48 JOURNAL OF BIOSCIENCE AND BIOENGINEERING © 2008, The Society for Biotechnology, Japan Vol. 105, No. 1, 48–54. 2008 DOI: 10.1263/jbb.105.48 Comparison of Thermophilic Anaerobic Digestion Characteristics between Single-Phase and Two-Phase Systems for Kitchen Garbage Treatment YongJin Park, 1 Feng Hong, 1 JiHoon Cheon, 1 Taira Hidaka, 1 * and Hiroshi Tsuno 1 Department of Urban and Environmental Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan 1 Received 4 July 2007/Accepted 27 October 2007 Lab-scale single-phase and two-phase thermophilic methane fermentation systems (SPS and TPS, respectively) were operated and fed with artificial kitchen waste. In both SPS and TPS, the highest methane recovery ratio of 90%, in terms of chemical oxygen demand by dichromate (CODcr), was observed at an organic loading rate (OLR) of 15 gCODcr/(l ⋅ d). The ratio of particle CODcr remaining to total CODcr in the influent was 0.1 and the ratio of NH 4 -N concentration to the input total nitrogen concentration was 0.5 in both SPS and TPS. However, the propionate con- centration in the SPS reactor fluctuated largely and was 2 gCODcr/l higher than that in TPS, in- dicating less stable digestion. Regardless, efficient kitchen waste degradation can be accomplished in both SPS and TPS at an OLR of <20 gCODcr/(l ⋅ d), even though TPS may be more stable and easier to maintain. Bacillus coagulans predominated with an occupied ratio of approximately 90% in the acid fermentation reactor of TPS, and then a richer microbial community with a higher Shannon index value was maintained in the methane fermentation reactor of TPS than in the SPS reactor. [Key words: thermophilic, single phase, two phase, kitchen waste, methane fermentation, microbial community] Resource recycling and energy-saving systems for proc- essing organic solid waste in urban areas need to be estab- lished. Anaerobic digestion (AD) has been considered to be a promising energy saving and recovery process for the treatment of organic solid waste with a high water content such as kitchen waste. AD is considered to take place in two steps: the acidogenic phase and the methanogenic phase. In AD processes, the single-phase system (SPS), in which these two steps proceed simultaneously in one reactor, is com- monly used because of simplicity in configuration and oper- ation. However, the two-phase system (TPS), which con- sists of separate acidogenic and methanogenic reactors, has several advantages over conventional single-phase systems, such as a higher organic degradation rate and a higher meth- ane production rate, which it achieves by optimizing envi- ronmental conditions for each phase, such as pH, substrate concentrations, and its loading (1, 2). In addition, tempera- ture is also an important parameter that directly influences AD. Thermophilic AD (temperature optimum at 55°C) shows several advantages over mesophilic AD (temperature opti- mum at 35°C), such as an increased degradation rate for organic solids, a high gas production rate, improved solid- liquid separation and increased disinfection of pathogenic organisms (3, 4). The AD of kitchen waste has been studied widely. Sasaki et al. (5) conducted a thermophilic SPS experiment and sug- gested that the appropriate organic loading rate (OLR) is 20 gCODcr/(l ⋅ d) for an influent total solids (TS) concentra- tion of 10%. Li et al. (6) developed a TPS that was com- posed of thermophilic acidogenic and mesophilic methano- genic reactors and suggested that the appropriate OLR is 20 gCODcr/(l ⋅ d) for an influent TS concentration of 20%. Sakamoto et al. (7) proposed a mesophilic TPS combined with an aerobic solubilization process before using the TPS, and stable operation was obtained at an OLR of 9 gCODcr/ (l ⋅ d). However, little research and scientific information is available concerning a direct performance comparison be- tween SPS and TPS, and the microbial community in the application of thermophilic AD to kitchen waste, whose TS concentration is higher than 10%. Furthermore, once-a-day- feeding operation has some advantages, such as ease of maintenance and low labor costs; feeding frequency in other papers is usually several times a day. The purposes of this study are to develop a once-a-day-feeding AD processes for the treatment of kitchen waste and the recovery of methane and to compare the advantages of thermophilic SPS and TPS. To obtain fundamental information, lab-scale SPS and TPS reactors were operated continuously with once-a-day feed- ing using artificial kitchen waste, whose components were kept constant. Treatment performances, such as methane pro- duction and the stability of the operation, and the microbial community are discussed. * Corresponding author. e-mail: hidaka@water.env.kyoto-u.ac.jp phone: +81-(0)75-383-3350 fax: +81-(0)75-383-3351