High-rate denitrification using polyethylene glycol gel carriers entrapping heterotrophic denitrifying bacteria Kazuichi Isaka a, *, Yuya Kimura a , Toshifumi Osaka b , Satoshi Tsuneda b a Hitachi Plant Technologies, Ltd., Kami-Hongo 537, Matsudo-shi, Chiba 271-0064, Japan b Waseda University, Department of Life Science and Medical Bioscience, 2-2 Wakamatsu-cho, Shinjuku-ku, Tokyo 162-8480, Japan article info Article history: Received 30 September 2011 Received in revised form 23 May 2012 Accepted 27 May 2012 Available online 4 June 2012 Keywords: Denitrification Nitrate Immobilization Activation energy Hyphomicrobium abstract This study evaluated the nitrogen removal performance of polyethylene glycol (PEG) gel carriers containing entrapped heterotrophic denitrifying bacteria. A laboratory-scale denitrification reactor was operated for treatment of synthetic nitrate wastewater. The nitrogen removal activity gradually increased in continuous feed experiments, reaching 4.4 kg N m 3 d 1 on day 16 (30  C). A maximum nitrogen removal rate of 5.1 kg N m 3 d 1 was observed. A high nitrogen removal efficiency of 92% on average was observed at a high loading rate. In batch experiments, the denitrifying gel carriers were characterized by temperature. Nitrate and total nitrogen removal activities both increased with increasing temperature, reaching a maximum at 37 and 43  C, respectively. Apparent activation energies for nitrate and nitrite reduction were 52.1 and 71.9 kJ mol 1 , respectively. Clone library analysis performed on the basis of the 16S rRNA gene revealed that Hyphomicrobium was mainly involved in denitrification in the methanol-fed denitrification reactors. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction Several types of wastewater, such as that from semiconductor fabrication plants, metal refinery plants, and landfill leach- ates, have high concentrations of ammonium but low concentrations of biodegradable organic compounds. Biolog- ical nitrogen removal is generally achieved by a nitrifica- tionedenitrification process. However, to achieve complete biological nitrogen removal from these types of wastewater, denitrification requires an additional organic carbon source. Various organic compounds, such as methanol, ethanol, sugars, methane, and acetate, have been used as hydrogen donors for heterotrophic denitrification (Akunna et al., 1993; Thalasso et al., 1997). Methanol was chosen because of its relatively low cost and low sludge production rates compared with other organic compounds, although only some types of bacteria can utilize methanol as a carbon source. It is important to develop a denitrification process which ensures a high nitrogen removal rate for practical applica- tions. With a higher nitrogen removal rate, we can reduce the footprint of wastewater treatment plants (WWTPs) by reducing reactor volume of the denitrification process. Therefore, it is important to develop a denitrification process with a high nitrogen removal rate. It is also preferable if the operation of WWTPs involves low maintenance and no sludge concentration control. Many types of biomass carriers, including microporous cellulose carriers, silicone membrane tubes, and porous polyvinyl alcohol (PVA) cubes, have been developed for the immobili- zation of denitrifying bacteria (Matsumura et al., 1997; Hsieh et al., 2003; Komori and Sakakibara, 2008). In addition, gel entrapment techniques have been used for immobilizing nitrifying and anammox bacteria in a reactor (Sumino et al., 1992; Vogelsang et al., 1997; Chen and Lin, 1994; Isaka et al., * Corresponding author. Tel.: þ81 47 361 6103; fax: þ81 47 361 6107. E-mail addresses: k-isaka@hitachiplant.co.jp, kazuichi.isaka.mp@hitachi-pt.com (K. Isaka). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 4941 e4948 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.05.050