Ultrasonic and Thermal Pretreatment of Apple Pomace to Improve Biochemical Methane Potential S¸evket Tulun and Melayib Bilgin Department of Environmental Engineering, Aksaray University, Aksaray, 68100, Turkey; sevkettulun@gmail.com (for correspondence) Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12836 This study evaluates the effects of ultrasonic and thermal pretreatment on apple pomace solubilization and on the per- formance of the subsequent mesophilic anaerobic digestion. Ultrasonic frequency (35 and 53 kHz) and thermal methods are studied for pretreatment strategies. For each pretreatment method, four different temperatures are selected such as 25, 40, 50, and 608C, and operation times are selected as 5, 15, 30, and 45 minutes. The highest increase in organic solubilization was observed at 608C with 53 kHz in the 45 minutes. The highest biochemical methane productivity was achieved by the 53 kHz ultrasonic pretreatment at the 45 min. In addition to this, the solution with 1481.4 mL CH 4 /g-VS was 1.74 times higher than the non- pretreatment condition 852.8 mL CH 4 /g-VS. The pretreatment tests showed that the increasing biochemi- cal methane potential could be obtained by using the apple pomace. V C 2017 American Institute of Chemical Engineers Environ Prog, 00: 000–000, 2017 Keywords: apple pomace, pretreatment, biochemical meth- ane potential, anaerobic digestion INTRODUCTION Waste is an important issue that affects all the species in the world. As the global population continues to increase the waste production also continues to grow. As a result, severe environmental problems concerning the traditional waste management system have arisen [1]. Due to the increasing energy demands and eventual depletion of limited nonre- newable energy resources, the world is now looking toward the renewable energy sources [2], which could be sustained over long time periods. Organic waste biomass, one of the most important renew- able resources available today, is considered to be a main alternative energy source obtained from biomass. Among these sources, AP represents the largest portion of available organic waste biomass in the Turkey. Apple is one of the most popular fruits in Turkey. A great amount of apple pom- ace (AP) is produced every day from bazaar, agro-industries, and kitchens [3]. AP is the solid residue that remains following extraction of juice from ground apples [4]. About 2480 million tons of fresh apple are produced per year in Turkey [5]. Pomace dis- posal methods are expensive and pose some environmental concerns due to the fact that AP is highly biodegradable and rich in sugars and fibers [6]. But when the pomace wastes are abandoned in fields, it may lead to many unpleasant environmental consequences. AP has a potential to be a good substrate for microbial fermentation due to its chemical and physical properties since it has around 70–75% water and 250–300 g/kg chemical oxygen demand (COD) [7]. Moreover, because of the political and technological con- siderations that have arisen from the consideration of other energy alternatives, e.g., biogas production via anaerobic digestion (AD) has been continuously developing [8]. AD is a widely applied in solids stabilization technology to degrade a complex variety of organic wastes in the absence of free or dissolved oxygen [9]. However, anaerobic degradation is a slow process and very long retention times are required due to the rate-limiting step of biological sludge hydrolysis [10]. Anaerobic degradation process occurs in three stages. These stages are hydrolysis, acid forming (acidogenesis and acetogenesis), and methanogenesis [11]. If the first stage is inhibited, the second and third stages are limited and meth- ane production decreases due to substrate deficiency from the first stage. This step can be accelerated by pretreatment of waste prior to AD [12–15]. Pretreatment is often designed to increase the solubilization of organic waste and improve the efficiency of the anaerobic decomposition of the waste by breaking down cell walls [16]. Many pretreatment methods have been studied, such as mechanical treatment, freeze-thawing treatment [17], thermal treatment [18], ultrasonic treatment [19], microwave treatment [20], chemical treatment, and biological treatment [21]. The term ultrasound is used to define sound energy in a frequency range of 20–100 kHz that is above human hearing. The principle of ultrasonic treatment relies on the cavitation process to disintegrate cell walls [22]. Researchers found that high-energy intensity enhances the disintegration of particulate matter, which is evidenced by a reduction in particle size and increasing soluble matter frac- tion [23]. Tiehm et al. demonstrated that the pretreatment of waste activated sludge by ultrasonic disintegration signifi- cantly improved microbial cell lysis and increased the volatile solids degradation as well as biogas production [24]. Produced heat during thermal treatment disrupts chemical bonds of the cell wall and cell membrane resulting in the release of intracellular components and enhancing anaerobic digestibility [25]. Climent et al. found that only low- temperature thermal treatment increased biogas production by 50% and found no effect of high-temperature treatment in that regard [26]. Bonmati et al. investigated the application of V C 2017 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2017 1