biogas yields (Biogas Yield)

Biogas yield is the volume of biogas produced per unit of feedstock processed — the single most important performance metric for a CBG plant because it directly determines revenue per tonne of input. It is expressed in two complementary ways: per kg of volatile solids (Nm³/kg VS, used for technical comparison across feedstocks) and per tonne of fresh feedstock (Nm³/tonne, used for commercial planning and feedstock contract pricing).

Typical Indian yields by feedstock are well-established benchmarks. Cattle dung: 0.20-0.35 Nm³/kg VS, equivalent to 30-60 Nm³/tonne fresh. Press mud from sugar mills: 0.35-0.50 Nm³/kg VS, equivalent to 70-110 Nm³/tonne fresh. Food waste: 0.45-0.65 Nm³/kg VS, equivalent to 100-160 Nm³/tonne fresh. Paddy straw with pretreatment: 0.30-0.45 Nm³/kg VS, equivalent to 230-380 Nm³/tonne fresh because straw is mostly dry matter. Distillery spent wash: 0.40-0.55 Nm³/kg VS. Poultry manure: 0.30-0.45 Nm³/kg VS.

Realised yield in a continuous plant is always lower than the theoretical Biochemical Methane Potential (BMP) of the feedstock — typically 70-85% of BMP at well-tuned operation, falling to 50-60% in poorly managed plants. The 15-30% gap is the cumulative result of incomplete hydrolysis of lignocellulose, mass-transfer limitations in viscous slurry, settled solids in dead zones, short-circuiting where some feed exits the digester faster than HRT suggests, and partial inhibition from VFA, ammonia or sulphide.

Improving biogas yield is the highest-leverage operational lever in a CBG plant because every additional 5% of yield translates almost directly into 5% more revenue at constant cost. The interventions, in order of typical cost-benefit, are: ensure complete pretreatment shredding and homogenisation; tune HRT and OLR through 5-10% step-changes during commissioning; supplement trace elements (cobalt, nickel, selenium at micro-gram levels) for monoculture feedstocks; verify digester mixing and check for settled solids quarterly; eliminate inhibitors through feedstock blending; monitor VFA-to-alkalinity ratio weekly to catch stress early. Plants that compare actual yield against BMP monthly and treat the gap as an improvement target typically converge to 80%+ within two years; plants that do not measure rarely exceed 60% and never know why.