Impact of iron salts on activated sludge and interaction with nitrite or nitrate Sarah Philips, Korneel Rabaey, Willy Verstraete * Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, B-9000 Ghent, Belgium Received 19 April 2002; received in revised form 24 September 2002; accepted 28 November 2002 Abstract Iron salts are often used in activated sludge treatment plants as coagulants or to improve reactor performance. Previous studies have indicated that iron itself has an impact on the activated sludge process. However, the interaction of iron with nitrite or nitrate present in the sludge has received little attention. In this research, the influence of addition of Fe(II) or Fe(III), alone or together with NO  2 or NO  3 on bench-scale activated sludge reactors was examined. Large differences were established between the distinct treatments, regarding reactor performance, sludge characteristics as well as microbial community. Ferric iron was more detrimental than ferrous iron. In some cases, nitrite was found to enhance inhibitory effects of the added iron, whereas nitrate had more a neutralizing effect. It was found that precipitation of phosphate by the iron was not responsible for the observed inhibition. Decrease in pH upon formation of iron hydroxides and the impairment of the floc structure could partially explain the toxicity of the iron dosages. The formation of toxic nitrogen oxides, such as nitric oxide, can also be of importance. The observed positive effect of nitrate on the floc activity is of interest and warrants further elucidation. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ferric iron; Ferrous iron; Nitrite; Nitrate; Activated sludge 1. Introduction Legislation concerning effluent quality of wastewater treatment plants becomes more and more stringent. For example, according to the European legislation (1991) phosphorous concentrations in the effluent should be decreased to below 2 mg/l. Although phosphorous can be removed biologically, chemical removal by precipi- tation or coprecipitation of non-soluble phosphates has been wide spread (Yeoman et al., 1988). Iron sulfates and chlorides are effective precipitants for phosphorus removal. Both ferric (Fe(III), e.g. FeCl 3 , FeClSO 4 ) as ferrous (Fe(II), e.g. FeSO 4  7H 2 O) iron salts are used. Next to phosphorous removal, iron salts are also added to improve settling properties (Bowen and Dempsey, 1992) or for improving general plant performance, by the removal of organic compounds (e.g. humic sub- stances) (Lefebvre and Legube, 1990) and other nutri- ents (Crozes et al., 1995). Chemicals can be added to the wastewater treatment plant at different stages, classified as pre-precipitation, coprecipitation and post-precipitation. In this research, attention focuses on the coprecipitation. As in that case the iron salt is added directly to the ML in the aeration basin, effects on the microbial community are inevitable. Iron is both a nutrient and a toxin to microorgan- isms, due to the formation of reactive oxygen species (De Freitas and Meneghini, 2001). In addition, iron ions have been known to react, both chemically and biologically, with other compounds possibly present in the activated sludge, which could in turn affect the sludge biology. Ferrous iron is very important to the stability of nitrite (Brons et al., 1991; Van Cleemput and Samater, 1996). The reaction of Fe(II) with nitrite, chemodenitrification, produces ferric iron and nitric oxide (NO), which is toxic to microorganisms. Nitrate also serves as electron acceptor in the oxidation of Fe(II) to Fe(III) under anoxic conditions. Biologically, Fe(II) can be oxidized to Fe(III) with nitrite or nitrate as electron acceptor under anoxic conditions at neutral pH. According to Straub et al. (1996) and Nielsen and Nielsen (1998), this microbial * Corresponding author. Tel.: +32-9-264-5976; fax: +32-9-264-6248. E-mail address: willy.verstraete@rug.ac.be (W. Verstraete). 0960-8524/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0960-8524(02)00314-0 Bioresource Technology 88 (2003) 229–239