Microwave and thermal pretreatment as methods for increasing the biogas potential of secondary sludge from municipal wastewater treatment plants Mariusz Kuglarz a , Dimitar Karakashev b , Irini Angelidaki b,⇑ a Department of Environmental Sciences and Materials, University of Bielsko-Biala, Willowa 2, Bielsko-Biala, Poland b Department of Environmental Engineering, Technical University of Denmark, Building 115, DK-2800, Lyngby, Denmark highlights " Microwave irradiation and thermal heating as methods for sludge pre-treatment. " Microwave irradiation turned out to be superior over thermal pre-treatment. " 900 W irradiation showed to be less energy demanding compared to 700 W. " Pre-treatment at 70 °C allowed to achieve the highest energy profit. " Microwave pre-treatment ensured high degree of sludge sanitation. article info Article history: Received 26 November 2012 Received in revised form 1 February 2013 Accepted 2 February 2013 Available online 9 February 2013 Keywords: Microwave irradiation Pre-treatment Anaerobic digestion Sanitation Secondary sludge abstract In the present study, the sludge was pretreated with microwave irradiation and low-temperature thermal method, both conducted under the same temperature range (30–100 °C). Microwave pretreatment was found to be superior over the thermal treatment with respect to sludge solubilization and biogas produc- tion. Taking into account the specific energy demand of solubilization, the sludge pre-treated at 60–70 °C by microwaves of 900 W was chosen for further experiments in continuous mode, which was more ener- getically sustainable compared to lower value (700 W) and thermal treatment. Continuous biogas reactor experiments indicated that pre-treated sludge (microwave irradiation: 900 W, temperature: 60–70 °C) gave 35% more methane, compared to untreated sludge. Moreover, the results of this study clearly dem- onstrated that microwave pretreated sludge showed better degree of sanitation. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Treatment of wastewater in municipal treatment plants (WWTP) is removing the majority of the organic matter, and a part of the or- ganic matter is ending up as sludge. It contains a significant amount of water, organic matter, un-degradable particles and living organ- isms. Primary sludge consists of the easily-biodegradable matter, which is a suspended particle. While secondary sludge, which is mainly microbial cell biomass, formed by aerobic bioconversion of organic matter in the activated sludge process. The latter process is commonly used as a core process in a majority (approx. 90%) of municipal wastewater treatment plants (WWTP) (Bordeleau and Droste, 2011; Tyagi and Lo, 2011). It is estimated that in WWTPs lo- cated in European Union, about 10 million tons of sludge dry matter is generated annually (Tyagi and Lo, 2011). Sludge is a burden to society and methods for its disposal and treatment are of high prior- ity. Among the various treatment methods, anaerobic digestion (AD) is the most frequently used. By anaerobic treatment not only stabil- ization of the sludge but also utilization of the residual organic mat- ter for production of biogas is achieved. In this process, organic matter (lipids, proteins and carbohydrates) is microbiologically con- verted under anaerobic methanogenic conditions into biogas (Angelidaki et al., 2011; Mata-Alvarez et al., 2000). The secondary sludge has low biodegradability, as it consists microbial cells agglomerated by extracellular polymeric substances (EPS) and cat- ions resulting in compact flocs. On the contrary, primary sludge is relatively easily biodegradable. Hydrolysis is widely regarded as the rate-limiting step of degradation of particulate organic matter in the secondary sludge. Pre-treatment is required in order to dis- rupt cell membranes and thus lyse microbial cells in the sludge (Tang et al., 2010; Tyagi and Lo, 2011). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.02.001 ⇑ Corresponding author. Tel.: +45 4525 1429; fax: +45 4593 2850. E-mail address: iria@env.dtu.dk (I. Angelidaki). Bioresource Technology 134 (2013) 290–297 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech