< 15% (below 15% DM)

The threshold of less than 15% dry matter (DM) characterises digestate that behaves as a liquid or semi-liquid slurry rather than a stackable solid. Below this DM level, material cannot be heaped without losing free liquid as leachate, must be handled by tanker rather than truck, requires pumps and pipework rather than conveyors, and is too dilute to be handled economically as fertiliser at long distance.

Raw digestate leaving an anaerobic digester typically sits at 4–10% DM, depending on the feedstock and the reactor type. Cattle-dung-fed CSTR digesters produce 5–8% DM digestate; food-waste digesters at higher organic loading rates produce 4–6% DM; semi-dry plug-flow digesters can hold 8–12% DM. All of these fall into the under-15%-DM category and share common handling characteristics: pumpability with standard centrifugal pumps, flowability through 4-inch and larger pipes, and the need for storage in closed tanks or lined lagoons rather than open heaps.

The practical implications of the under-15%-DM threshold define digestate logistics. Transport economics: at 8% DM, a 20-tonne tanker carries only 1.6 tonnes of dry matter — the rest is water. Freight cost per kg of nutrient delivered makes any haul over 20–30 km uneconomical. Storage: under-15%-DM digestate must be stored in sealed tanks or HDPE-lined lagoons to prevent ammonia evaporation, surface-water contamination, and odour. Storage volume for 3–6 months of production at 80–95% moisture creates significant land requirements — a 5 TPD CBG plant needs 3,000–5,000 m³ of digestate storage. Application: under-15%-DM digestate can be field-applied by tanker with trailing-hose injectors or by fertigation through drip lines — both effective but limited to seasonal windows and short distances. The strategic response in any project that cannot meet hyper-local digestate off-take is mechanical separation: a screw press or decanter centrifuge moves the liquid fraction up to 4–8% DM (still liquid but lower volume) and the solid fraction up to 20–35% DM (stackable, transportable, marketable). Cost of separation is the tax paid to escape the under-15%-DM trap.