10%–20% (10–20% TS)

The total solids range of 10-20% defines the operating window for semi-dry anaerobic digestion — the middle band between wet digestion (below 10% TS) and dry digestion (above 20% TS). Total Solids is the mass of dry matter in the feed slurry expressed as a percentage of total wet mass; the balance is water. At 10-20% TS the slurry is a thick, porridge-like consistency that can be pumped with positive-displacement pumps but not with conventional centrifugal pumps.

Each digestion mode has trade-offs that 10-20% TS bridges. Wet digestion below 10% TS uses conventional CSTR reactors with simple agitation, suits liquid feedstocks like food waste, distillery slop and slurry-form dung, but requires large digester volumes per kg of dry matter processed (typically 3-5× the volume of dry digestion) and produces large volumes of digestate that need handling and dewatering. Dry digestion above 20% TS uses plug-flow or batch reactors, suits stackable feedstocks like crop residue and source-segregated food waste, gives 30-50% higher biogas yield per reactor volume, but is more sensitive to clogging and inhibition from local acid build-up. Semi-dry at 10-20% TS blends the advantages: medium reactor volume, pumpable slurry, manageable digestate volume, with somewhat better gas yield than wet but lower than dry.

In Indian CBG plants the choice depends primarily on feedstock. A plant taking 70% cow dung at 80% moisture (20% TS) and 30% chopped agricultural residue at 15% moisture (85% TS) can blend to around 30-40% TS — dry mode. A plant taking food waste at 25% TS plus dilution water lands at 10-15% TS — semi-dry. A plant taking only sugarcane press mud at 30% TS plus recycled liquid digestate lands at 12-18% TS — semi-dry.

Process-control considerations at 10-20% TS include pump and pipe sizing for thick slurry, more powerful mixers, and careful temperature control because thicker slurry conducts heat less efficiently. The reward is roughly 15-25% smaller digester footprint than equivalent wet-mode capacity, lower water and pumping energy use, and lower volume of digestate to handle downstream.