composted (composted digestate)

Composted refers to organic material that has been processed through controlled aerobic decomposition — typically 4–12 weeks of turning, moisture management, and temperature regulation — until it reaches a stable, mature, pathogen-free state suitable for application to soil. Composted digestate from CBG plants is the solid post-treatment product with the broadest agricultural market in India, qualifying for FCO City Compost certification when CPCB and FCO specifications are met.

The composting process passes through three classical phases:

• Mesophilic phase (1–3 days) — easily degradable sugars and proteins consumed by mesophilic bacteria at 20–40 °C
• Thermophilic phase (1–4 weeks) — pile temperature rises to 55–70 °C, killing pathogens and weed seeds; thermophilic actinomycetes and fungi degrade proteins, fats, and hemicellulose
• Maturation phase (2–8 weeks) — temperature falls back, mesophilic organisms re-colonise, lignocellulose breakdown continues at slower rate, humic substances form

Indian FCO City Compost specifications (Schedule III, Part C):

• Moisture — 15–25%
• Organic carbon — minimum 12%
• Total NPK — minimum 1.6% (typically 0.8% N, 0.4% P₂O₅, 0.4% K₂O)
• C/N ratio — under 20:1
• Heavy metals — Pb under 100 ppm, Cd under 5 ppm, Cr under 50 ppm, Hg under 0.15 ppm
• Pathogen — faecal coliform under 1,000 MPN/g; Salmonella absent in 25 g

For CBG plants, composting digestate (rather than selling it wet or dried directly) adds value but also cost. Capex for a covered windrow composting yard with turners and screen runs ₹50 lakh–1 crore for a 5 TPD CBG plant. The trade-off is shelf life and customer acceptance: raw digestate has limited shelf life and farmer reluctance, while finished compost stores 6–12 months in bags and commands ₹3,000–6,000 per tonne at the farm gate, plus the ₹1,500 per tonne Market Development Assistance subsidy from the Department of Fertilizers.