Peracetic Acid (PAA) in Wastewater Disinfection

Peracetic Acid (PAA) in Wastewater Disinfection
October 18, 2019
By Jill
Wallitschek Technology, Wastewaterchlorination, disinfection, peracetic
acid, ppa, wastewater disinfection, wastewater treatment 0
Source: WaterOperator.org Blog | Peracetic Acid (PAA) in Wastewater Disin
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 diseasecausing 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 (CH3CO3H) is purchased in an equilibrium mixture of acetic acid
(H3CO2H), hydrogen peroxide (H2O2), and water (H2O). Manufacturers typically add
a stabilizer as well. The following formula represents the equilibrium equation:
CH3CO2H + H2O2 ←→ CH3CO3H + H2O.
PAA can generally be purchased in concentrations of 5% to 22%. When PAA
decomposes in water, free hydrogen peroxyl (HO2) and hydroxyl (OH) radicals are
formed. These radicals have significant oxidizing capacity that play an active role in
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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 byproducts (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.
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