Biochemical Engineering Journal 37 (2007) 49–55 Nitrification of landfill leachate using immobilized nitrifying bacteria at low temperatures Kazuichi Isaka a,∗ , Sachiko Yoshie b , Tatsuo Sumino a , Yuhei Inamori c , Satoshi Tsuneda b a Hitachi Plant Technologies Ltd., Kami-Hongo 537, Matsudo, Chiba 271-0064, Japan b Department of Chemical Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan c National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, Ibaraki 305-0053, Japan Received 15 December 2006; received in revised form 17 March 2007; accepted 17 March 2007 Abstract A technology to achieve stable and high rates of nitrification of landfill leachate at low temperatures has been desired. Nitrifying bacteria entrapped in a polyethylene glycol (PEG) gel carrier produced high nitrification rates of 0.71 kg N/m 3 /day at 10 ◦ C for more than 1 year. As a characteristic of nitrification, ammonium nitrogen at 16–35 mg/L remained in effluent water irrespective of nitrogen load and nitrite accumulation was observed. Batch experiments clearly showed that the relationship between ammonium concentration and ammonium removal rate followed a Monod-type equation. It was also revealed that ammonium-oxidizing bacteria cultivated in a gel carrier had a low affinity for ammonium, leading to incomplete nitrification. Moreover, it was suggested that the remaining ammonium in the reactor produced free ammonium, which inhibited the activities of nitrite-oxidizing bacteria. Thus, only nitritation was observed. Molecular biological methods, such as denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH), revealed that Nitrosomonas sp. was the dominant ammonium-oxidizing bacteria in the gel carrier at low temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Aerobic processes; Bioreactor; Immobilization; Wastewater treatment; Nitrification; Leachate 1. Introduction Landfill leachate includes high concentrations of ammo- nium, primarily originating from the leaching of municipal solid waste. To remove ammonium nitrogen, biological nitri- fication/denitrification systems are effective and economical. However, biological systems, especially nitrification processes, are strongly affected by water temperature [1]. Because land- fills exist in a wide range of environments, including areas with cool climate, and because landfill leachate temperature strongly depends on ambient temperature, an effective technology to remove ammonium from landfill leachate at low temperatures is often necessary. There have been a few reports on biological nitrification of landfill leachate at low temperatures. Welander et al. [2] con- ducted nitrification of landfill leachate at 10 ◦ C using a plastic ∗ Corresponding author. Tel.: +81 47 361 6103; fax: +81 47 361 6107. E-mail address: kazuichi.isaka.mp@hitachi-pt.com (K. Isaka). carrier to which surface nitrifying bacteria adhered. In these experiments, complete nitrification was attained, but the nitrifi- cation rate was as low as 0.13 kg N/m 3 /day. Similarly, Jokela et al. [3] confirmed a 95% nitrification efficiency and a nitrifica- tion rate of 0.05 kg N/m 3 /day at 5–10 ◦ C using crushed bricks to which nitrifying bacteria adhered. These studies suggested the applicability of biological treatment to the removal of ammo- nium from landfill leachate at low temperatures. However, the nitrification rates reported by these studies are so low that huge treatment plants would be required in practice. To obtain higher nitrification rates at low temperatures, we believed that ammonium-oxidizing bacteria must be immobi- lized at high densities, because the growth rates and activities of ammonium-oxidizing bacteria are extremely low at low temper- atures. Because of this, immobilization of ammonium-oxidizing bacteria would be a key factor in developing nitrification pro- cesses at low temperatures in practice. To immobilize the ammonium-oxidizing bacteria, gel entrap- ment technologies have been reported. Sumino et al. [4] developed an immobilization method by entrapping ammonium- 1369-703X/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2007.03.008