The relative acute toxicity of continuous and intermittent exposures of chlorine and bromine to aquatic organisms in the presence and absence of ammonia

Sodium bromide can be used to convert hypochlorous acid into hypobromous acid. An alternative strategy to biofouling control uses the simultaneous addition of sodium bromide and chlorine to water used for power plant condenser cooling. This approach can significantly reduce the total disinfectant and halogen application rates because the bromine oxidants that are generated are more effective for controlling biofouling than their chlorine counterparts, especially at higher pHs. Since such a change in biofouling control strategy could adversely impact the environment, the acute toxicity of bromine oxidants were evaluated in both continuous and intermittent exposure scenarios. Decay properties of bromine oxidants were compared to those of chlorine oxidants. In addition, two tests were conducted to investigate the relative toxicities of chloramines and bromamines. For the six species tested, bromine oxidants were two to five times more toxic than chlorine oxidants. For continuous exposure to bromine oxidants, LC50 values for daphnids (Dapnia magna) and amphipods (Hyalella azteca) could not be calculated because significant mortality occurred at the oxidant quantitation limit. Both chlorine and bromine proved to be more toxic to daphnids and mysids (Mysidopsis bahia) in the presence of ammonia. While the toxicity data show that bromine oxidants are more toxic than chlorine oxidants, bromine oxidants decayed two to five times faster than chlorine oxidants in freshwater and saltwater, respectively. With regard to potential environmental impact, it is important that one consider the more efficacious biocidal characteristics of bromine (i.e. higher pHs) in the context of more rapid decay relative to chlorine. The strategy of using simultaneous addition of sodium bromide and chlorine could reduce environmental impact potential, although insufficient data exists to prove this.