8–15% (8–15% TS)

The total solids (TS) range of 8–15% defines the feedstock concentration window for wet anaerobic digestion, the most common digester configuration in Indian CBG plants. TS is the dry-matter fraction of slurry after evaporation — at 10% TS, every 100 kg of feed contains 10 kg of solids and 90 kg of water. This range distinguishes wet AD from two adjacent operating regimes: low-solids slurry digestion (under 5% TS, typical of sewage sludge) and dry/semi-dry AD (above 20% TS, used for source-separated food waste and crop residues).

The 8–15% TS window is preferred for several technical reasons:

• Pumpability — slurry below 12% TS can be moved with standard centrifugal pumps; between 12–15% TS requires positive-displacement progressing-cavity or lobe pumps
• Mixability — mechanical agitators handle 8–15% TS slurry comfortably at 5–10 W/m³ specific power; above 15%, agitator load and shear damage increase sharply
• Heat transfer — slurry in this range has thermal properties similar to water, allowing standard heat exchangers and digester heating coils to maintain temperature
• Microbial activity — the high water content gives methanogens unrestricted access to dissolved substrates, supporting steady gas production

Indian CBG feedstocks naturally fall within or near this range with appropriate pre-treatment:

• Cattle dung — fresh at 12–18% TS, typically diluted to 10–12% with process water
• Press mud — 22–28% TS as received, diluted with vinasse or water to 10–12%
• Food waste — 18–25% TS, diluted to 12% after pulping and screening
• Napier grass / paddy straw — chopped and wetted to 8–12% TS

The trade-off against dry AD (20–35% TS) is reactor volume versus water management. Wet AD systems need larger digester tanks for the same dry-matter throughput (because most of the volume is water) but produce a thinner, more easily handled digestate stream. Dry AD systems are more compact but require batch garage-style operation and generate concentrated digestate that may need composting before sale.