0.1 1 10 100 0.1 1 10 Lundtofte STP effluent Lynetten STP effluent time (min) [ClO 2 ] mg/L Figure 1: Concentration profile of ClO 2 added to the two effluents. Treatment of estrogenic chemicals in biologically treated sewage by chlorine dioxide oxidation Andersen H. R. 1 & Ledin A. 1 1 Department of environmental engineering, Technical University of Denmark, Denmark Key words: Chlorine dioxide, estrogens, sewage, cost of treatment Introduction Ozonation of effluents from wastewater treatment plants (WWTPs) has recently emerged as a promising polishing treatment for biologically treated municipal wastewater to remove bioactive xenobiotic micropollutants like estrogens, xeno-estrogens and pharmaceuticals (Huber et al., 2005a). The cost of this is acceptable for large sewage treatment plants (Ried et al., 2007). However, in smaller treatment plants (e.g. less than 10.000 person equivalents) treatment cost is strongly influenced by the high initial investment cost of the ozone generator, reaction tank and safety equipment required (Reid et al., 2007). It is known that chlorine dioxide reacts similar to ozone with some estrogens and pharmaceuticals (Huber et al., 2005b), but since the cost of producing ClO 2 is higher than the cost for the same amount of ozone, this has not been considered as a treatment alternative for wastewater. ClO 2 is considerably easier to handle and the commercially available ClO 2 generators are markedly less expensive than ozone generators compared on production capacity. Therefore it was worth investigating the use of ClO 2 as replacement of ozone for smaller WWTPs. In this study the removal efficiency by chlorine dioxide of some estrogenic compounds in biologically treated sewage from two WWTPs was investigated. The compound investigated included parabens, industrial phenols, sunscreen chemicals, and steroid estrogens. Materials and methods The investigated chemicals were some parabens [methyl 4-hydroxybenzoate (MP), ethyl 4- hydroxybenzoate (EP), propyl 4-hydroxybenzoate (PP), isoButyl 4-hydroxybenzoate (isoBP), butyl 4- hydroxybenzoate (BP)], some industrial phenols [bisphenol A (BPA), 4-isononylphenol (isoNP), 4-tert- octylphenol (OP)], sun screen agents [2-ethylhexyl trans-4-methoxycinnamat (OMC), 2-hydroxy-4- methoxybenzophenon (BP-3), 5-Chloro-2-hydroxybenzophenone (BP-7), homosalate (HMS), 2- ethylhexyl 4-(dimethylamino)benzoate (OD-PABA), 3-(4’-methylbenzylidene)camphor (4-MBC)], and three steroid estrogens [estrone (E1), 17β-estradiol (E2), 17α-ethynyl estradiol (EE2)]. Biologically treated sewage from two different wastewater treatment plants, Lundtofte WWTP and Lynetten WWTP, were spiked with the investigated chemicals to a concentration of each xeno- estrogen and steroid estrogens of 1.0 μg/L and 40 ng/L, respectively. Treatment experiments were performed to characterise the kinetic of ClO 2 consumption by oxidizable material in wastewater and the kinetic of removal of estrogenic chemicals. Concentrations of the investigated estrogenic chemicals and estrogenic potency were quantified by SPE followed by GC-MS- MS and YES-assay analysis. Results and Discussion Concentration profiles of ClO 2 consumption for both effluents (figure 1) shows an initially a rapid consumption of ClO 2 around 4.5 mg/L within the first 30 seconds followed by a slower removal in the next 120 min. The effluent from Lynetten consumed ClO 2 faster than the effluent from Lundtofte which is consistent with that it has higher a COD of ~40 mg/L versus ~15 mg/L. It can be calculated that the very fast initial reaction with ClO 2 in the first 0.5 min after addition represents a very small fraction of the theoretical oxidant demand of the water around 1.2 % and 3.5 % respectively for Lynetten and Lundtofte STP effluents. Concentrations of steroid estrogens after treatment with different doses of ClO 2 in the effluents are shown in figure 2. In Lundtofte STP effluent steroid estrogens were removed to about 50% already by