Simultaneous sulfide and nitrate removal in anaerobic reactor under shock loading Cai Jing a , Zheng Ping a, * , Qaisar Mahmood b a Department of Environmental Engineering, Zhejiang University Hangzhou, Hangzhou, China b Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad, Pakistan article info Article history: Received 12 September 2008 Received in revised form 22 December 2008 Accepted 23 December 2008 Available online 23 February 2009 Keywords: Anaerobic sulfide and nitrate removal Substrate shock loading Operation performance abstract The performance of anaerobic reactor for simultaneous sulfide and nitrate removal under substrate shock loading was studied. The response to the shock loading could be divided into three stages i.e. disturbance, inertial and recovery periods. The effect of the shock loading was directly proportional to the intensity of the shock loads. The reactor performance was stable at a relatively lower intensity (1.5 times shock load), while it was considerably affected by higher intensity (higher than 2.0 times shock load). Nevertheless, the reactor performance recovered from disturbances at all the tested shock loads. The effluent sul- fide-sulfur concentration was found as sensitive parameter, which increased up to 18 times of that at steady state; it could be used as an indicator of the reactor’s performance. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction A number of industries such as petrochemical plants, tanneries, viscose rayon factories etc., generate sulfides containing waste streams. Various toxicological effects of sulfide on human health have been described elsewhere (Mahmood et al., 2007a). Large vol- umes of wastewaters containing nitrogenous compounds are pro- duced from industry, agriculture and housing settlements. Untreated discharge of these wastewaters to the environment can lead to serious environmental problems. Nitrate or nitrite can induce multidimensional hazards, such as the eutrophication of water bodies (Zheng et al., 2004). As alternatives for oxygen, nitrate and nitrite can be used to control sulfide generation during treatment of S-containing waste- waters (Cirne et al., 2008). Nitrate and nitrite are usual constitu- ents of many wastewaters, or can be generated separately, in a nitrification reactor. Compared to oxygen, nitrate and nitrite have the advantage of being highly soluble. This means that their use does not require applying an external gas flow and, consequently, there will be less stripping of gaseous sulfide. Reduction of nitrate and nitrite follows either one of two possible mechanisms, denitri- fication or ammonification, yielding nitrogen gas or ammonia, respectively (Cirne et al., 2008). The feasibility of using nitrate and nitrite as electron acceptors for sulfide oxidation has been demonstrated in several reactor studies (Kleerebezem and Meen- dez, 2002; Reyes-Avila et al., 2004; Sierra-Alvarez et al., 2005; Vai- opoulou et al., 2005; Cardoso et al., 2006; Gadekar et al., 2006; Lau et al., 2006; Cai et al., 2007; Mahmood et al., 2007a). A common feature of these studies was that the reaction was studied under fully anoxic conditions and with sulfide as a main, usually the sole, electron donor. In many of these studies, the concentration of ni- trate relative to that of sulfide determined whether sulfide was oxidized to elemental sulfur or sulfate (Cirne et al., 2008). Wastewater treatment processes are subjected to variations in one or more operational parameters that affect the reactor perfor- mance, such as influent concentration and flow rate. Though some of these variations can be predicted or controlled; the reactor’s per- formance would still be deteriorated due to extreme disturbances. Literature review suggested that a few reports highlighted the role of substrate shock load in affecting the hydrogen sulfide oxidation (Wani et al., 2008). The present study was designed to elucidate the effect of shock loading on the simultaneous sulfide and nitrate removal in anaerobic reactor. 2. Methods 2.1. Inoculum and enrichment of microbial communities Inoculum was taken from the anaerobic methanogenic reactor of Dengta wastewater treatment plant (WWTP) located in Hangz- hou City of China. Its total solids (TSs) and volatile suspended sol- ids (VSSs) were 95.03 g l 1 and 68.68 g l 1 , respectively, with VSS/ TS ratio of 0.72. The anoxic sulfide oxidizing reactor was operated under autotrophic conditions using sulfide as electron donor and nitrate as electron acceptor to accomplish denitrification. For the initial one month the reactor was fed with synthetic wastewater in order to acclimatize the bacteria to the new substrates and en- rich the sludge. 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.12.041 * Corresponding author. Tel.: +86 571 86971709; fax: +86 571 86949320. E-mail address: pzheng@zju.edu.cn (Z. Ping). Bioresource Technology 100 (2009) 3010–3014 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech