Anaerobic digestion of dairy manure in a hybrid reactor with biogas recirculation Go¨ksel N. Demirer 1, * and Shulin Chen 2 1 Dept. of Environmental Eng., Middle East Technical Univ., 06531, Ankara, Turkey 2 Dept. of Biological Systems Eng, Washington State Univ, 99163, Pullman, WA, USA *Author for correspondence: Tel.: +90-312 210 58 67, Fax: +90-312 210 12 60, E-mail: goksel@metu.edu.tr Received 17 March 2005; accepted 12 May 2005 Keywords: Anaerobic digestion, biogas recirculation, biomass immobilization, dairy manure, hybrid reactor Summary A novel anaerobic hybrid reactor (AHR) configuration incorporating floating support media for biomass immo- bilization and biogas recirculation for enhanced mixing was used for anaerobic digestion of dairy manure. No pretreatment or solid liquid separation was applied. The reactor was operated at high influent volatile solids (VS) and organic loading rates (OLR) of up to 9.87% and 7.30 g VS/l day, respectively. After 149 days of continuous operation the results revealed that a high amount (38.1 g VSS) of biomass was able to attach itself to the support medium being used. The investigated AHR configuration achieved COD, BOD, TS, and VS removal efficiencies of 48–63, 64–78, 55–65, and 59–68%, respectively, at a hydraulic retention time (HRT) of 15 days. The corresponding average methane production value obtained in this study was 0.191 l/g VS added. Introduction Anaerobic digestion is an established bioconversion technology for high strength wastewater treatment. Interest in using anaerobic digestion for manure man- agement is rapidly growing as farmers and governments are faced with mounting economic and environmental concerns. The growing interest in the technology can be attributed to anaerobic digestion’s potential for reduc- ing pathogen levels and weed seeds, controlling odour, improving fertilizer value, and producing value-added products such as energy-rich methane; all of which can help remedy or alleviate many of the economic and environmental issues facing the growers and society. Extensive research has been conducted and well docu- mented on the feasibility of anaerobic treatment of farm animal manure and its advantages, reactor types used, performance, etc. (Lo & Liao 1985; Hobson & Wheatley 1993; Chynoweth et al. 1999, 2001; Demirer et al. 2000; Demirer & Chen 2004; Gu¨ngo¨r-Demirci & Demirer 2004). There are, though, several key areas of research that must be pursued if anaerobic digestion technology is to be made more economically advantageous. These include investigating anaerobic digestion’s relatively low digestion rates (or high retention time requirements) and difficulties with biodegradation of lignocellulosic mate- rial. Conventional high-rate anaerobic reactors such as anaerobic filters, upflow anaerobic sludge blanket reac- tors, etc. cannot effectively process wastes containing more than 2–3% solids and are thus inappropriate for use with animal manures which have much higher solids content. Thus, demonstrating innovative anaerobic process configurations that can process high solid ani- mal waste at relatively short retention times will be an innovative step towards achieving effective exploitation of anaerobic digestion for animal manure. Hybrid reactor configurations combine the positive features of both suspended and attached growth systems and constitute a promising option in anaerobic treat- ment of different wastes. The anaerobic hybrid reactor (AHR) which consists of a sludge bed in the lower part and an anaerobic filter (AF) in the upper part combines the advantages of upflow anaerobic sludge blanket (UASB) and AF reactors (Suvajittanont & Chaiprasert 2003) while minimizing their limitations (Tilche & Vieira 1991). The application of AHRs in anaerobic treatment include molasses (Boopathy & Tilche 1991), baker’s yeast (Britz & van der Merwe 1993), coffee processing (Bello-Mendoza & Castillo-Rivera 1998), slaughter- house (Borja et al. 1995), pharmaceutical (Henry et al. 1996), ice-cream wastewater (Hawkes et al. 1995), cheese whey (Malaspina et al.1996), phthalic waste (Tur & Huang 1997), landfill leachate (Chang 1989), and domestic sewage at low temperatures (Elmitwalli et al. 2002), as well as some studies focusing on improving reactor design and operational parameters (Suvajitt- anont & Chaiprasert 2003; Pender et al. 2004). Increased biomass inventory and ability to process high solid-containing wastes are two features of the hybrid configuration which are significant for anaerobic World Journal of Microbiology & Biotechnology (2005) 21:1509–1514 Ó Springer 2005 DOI 10.1007/s11274-005-7371-6