Detoxification rates of chlorinated solutions as determined by the half‐life of biological activity withDaphnia magna

Abstract

AbstractData are presented that discourage the use of total residual chlorine (TRC) and free residual chlorine (FRC) levels in setting water quality criteria for chlorine and chlorinated carrier molecules. Repetitive static acute toxicity tests for aging solutions of chlorinated complexes withDaphnia magnapermit generation of biological half‐lives for direct comparison with analytically derived concentrations of TRC and FRC. The results indicate a strong similarity in the decline of toxic effects onD. magnaand TRC for an aging solution of 1,3‐dichloro‐5,5‐dimethylhydantoin (DCDMH), although no such similarity exists between these two measures for aging solutions of chlorine and trichloro‐S‐triazinetrione (TCTT). Repetitive toxicity tests with aging DCDMH generated toxic decay curves with coefficients of determination (r2) exceeding 73% while exhibiting a minimum decay rate of ‐0.10. TRC data for DCDMH exhibited anr2of 93% and a decay rate of ‐0.07. Ther2values for the chlorine series ranged from 15 to 60% for the toxic effects decay curve, as compared with 76% for analytically derived data. The TCTTr2values for these same parameters were 23 to 64 and 84%, respectively. Both chlorine and TCTT presented definable, consistent decreases in TRC levels over the course of the studies, while the observed biological effects oscillated widely from test to test within a series. These results demonstrate that the standard reported parameters of TRC and FRC are inadequate for setting effluent limitations on chlorinated discharges because they are inconsistent in measuring the toxic constituents of these solutions.