Ultrasonically enhanced anaerobic digestion of waste activated sludge

Anaerobic digestion is increasingly applied to stabilise sludge and to reduce operating costs of the wastewater treatment plant by generating biogas, considered as a sustainable energy source. The process suffers from many drawbacks such as slow and incomplete degradation rates, the result of the slow and rate-limiting sludge hydrolysis step, due to the low biodegradability of the cell walls and the presence of extra-cellular biopolymers. Methods to enhance the biogas formation should hence make the substrate more accessible to the anaerobic micro-organisms. Research on increasing the biogas production is extensive, with low-frequency ultrasound treatment emerging as the technical and economic most appropriate technology. Although ultrasound is now widely applied in practice, underlying phenomena and optimum operating conditions are still open to debate. Laboratory ultrasound treatment was coupled with digestion experiments (at 37 degrees C) using 12 parallel digesters, two of them being used as control digesters with untreated sludge. The present paper demonstrates that ultrasound treatment can - within specific ranges of energy-input achieve (i) an increased disintegration of the sludge, as witnessed by an increasing soluble COD-fraction accompanied by an increasing presence of BOD; (ii) an increased release of volatile fatty acids as a result of the oxidising radicals being formed through cavitation; and (iii) a slightly reduced dewaterability of the residual sludge, needing an increased dosage of poly-electrolyte to obtain a high degree of cake dryness. As pre-treatment to digestion, ultrasound enhances the biogas production by more than 40% at low specific energy-inputs (SE), and approximately 15% at higher SE-values.