Chlorination or chloramination: Balancing the regulated THM formation and microbial inactivation in marine aquaculture waters

Disinfection methods like chlorination are increasingly used to improve water quality in aquaculture facilities and to maintain a sanitary environment for the well-being of the animals. However, chlorination can result in formation of disinfectant byproduct (DBP) that can be carcinogenic and toxic. This study aims to evaluate if an optimal balance can be achieved between minimal DBP formation and effective microbial inactivation with either chlorination or chloramination for application in the Red Sea aquaculture waters. The efficiency in removing ARB and their associated antibiotic resistance genes (ARG) were examined alongside the concentrations of regulated DBPs that occurred as a result of disinfection. The concentration of total trihalomethanes, primarily bromoform, exceeded the USEPA regulatory limit of 80 µg/L even at the lowest tested concentration of chlorine (1 mg/L) and contact time (1 h). Comparatively, THMs concentration was only detectable in chloraminated Set 1 samples at an approximately 30 µg/L concentration. The average log reduction of antibiotic-resistant bacteria by chlorine ranged from 2.3-log to 3.2-log with different contact time. The average log reduction of antibiotic-resistant bacteria by monochloramine were comparatively lower at 1.9 to 2.9-log. Viable Staphylococcus aureus was recovered from monochloraminated samples and not chlorinated samples, but the abundance of S. aureus were not high enough to result in any significant microbial risks. Both chlorination and chloramination did not provide any significant improvement in the reduction of ARGs although the abundance of these genes result in inconsequential detrimental impact. This study demonstrates that a systematic evaluation is needed to determine the optimal disinfectant required to balance both microbial and chemical risks. Compared to chlorine, chloramine may be a more appropriate disinfection strategy for the treatment of aquaculture waters prior to discharge or for recirculatory use in the aquaculture facility.