There must be suitable moisture content of the feedstock as the microorganisms’ excretive and other metabolic processes require water. The moisture content should normally be around 90% of the mass of the total contents (Buren 1983). Both too much and too little water are harmful, with too much water the rate of production per unit volume in the digester will fall, preventing optimum use of the digester. If the moisture content is too low, acetic acids will accumulate inhibiting the digestion process and hence production. Furthermore, a rather thick scum will form on the surface of the substrate. This scum may prevent effective mixing of the charge in the digester.