Opportunities and Challenges Associated with PAA Disinfection
- Thomas Worley-Morse, Melanie Mann, Saya Qualls, Rob Sharp, and Jason Beck
This paper will provide an overview of the key considerations associated with utilizing peracetic acid (PAA) for disinfection at water resource reclamation facilities (WRRFs). We will review data from PAA pathogen inactivation studies from the City of New York, NY and review PAA wet weather disinfection from the City of Sidney, OH and the City of New York, NY. We will also present regulatory considerations including establishing permit limits for PAA residual, discuss concerns with PAA oxidant residual on effluent dominated streams, and discuss the implications of the EPA’s plans to establish water quality criteria for bacteriophages, which could influence the adoption of PAA.
PAA is an equilibrium product of hydrogen peroxide and acetic acid, and has been identified as an emerging biocide in the United States for inactivation of fecal bacteria. Interest in PAA as a primary disinfectant has grown due to its reputation as a green, affordable, effective alternative to chlorine that does not produce chlorinated disinfection byproducts. In many studies, PAA has demonstrated equivalent disinfection as chlorine on secondary effluents but at lower dosage and neutralization requirements. The lower dosage and lack of neutralization requirements can offer economic benefits compared to chlorine in some cases.
Despite these reported benefits, there are few full-scale water resource reclamation facilities (WRRFs) in the US utilizing PAA for disinfection. Because of this limited application, operational experience associated with PAA disinfection is limited. Additionally, regulatory agencies have limited guidance on setting the required doses and contact times for PAA and on setting permit limits for PAA residual, especially for discharges to effluent-dominated streams. Further information is needed to understand how PAA performs as a primary disinfectant for dry and wet weather flows.
This paper will provide a summary of PAA’s opportunities and risks. We will review data from PAA pathogen inactivation studies performed for Utility A, review recent work on wet weather disinfection for the City of Sidney, OH and the City of New York, NY, and review primary disinfection and PAA residual permitting for the Tullahoma, TN Utilities Board. We will also discuss the implications of recent EPA efforts to establish coliphage-based recreational water quality criteria.
PAA Pathogen Inactivation Studies: Pathogen inactivation studies were performed for utility A to compare the effectiveness of PAA to chlorine. These tests used secondary samples from two different facilities. These studies were conducted using bench scale jar testing, and dose response curves were developed for both fecal bacteria and enterococci. In addition, residuals were monitored over time to determine the decay of each disinfectant.
Wet Weather Disinfection Performance: Jar testing was performed to determine the effectiveness of using chlorine or PAA as a disinfectant for wet weather flows. For the City of Sidney, OH, wet weather flows were simulated using 75% secondary effluent and 25% raw influent. Wet weather testing is currently underway for the City of New York, NY, will be finished during the first quarter of 2016, and reported on in the final paper.
Results and Discussion
Effectiveness of PAA on fecal coliform and enterococci: To compare the effectiveness of PAA on fecal coliforms and enterococci, bench scale tests were conducted with the secondary effluent from Facility A and Facility B. Results to date illustrate that the effectiveness of either hypochlorite or PAA in meeting the new enterococci criteria is plant specific. At Facility A, PAA required a significantly higher CT to achieve both the enterococci and fecal coliform criteria when compared to chlorine. PAA was more effective at inactivating fecal coliforms compared to enterococci in Facility B effluent, while chlorine appears to be more effective or equally as effecting inactivating both fecal coliforms and enterococci. For both Facility A and B, neither plant can meet their proposed TRC permit levels (0.05 and 0.07 mg/l respectively) and the proposed enterococci criteria without dechlorination or change in disinfection process (UV or PAA). So far the data indicates that aside form issues associated with health and safety, handling, storage and transport issues, PAA is a viable disinfectant that can meet the current fecal coliform and future (proposed) enterococci permit criteria. However, it is likely that PAA would require higher doses compared to chlorine and high CTs to consistently meet these criteria at all of Utility A’s WWRF.
Effectiveness for wet weather disinfection: For both PAA and chlorine, a CT of 150 mg-min/L was required to meet the proposed bacterial limits and resulted in a design effluent residual of 7.5 mg/L at a total contact time of 20 minutes (Figure 1). An effluent residual permit limit of 1 mg/L or less is anticipated for both chemicals and will require neutralization. An economic evaluation was completed assuming use of sodium bisulfite for neutralization. Based on these tests, chlorine was selected for the City of Sidney’s wet weather disinfection.
Regulatory considerations: At the national level, the EPA initially approved certain blends of PAA as a disinfectant for wastewater use in the United States with a residual of less than 1 ppm, but has since revised this restriction to allow higher residuals. However individual states’ attitudes towards PAA remain variable. Since only a few utilities are using PAA, most regulatory agencies are adopting a case-by-case approach on how to regulate PAA use. For example, the Tennessee Department of Environment and Conservation (TDEC) modified the NPDES permit for the Tullahoma WWRF that included a water quality-based effluent limit (WQBEL) for PAA residual and monitoring for hydrogen peroxide residual. The WQBEL was based on a laboratory determined IC25 of 0.38 mg/l PAA for Ceriodaphnia dubia. The bioassay was performed using PeroxyChem’s VigorOx® WWT II PAA solution which contains 15% PAA by weight and 23% hydrogen peroxide as delivered. TDEC did not specifically require a chlorination/dechlorination system as back-up but did require the Tullahoma Utilities Board to develop a back-up disinfection plan.
As a disruptive long-term driver, the EPA is considering adding bacteriophage Recreational Water Quality criteria that may limit the overall adoption of PAA. For example, since PAA is less effective on viruses, any future regulatory framework that specifically addresses viruses will drive utilities and the industry either away from PAA or a towards multi-disinfectant approach (Figure 2). Paradoxically, even though PAA poorly inactivates viruses and phages, the uncertainty in the upcoming EPA’s water quality criteria may favor a short-term shift to PAA because utilities can use PAA to delay larger upgrades until the EPA finalizes the upcoming water quality criteria.
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