feedstock composition (feedstock composition)

Feedstock composition is the specific mix of organic waste materials — and their proportions, moisture levels, particle sizes and biochemical compositions — that is fed into a biogas digester or pyrolysis reactor. It is the single biggest determinant of biogas yield, methane content, process stability and digestate quality, and the single most common reason CBG plants fail to meet their design output.

The relevant compositional variables form a structured hierarchy. Physical: total solids percentage (determining wet, semi-dry or dry digestion mode), volatile solids fraction (determining gas yield potential), particle size, density. Biochemical: carbohydrate, protein, lipid and lignin content, which set the theoretical methane potential — lipids yield 1,000-1,400 L methane per kg VS, proteins 600-700, carbohydrates 350-450, lignin essentially nothing. Chemical: C:N ratio (target 20-30:1; below 15 ammonia builds up, above 35 nitrogen limits microbial growth), pH, trace elements (cobalt, nickel, selenium, molybdenum needed at micro-gram levels for methanogen enzymes). Contaminant: sulphur compounds, antibiotics in animal manure, salts in distillery effluent, plastic and grit in food waste.

Indian CBG plants typically run on a blend rather than a single feedstock. A well-designed plant may take 50% cow dung (for buffering and microbial inoculum), 30% pressmud or distillery slop (for high VS yield), and 20% food waste or chopped agricultural residue (for additional methane potential), with the blend tuned to hit C:N around 25:1, TS around 8-12%, and stable buffering. Single-feedstock plants exist — pure paddy-straw digesters, pure press-mud plants — but require active monitoring of trace elements, buffering and inhibition.

The strategic trade-off is between feedstock security and process simplicity. Long-term contracts with multiple suppliers across multiple feedstock types provide resilience against supply shocks (seasonality, monsoon disruption, price spikes) but require flexible pretreatment lines that can handle different particle sizes and moisture levels. Locking into a single low-cost feedstock simplifies pretreatment but creates concentration risk — a 25% rise in press-mud price or a drought-driven cattle-dung shortage can convert a profitable plant into a loss-making one. Compositional analysis at DPR stage should rely on actual lab samples of each candidate feedstock, not on textbook ranges, with sensitivity analysis across plausible blend variations.