Final Effluent Disinfection Using Peracetic Acid (PAA)

Project Scope

For over 12 years, the NW Langley wastewater treatment plant (Vancouver, BC) has been using USP Technologies peracetic acid (PAA) application program for disinfection of their final effluent to successfully meet and exceed strict discharge permit conditions including fecal coliform concentrations, total residual chlorine (TRC), and toxicity threshold (96-hr LC50). Comparing PAA disinfection to the previously used chlorination and dechlorination disinfection process provided notable benefits for the plant in both final effluent quality, and a reduced risk profile for breaching effluent discharge limits.
• Zero chlorine residual (TRC)
• No quenching required (only requires one chemical system instead of two for hypochlorite)

Project Background:

In efforts to address risks associated with noncompliance of total residual chlorine (TRC), the NW Langley wastewater treatment plant (Vancouver, BC) was looking for a safer and more reliable method for chemical disinfection of their final effluent. Initially, USP Technologies piloted a full-service PAA disinfection program to replace the existing chlorination and dechlorination process in order to demonstrate the performance improvements that can be achieved by using PAA for final effluent disinfection.

USP’s program application using PAA was chosen to replace the previous process as it showed improved final effluent quality; removed the risk associated with breaching TRC limits that had been observed when previously adding chlorine to the system for disinfection; improved process control compared to chlorine because of the issues associated with nitrite formation as the result of partial nitrification; and harnessed the ability of PAA to address contaminants of emerging concern such as endocrine disrupting compounds. Additionally, in a life-cycle cost comparison, the municipality determined that disinfection using PAA would yield savings of greater than 12% per year vs the existing chlorination program.

Technology:

Peracetic Acid (PAA) is an organic peroxide compound used for disinfection with the formula CH₃CO₃H. It is environmentally-friendly as it hydrolyzes to acetic acid and hydrogen peroxide in water.
It is a broad-spectrum biocide with a fast reaction time (minutes rather than hours), and leaves no harmful by-products. PAA is also a strong oxidant and disinfectant with oxidation potential higher than that of chlorine or chlorine dioxide.
PAA is available in the form of equilibrium mixture which also contains hydrogen peroxide, acetic acid (vinegar) and water as shown by the following equation:

CH3CO2H + H2O2 ⇌ CH3CO3H + H2O
Acetic Acid + Hydrogen Peroxide ⇌ Peracetic Acid + Water

Solution:

USP Technologies was asked to conduct pilot testing to determine required PAA doses for permit compliance and evaluate any possible toxicity. Results of the pilot testing indicated that an initial dose of 2 mg/L of PAA (as active ingredient) was sufficient to reduce fecal coliform concentrations to less than 700 MPN/100 ml. Further, full-scale, testing was then conducted during the disinfection season (April 1 through October 31) demonstrating that the average PAA demand of effluent at the NW Langley WWTP was 1.7 mg/L.

Additionally, testing was conducted to establish the maximum PAA residual that would not be toxic. The results of the testing showed that the 96-hour LC50 could be estimated to be 4.2 mg/L (as PAA). It is important to note that these results do not mean that PAA doses greater than 4 mg/L are toxic. When PAA is added to effluent, its reaction with constituents in the water, combined with its auto-decay, results in reduced concentration below LC50 levels. Based on the data collected in the full-scale trial, it was demonstrated the expected maximum dose of PAA that would need to be applied is approximately 3.0 mg/L, which would result in a residual of only 1.3 mg/L, much lower than the established threshold value for the 96-hour LC50 toxicity limit. In practice, final effluent PAA residual from the NW Langley WWTP to the Fraser River is <0.1 mg/L.

Following the successful evaluation of PAA toxicity and final effluent quality, Metro Vancouver determined the following benefits were realized by switching from chlorination/dechlorination using bulk liquid sodium hypochlorite and sodium bisulphite compared with disinfection using PAA:

Discharge effluent risks significantly reduced, to almost zero

•Zero chlorine residual (TRC)
•Initial dosage rate of PAA required was already lower than the effluent discharge limit

Reduced chemical footprint

•PAA disinfection only requires one chemical system instead of two for hypochlorite
•Less deliveries, less safety risk, less labor/manpower required to manage & operate

Potential for life-cycle cost-savings

•Per the WEF Special Publication in 20201, Metro Vancouver calculated they would save >12% over a 20-year life-cycle including construction costs and operational costs
•This would need to be evaluated on a case-by-case basis for other plants

Thus, in addition to allowing Metro Vancouver to manage risks associated with discharges of TRC, which are strictly limited into the Fraser River, the NW Langley WWTP asked USP Technologies to convert the full-scale pilot facilities into a permanent installation. The plant has been using USP’s full-service supply and application of PAA to meet its disinfection compliance without issues of toxicity or concerns related to TRC noncompliance risks ever since.

More Resources

Peracetic Acid Disinfection – Implementation Considerations for Water Resource Recovery Facilities (WEF Special Publication, First Edition, 2020 – Pages 274-280)

Disinfection https://staging.usptechnologies.com/disinfection/