Peracetic acid (PAA) has grown in popularity over the last several years for its use in the disinfection of wastewater and stormwater. Utilities use disinfectants as the primary mechanism to inactivate and destroy pathogenic organisms that spread waterborne disease. An appropriate disinfectant will sufficiently treat any disease-causing microbes including bacteria, spores, helminthes, and protozoa. While PAA technology has been employed in Canada and Europe for the last 30 to 40 years, this disinfectant has only become noticed in U.S. municipal wastewater treatment within the last 10 years. Competing with chlorine, an already well-established disinfectant, its use is still slow growing, however systems are discovering that PAA offers several benefits to wastewater treatment that chlorination does not. What is peracetic acid? The alternative disinfectant is a clear, organic peroxide compound that readily hydrolyzes to acetic acid and hydrogen peroxide in water. It’s characterized as a strong oxidant and fast reacting disinfectant. Commercially available peracetic (CH 3 CO 3 H) is purchased in an equilibrium mixture of acetic acid (H 3 CO 2 H), hydrogen peroxide (H 2 O 2 ), and water (H 2 O). Manufacturers typically add a stabilizer as well. The following formula represents the equilibrium equation: CH 3 CO 2 H + H 2 O 2 ←→ CH 3 CO 3 H + H 2 O. PAA can generally be purchased in concentrations of 5% to 22%. When PAA decomposes in water, free hydrogen peroxyl (HO 2 ) and hydroxyl (OH) radicals are formed. These radicals have significant oxidizing capacity that play an active role in microbial disinfection. According to the EPA, bacteria are destroyed through cell wall lysis and leakage of any cellular constituents. Wastewater systems consider moving to peracetic acid for several reasons . Unlike chlorine, PPA decomposes into biodegradable residuals of vinegar (acetic acid) and hydrogen peroxide that can pass fish toxicity tests without removal. These residuals are not toxic, mutagenic, or carcinogenic. Bioaccumulation in aquatic organisms is also highly unlikely. Neither chlorinated compounds nor harmful disinfection by-products (DBPs) are produced with its use. As such, PAA has been considered the potential answer to tough DBP regulations. Peracetic acid can also disinfect over a wide range of pH and is unaffected by nitrate and ammonia concentrations. Chemical handling of PPA is toted for being easier and safer than chlorination. The disinfectant can be stored for long periods of time exhibiting less than 1% decrease in activity per year when properly stored. Its use does not require any special risk management plans (RMPs) required by the EPA when handling certain toxic chemicals. For systems that operate under cooler conditions to prevent contamination or elevated temperatures, PAA has a low freezing point. Switching to PAA requires minimal retrofitting with the chemical itself being offered at prices competitive to other disinfectants. There can be some disadvantages to peracetic acid. Depending on the formula purchased, PAA introduces varying amounts of acetic acid into the wastewater effluent. This can contribute to biological oxygen demand (BOD) and may not be appropriate for systems that are struggling to meet these limits. The biggest challenge wastewater systems face is regulatory approval. While PAA has been approved by the EPA as a primary disinfectant, each state regulatory agency must also approve its use. A WaterOnline guest column includes an infographic of states that have approved PPA as of 2017. The guest column discusses how systems can approach local regulatory agencies to seek approval on a case-by-case basis. The overall effectivity of PPA will depend on wastewater characteristics, the PAA concentration, contact time, and the reactor configuration. Dosage will depend on the target organisms, wastewater quality, and level of inactivation required. When monitoring PAA residuals, operators can use the same analyzer and method as for chlorine residuals. A standard EPA sampling method does not yet exist. The lack of established methods and protocols for PAA makes approval difficult for local regulatory agencies. To help investigate the use and implications of PAA in wastewater, the Water Research Foundation (WRF) completed a study to evaluate effluent toxicity as well as dosage and contact times required to meet compliance. Metro Vancouver’s Northwest Langley WWTP in Canada has also published findings from a multi-year pilot program that used PAA as a disinfectant. More studies will have to expand on existing research until peracetic acid can become easily and widely adopted.