Digestate quality (digestate quality)

Digestate quality refers to the chemical composition, physical properties, and microbiological status of the residue produced after anaerobic digestion — the bundle of parameters that determines whether digestate can be sold as organic fertiliser, applied to specific crop types, or must be treated further before disposal. For a CBG or biogas plant, digestate quality is the single most important determinant of whether the secondary product line — fertiliser revenue — adds meaningfully to project economics.

Quality is characterised across four parameter groups. Nutrient content: total nitrogen (typically 1.5–3.5% DM), ammoniacal nitrogen (50–80% of TN), phosphorus as P₂O₅ (0.5–1.5% DM), potassium as K₂O (1.0–3.0% DM), organic carbon (40–60% DM), and micronutrients (Zn, Cu, Fe, Mn). The Fertiliser Control Order, 1985 (FCO) and its 2022 amendments specify minimum thresholds for Fermented Organic Manure (FOM) and Phosphate-Rich Organic Manure (PROM). Physical properties: dry matter content, bulk density, pH (typically 7.5–8.5), electrical conductivity (1.0–10.0 dS/m), and particle size — drivers of handling, transport, and application. Pathogen status: E. coli below 1,000 MPN/g DM, Salmonella absent in 25 g, and faecal coliforms below 1,000 MPN/g — required for fertiliser sold for food-crop application. Contaminants: heavy metals (Cd under 5 mg/kg, Pb under 100 mg/kg, Cr under 50 mg/kg, Ni under 50 mg/kg as specified in FCO), and visible impurities (plastic, glass, metal).

Quality outcomes depend heavily on feedstock selection and process design. Pure cattle-dung digestate scores high on all FCO parameters and is the standard for premium-priced retail fertiliser. Mixed agro-residue and dung digestate is workable. Food-waste digestate is high in nutrients but risks heavy metal and pathogen issues if sourcing is not controlled. Sewage-sludge digestate is non-FCO-compliant in most cases and limited to non-food applications. Process design also matters — mesophilic digestion delivers lower pathogen kill than thermophilic, requiring post-treatment for food-crop applications. Quality monitoring is mandatory under FCO registration: every 1,000 tonnes of product sold needs lab certification across all FCO parameters. The trade-off in design is consistency versus yield — pushing biogas output via aggressive loading or unusual feedstock often compromises digestate quality, eroding the secondary revenue line.