Influence of microwave pre-treatment on sludge solubilization and pilot scale semi-continuous anaerobic digestion

► The effects of a microwave pre-treatment on anaerobic digestion were studied. ► The microwave treatment resulted in a significant solubilization of organic matter. ► Volatile fatty acid concentration increased during the treatment. ► A 50% increase in biogas production was observed for a retention time of 20days. Anaerobic digestion is widely applied for the recovery of energy from waste activated sludge. Pre-treatment methods are of high interest to increase the biodegradability of the sludge and to enhance the digestion efficiency. This paper studies the application of a microwave pre-treatment. An experimental set-up of two pilot scale semi-continuous digesters was used. During a long term experiment, one of the reactors was fed with untreated sludge, while microwave pre-treated sludge (336kJ/kg sludge) was introduced in the second one. A solid retention time of 20days was kept during the experiments. (Organic) dry solids, carbohydrates, proteins and volatile fatty acids were monitored during digestion. It was seen that the microwave pre-treatment resulted in an effective solubilization of the organic matter in the sludge. The changes to the sludge composition resulted in an increase in biogas production by 50%, while the methane concentration in both reactors remained stable.