Peracetic Acid Disinfection of Wastewater with Class A Reclaimed Water Production

Introduction
A temporary full-scale peracetic acid disinfection system was installed at a moderately sized wastewater treatment plant that includes Class A Reclaimed Water production. This presentation will focus on the performance of the system, lessons learned while operating the plant, and the planning and analysis of the interactions between peracetic acid and chlorine to meet the reclaimed water requirements.
Background
The Budd inlet Treatment Plant, owned and operated by the LOTT Clean Water Alliance, is a wastewater treatment and reclaimed water plant serving the municipalities of Lacey, Olympia, and Tumwater and Thurston County. The facility is am activated sludge plant including nitrogen removal, with a design flow of the facility is 28 MGD. Additionally, it has a 1.5 MGD Class A reclaimed water production facility which provides water for external clients, in addition to internal plant use. In April, 2019, a full-scale temporary disinfection system using peracetic acid was installed to provide disinfection during an upgrade to the permanent ultraviolet (UV) disinfection system. Due to the nature of the UV system upgrade, that system would be completely offline for the duration of the project necessitating an alternate disinfection system, and peracetic acid was chosen.
Disinfection Performance
Bench testing conducted before installation showed that peracetic acid was effective against fecal coliforms (the permit limited constituent), e. coli, and enterococci with sufficient dose. During full scale operation, however, the treatment effectiveness was good for fecal coliforms and e. coli (average values of 8 and 24.5 MPN/100 ml respectively), but poor for enterococci (366 MPN/100 mL) and very poor for total coliforms (>1900 MPN/100 mL). Note that the dose was controlled to meet the fecal coliform permit limit, and the other microbes were measured for information only.
Reactions with Chlorine and Reclaimed Water
A detectable chlorine residual is required in the reclaimed water distribution system. However, due to the configuration of the effluent system, peracetic acid needed to be dosed upstream of the point where water is pulled off for reclaimed water treatment. Peracetic acid or its components are known to react with hypochlorite resulting in mutual destruction, however, we were unaware of any precise measures of this. An additional complication is the fact that the standard test for chlorine residual, the DPD test, also detects chlorine. We were not aware of a way to selectively test for chlorine or peracetic acid. A bench testing plan was developed, where peracetic acid was dosed to effluent samples followed by varying amounts of hypochlorite, and the DPD residual and fecal coliform concentration were measured after up to 20 hours. This testing showed there was a breakpoint at approximately a 1:1 ratio of hypochlorite to peracetic acid (see Figure 1). Below that hypochlorite dose, the DPD measurement and disinfection performance (measured as fecal coliform concentration) was at a minimum. At higher doses, the DPD measurement and performance increased. From this, we concluded that the hypochlorite dose needed to be greater than the peracetic acid dose to maintain a chlorine residual. Additionally, this testing showed that there is an initial fast reaction where peracetic acid and hypochlorite neutralize each other, but that the neutralization continues for some time. Hydrogen peroxide is known to neutralize hypochlorite, and is one of the main stabilizers/peracetic acid byproducts. We believe it is likely the hydrogen peroxide produced by the degradation of peracetic acid continued to neutralize hypochlorite long after the initial reaction period. However, this effect was not observed in the full-scale system. In the full-scale system, typical chlorine residuals are shown in Table 1 before and during peracetic acid addition. With peracetic acid present, a higher initial dose of hypochlorite was needed to maintain a residual. However, it rapidly decayed to a more typical level through the sand filters, then proceeded to decay at a typical slow rate in the distribution system. This experience shows that it is possible to maintain a chlorine residual in combination with peracetic acid disinfection, but careful attention should be paid to the interactions between the two chemicals. Operators did note that the sand filters were the cleanest they had ever seen them following peracetic acid addition.
Operational Lessons Learned
During full scale operation, the peracetic acid dose was initially set at a conservative dose of 1.7 mg/L based on the bench testing results. There was an initial period of satisfactory though variable disinfection before the effluent numbers stabilized at very low levels (<15 CFU/100 mL). Then the peracetic acid dose was reduced over time, eventually reaching 1.3 mg/L while maintaining excellent disinfection performance. However, after operating at the lower dose for several weeks, disinfection performance decreased. Simultaneously, operators noticed increased biofilm in the effluent sample location and in the effluent pumping wetwell. Also, increasing levels of fungus and glycogen accumulating organisms which thrive on acetate, a peracetic acid byproduct, were noted in microscope samples. Increasing the peracetic acid dose back to previously effective levels did not improve disinfection performance. We believe that though peracetic acid was a very effective disinfectant, at low levels it promoted the increased growth of (non-pathogenic) biofilms, which eventually began to shield fecal coliforms from the chemical. To restore system performance, an elevated 'shock' dose was employed for a limited time followed by resuming peracetic acid dosing at more conservative normal levels. While peracetic acid was very effective, we found that immediate fecal coliform concentration is not a good indicator of the ideal dose. Instead, a dose which prevents the overgrowth of biofilms is needed to maintain long term disinfection stability.
Summary
Peracetic acid was employed at full scale at a treatment and provided good performance for fecal coliform and e. coli treatment. However, there are challenges with treatment performance for enterococci and total coliforms. Additionally, biofilm growth can be promoted by peracetic acid. Finally, interactions with chlorine are complex but manageable, and reclaimed water production is possible.