1. Peracetic and Performic Acid as a Wastewater Disinfectant
Although US municipal treatment plants commonly use chlorination to disinfect wastewater, concerns about chlorinated disinfection byproducts (DBPs) are prompting utilities’ managers to consider other disinfection technologies. Two disinfection alternatives are peracetic acid (PAA)1–5 and performic acid (PFA),6–9 which do not form chlorinated DBPs. Both PAA and PFA are aliphatic peracids. Their effectiveness as a disinfectant depends on their 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). PAA and PFA dose rates may vary. PFA must be generated on-site.
When PAA or PFA react with domestic wastewater, they rapidly decompose to oxygen, water, and acetic acid or formic acid, respectively. The decay rates of PAA and PFA due to oxidant demand are rapid, usually occurring within the first few minutes of contact time. Depending on the dose, the disinfected effluent may not contain any PAA or PFA residual, and therefore quenching may not be required before the effluent is discharged.
This method describes the measurement of PAA and PFA residuals. Use measurement methods that are rugged, accurate, fast, and operator-friendly so that residual PAA and PFA levels can be monitored efficiently and effectively.
Commonly, both peracids are prepared by reacting either acetic or formic 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 or formic 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. Solutions of PFA are not shelf-stable and must be generated on-site immediately before use.
When microorganisms are exposed to PAA or PFA, highly oxidative radical species are produced that cause catastrophic cellular damage.
2. 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).8 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.
3. Sampling and Storage
To minimize PAA or PFA decomposition during sampling, avoid agitation and exposure to sunlight and other strong light. Analyze immediately after sampling; do not store PAA or PFA samples.