Abstract
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, Australia. Effluents from 18 sewage treatment plants were evaluated for acute and 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) accounted for toxicity in the majority of samples. Ammonia also contributed to toxicity in one sample, an unidentified OP was responsible for toxicity in another sample, and unidentified transient toxicant(s) were present in four samples. Transient toxicity was not due to surfactants (methylene blue active substances, cobalt thiocyanate active substances, or nonyl‐ or octylphenolethoxylates) or to a polymer used to dewater the sludge. The most likely cause of the transient toxicity was chlorpyrifos, which rapidly dissipated when stored in polyethylene containers. Only two effluent samples reduced the cell numbers of S. capricornutum. In both cases, toxicity dissipated too rapidly to identify its cause.