Ecotoxicological Properties of Wastewater Treated Using Tertiary Methods M. Petala, 1 P. Samaras, 2 A. Zouboulis, 3 A. Kungolos, 4 G. Sakellaropoulos 1 1 Department of Chemical Engineering, Chemical Process Engineering Laboratory, Aristotle University of Thessaloniki and Chemical Process Engineering Research Institute, 6th km Harilaou Thermi Road, 57001 Thessaloniki, Greece 2 Department of Pollution Control Technologies, Technological Educational Institute of West Macedonia, 50100 Kozani, Greece 3 Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 4 Department of Planning and Regional Development, University of Thessaly, 38334 Volos, Greece Received 17 June 2005; accepted 21 March 2006 ABSTRACT: The objective of this work was the examination of the efficiency of coagulation and ozonation processes for the production of reclaimed wastewater with low toxicity. Municipal secondary effluents were treated by FeCl 3 , Al 2 (SO 4 ) 3 (alum), and a commercial substance at metal ion concentrations of 0.5 and 1 mmol/L. Alternatively, the effluents were treated by ozonation in a semibatch ozone reactor. The feed gas was introduced at a flowrate of 3 L/min containing ozone at various concentrations, ranging between 2.5 and 8 mg/L; ozone residence times were 2, 5, 15, and 30 min. The toxic effects of the advanced treated effluents were examined by a battery of tests using the marine bacteria Vibrio fischeri, the freshwater crustaceans Daphnia magna, Daphnia pulex, and Thamnocephalus platyurus, and the roti- fers Brachionus calyciflorus. The addition of alum decreased the toxic effect of reclaimed wastewater on immobilization of D. pulex, from 90 to 60%. Ozonation was also effective for toxicity removal, which decreased to 25% effect on D. pulex after treatment by 2.5 mg O 3 /L for 2 min. However, acute toxic effects after ozonation, were observed on V. fischeri and were related to ozone gas concentration and contact time. At the highest ozone dosage, the toxicity reached almost 100% inhibition of biolumines- cence after 15 min. The toxicity of the ozonated effluents to bacteria decreased with sample storage time and was almost negligible after 48 h, indicating that the potential adverse effect of reclaimed wastewaters on receiving waters might be reduced by storage for a certain time. # 2006 Wiley Periodicals, Inc. Environ Toxi- col 21: 417–424, 2006. Keywords: wastewater; reclamation; coagulation/flocculation; ozonation; toxicity; microbiotests INTRODUCTION Wastewaters are usually subjected to biological treatment and the quality of the treated effluents is assessed mainly by physicochemical parameters. Although in several cases the effluents of such plants fulfill the permission limits set by regional regulations, it has been reported that these efflu- ents may cause adverse effects on several organisms of aquatic biota (Dizer et al., 2002; Pessala et al., 2004). The physicochemical characteristics did not reflect the potential hazard of the effluent, revealing that short-term biotests could be a practical, cost-effective, and reliable toxicity- Correspondence to: P. Samaras; e-mail: psamaras@kozani.teikoz.gr Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/tox.20188  C 2006 Wiley Periodicals, Inc. 417