Novel nitritation process using heat-shocked nitrifying bacteria entrapped in gel carriers Kazuichi Isaka a, * , Tatsuo Sumino a , 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 Received 9 September 2007; received in revised form 21 November 2007; accepted 11 December 2007 Abstract Nitritation (ammonium being oxidized to nitrite) is a cost-effective method for treating wastewater having high ammonium concentrations or low C/N ratios. We developed a novel nitritation process based on the observation that nitrite-oxidizing bacteria (NOB) in sewage sludge can be killed by heat shock, but ammonium-oxidizing bacteria (AOB) may survive. The effects of maximum heat-shock temperature and heat-shock duration on populations of AOB and NOB in gel carriers were measured. No NOB were detected after a heat-shock treatment higher than 60 8C for 20 min. However, the population of AOB continued to exist at above 10 8 MPN/mL-carrier even after heat shock at 80 8C for 1 h. To evaluate the nitritation performance, continuous feeding tests were conducted using heat-shocked gel carriers treated at three temperatures. Stable nitritation was observed for 49 days when gel carriers were heat shocked at 60–90 8C for 1 h. However, because nitrate production, i.e., nitratation, was observed after 77 days, the gel carriers were heat shocked again. Consequently, nitratation stopped immediately and nitritation restarted after 14 days. These results clearly show that this technique is effective for suppressing nitratation. # 2007 Elsevier Ltd. All rights reserved. Keywords: Nitritation; Nitrite; Inhibition; Nitrification; Immobilization; Heat shock 1. Introduction Several kinds of wastewater, such as wastewater from the dewatering of digested sludge and landfill leachate, have high concentrations of ammonium but low concentrations of biodegradable organic compounds (low C/N ratio). Generally, biological nitrogen removal is carried out by a nitrification– denitrification process. However, to achieve complete biolo- gical nitrogen removal from these kinds of wastewater, the denitrification process must be supplied with an additional organic carbon source, entailing higher operating costs. Therefore, the conventional nitrification–denitrification pro- cess is not ideal for ammonia-containing wastewater with low C/N ratios. In the 1990s, a novel metabolic pathway, anaerobic ammonium oxidation (anammox), was discovered [1,2]. In the anammox reaction, ammonium is oxidized to nitrogen gas using nitrite as the electron acceptor. The anammox process can achieve high nitrogen removal rates of up to 11.5 kg N m 3 d 1 [3]. In addition, because the reaction does not require the addition of a hydrogen donor such as methanol, the amount of sludge can be reduced through denitrification. In view of these advantages, the anammox process is suitable for treating ammonia-containing wastewater with a low C/N ratio. However, because ammonium is oxidized using nitrite as the electron acceptor in the anammox reaction, a pretreatment process that produces nitrite from the influent ammonium is essential for the anammox process. There are two steps in a conventional nitrification process. First, in a reaction called ‘‘nitritation’’, ammonium is oxidized to nitrite by ammonium-oxidizing bacteria (AOB). Second, in a reaction called ‘‘nitratation’’, nitrite is oxidized to nitrate by nitrite-oxidizing bacteria (NOB). Nitritation : NH 4 þ þ 1:5O 2 ! NO 2  þ 2H þ þ H 2 OðAOBÞ Nitratation : NO 2  þ 0:5O 2 ! NO 3  ðNOBÞ Because the growth rate of AOB is low, maintaining AOB in a reactor and inhibition of the growth of NOB are both necessary in order to establish a stable nitritation process. www.elsevier.com/locate/procbio Process Biochemistry 43 (2008) 265–270 * Corresponding author. Tel.: +81 47 361 6103; fax: +81 47 361 6107. E-mail address: kazuichi.isaka.mp@hitachi-pt.com (K. Isaka). 1359-5113/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2007.12.004