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challenges of high capital costs, even in light of advances in ozone generation technologies, can make implementation of this technology for wastewater disinfection unfeasible. An Alternative Disinfectant Peroxyacetic acid or peracetic acid (PAA) is a chemical gaining a great deal of interest due to its ability to provide bacterial inactivation performance at costs competitive with other mature disinfection technologies. PAA is a chemical oxidant that has been applied to the food, beverage, medical, and pharmaceutical industries as a disinfectant for many years; and, because of its oxidizing power, it can address at least some constituents of emerging concern. PAA has the chemical formula CH 3 CO 3 H and is produced as an equilibrium solution as shown in Figure 1. Commercial preparation includes reacting acetic acid with hydrogen peroxide in the presence of a catalyst; specific grades of PAA are formulated by controlling the concentration and amounts of reagents during the manufacturing process. The oxidation potential of PAA is higher than other chemicals used in water and wastewater treatment, nearly as high as ozone (Figure 2), which accounts for its ability to both disinfect and oxidize organic chemicals. Because of its reactivity, PAA does not persist in the environment and breaks down into acetic acid (vinegar) and hydrogen peroxide, which subsequently decomposes to oxygen and water. This is an important consideration in selection of a PAA product for wastewater applications, because each formulation introduces a slightly different amount of acetic acid (Table 1), which has an associated biological oxygen demand (BOD) for each mg/L of PAA dosed. Peracetic acid, hydrogen peroxide, and acetic acid have low octanol-water partition coefficients (K OW ) (0.3, 0.4, and 0.68, respectively) and low sediment adsorption coefficients, so bioaccumulation in aquatic organisms or in sediments is highly unlikely. In addition to the facts that PAA does not produce halogenated DBPs and has low aquatic toxicity relative to chlorine, its use on-site does not require special risk management plans (RMPs), as required by the EPA when handling certain toxic chemicals. According to the Code of Federal Regulations, if "the partial pressure of the regulated substance in the mixture (solution) under handling or storage conditions in any portion of the process is less than 10 millimeters of mercury (mm Hg), the amount of the substance in the mixture in that portion of the process need not be considered when determining whether more than a threshold quantity is present at a stationary source." The commercial products used for wastewater disinfection have low vapor pressures of PAA in the mixture, and PAA may be excluded from the RMP; facilities are required only to adhere to the general duty clause. This is similar to the requirements for the E2 regulations in Canada. PAA Applications In Municipal Wastewater PAA has been demonstrated to be an effective wastewater disinfectant over a wide range of effluent qualities, requiring low doses of chemical to achieve bacterial inactivation. PAA has fast kinetics, requiring short contact times for disinfection. Capital costs for retrofit of existing chlorine facilities are very low, and PAA is able to provide treatment for challenging effluent while meeting stringent limits for halogenated DBPs such as THMs, cyanide, and dioxins. There are a handful of full- scale installations that have been implemented where PAA was selected for its benefits; some of these facilities are listed here: • NW Langley WWTP in Metro Vancouver, British Columbia • St. Augustine WWTP in St. Augustine, FL • City of Steubenville WWTP in Steubenville, OH • Mayport Naval Facility in Jacksonville, FL • Greenville WWTP in Greenville, KY • Whitehouse WWTP in Whitehouse, TN • Flagler Beach WWTP in Flagler Beach, FL • Three Rivers Regional WWTP in Longview, WA • Tri Cities WWTP in Clackamas, OR Further, the Stiles WWTP in Memphis, TN is currently under design for implementa- tion of a PAA disinfection system after a full-scale study conducted in 2014 (Figure 3). This facility, when completed, will be the largest municipal PAA disinfection system glob- ally, with a peak hour treatment capacity of 250 MGD, exceeding the capacity of the Nosedo WWTP in Milan, Italy with a capacity of approx- imately 110 MGD. Typical PAA installations are simple chemical storage tanks and pump skids, as shown in Figures 3 and 4. Work To Be Done This list of PAA installations in North America is not comprehensive, and there are a number of ongoing pilots that are providing scientific data to deepen the industry understanding of this technology. While the roles of factors such as suspended solids, temperature, pH, and wateronline.com n Water Innovations DISINFECTIONBYPRODUCTS 17 Figure 1. PAA is commercially available in an equilibrium solution Figure 2. Electrochemical oxidation potential (EOP) for several disinfectants Figure 3. Temporary bulk PAA tank installation used for full-scale pilot testing at the Stiles WWTP in Memphis, TN. (Credit: PeroxyChem)