Kinetics of nitrate and perchlorate removal and biofilm stratification in an ion exchange membrane bioreactor Ana R. Ricardo a , Gilda Carvalho a,b , Svetlozar Velizarov a , Joa ˜o G. Crespo a , Maria A.M. Reis a, * a REQUIMTE/CQFB, Department of Chemistry, FCT, Universidade Nova de Lisboa, Campus de Caprarica, P-2829-516 Caparica, Portugal b Instituto de Biologia Experimental e Tecnolo ´gica (IBET), Av. da Repu ´ blica (EAN), 2784-505 Oeiras, Portugal article info Article history: Received 4 December 2011 Received in revised form 18 April 2012 Accepted 22 May 2012 Available online 13 June 2012 Keywords: Ion exchange membrane bioreactor Drinking water Nitrate and perchlorate Competitive inhibition Biofilm stratification FISH abstract The biological degradation of nitrate and perchlorate was investigated in an ion exchange membrane bioreactor (IEMB) using a mixed anoxic microbial culture and ethanol as the carbon source. In this process, a membrane-supported biofilm reduces nitrate and perchlorate delivered through an anion exchange membrane from a polluted water stream, containing 60 mg/L of NO 3  and 100 mg/L of ClO 4  . Under ammonia limiting conditions, the perchlorate reduction rate decreased by 10%, whereas the nitrate reduction rate was unaffected. Though nitrate and perchlorate accumulated in the bioreactor, their concen- trations in the treated water (2.8  0.5 mg/L of NO 3  and 7.0  0.8 mg/L of ClO 4  , respectively) were always below the drinking water regulatory levels, due to Donnan dialysis control of the ionic transport in the system. Kinetic parameters determined for the mixed microbial culture in suspension showed that the nitrate reduction rate was 35 times higher than the maximum perchlorate reduction rate. It was found that perchlorate reduction was inhibited by nitrate, since after nitrate depletion perchlorate reduction rate increased by 77%. The biofilm developed in the IEMB was cryosectioned and the microbial population was analyzed by fluorescence in situ hybridization (FISH). The results obtained seem to indicate that the kinetic advantage of nitrate reduction favored accumulation of denitrifiers near the membrane, whereas per(chlorate) reducing bacteria were mainly positioned at the biofilm outer surface, con- tacting the biomedium. As a consequence of the biofilm stratification, the reduction of perchlorate and nitrate occur sequentially in space allowing for the removal of both ions in the IEMB. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction Perchlorate (ClO 4  ) contamination of surface and ground water is a relevant problem due to its negative impact on human health, particularly on the thyroid gland (Greer et al., 2002). The main source of contamination is military facilities, where synthetically manufactured ammonium perchlorate was used as a rocket fuel (Srinivasan and Sorial, 2009). The US Envi- ronmental Protection Agency (EPA) recommends a maximum of 15 mg/L of ClO 4  in drinking water sources (USEPA, 2011). The most common technology for perchlorate removal from drinking water is ion exchange (Srinivasan and Sorial, 2009). However, the presence of other anions in higher concentrations, e.g. nitrate and sulphate, can reduce the resin * Corresponding author. Tel.: þ351 212 948 385; fax: þ351 212 948 550. E-mail address: amr@fct.unl.pt (M.A.M. Reis). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 4556 e4568 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.05.045