Slurry (feed slurry)

Slurry is a semi-liquid mixture of organic feedstock and water that has been prepared for input into an anaerobic digester. It typically contains 4–12% total solids (TS) for wet digestion processes, 12–20% TS for semi-dry, and 20–35% TS for dry digestion. The slurry must be pumpable, sufficiently homogeneous to allow consistent biological breakdown, and free of contaminants that could damage downstream equipment — stones, metals, plastics, fibres longer than 50 mm — which means upstream pre-processing (shredding, screening, magnetic separation) is integral to slurry preparation.

The TS target determines almost every design choice downstream of slurry preparation. Wet digestion at 8–12% TS is the dominant Indian CBG technology, suited to dilute or high-moisture feedstocks (dairy manure, food waste, fruit waste, fresh energy crops, sugar mill effluent). It uses CSTR digesters with mechanical mixing, slurry pumps for transfer, and standard centrifugal or progressive cavity pumps. Semi-dry digestion at 12–20% TS suits press mud, partially dewatered MSW, and chopped lignocellulosic biomass — requires more robust positive-displacement pumps and stronger mixing, but yields higher biogas per reactor volume. Dry digestion at 25–35% TS uses batch garage-type digesters or continuous plug-flow with auger feed, suited to woody biomass and very high-fibre crops; dispenses with pumping entirely.

Slurry preparation in Indian CBG plants typically takes place in a mixing pit (concrete tank, 10–50 m3 working volume) fitted with high-shear chopper-mixer, dilution water inlet, and macerator pumps. Process water comes from three sources: dewatering centrate from digestate separation (reused at 60–80% of total), rainwater harvesting from plant roof areas, and fresh borewell water. Recycling centrate is critical because each tonne of feedstock requires 4–8 tonnes of dilution water — buying or pumping that volume of fresh water is uneconomical and unsustainable in most Indian locations. Slurry quality monitoring includes daily measurement of TS, VS, pH, particle size distribution, and visual checks for contaminants. Poor slurry preparation is among the most common causes of digester upset events in commissioning-phase Indian plants — under-shredded fibrous material clogs pumps, over-diluted slurry under-loads the digester, and contamination by sand or plastic accumulates as scum or sediment that requires costly cleanout.