suboptimal mixing, undigested fibrous material exiting before breakdown, and inhibition events. Pre-treatment options — mechanical pulping, thermal hydrolysis, or enzymatic pre-digestion — can lift actual yield by 10-30% but add 2-5% to operating cost. Improving yield by even 10% can shift plant payback by a full year, making this metric the central focus of process optimisation.