Effect of hexavalent chromium on the activated sludge process and on the sludge protozoan community P. Samaras a, * , C.A. Papadimitriou b , D. Vavoulidou c , M. Yiangou c , G.P. Sakellaropoulos b a Department of Pollution Control Technologies, Technological Educational Institute of W. Macedonia, Koila, 50100 Kozani, Greece b Chemical Process Engineering Research Institute and Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 1520, 54 124 Thessaloniki, Greece c Department of Genetics, Development and Molecular Biology, School of Biology, Aristotle University of Thessaloniki 54 124 Thessaloniki, Greece article info Article history: Received 23 January 2008 Received in revised form 22 May 2008 Accepted 23 May 2008 Available online 23 July 2008 Keywords: Activated sludge Chromium Protozoan community Nutrient removal SBI abstract The objectives of this study were the determination of chromium effects to the performance of an acti- vated sludge unit and the investigation of the response of the activated sludge protozoan community to Cr(VI). Two bench scale activated sludge reactors were supplied with synthetic sewage containing Cr(VI), at concentrations from 1 up to 50 mg L À1 . Protozoan species were identified and were related to the sys- tem efficiency. Variations in the abundance and diversity of the protozoan species were observed under various chromium concentrations. High removal rates of organics and nutrients were observed after the acclimatization of the activated sludge, which were related to the initial chromium(VI) concentration. Chromium(VI) removal efficiency was high in all cases. The protistan community was affected by the influent chromium content. Dominance of sessile species was observed in the reactor receiving 5 mg L À1 influent chromium, whereas co-dominance of sessile and carnivorous species was observed in the reac- tors receiving higher chromium concentrations. Ó 2008 Published by Elsevier Ltd. 1. Introduction Activated sludge is a widely used process, based on the develop- ment of appropriate bacterial aggregates and other associated organisms in an aeration tank; these organisms are easily sepa- rated from the aqueous phase during the subsequent sedimenta- tion. Ciliated protozoa play an essential role in the whole process by removing dispersed bacteria through grazing, which otherwise may result in high turbidity effluents (Salvado and Gracia, 1993; Lee et al., 2004). The presence of toxic substances in the influent may induce changes in the whole food web of the activated sludge ecosystems affecting their activity and the performance of the wastewater treatment plant. Chromium is a common pollutant found in industrial effluents; chromium salts are extensively used in several industrial processes such as tanneries, electroplating, textile, dyeing, and metal finish- ing industries. Chromium may exist in the trivalent [Cr(III)] and hexavalent [Cr(VI)] state. Hexavalent chromium compounds (chro- mates and dichromates) are highly toxic and are considered as mutagens and carcinogens (USEPA, 1998). Due to the severe toxic- ity of Cr(VI), the Agency for Toxic Substances and Diseases Registry (ATSDR) classifies it as the top sixteenth hazardous substance (ATSDR, 2000). Indicative limits for total chromium concentrations in drinking water and reclaimed wastewater for irrigation are 0.05 (WHO, USEPA) and 0.1–1 mg L À1 , respectively (EPA, 2004). Nicolau et al. (2005) investigated the effect of copper in the acti- vated sludge protistan community of a conventional bench scale reactor and found that low concentrations of the metal, 4–8 mg L À1 , seemed to stimulate the growth of crawling species. On the other hand, the sessile Opercularia sp. was exceptionally toler- ant to copper and was related to the low quality of the activated sludge. Abraham et al. (1997a) investigated the ciliate populations in an activated sludge plant, and found significant positive and negative correlations between ciliate species and metal content. Schlenk and Moore (1994) investigated the effect of pH and time on the acute toxicity of copper sulfate to the ciliated protozoan Tet- rahymena thermophila and found that this protozoan was resistant to copper sulfate toxicity attributed to an intracellular mechanism. Madoni et al. (1996) investigated the toxic effect of heavy metals on the activated sludge protozoan community in a batch reactor and found variability between the sensitivities of the protozoan species to the tested metals. Ciliated species such as Chilodonella uncinata and Trochilia minuta showed the highest sensitivity to all studied metals, while Opercularia coarctata and Opercularia min- ima were the most tolerant species. Abraham et al. (1997a,b) inves- tigated seasonal population variations and significant interspecies correlations, and found that the major ciliate species were able to tolerate sudden rises in metal concentrations, including Fe (>2000 ppb), Zn (>500 ppb), Cu (>60 ppb) and Cr (>100 ppb). 0960-8524/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.biortech.2008.05.036 * Corresponding author. Tel.: +30 2461068023; fax: +30 2461039682. E-mail address: psamaras@kozani.teikoz.gr (P. Samaras). Bioresource Technology 100 (2009) 38–43 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech