Energy-positive sewage sludge pre-treatment with a novel ultrasonic flatbed reactor at low energy input

The performance of a novel ultrasonic flatbed reactor for sewage sludge pre-treatment was assessed for three different waste activated sludges. The study systematically investigated the impact of specific energy input (200 – 3,000 kJ/kg_TS) on the degree of disintegration (DD_COD, i.e. ratio between ultrasonically and maximum chemically solubilized COD) and methane production enhancement. Relationship between DD_COD and energy input was linear, for all sludges tested. Methane yields were significantly increased for both low (200 kJ/kg_TS) and high (2,000 – 3,000 kJ/kg_TS) energy inputs, while intermediate inputs (400 – 1,000 kJ/kg_TS) showed no significant improvement. High inputs additionally accelerated reaction kinetics, but were limited to similar gains as low inputs (max. 12%), despite the considerably higher DD_COD values. Energy balance was only positive for 200 kJ/kg_TS-treatments, with a maximum energy recovery of 122%. Results suggest that floc deagglomeration rather than cell lysis (DD_COD = 1% – 5% at 200 kJ/kg_TS) is the key principle of energy-positive sludge sonication.