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 ac id. An alternative strategy to biofouling control uses the simultaneous add ition of sodium bromide and chlorine to water used for power plant condense r cooling. This approach can significantly reduce the total disinfectant an d halogen application rates because the bromine oxidants that are generated are more effective for controlling biofouling than their chlorine counterp arts, especially at higher pHs. Since such a change in biofouling control s trategy could adversely impact the environment, the acute toxicity of bromi ne oxidants were evaluated in both continuous and intermittent exposure sce narios. Decay properties of bromine oxidants were compared to those of chlo rine oxidants. In addition, two tests were conducted to investigate the rel ative 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 (Dap nia magna) and amphipods (Hyalella azteca) could not be calculated because significant mortality occurred at the oxidant quantitation limit. Both chlo rine and bromine proved to be more toxic to daphnids and mysids (Mysidopsis bahia) in the presence of ammonia. While the toxicity data show that bromi ne oxidants ore more tonic than chlorine oxidants, bromine oxidants decayed two to five times faster than chlorine oxidants in freshwater and saltwate r, respectively. With regard to potential environmental impact, it is impor tant that one consider the more efficacious biocidal characteristics of bro mine (i.e. higher pHs) in the context of more rapid decay relative to chlor ine. The strategy of using simultaneous addition of sodium bromide and chlo rine could reduce environmental impact potential, although insufficient dat a exists to prove this. (C) 1998 Elsevier Science Ltd. All rights reserved.