Whole digestate may be applied directly to land however dewatering or further processing of digestates may be necessary in certain situations. Dewatering separates the material into two fractions: a solid fraction (typically 25-35% dry matter) which can be used as a soil improver and a liquid fraction (typically ≤ 6% DM) that can be used as a liquid fertiliser.
The main advantages of dewatering digestate are to improve its manageability e.g. when using land injectors by avoiding blockages and to reduce transportation and storage costs. For example, dewatering produces an easily stackable fibrous fraction (potentially rich in phosphorus) and a liquid fraction (potentially rich in nitrogen and potassium). In many cases these digestates can be applied directly to farmland via traditional irrigation equipment. Another advantage of separating the liquor for use as a fertiliser is it will run easily from foliage leaving little residue.
Dewatering can be achieved using biological, mechanical processes, or thermal processesor a combination of these. For example, biological dewatering involves biodrying, which utilises the heat produced by the exothermic reactions in aerobic decomposition. Dewatering can also be performed through evaporation of the moisture. Mechanical dewatering devices for solid-liquid fraction separation can take the form of:
• Screw presses
• Belt presses
Flocculants/polymers are often added to aid the efficiency of dewatering. The capital and running costs, throughput, energy requirements, and separation efficiency in terms of solid or mineral removal are important considerations when deciding which method of separation to employ. For further descriptions of these methods of dewatering please see below.
As an alternative to direct land application, further stabilisation treatments e.g. composting can be carried out to improve the stability of the digestate. The resulting compost may then be applied to land. Other applications of digestates are currently being explored e.g. drying and pelletizing for use as a solid fuel.
Further digestate processing may be required e.g. ultrafiltration and reverse osmosis technologies to purify the liquid fraction prior to discharge to sewer or for direct discharge to receiving environment.
METHODS OF DEWATERING DIGESTATES
A screw press is a simple, slow moving mechanical device. It is based around a screw surrounded by a fluid permeable mantle. The screw shaft and the mantle form between them a screw channel with a cross-section that decreases towards the transport direction of the screw. As material is forced into the narrowing channel, high pressure is built up and water is squeezed from the solid fraction through the fluid permeable mantle.
Belt presses dewater digestate by applying mechanical pressure to digestate. Digestate is sandwiched between two tensioned belts, which are passed through decreasing diameter rolls to squeeze out water.
Centrifuges utilise high speed rotation to separate the solid and liquid fractions of the digestate.
Biological dewatering involves biodrying, which utilises the heat produced by the exothermic reactions in aerobic decomposition.