This study examines the biodegradation of 3,3′,4′,5-tetrachlorosalicylanilide (TSCA) by activated sludge and investigates its effects to different aquatic organisms. According to the experimental results, the fate of the target compound in activated sludge systems is governed by the mechanisms of sorption and biodegradation, while almost 90% of TCSA is expected to be removed in an aerobic activated sludge system operating with hydraulic residence time of 10 h. Ecotoxicity experiments shown that TSCA toxicity decreased from Daphnia > Vibrio > Lemna, while an ecological threat is possible in rivers where treated wastewater is diluted up to 100-fold.