composting (composting)

Compost is the dark, friable, stable, humus-rich soil amendment produced when organic waste is decomposed by aerobic microorganisms — primarily bacteria, fungi, and actinomycetes — under controlled moisture (50–60%), oxygen (above 5% in the pile), temperature (55–65°C for at least 3 days), and C:N ratio (25:1 to 35:1). The composting process typically runs 6–12 weeks from raw feedstock to mature compost, with bulk volume reduction of 40–60% and significant pathogen and weed-seed kill at thermophilic temperatures.

For Indian CBG plants, composting of the digestate solid fraction is the standard route to convert wet, odorous, low-density material into a stable, marketable fertilizer. The solid fraction (25–30% DM after screw-press separation) is mixed with bulking agents — paddy straw, sugarcane bagasse, or sawdust — to raise C:N to 30:1 and create air channels. Windrow turning every 5–10 days maintains aerobic conditions; closed in-vessel composting in tunnels cuts the cycle to 14–21 days but adds ₹40–80 lakh of capex. Mature compost is screened to 4–8 mm and bagged for sale.

Composted CBG digestate sells under FCO 1985 as City Compost or Fermented Organic Manure at ₹3,500–6,500 per tonne in Indian agricultural markets, compared to ₹500–1,500 per tonne for raw digestate. The trade-off is significant nitrogen loss — 25–45% of the original ammoniacal nitrogen is lost as ammonia gas during the thermophilic phase, dropping total N from 2.5–3.5% to 1.2–1.8% on dry basis. Plants that prioritise nitrogen retention use pelletisation of dewatered digestate directly (skipping composting), accepting lower microbiological stability in exchange for higher nutrient density.

• Aerobic biological decomposition at 55–65°C, 50–60% moisture, C:N 25–35:1.
• Cycle time: 6–12 weeks windrow, 2–3 weeks in-vessel.
• Nitrogen loss of 25–45% as ammonia is the main trade-off versus direct pelletisation.
• FCO 1985 sale price typically ₹3,500–6,500/tonne for compost vs ₹6,000–10,000/tonne for pellets.