Characterization of the acute and chronic toxicity of effluent samples from the Patapsco Wastewater Treatment Plant (WWTP), Baltimore, Maryland, was performed approximately twice weekly from April 1986 to January 1987. Acute toxicity of both the primary and secondary effluents was evaluated using Ceriodaphnia dubia, Mysidopsis bahia, and Microtox [a luminescent marine bacteria Photobacterium phosphoreum (Beckman Instruments, Inc.)]; chronic toxicity was evaluated using the C. dubia seven-day chronic test. The data indicate that C. dubia was the most sensitive indicator of acute toxicity for both primary and secondary effluent wastewaters. The acute responses for M. bahia and Microtox were considerably less sensitive than those for C. dubia. The 48-h median lethal concentration (LC₅₀) values for C. dubia ranged from 0.5 to 23.4% secondary effluent and 0.1 to 6.5% primary effluent wastewaters. Microtox 5-min median effective concentration (EC₅₀) values ranged from 9.4 to 100% secondary effluent and 0.6 to 19.5% primary effluent, respectively. _{M. bahia} 96-h LC₅₀ values were 8.9 to 100% secondary effluent and 14.8 to 35.4% primary effluent, respectively. The seven-day C. dubia chronic values (ChV) for the primary and secondary effluents were only slightly lower than the acute responses, ranging from <0.1 to 9.4% secondary effluent and <0.1 to 6.5% primary effluent, respectively.

As an additional characterization of the plant's treatment process effectiveness, influent and effluent samples were fractionated in an attempt to identify specific components of acute toxicity entering and leaving the WWTP. As a result of the fractionation study, nonpolar organic compounds appeared to be the major toxic component of the samples fractionated. While identification of the specific compounds causing toxicity in the nonpolar organic fractions was not possible, the octanol/water partition coefficient (Log P) values for the most toxic fractions were estimated to be approximately 6 to 9 in the summer samples and approximately 3.6 to 8.3 in the winter samples. Cations, anions, and polar organic compounds do not appear to be major contributors to the toxicity of the effluent samples fractionated. The percentage of toxicity reduction across the plant was approximately 50 to 90% for the three species tested.