ORIGINAL ARTICLE Volatile Fatty Acid Production from Organic Fraction of Municipal Solid Waste Through Anaerobic Acidogenic Digestion Eylem Dog ˘an and Go ¨ ksel N. Demirer* Middle East Technical University, Department of Environmental Engineering, Ankara, Turkey. Received: February 12, 2009 Accepted in revised form: June 5, 2009 Abstract There are numerous studies on two-phase anaerobic digestion of municipal solid waste (MSW) in the literature. Yet, in the majority of these studies, applied organic loading rates (OLR) were relatively low. This study differs from those in the literature in terms of high OLR application and focuses on the optimization of the acidification phase. Effect of high OLRs and pH values were investigated and optimum operational conditions for acidifi- cation were determined. Although conversion of particulate organics to soluble chemical oxygen demand (COD) was achieved in the reactors, total COD values remained the same. Various types of organic acids were produced in the reactors and the selective production of organic acids in the anaerobic acidogenesis process was possible by controlling the organic loading rate and pH. Optimum OLR and pH values were determined as 15 g volatile solid (VS)=L Á day and 5.5 Æ 0.1, respectively, for the MSW composition investigated. Key words: acidification; anaerobic; organic loading rate; organic fraction of municipal solid waste; pH Introduction O rganic fraction of municipal solid waste (OFMSW) is semisolid waste that brings about significant envi- ronmental and public health problems. These problems include contamination of drinking water supplies, global warming en- hancement, and odors (Ostrem, 2004). The total production of municipal solid waste in Turkey was 25 million tones per year for 2004, and only 28% of this waste was disposed in landfills. On the other hand, no engineered waste management practices were applied to the remaining percent, which represents a significant danger to the environment. Various approaches to municipal solid waste (MSW) management problem such as landfilling, composting, incineration exist; however, anaerobic digestion (AD) has advantages over these techniques in terms of process control and efficiency, operation time, and production of value added products, such as biogas, etc. AD can be operated in single- or two-phase configurations. Single-phase incorporates both acid formation and methane production in the same reactor, whereas a two-phase opera- tion separates acid formation from methane production (Speece, 1996). Two-phase processes have some advantages over one-phase systems. First of all, the selection and enrich- ment of different bacteria is achieved, and the control of acidification phase enhances the stability of the system by preventing overloads that may affect methanogens. Another advantage is that the volume requirement can be smaller due to applicability of short hydraulic retention times, and there- fore, the system is more cost effective. Last, through acidifi- cation high solid containing wastes are liquefied, and this increases the efficiency of the system. Furthermore, conven- tional one-phase digestion is not effective for wastes with high solid content because a significant increase in fluid and di- gester volume is observed during one-phase operation sys- tems (Demirer and Chen, 2005). There are numerous studies (Virturtia et al., 1995; Raynal et al., 1998; Vieitez and Ghosh, 1999; Wang et al., 2002; Bouallagui et al., 2004; Gioannis et al., 2008) on anaerobic di- gestion of MSW with two-phase systems in the literature. Yet, in the majority of these studies, applied organic loading rates (OLRs) were generally lower than 10 g volatile solid (VS)=L Á day because they were lab-scale applications (Diaz et al., 1981; Viturtia et al., 1995; Hartmann and Ahring, 2005). In addition, these studies did not focus on the optimum op- erational conditions for the acidification of OFMSW. How- ever, acidification should be controlled to obtain maximum efficiency in the methane producing reactor in a two-phase system. Therefore, this study differs from the literature in terms of high OLR application and focuses on acidification phase optimization. The effect of high OLRs and pH values was studied and optimum operational conditions for acidifi- cation were determined in terms of load and pH value. Materials and Methods Organic fraction of MSW and anaerobic seed culture The OFMSW used in this study was composed of food wastes collected from houses and vegetable and fruit wastes *Corresponding author: Department of Environmental Engineering, Middle East Technical University, Ankara, Turkey 06541. Phone: 90-312 210 58 67; Fax: 90-312 210 26 46; E-mail: goksel@metu.edu.tr ENVIRONMENTAL ENGINEERING SCIENCE Volume 26, Number 9, 2009 ª Mary Ann Liebert, Inc. DOI: 10.1089=ees.2009.0062 1443