The aim of this study was to elucidate the role of total suspended solids (TSS, which are claimed to be responsible for protecting embedded microorganisms) during disinfection of wastewater with peracetic acid (PAA). In particular, the focus was on a physicochemical effluent, having been treated with ferric chloride for TSS and phosphorus removal. Batch disinfection tests with various PAA dosages and contact times were carried out on the effluent from Montreal's wastewater treatment plant which uses only chemical precipitation and primary sedimentation. In addition to these samples, disinfection of “medium” and “highly” filtered effluents, obtained by sequential filtration through 120- and 10-µm nominal pore size membrane laboratory filters, was investigated. Modified second-order and Selleck model kinetics were used with moderate success to describe disinfectant consumption and microbial inactivation rates, respectively. This study showed that the overall amount of protection afforded by TSS to the microbial indicator considered (i.e., fecal coliforms in this case) was approximately 1.9 logs. TSS size was a key variable in this protection; approximately 1.3 logs and an additional 0.6 logs was the protection afforded by TSS greater than 120 µm, and between 10 µm and 120 µm, respectively. Fecal coliform inactivation of 3.8 to 7.3 logs after 40 minutes contact time could be achieved with a PAA dosage of 2 or 8 mg/L, respectively, in a highly filtered effluent.

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