Effect of Combined Microwave-Ultrasonic Pretreatment of Real Mixed Sludge on the Enhancement of Anaerobic Digester Performance Anteneh Mesfin Yeneneh & Ahmet Kayaalp & Tushar Kanti Sen & Ha Ming Ang Received: 21 June 2015 /Accepted: 14 August 2015 /Published online: 28 August 2015 # Springer International Publishing Switzerland 2015 Abstract The anaerobic biodegradability of combined microwave-ultrasonic pretreated thickened excess acti- vated sludge (PTEAS) mixed with raw primary sludge (PS) was investigated in this study. The pretreatment resulted in the enhancement of mesophilic anaerobic digester performance which in turn improved biogas production capacity and quality, total and volatile solid reduction, dewaterability, protein solubilisation and sig- nificant reduction of pathogens to produce class A bio- solid. This study presented the results of two continu- ously stirred mesophilic anaerobic digesters charged with various proportions of a mixture of PTEAS and PS similar to the large-scale industrial practice. Digester 1 was charged with 75 % PTEAS and 25 % PS, while digester 2 was fed with 25 % PTEAS and 75 % PS. The methane production was 122 mL CH 4 /g total chemical oxygen demand for digester 2 after 20 days of anaerobic digestion. This amount further increased for both di- gesters with digestion time. The biogas quality in terms of methane to carbondioxide ratio (CH 4 /CO 2 ) was sig- nificantly improved for digester 1 compared with digester 2 after 20 days of digestion. Volatile solid reduction of 76 and 57 % was achieved for digester 1 and digester 2 respectively after the same 20 days of digestion. The CH 4 /CO 2 ratio reached 2.2:1 and 1.1:1 after 20 days of digestion for digester 1 and digester 2, respectively. Higher percentage of PTEAS increases the digestion kinetics, the methane production capacity and the biogas quality. Furthermore, total coliform reduction of 84 and 44 % was achieved for digester 1 and digester 2 respec- tively after 22 days of digestion. Hydrolysis rate and biochemical methane production were improved for both digesters based on the results of Gompertz kinetic model and the hydrolysis rate constants as determined by model fitting of the experimental data. Keywords Anaerobic digestion kinetics . Biodegradability . Combined microwave-ultrasonic pre- treatment . Dewaterability 1 Introduction Sewage sludge handling and processing for ultimate disposal is one of the major challenges in the operation of municipal wastewater treatment plants. The produc- tion of waste activated sludge has significantly increased as a result of increase in the number and capacity of wastewater treatment plants over time. Sludge disposal is subjected to more stringent environmental policies and regulations and social constraints. Sludge treatment technologies like incineration are also quite expensive (Navaratnam 2007). Biogas production through anaerobic digestion has recently captured global attention. This is because of various advantages of the anaerobic digestion process. Water Air Soil Pollut (2015) 226: 314 DOI 10.1007/s11270-015-2586-0 A. M. Yeneneh : T. K. Sen (*) : H. M. Ang Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia e-mail: t.sen@curtin.edu.au A. Kayaalp Water Corporation of Western Australia, West Leederville 6007 WA, Australia