Effects of ultrasonic pretreatment on the solubilization and kinetic study of biogas production from anaerobic digestion of waste activated sludge Alfredo C  ordova Lizama, Cristian Carrera Figueiras, Rafael Rojas Herrera, Alejandro Zepeda Pedreguera, Juan Enrique Ruiz Espinoza * Facultad de Ingeniería Química, Universidad Aut onoma de Yucat an, Campus de Ciencias Exactas e Ingenierías, Periferico Norte, Km. 33.5, Tablaje Catastral 13615, Col. Chuburn a de Hidalgo Inn, C.P. 97203, Merida, Yucat an, Mexico article info Article history: Received 4 March 2017 Received in revised form 19 May 2017 Accepted 19 May 2017 Keywords: Ultrasonic pretreatment Anaerobic digestion Biochemical methane potential Waste activated sludge Biogas production abstract Waste activated sludge (WAS) is a polluting waste with severe management problems that must be treated to prevent pollution and human health risks. Anaerobic digestion (AD) is the most used process to stabilize sludge; however, it must be improved because the biomethanation of sludge entails low biodegradability, pathogen inactivation, and biogas production. This study investigated the effects of ultrasonic pretreatment (USp) of WAS as a strategy to improve AD. Macromolecule solubilization, heavy metal behavior, pathogen inactivation and biogas production were evaluated. USp was applied at a range of 5000e35000 kJ/kg TS (total solids). The maximum solubilization degree of soluble chemical oxygen demand was 26%, and 22.9% of proteins at 35000 kJ/kg TS. The highest USp reduced only 2 log units of pathogens; nevertheless, a high inactivation was obtained when TS were reduced to 2% and continuous stirring was applied. AD of raw and sonicated WAS were compared with biochemical methane potential tests, and a biogas overproduction of 31.43% (219.5 mL/g VS) was achieved at highest USp. A modified Gompertz model was used for kinetic study of biogas production. The kinetic parameters at highest energy were: biogas production potential, G 0 ¼ 634.2 mL; maximum biogas production rate, R max ¼ 57.23 mL/day. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Waste activated sludge (WAS) is a polluting and hazardous waste generated in wastewater treatment plants (WWTP) and has serious management problems. The concentration of heavy metals and pathogens limit the implementation of standard disposal methods (Martín et al., 2015). Therefore, WAS must be adequately treated to prevent environmental pollution and human health risks. Direct applications of WAS in agriculture, landfill disposal, and composting are losing popularity due to increasingly stringent restrictions (CEC, 1986; U.S. EPA, 1994). Anaerobic digestion (AD) is a biological process whereby organic matter from the substrate is degraded by microorganisms in the absence of oxygen and is the most widely used method to stabilize sewage sludge (Liao et al., 2016). AD has the advantages of (1) reducing the sludge volume, (2) generating energy-rich biogas, and (3) yielding a nutrient-containing final biosolid (Kim et al., 2010). It is also the preferred stabilization method of WAS because of its low cost and low energy footprint (Kim et al., 2015). The anaerobic process involves four stages: hydrolysis, acido- genesis, acetogenesis and methanogenesis. During hydrolysis, both solubilization of particulate matter and biological decomposition occurs via exoenzymes. This happens slowly, making it the rate- limiting step of the process (Alag€ oz et al., 2015; Lin et al., 2016; Zhen et al., 2017). However, methane production during the last stage may be adversely affected if the conditions are not adequate, such as pH, the type of inhibitors present (ammonia, sulfide, heavy metals, and organics), and overproduction and accumulation of volatile fatty acids during hydrolysis and acidogenesis (Liu et al., 2014; Lin et al., 2014). In the AD of WAS, the hydrolysis is limited due to the low biodegradability of the cell walls and extracellular polymeric substances of microorganisms; however, it can be enhanced by a pretreatment step. For this reason, many * Corresponding author. E-mail address: juan.ruiz@correo.uady.mx (J.E. Ruiz Espinoza). Contents lists available at ScienceDirect International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod http://dx.doi.org/10.1016/j.ibiod.2017.05.020 0964-8305/© 2017 Elsevier Ltd. All rights reserved. International Biodeterioration & Biodegradation 123 (2017) 1e9