Nutrient-rich (nutrient-rich digestate)

Nutrient-rich is a descriptive property of digestate, compost, and digestate-derived fertilisers indicating they contain significant concentrations of the three primary plant macro-nutrients — nitrogen (N), phosphorus (P), and potassium (K) — typically alongside meaningful levels of secondary nutrients (calcium, magnesium, sulfur) and micronutrients (iron, zinc, manganese, boron). For CBG plant economics, the nutrient-rich digestate by-product is the second revenue stream after biomethane sales and increasingly the difference between marginal and healthy plant returns.

Typical Indian CBG digestate N-P-K composition (as percentage of dry matter):

• Total nitrogen (N): 2.0-4.0%.
• Total phosphorus (P2O5): 0.8-2.0%.
• Total potassium (K2O): 1.5-3.5%.
• Organic carbon: 25-40%.
• pH: 7.5-8.5.
• C:N ratio: 12-18 (well-mineralised).

Critically, much of the nitrogen in digestate is in ammoniacal form (NH4+) which is immediately available to plants — unlike fresh manure where most nitrogen is locked in slow-releasing organic forms. This makes digestate nutritionally closer to a chemical fertiliser in availability but with the additional benefits of organic carbon (soil-conditioning), microbial inoculum, and slow-release organic nitrogen reserves.

Indian regulatory recognition of nutrient-rich digestate comes through the Fertiliser Control Order (FCO) 1985 under the Essential Commodities Act 1955, which classifies digestate-based products as:

• Fermented Organic Manure (FOM): minimum N+P2O5+K2O of 1.2%; minimum organic carbon 14%.
• Liquid Fermented Organic Manure (LFOM): minimum N+P2O5+K2O of 0.4%.
• Phosphate-Rich Organic Manure (PROM): minimum P2O5 of 10.4% (blended with rock phosphate).

Under SATAT, FOM and LFOM sales typically generate 1,500-4,000 INR per tonne, contributing 15-25% of total plant revenue. The trade-off in monetising the nutrient-rich claim is processing cost: liquid digestate as-is sells at low prices because of transport economics, but drying and pelletising into bagged FOM lifts price 3-5x while adding 800-1,800 INR per tonne in processing cost. Most Indian CBG plants now bundle a separator-dryer-pelletiser train to capture the FOM premium and bring the by-product to a shelf-stable, FCO-compliant product.