A perspective on the environmental risk of halogenated by-products from uses of hypochlorite using a whole effluent toxicity based approach

An assessment has been made of the potential for toxic effects to aquatic life that may arise from halogenated organic by-products formed by reaction of hypochlorite with organic matter in various use scenarios as a contribution to the risk assessment of sodium hypochlorite under the European Union Existing Chemicals legislation. In the study, samples that would represent worst-case models for effluents containing halogenated by-products from a range of uses of hypochlorite were prepared by chlorinating and then dechlorinating (removing residual chlorine) raw settled sewage (C/D-RSS). This sample was then compared to raw settled sewage that had not been treated (RSS) to assess whether halogenated by-products formed in the chlorination process were toxic or bioaccumulable and persistent. The relative toxicity of the samples was assessed using a series of tests with representatives of different taxonomic groups (bacteria, algae, and invertebrates). The persistence and potential for bioaccumulation of chlorinated by-products was assessed by exposing solid-phase microextraction fibers to samples of RSS and C/D-RSS before and after degradation in a Zahn-Wellens test. For all the taxa tested in the study, the mixture of by-products formed in the C/D-RSS sample did not increase toxicity relative to that measured in the RSS sample. Chlorination of the raw settled sewage did produce additional potentially bioaccumulable halogenated substances compared to the raw settled sewage. However, after degradation, the amount of potentially bioaccumulable halogenated substances in the RSS and C/D-RSS samples was comparable, indicating that these substances were degradable. The results are discussed in the context of the overall risk assessment for sodium hypochlorite.