Use of Microtox® and Ceriodaphnia bioassays 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, C18 solid-phase extraction columns, and air stripping. Microtox required less time than Ceriodaphnia dubia bioassay 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 of C. dubia, were nontoxic to Microtox. Diazinon was tentatively identified as one of the causative toxicants present. Its LC50 to C. dubia (0.5 μg/L) is within the range of concentrations detected (0.1–0.6 μg/L), whereas the EC50 of diazinon to Microtox is much higher (> 18,000 μg/L).