Whole effluent toxicity of sewage treatment plants in the Hawkesbury-Nepean watershed, New South Wales, Australia, to Ceriodaphnia dubia and Selenastrum capricornutum

This paper describes the results of whole effluent toxicity tests conducted with Ceriodaphnia dubia and Selenastrum capricornutum on sewage treatment plant effluents in the Hawkesbury-Nepean watershed in New South Wales, Aust ralia, Effluents from 18 sewage treatment plants were evaluated for acute a nd chronic toxicity. Toxicity identification evaluations were performed on toxic samples to determine the cause of toxicity. Fifteen of the facilities sampled exhibited acute or chronic toxicity to C. dubia. Organophosphorus pesticides (OPs) (e.g., diazinon, chlorpyrifos, and chlorfenvinphos) accoun ted for toxicity in the majority of samples. Ammonia also contributed to to xicity in one sample, an unidentified OP was responsible far toxicity in an other sample, and unidentified transient toxicant(s) were present in four s amples. Transient toxicity was not due to surfactants (methylene blue activ e substances, cobalt thiocyanate active substances, or nonyl- or octylpheno lethoxylates) or to a polymer used to dewater the sludge. The most likely c ause of the transient toxicity was chlorpyrifos, which rapidly dissipated w hen stored in polyethylene containers. Only two effluent samples reduced th e cell numbers of S. capricornutum. In both cases, toxicity dissipated too rapidly to identify its cause.