4–8% solids (4–8% TS liquid fraction)

The 4–8% solids range describes the typical total solids content of the liquid fraction produced when digestate is mechanically separated using a screw press or decanter centrifuge. The liquid fraction — also called centrate (from centrifuges) or filtrate (from screw presses) — is mostly water carrying dissolved nutrients, fine suspended solids that escaped the separator screen, and dissolved organic matter.

At 4–8% TS, the liquid fraction has well-defined physical properties: it pumps easily through standard centrifugal pumps, flows by gravity in 4-inch and larger pipework, and remains pumpable across a wide temperature range (4–40°C). It is too dilute to handle as a solid but too concentrated for direct discharge to surface water under the Water (Prevention and Control of Pollution) Act, 1974, which sets BOD limits of 30 mg/L for inland discharge and TSS limits of 100 mg/L. Liquid digestate typically has BOD of 5,000–25,000 mg/L and TSS in the percent range, making direct release a non-starter.

Three end-use pathways are common. Direct agricultural application via fertigation — the liquid fraction is injected into drip or sprinkler irrigation lines to deliver nitrogen and potassium (which are highly soluble and concentrate in the liquid fraction) directly to crops. This is the highest-value use but requires a piped network or tankering to the field. Storage and tankering to nearby farms — lined lagoons sized for 3–6 months of production allow seasonal application during pre-sowing windows. Recirculation to the digester inlet — sending part of the liquid fraction back as digester dilution water reduces fresh-water demand by 30–50% and saves the cost of further treatment. The trade-off is potential build-up of dissolved salts (electrical conductivity rising above 8 dS/m can inhibit methanogens), so blowdown is typically required. If none of these are feasible, the liquid fraction must be further treated by evaporation, reverse osmosis, or constructed-wetland polishing before discharge — each adding significant capex and opex that erodes project economics.