Disinfection efficiency of peracetic acid, UV and ozone after enhanced primary treatment of municipal wastewater
Introduction
The City of Montreal Wastewater Treatment Plant (CMWTP) uses physicochemical processes to treat up to 7.6×106 m3/d of combined domestic and industrial wastewater. These processes include screening, grit removal, addition of ferric chloride and/or alum as well as a polyelectrolyte to precipitate phosphorus and improve solids settling, and sedimentation prior to discharge into the St. Lawrence River. At present there is no disinfection. Since the use of chlorine for wastewater disinfection has been banned in the Province of Quebec, the City of Montreal is exploring alternative disinfectants in order to produce water which could be suitable for contact aquatic sports, and as a raw potable water source to communities downstream. The discharge permit for this wastewater treatment plant would allow for an effluent containing 9000 CFU (colony forming units) of fecal coliforms (FC) per 100 mL, and 1000 CFU/100 mL to allow for photoreactivation if ultraviolet radiation (UV) is used [1]. However concerns have been raised regarding the adequacy of indicator organisms such as FC to predict the performance of various disinfection processes against pathogens. Accordingly, the objectives of this study were (a) to assess the doses required for three disinfection processes—peracetic acid (PAA), UV or ozone—to reach the target FC standard, and (b) to compare the responses of three other indicators—Enterococci (EC), Clostridium perfringens (CP) and MS-2 coliphage—to these disinfectants. The data will be used to determine the level of disinfection required at this wastewater treatment plant to attain a level of risk (using a model proposed by Haas et al. [2]) that is acceptable to public health and to the population, and is economically feasible. The risks examined will be those associated with recreational activities during which direct and indirect contact occurs with river water impacted by this effluent.
Section snippets
Peracetic acid (PAA)
Of the three disinfectants being studied, PAA is the newest alternative for applications in North America, though it has been used in Europe for wastewater disinfection for many years. Interest in the use of PAA as a disinfectant for wastewaters began in the late 1980s with publications by Baldry and coworkers [3], [4]. Grantham [5] presented results from many internal studies of the National River Authority (NRA) of England and Wales. He pointed out that since PAA is an equilibrium mixture,
Materials for disinfection
Materials for disinfection and analysis of disinfectants are shown in Table 1.
PAA
Residual concentrations of peroxycompounds (sum of H2O2 and PAA) were measured in terms of the absorbance of the radical cation of ABTS, formed when H2O2 or PAA interact with ABTS and horseradish peroxidase at pH 6.0 [31]. The colorimetric assay was composed of 1.7 mM ABTS and 4.2 mg/L horseradish peroxidase (HRP) in 67 mM sodium phosphate buffer at pH 6.0 [32]. When peroxycompounds (peracetic acid and/or H2O2) are added
Wastewater characteristics
Due to the wide variability in the quality of the effluent samples from the wastewater treatment plant, the data are presented in Fig. 2 on a log scale showing the mean as well as the range. An attempt was made to correlate these data with disinfection performance, but this was largely unsuccessful. It is clear, however, that this wastewater would not be “easy” to disinfect, and this is typical of wastewaters from purely physicochemical treatment plants [17]. In particular, the high
Summary and conclusions
PAA was not a suitable disinfectant for the City of Montreal's Wastewater Treatment Plant. Based on a fecal coliform (FC) target level of 9000 CFU/100 mL, over 6 mg/L would be required, whereas 1.5–2 mg/L is considered economically viable. For ozone, the required dose exceeded 30 mg/L; this was also considered excessive. On the other hand, for a more stringent FC target of 1000 CFU/100 mL to account for photoreactivation, the required UV fluence was 20 mJ/cm2, which may be economically acceptable.
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