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Toxicity to microorganisms

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Toxicity of CBS to activated sludge was evaluated by Bayer AG (1989) according to the ISO 8192 -1986/B yielding an EC50 value of > 10000 mg/l after 3 hours exposure at a pH 7.2 - 8.1 and temperature of 20.6 - 22.8 °C.

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In sewage treatment plant, large amount of test substance might be present in both dissolved and undissolved form. To represent a more realistic situation, the tested concentrations are much higher than the water solubility of the test substance (i.e. 10.3 mg/l). Therefore the reported concentrations in Bayer AG study (1989) represent the combined effect of dissolved and undissolved test substance on the activity of microorganism in the sewage treatment plant.

CBS can be rapidly degraded by hydrolysis and benzothiazole (BT) is the main degradation product appearing during the time frame of acute tests. In a respiration inhibition test using activated sludge according to OECD 209, a 3h-EC50 of 650 mg/l was obtained (Yoshioka et al., 1986) for BT. A similar test according to a national guideline resulted in a 3h-EC50 of 635 mg/l and an EC10 of 274 mg/l (Bayer AG, 1990). A nitrification inhibition test was performed with Nitrosomas isolated from wastewater. NH4 removal was analysed in a model of sediment columns charged with quartz sand. Effects were found with BT concentration of 0.2 mg/l (Reemtsma, 1994). Results from 2 growth inhibition tests for Tetrahymena pyriformis are available. EC50 values of 135 mg/l (Yoshioka et al., 1986) and 160 mg/l (Schultz, 1997) were obtained. Some further data are available for the two other main metabolites from CBS, like MBT and MBTS. Toxicity of MBT to activated sludge was evaluated by Bayer AG (1990) according to the ISO 8192 -1986/B yielding an EC50 of 3301 mg/l and an EC10 of 279 mg/l after 3 hours exposure at a pH 7.5 - 8.2 and temperature of 19.6 - 20 °C. Tomlinson (1966) studied the inhibition of the first nitrification step (oxidation of NH4 to NO2) and obtained after 2-4 h exposure EC75 values of 3mg/l for non-adapted sludge. With the protoza Tetrahymena pyriformis a 24-hour EC50 of 10 mg/l related to growth rate was determined for MBT (Yoshioka, 1985). A large number of experimental results were reviewed already in the appendix I of EU-risk assessment of CBS (95-33-0), which reveal the effect concentrations ranging from 0.25 mg/l for Nitrosomas (Reemtsma, 1994) to 500 mg/l for Mycobacterium tuberculosis (Foltinova Blöckinger, 1970). In a respiration inhibition test of MBTS using activated sludge similar to OECD 209, no effects were observed in concentrations up to 10 g/l after 3 h of incubation (Bayer AG, 1988). Tomlinson (1966) studied the inhibition of the first nitrification step (oxidation of NH4 to NO2) and obtained a 4 h-EC75 value of 38 mg/l.