Peracetic acid: an emerging wastewater disinfectant
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Peracetic Acid (PAA), due to its lower toxicity and inability to form halogenated disinfection byproducts, has the potential to be an alternative wastewater disinfectant. Disinfection efficiency of PAA has been previously studied using synthetic wastewater or samples from a single wastewater treatment plant. This study bridges this gap by evaluating disinfection efficacy of PAA on multiple samples from different wastewater treatment plants. First, the efficacy of PAA was assessed using E.coli and Enterococus spp. as model indicator bacteria, at different exposure (C•t values) and temperature. The potential effects of wastewater quality parameters, including pH, total suspended solids, UV absorbance at 254 nm, and oxidant demand were also considered. Both PAA and chlorine achieved similar log-removal of E.coli and Enterococus spp at a high dose and long contact time (2 mg/L, 30 minutes), A PAA dose of 2mg/L and a contact time of 30 minutes achieved 1.5-3.5 log-removal of Enterococus spp. and 2.0-3.5 log-removal of E.coli in five out of six experiments. Inactivation of bacteria by low PAA dose and long contact time (0.5 mg/L, 30 minutes) and high PAA dose and short contact time (2 mg/L, 10 minutes) were comparable. However, these two C•t values could only achieve up to 1 log-removal for Enterococus spp. and 0.5-3.0 log-removal for E.coli . PAA disinfection experiments at room temperature and at 4 °C did not show significantly different results. Overall, disinfection performance of PAA across different wastewater matrices was variable on E.coli whereas it was consistent on Enterococus spp. Initial disinfectant demand of different wastewater samples showed higher variability for chlorine than for PAA, suggesting that PAA decay may be less sensitive to wastewater characteristics. Second, the potential synergy of UV and PAA treatment, decay of PAA in the presence of UV, and the possibility of dark repair of bacteria post UV treatment were investigated. UV/PAA synergy was evaluated by comparing the disinfection efficiency achieved by using UV and PAA together with that achieved by UV or PAA individually. UV/PAA synergy was not observed at the tested doses (PAA 0.5 mg/L, UV 10 and 16 mJ/cm 2 ) regardless of the PAA contact time (30 seconds or 10 minutes) pre-UV treatment. Regarding dark repair, an increase in the concentrations of both indicator bacteria was observed over a period of 24 hours in samples that were stored in the dark post UV treatment, suggesting the possibility of dark repair in bacteria. However, the concentrations of both indicator bacteria did not increase when a low dose of PAA (0.5 mg/L) was added to the samples prior to UV treatment. In fact, the residual concentrations of E.coli and Enterococus spp. after 24 hours in the presence of PAA were substantially lower than those immediately after UV, suggesting the prolonged disinfection by PAA. Lastly, preliminary cost analysis comparing PAA with chlorination (with and without dechlorination) indicated promising results in favour of PAA use. For instances, when chlorine residual is more than 35% of the initial chlorine dose, PAA would be a cheaper option than chlorination (with dechlorination). It is acknowledged, however, that the economics of this comparison would highly depend on future regulations and consumer demand.