Evaluation of Site-Generated Peracetic Acid for Secondary Effluent Disinfection

Peroxyacetic acid (PAA) is an EPA-approved, broad spectrum biocide made from acetic acid (CH3OOH) and hydrogen peroxide (H2O2). Currently, chlorine (gas or bleach) is the most common biocide used for disinfection due to its low cost and efficacy. However, chlorine has several disadvantages which can present challenges to wastewater treatment plants. Chlorine reactions with certain organic matter produce carcinogenic, bio-accumulative disinfection byproducts (DBPs), which include halo-acetic acids (HAAs), trihalomethanes (THMs), and absorbable organic halides (AOX). High levels of chlorine are also toxic to aquatic life, which require chlorine residuals to be monitored and/ or quenched before release to the environment. Costly safety and security plans are required by OSHA and the Department of Homeland Security, especially for gaseous chlorine. As an alternative to chlorine, PAA is gaining traction in the municipal wastewater market owing to its effectiveness, cost, and minimal environmental impact. In the past few decades, global environmental protection agencies (e.g., Water Environment Federation [WEF], EPA, European Environment Agency) have considered PAA as a replacement for chlorine in the disinfection of municipal wastewater. EPA performed a pilot case study in collaboration with the Metropolitan Sewer District of Greater Cincinnati (Garg, 2019). Numerous studies by these agencies have also confirmed the efficacy of the chemistry for effluent disinfection, and recommendations have been made for its use.
This study highlights a new method for using PAA: a safe, economical, and unique three-precursor system for the on-site generation of PAA (PAA-OSG). In contrast to traditional or 'equilibrium' acidic PAA, the PAA-OSG product is alkaline, non-corrosive, and odor free. Testing was conducted under bench-scale batch disinfection and pilot-scale flow-through disinfection. Oxidant demand tests were performed as a function of time, for PAA-18, PAA-OSG, and sodium hypochlorite. Results indicate that the wastewater demand for PAA-18 was approximately 0.5 ppm while the demand for PAA-OSG was between 1.1 and 1.5 ppm (higher demand). The demand reactions are non-specific so it is hypothesized that H2O2 satisfies part of the oxidant demand. For bench scale disinfection tests, PAA products, H2O2, and free available chlorine were spiked into 300 mL biochemical oxygen demand (BOD) bottles with secondary effluent and evaluated for disinfection efficacy at contact times of 5, 15, and 40 minutes. PAA was observed as the primary disinfecting agent in the PAA products. H2O2 by itself was not effective at inactivating TC and EC in secondary effluent. However, the peroxide levels in the higher H2O2:PAA (PAA-18) formulations would meet some of the overall oxidant demand that would otherwise consume PAA, making the PAA unavailable for disinfection. Additional pilot scale tests were conducted using a flow through system. Overall, PAA was observed to likely be more effective in inactivation of E. coli than total coliform. It was also observed that, at the same PAA dosing rates, the flow-through system showed lower kill rates for both TC and EC compared to the bench-scale run. Again, these data will help determine real-world treatment parameters. The efficacy of the PAA disinfectants, PAA-18 and PAA-OSG, was similar to chlorine in the ability to deactivate TC and EC reference organisms in real wastewater. PAA products contain both PAA and H2O2, and PAA was determined as the primary disinfectant. However, the peroxide did consume some of the oxidant demand, conserving the PAA for disinfection action. Of the products tested in this study, chlorine showed the highest level of efficacy at higher dosing rates and longer contact times, ostensibly due to the formation of stable and persistent chloramines. In practice, these chloramine species would need to be tracked and removed prior to discharge. PAA can serve as a potential alternative to chlorination and UV light under certain circumstances, and it does not form eco-toxic disinfection byproducts or require costly and environmentally intrusive infrastructure modifications to POTWs. The advent of PAA-OSG now facilitates the on-site generation of non-acidic and odor-free PAA solutions, eliminating the need to ship dilute PAA solutions in favor of concentrated precursors.