Organotin compounds in municipal wastewater and sewage sludge: contamination, fate in treatment process and ecotoxicological consequences

Organotin compounds find various applications in industry and agriculture, which may lead to contamination of municipal wastewater and sewage sludge. Here, an overview on the contamination of these media is given, and the behavior of organotins in the treatment process is described. In raw municipal wastewater of the city of Zürich, Switzerland, mono- (MBT), di- (DBT) and tributyltin (TBT) were detected in the range of 140–560, 130–1030, and 60–220 ng/l, respectively. These compounds were primarily associated with suspended particulates, and transferred from wastewater to the sludge in the primary clarifier of the treatment plant. Both aerobic and anaerobic degradation were found to be insignificant. This has also been shown by a laboratory-scale sludge treatment system. After secondary and tertiary treatment, organotin concentrations were in the range of < 1–17 ng/l in the effluent of the plant. In digested sludge, concentrations of MBT, DBT and TBT were in the range of 0.3–0.8, 0.5–1.0 and 0.3–1.0 mg/kg (dry weight), respectively, in 1988–1990. A survey in 1995 in 25 treatment plants showed a similar contamination pattern with averages ± S.E.M. of 0.5 ± 0.2 mg/kg MBT, 1.5 ± 0.5 mg/kg DBT and 1.1 ± 0.4 mg/kg TBT. In seven sludges, phenyltins were found with averages of 0.1 ± 0.04 mg/kg monophenyltin, 0.1 ± 0.06 mg/kg diphenyltin and 0.5 ± 0.2 mg/kg triphenyltin. The ecotoxicological consequences of organotin-polluted wastewater and sludge should be regarded concerning both the discharge of wastewaters into aquatic systems and the use of digested sludge as a soil amendment. Adverse effects on the most sensitive aquatic biota (gastropods) in receiving waters were shown at the concentrations found in plant effluents. The ecotoxicological implications of sludge derived organotin pollution on soils are not well understood, but bioaccumulation of these compounds in the terrestrial food web may occur.