Impact of transformation, photodegradation and interaction with glutaraldehyde on the acute toxicity of the biocide DBNPA in cooling tower water

The reuse of cooling tower water (CTW) can substantially lower the freshwater footprint of cooling towers. CTW requires desalination before its reuse, and pre-treatment before desalination enhances the desalination efficiency. Constructed wetlands (CWs) have been shown to remove fractions from CTW that hamper physico-chemical desalination technologies, and thus provide adequate pre-treatment. However, CTW contains biocides, and these and their transformation products are potentially toxic to aquatic fauna in surface flow CWs. Therefore, we assessed the acute toxicity to daphnids of CTW which contained the biocide 2,2-dibromo-3-nitrilopropionamide (DBNPA), and the impact of photodegradation and the presence of biocide glutaraldehyde on its toxicity. It was observed that the toxicity of DBNPA in CTW was lower than that reported in the literature, and non-target screening showed that this was due to rapid DBNPA transformation, via 2-monobromo-3-nitrilopropionamide (MBNPA) and nitrilopropionamide (NPA). Photodegradation resulted in an increased CTW toxicity after 1 h of illumination as a result of the formation of new DBNPA transformation products, and subsequent decrease in toxicity after 48 h. The simultaneous presence of DBNPA and glutaraldehyde resulted in the formation of interaction products. Photodegradation did not result in unique interaction products, but increased the rate of interaction product formation, leading to a decrease in toxicity. Due to the toxicity of DBNPA in CTW and the effect of photodegradation on this toxicity, it is not recommended to use a surface flow CW as first treatment step in a multistage CW-system for the treatment of CTW.