Effect of ammonia on acute toxicity and disinfection byproducts formation during chlorination of secondary wastewater effluents.

Ammonia nitrogen (NH3-N) significantly affects the occurrence of disinfection byproducts (DBPs) and residual chlorine in chlorinated wastewater, thereby affecting the acute toxicity to aquatic organisms. In this paper, the formation of thirty-five halogenated DBPs and the changes in acute toxicity of luminescent bacteria and zebrafish embryos were evaluated after chlorination of seven secondary wastewater effluents with different NH3-N concentrations. Results showed that NH3-N significantly reduced the formation of most DBPs by 82-100%. The acute toxicity was enhanced after chlorination and increased linearly with increasing NH3-N concentration for luminescent bacteria (r = 0.986, p < 0.05) and zebrafish embryos (r = 0.972, p < 0.05) due to the coexistence of DBPs and monochloramine. According to the toxicity classification system of wastewater, the fitting results indicated that the toxicity level was acceptable for chlorinated wastewater with NH3-N concentration below 1.00 mg-N/L. DBPs might be the main toxicant to luminescent bacteria in the wastewater with low NH3-N concentrations (0.06-0.31 mg-N/L), which accounted for 68-97% of the toxicity contribution. By contrast, monochloramine contributed over 80% to the toxicity of luminescent bacteria and zebrafish embryos in the wastewater with high NH3-N concentrations (2.66-7.17 mg-N/L). Compared to chlorination, chlorine dioxide and ultraviolet disinfection unaffected by NH3-N could reduce acute toxicity by nearly 100%, primarily due to the lack of residual disinfectant. In view of the high toxicity caused by chlorination, chlorination-dechlorination or chlorine dioxide and UV disinfection are highly recommended for the treatment of wastewater with high NH3-N concentration.