Short communication Empirical model of the pH dependence of the maximum specific nitrification rate Seongjun Park a , Wookeun Bae a, * , Jinwook Chung b , Seung-Cheon Baek a a Department of Civil & Environmental Engineering, Hanyang University, Sa 1-Dong, Ansan, Gyeonggi-Do, Republic of Korea b R&D Center, Samsung Engineering Co. Ltd., 39-3 Sungbok-Dong, Yongin, Gyeonggi-Do, 449-844, Republic of Korea Received 19 April 2007; received in revised form 19 September 2007; accepted 26 September 2007 Abstract A bell-shaped empirical model was proposed to describe the pH-dependent behavior of the maximum specific substrate utilization rates (MSSUR, ˆ q pH ) of ammonium and nitrite oxidations. The model was developed using a cosine function. The model satisfactorily captured the bell- shape decrease of the MSSUR as the pH deviated from the optima. Analysis of 7 experimental data sets gave that the optimum pH (pH opt ) lied in the range of 8.2  0.3 for ammonium oxidizing bacteria (AOB) and 7.9  0.4 for nitrite oxidizing bacteria (NOB). The pH range (w) within which the MSSUR maintained above a half of the greatest MSSUR ( ˆ q max ) was 3.1  0.4 for AOB and 2.2  0.4 for NOB, which means that NOB is more sensitive to pH changes. Since the pH opt and the pH range (w) for NOB were smaller than those for AOB, it was inferred that a selective enrichment of AOB and consequent nitrite accumulation could be possible at an alkaline pH near or greater than the pH opt of AOB. # 2007 Elsevier Ltd. All rights reserved. Keywords: Ammonium oxidizing bacteria (AOB); Empirical model; pH; Maximum specific substrate utilization rate; Nitrite oxidizing bacteria (NOB) 1. Introduction The pH drops significantly during nitrification. For instance, a complete nitrification of 5 mM of ammonium (70 mg N/L) produces 10 mM of proton that could give a pH value as low as 2 unless the system is buffered. Although the pH change is not so drastic in actual treatment systems, it is not uncommon that a pH value above 8 drops to below 6 in many batch nitrification reactors. The pH critically affects the nitrite accumulating processes, such as single reactor high activity ammonium removal over nitrite (SHARON) [1] and shortcut biological nitrogen removal (SBNR) [2–6]. The nitrite accumulating processes are advantageous because it can save the demand of oxygen in nitrification and external carbon source in denitrification [3,5], or provides reactants (ammonium and nitrite) for anaerobic ammonium oxidation (ANAMMOX) [7]. A key factor for successful nitrite accumulation is a selective suppression of nitrite oxidation, which is achieved at certain concentration of free ammonia (NH 3 , FA) (1–10 mg NH 3 /L) [8,9], or free nitrous acid (HNO 3 , FNA) (approximately 0.011–0.10 mg HNO 2 -N/L) [10,11]. The formation of FA is favorable at a basic pH, whereas that of FNA at an acidic pH. High nitrite accumula- tions were commonly observed at pH 8–9 [3,9] which is significantly basic compared to common biological treatment systems. The pH affects the maximum specific substrate utilization rate (MSSUR). Since the previous models such as WRC [12], EPA US [13], Siegrist and Gujer [14], etc. (refer to [15]) focus on ammonium oxidizing bacteria (AOB) and neutral to acidic conditions (which are, in fact, most common in conventional nitrification systems), those models fail to describe the rate of nitrification at a significantly basic environment. Because an alkaline pH above 9 is unavoidable in high-strength ammonium wastewater treatments [2–5,16], a proper prediction of ˆ q under basic conditions is essential. In addition, those models do not address the nitrite oxidation by the nitrite oxidizing bacteria (NOB). Flora et al. [17,18] modified the Siegrist and Gujer’s model and attempted to explain the nitrification in biofilm by both AOB and NOB. Still their model for AOB focused on neutral to acidic conditions only, while the model for NOB focused on neutral to alkaline conditions. However, www.elsevier.com/locate/procbio Process Biochemistry 42 (2007) 1671–1676 * Corresponding author. Tel.: +82 31 400 4030; fax: +82 31 407 0697. E-mail addresses: jayenv93@chol.com (S. Park), wkbae@hanyang.ac.kr (W. Bae), jin-wook.chung@samsung.com (J. Chung), baek.seungcheon@erm.com (S.-C. Baek). 1359-5113/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2007.09.010