Use of Microtox® andCeriodaphniabioassays in wastewater fractionation

Collection system and nonchlorinated secondary effluent samples from a large municipal wastewater system were fractionated using a scheme that included filration, EDTA treatment, C18solid‐phase extraction columns, and air stripping. Microtox required less time thanCeriodaphnia dubiabioassay for determining the toxicity of the numerous test samples generated by the fractionation procedure. Its usefulness was limited to collection system samples, however. Secondary effluent samples, which caused significant mortality ofC. dubia, were nontoxic to Microtox. Diazinon was tentatively identified as one of the causative toxicants present. Its LC50toC. dubia(0.5 μg/L) is within the range of concentrations detected (0.1–0.6 μg/L), whereas the EC50of diazinon to Microtox is much higher (> 18,000 μg/L).