Full-stream and part-stream ultrasound treatment effect on sludge anaerobic digestion S. I. Pe ´ rez-Elvira, L. C. Ferreira, A. Donoso-Bravo, M. Fdz-Polanco and F. Fdz-Polanco ABSTRACT S. I. Pe ´ rez-Elvira A. Donoso-Bravo M. Fdz-Polanco F. Fdz-Polanco Department of Chemical Engineering and Environmental Technology, University of Valladolid, 47011 Valladolid, Spain E-mail: sarape@iq.uva.es; adonosobravo@gmail.com; maria@iq.uva.es; ffp@iq.uva.es L. C. Ferreira Department of Chemical and Biological Technology and Management, Braganc ¸ a Politecnic Institute, Portugal E-mail: lcatarina.ferreira@gmail.com The use of ultrasound as pre-treatment to improve anaerobic digestion of secondary sludge has been established as a promising technology. There are great differences between lab scale and full-scale devices, regarding the relationship between the disintegration achieved and the energy supplied. Based on economic aspects, most of the full-scale plants use partial-stream instead of the full-stream sonication, which affects biogas production and digestate dewatering characteristics. A laboratory scale operation combining ultrasound and anaerobic digestion (batch tests) has been performed, determining the relationship between the ratio of sonicated sludge fed and the methane production, SCOD removal and capillary suction time after 20-day anaerobic biodegradation, in order to check the possible benefits of part-stream versus full-stream sonication. Additional incubation was also evaluated, searching for an optimum process combining ultrasound and 24-h incubation pretreatment. Results showed that by sonicating fresh WAS at 25,700 kJ/kg TS biogas yield increased linearly with the percentage of sonicated WAS in the substrate, from 248 (control reactor) to 349 mL CH 4 /g VS (41% increase in full-stream sonication). By incubation (24 h, 558C), 325 mL CH 4 /g VS were obtained (31% increase), but the digestion of the soluble compounds generated during incubation of sonicated sludge appeared to be less degradable compared to those solubilised by ultrasound or incubation alone, which showed no benefit in combining both treatments. Post-digestion dewatering deteriorated for both part-stream and full-stream sonication, and CST values were constant (74% higher than the control digestate) from 30% to 100% sonicated sludge. Key words | anaerobic digestion, biogas, dewaterability, disintegration, part-stream, sludge, ultrasound ACRONYMS AND ABBREVIATIONS COD chemical oxygen demand CST capillary suction time DD COD disintegration degree (Equation (3)) DS dry solids P methane productivity (mL CH 4 /g VS) PCOD particulate chemical oxygen demand R m biogas production rate (mL CH 4 /g VS d) SCOD soluble chemical oxygen demand S COD COD solubilisation (Equation (2)) TCOD total chemical oxygen demand TS total solids VS volatile solids WAS waste activated sludge WWTP wastewater treatment plant l lag time (d) doi: 10.2166/wst.2010.893 1363 Q IWA Publishing 2010 Water Science & Technology—WST | 61.6 | 2010