digestate (anaerobic digestate)

Digestate is the semi-liquid residue that remains after organic material has been processed through anaerobic digestion. It comprises three components: undigested organic matter (lignin, recalcitrant cellulose, fibres that resisted breakdown), mineralised nutrients (nitrogen, phosphorus, potassium, sulphur, and trace minerals released during decomposition and now in plant-available forms), and water (typically 90–95% of mass). For every tonne of feedstock fed to a digester, approximately 0.85–0.95 tonnes of digestate emerge — biogas accounts for the small remaining mass through conversion of organic carbon to methane and CO2.

Digestate is genuinely valuable as an organic fertiliser and soil amendment because anaerobic digestion mineralises organic nitrogen into ammonium (NH4+) and orthophosphate, the forms plants take up most efficiently. Typical Indian CBG plant digestate composition: 0.4–0.8% N (mostly NH4-N), 0.2–0.4% P2O5, 0.3–0.6% K2O, with secondary nutrients (Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu). This makes the NPK ratio comparable to dilute farmyard manure, but in a form 3–5x more rapidly plant-available. The Fertiliser Control Order 1985, amended in 2017 and again in 2021, recognises two digestate-derived products: Liquid Fermented Organic Manure (LFOM) — direct digestate liquid — and Phosphate Rich Organic Manure (PROM) — solid-fraction digestate composted with rock phosphate, sold under FCO specifications.

Digestate management is the second revenue stream of every CBG plant and often the difference between viable and unviable economics. A 10 TPD CBG plant produces 8–9 tonnes of digestate per day, which must either be applied locally (within 25–40 km radius for economical liquid transport) or processed into solid Fermented Organic Manure (FOM) for wider distribution. Standard processing: solid-liquid separation by screw press or decanter centrifuge produces solid cake at 20–30% TS and liquid centrate at 1–3% TS. The solid cake is composted, dried, pelletised, fortified with rock phosphate or microbial inoculants, and packaged as branded FOM selling at Rs 2,000–4,000 per tonne. The liquid centrate is either land-applied through pipelines or further treated for water recovery and reuse in the digester. Indian regulatory compliance for digestate disposal requires CPCB consent under Solid Waste Management Rules 2016 and Hazardous Waste Rules 2016 if the feedstock contains industrial streams; pure agricultural feedstock digestate is exempt from hazardous classification.