Nitrification resilience and community dynamics of ammonia-oxidizing bacteria with respect to ammonia loading shock in a nitrification reactor treating steel wastewater Kyungjin Cho, 1 Seung Gu Shin, 2 Joonyeob Lee, 2 Taewoan Koo, 2 Woong Kim, 3 and Seokhwan Hwang 2, * Center for Water Resource Cycle Research, Korea Institute of Science and Technology, 39-1 Hawolgok-Dong, Seongbuk-Gu, Seoul 136-791, Republic of Korea, 1 School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea, 2 and Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea 3 Received 3 November 2015; accepted 20 January 2016 Available online xxx The aim of this study was to investigate the nitrification resilience pattern and examine the key ammonia-oxidizing bacteria (AOB) with respect to ammonia loading shocks (ALSs) in a nitrification bioreactor treating steel wastewater. The perturbation experiments were conducted in a 4-L bioreactor operated in continuous mode with a hydraulic retention time of 10 d. Three sequential ALSs were given to the bioreactor (120,180 and 180 mg total ammonia nitrogen (TAN)/L. When the first shock was given, the nitrification process completely recovered after 14 d of further operation. However, the resilience duration was significantly reduced to w1 d after the second and third ALSs. In the bioreactor, Nitro- somonas aestuarii dominated the other AOB species, Nitrosomonas europaea and N. nitrosa, throughout the process. In addition, the population of N. aestuarii increased with ammonia utilization following each ALS; i.e., this species responded to acute ammonia overloadings by contributing to ammonia oxidation. This finding suggests that N. aestuarii could be exploited to achieve stable nitrification in industrial wastewaters that contain high concentrations of ammonia. Ó 2016, The Society for Biotechnology, Japan. All rights reserved. [Key words: Nitrification; Ammonia loading shock; Steel wastewater; Quantitative PCR; Denaturing gradient gel electrophoresis] Nitrogen compounds in wastewater must be removed because they cause environmental problems including dissolved oxygen depletion, eutrophication, odor, ammonia toxicity, and ground- water contamination (1,2). Biological nitrogen removal (BNR) involving nitrification and denitrification has been still adopted in wastewater treatment because it is inexpensive and causes little environmental damage, in contrast to physico-chemical treatments (3,4). Nitrification, the first step in BNR, involves two processes: ammonia oxidation to nitrite by ammonia oxidizing bacteria (AOB); and nitrite oxidation to nitrate by nitrite oxidizing bacteria (NOB) (5). Ammonia oxidation is thought to be the rate-limiting step in nitrification, because AOB have lower growth rates than NOB, and are more sensitive to inhibition by environmental factors (6,7). Therefore, to establish the stable nitrification reaction in waste- water treatment process the dynamics of AOB in response to the operating conditions must be understood. The substrate consistency is one of the important factors to ensure the process stability of biological system. However, sub- strate concentrations often fluctuate in full-scale wastewater treatment plants (8). Substrate overload reduces microbial activity, which results in the poor removal efficiency (9). Similarly, ammonia overload resulting from variations in substrate content can cause failure of ammonia oxidation in nitrification system. AOB are much more sensitive to substrate concentration rather than are hetero- trophic bacteria grown in wastewater treatment plants. For example, AOB can be inhibited only 1.0 mM of free ammonia (FA) concentration, although some heterotrophic bacteria can efficiently grow at 1.0% of glucose (55.5 mM) (10e12). The steel manufacturing industry produces large amounts of steel wastewater (SWW) that contains a high concentration of ammonia (>100 ppm) and inorganic salts as a byproduct of a process (13e15). In our survey, SWW has 6.4  pH  8.9 (14e17). This high concentration of ammonia can often cause ammonia overloading shock (ALS) to the BNR system, thus reducing its nitrification efficiency. In some SWW treatment plants, the high strength of ammonia wastewater used introduced into the nitrifi- cation system can change suddenly, thereby causing a drastic in- crease of ammonia concentration. Therefore, for nitrification of SWW to be successful, the effects of ALS on the nitrification resil- ience of the AOB community must be determined. Certain AOB species appear to adapt to BNR systems that are subject to ALS, and become the dominant species in the AOB communities (18,19). Hence, microbial community resilience following ammonia overloading can contribute to accelerating nitrification step in full-scale steel wastewater treatment facilities; however only a few studies have been conducted regarding the process resilience under ALS. For example, activated sludge pre- exposed to high ammonia level had higher resistance to ammonia than did un-acclimated activated sludge (20), and the dominant AOB community was changed after the ammonia * Corresponding author. Tel.: þ82 54 279 2282; fax: þ82 54 279 8299. E-mail address: shwang@postech.ac.kr (S. Hwang). www.elsevier.com/locate/jbiosc Journal of Bioscience and Bioengineering VOL. xx No. xx, 1e7, 2016 1389-1723/$ e see front matter Ó 2016, The Society for Biotechnology, Japan. All rights reserved. http://dx.doi.org/10.1016/j.jbiosc.2016.01.009 Please cite this article in press as: Cho, K., et al., Nitrification resilience and community dynamics of ammonia-oxidizing bacteria with respect to ammonia loading shock in a nitrification reactor..., J. Biosci. Bioeng., (2016), http://dx.doi.org/10.1016/j.jbiosc.2016.01.009