Enhancement of anaerobic digestion efficiency of wastewater sludge and olive waste: Synergistic effect of co-digestion and ultrasonic/ microwave sludge pre-treatment B. Aylin Alagöz ⇑ , Orhan Yenigün, Ays ßen Erdinçler Bogazici University, Institute of Environmental Sciences, 34342, Bebek, Istanbul, Turkey article info Article history: Received 21 April 2015 Revised 10 August 2015 Accepted 15 August 2015 Available online 25 August 2015 Keywords: Anaerobic co-digestion Biogas production Microwave pre-treatment Olive pomace Ultrasonic pretreatment Sludge disintegration abstract This study investigates the effect of ultrasonic and microwave pre-treatment on biogas production from the anaerobic co-digestion of olive pomace and wastewater sludges. It was found that co-digestion of wastewater sludge with olive pomace yielded around 0.21 L CH 4 /g VS added , whereas the maximum methane yields from the mono-digestion of olive pomace and un-pretreated wastewater sludges were 0.18 and 0.16 L CH 4 /g VS added . In the same way, compared to mono-digestion of these substrates, co-digestion increased methane production by 17–31%. The microwave and ultrasonic pre-treatments applied to sludge samples prior to co-digestion process led to further increase in the methane production by 52% and 24%, respectively, compared to co-digestion with un-pretreated wastewater sludge. The high- est biogas and methane yields were obtained from the co-digestion of 30 min microwave pre-treated wastewater sludges and olive pomace to be 0.46 L/g VS added and 0.32 L CH 4 /g VS added , respectively. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The substantial increase in the worldwide agricultural activities has recently resulted in the production of an important amount of agro-wastes. Among the existing agricultural activities, olive is one of the most economically important fruit species in the Mediter- ranean countries (Sensi et al., 2003). The Mediterranean region (Spain, Italy, Greece, Turkey, Tunisia, Morocco, Syria, and Portugal) is the major production area, constituting for more than 90% of world production (FAO, 2007). Turkey is the fourth biggest olive- producing country in the world and is ranked fifth in olive oil and second in table olive production. Most of the olive production in Turkey occurs in the Aegean, Marmara and Mediterranean regions accounting for approximately 65-Mton agricultural wastes production annually from the vegetable-processing activities (Mendilcioglu, 2001). The main environmental impacts produced by fruit processing activities such as wineries correspond to pomace generation (residuals solid). The olive pomaces possess very high levels of bio- chemical oxygen demand and if not treated properly; facing with the problems of odour and microbial contamination inside the winery is inevitable (Cáceres et al., 2012). Regarding the abundant sources of agricultural residues, the most common ‘‘waste-to energy” application is the anaerobic digestion (AD) process. The commonly used substrates in anaero- bic digestion are livestock manure, organic fraction of the munici- pal solid waste, food processing waste, energy crops, agricultural residues, etc. (Ralph and Keith, 1990; Mähnert and Linke, 2009; Murto et al., 2004; Neves et al., 2006). Preliminary studies had shown that the anaerobic digestion of the agricultural products alone was, however, not so efficient. In general, hydrolysis is the rate limiting step if the substrate is in particulate form (Veeken and Hamelers, 1999). However, when the substrate is cellulose-poor wastes like from fruit processing activities, the AD process is limited by methanogenesis rather than by hydrolysis. The major constraints for the anaerobic digestion of fruit and vegetable processing wastes like olive pomace, maize silage, etc. were rapid acidification of these wastes and production of high amounts of volatile fatty acids (VFA), which decrease the pH in the reactor and inhibit the activity of methanogenic bacteria (Misi and Forster, 2002; Bouallagui et al., 2005). Furthermore, the agricultural products have a carbon-to-nitrogen (C/N) ratio of about 40–70, whereas the suggested optimum C/N ratio for anaer- obic digestion is in the range of 20–30 (Parkin and Owen, 1986). Over the last 15–20 years, this consideration has driven several authors to study the performance of the anaerobic digestion pro- cess through the simultaneous treatment of solid and liquid organic wastes. This method known as ‘‘co-digestion” enhances http://dx.doi.org/10.1016/j.wasman.2015.08.020 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: aylin.zeren@boun.edu.tr (B. Aylin Alagöz). Waste Management 46 (2015) 182–188 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman