20–40% (20–40% DM)

The 20–40% dry matter range describes the typical solid fraction produced by mechanical separation of biogas digestate — the stackable cake output of a screw press or decanter centrifuge. Within this band, the material transitions from semi-plastic (20–25% DM) through firmly stackable (25–35% DM) to crumbly and friable (35–40% DM), and at the higher end approaches the moisture content of conventional compost.

This DM range unlocks the agronomic, logistical, and commercial value of digestate as an organic fertiliser. At 20–40% DM, the solid fraction can be heaped 2–3 metres high without losing leachate, loaded by front-end loader into trucks, transported by open-bed lorry over hundreds of kilometres without specialised tanker equipment, and stored in open windrows under tarpaulin for 3–6 months. Nutrient density per kg shipped becomes high enough to support commercial freight economics: a 20-tonne open truck carries 6–10 tonnes of dry matter against just 1–2 tonnes for raw 4–8% DM digestate.

The specific DM achieved depends on separator technology and feedstock. Screw presses typically deliver 18–28% DM cake without polymer addition — suitable for nearby agricultural application. Decanter centrifuges with polymer flocculant addition reach 25–35% DM — needed for pelletising or composting downstream. Composting windrows under aerobic decomposition bring 25% DM cake up to 50–65% DM stable compost over 8–12 weeks. Thermal drying reaches 85–90% DM but requires significant energy input — typically 1.0–1.5 kWh per kg of water evaporated. Under the Fertiliser Control Order, 1985 (and the FCO amendment 2022 covering FOM and PROM), digestate sold as bulk solid organic fertiliser must meet a moisture maximum of 25% (i.e., 75% DM) — pushing many SATAT plants to compost or thermal-dry separator output before retail packaging. Pelletising at 15–30% moisture (70–85% DM) allows machine-fill bagging, palletising, and distribution through agricultural retailers. The trade-off across the 20–40% DM range is process complexity and capex versus product value: each percentage point of DM improvement opens access to higher-value markets.