Acute toxicity and risk evaluation of the CSO disinfectants performic acid, peracetic acid, chlorine dioxide and their by-products hydrogen peroxide and chlorite.

The ecotoxicological evaluation of combined sewer overflow (CSO) disinfectants, with their degradation products, is important for ensuring safe use. For this form of toxicity, data for organisms representing different trophic levels are needed. We studied the toxicity of the alternative disinfectants peracetic acid (PAA), performic acid (PFA) and chlorine dioxide (ClO) and their degradation products hydrogen peroxide (HO) and chlorite (ClO) on Vibrio fischeri and Daphnia magna. ClO was more toxic to D. magna (EC < 0.09 mg/L) and PFA was most toxic to V. fischeri (EC 0.24 mg/L). EC of PFA, PAA, ClO, HO and ClO on D. magna were 0.85, 0.78, <0.09, 3.46 and 0.36 mg/L, respectively. Similarly, EC of PFA, PAA, ClO, HO and ClO on V. fischeri were 0.24, 0.42, 1.10, 5.67 and 30.93 mg/L, respectively. For both PFA and ClO, the degradation in water was faster than for PAA, HO and chlorite. Using these data together with literature values, we derived environmental quality standards. By combining these with typical concentrations of disinfectants used for CSOs, we estimated the dilution required for discharging CSOs after disinfection, which can be used for quick assessment of the environmental feasibility of disinfection systems at specific CSO sites. Minimal dilutions in the receiving water, in the orders of 44, 70 or 138-fold, are needed for ClO, PFA and PAA, respectively. This highlights PFA as the most widely applicable disinfectant, taking into account both its efficiency and the lower risk of unwanted environmental effects.