Mechanical disintegration - wet milling - was chosen to physically disrupt cellular material. By breaking up microbial or plant cell walls intracellular carbon sources as well as nutrients are made available for further anaerobic digestion. Ball milling consistently showed better disintegration results than high speed cutter milling. With respect to solubilisation of COD a high degree of cell disruption could be achieved in a variety of aerobic and anaerobic sewage sludges, where the fraction of soluble COD could be changed from 1–5% in original sludges to up to 47% after wet milling. In distillery slops and brewery residues milling failed to enhance soluble COD significantly.
Anaerobic digestion testing showed a good digestability of the solubilized intracellular material and consistently enhanced overall COD-degradation of the sludges by a factor of 1.2–1.5. Net biogas production was enhanced in the same order of magnitude. Stabilised sludges showed a higher beneficial effect of wet milling than raw excess sludge.