1. Peracetic Acid as a Wastewater Disinfectant
Although US municipal treatment plants still commonly use chlorination to disinfect wastewater, concerns about chlorinated disinfection byproducts (DBPs) are prompting utilities’ managers to consider other disinfection technologies. One disinfection alternative is peracetic acid (PAA), which does not form chlorinated DBPs.
The effectiveness of PAA as a disinfectant depends on its dose, contact time, and the susceptibilities of the target organisms. The oxidant demand in wastewater depends on the wastewater’s characteristics (e.g., natural organic matter, reduced metals, biological oxygen demand, chemical oxygen demand, and total suspended solids).
When PAA reacts with domestic wastewater, it rapidly decomposes to oxygen, water, and acetic acid. The decay of PAA due to oxidant demand generally is rapid, occurring within the first few minutes of contact time. Depending on the dose, the disinfected effluent may not contain any PAA residual and therefore quenching (PAA’s version of dechlorination) may not be required before the effluent is discharged.
This method describes the measurement of PAA residuals. Peracetic acid measurement methods should be rugged, accurate, fast, and operator-friendly so residual PAA levels can be monitored efficiently and effectively.
2. Peracetic Acid Disinfection Mechanism
Commonly, PAA is prepared by reacting acetic acid with hydrogen peroxide (H2O2) in the presence of an acid catalyst. A range of concentrations can be prepared by adjusting reactant concentrations during manufacturing. The final product is then distributed as a solution in equilibrium with H2O2, acetic acid, and water. Commercially available PAA products maintain their equilibrium as a result of proprietary stabilizers, and these products often claim shelf lives greater than 1 year.
CH3COOH + H2O2 ↔ CH3COOOH + H2O
When microorganisms are exposed to PAA, highly oxidative radical species are produced that cause catastrophic cellular damage.
3. Selection of Method
The N,N-diethyl–p-phenylenediamine (DPD) colorimetric method originally proposed by Dr. Arthur T. Palin in the 1950s is widely used to analyze free and combined chlorine (refer to Section 4500–Cl.G), as well as other oxidizing disinfection agents (e.g., bromine, chlorine dioxide, and ozone).1 Historically, the oxidizers measured by DPD have not been commercially available as stable standards, so analysts typically make the appropriate stoichiometric adjustment to the final calculations to obtain disinfectant-specific test results.
4. Sampling and Storage
To minimize PAA decomposition during sampling, avoid agitation and exposure to sunlight and other strong light. Analyze immediately after sampling; do not store PAA samples.