Performic Acid (PFA) Disinfection Pilot Program and Options for PFA Methods of Analysis

BACKGROUND
The disinfection of municipal wastewater (WW) is essential to ensure discharge quality and minimize potential human health risks. Effective treatment is particularly essential when discharges are near recreational water. Chlorine-based disinfectants are the most common disinfectants due to their low costs, but they produce harmful by-products. In this pilot study, an alternative disinfectant is evaluated, performic acid (PFA), and methods for the manual and online detection of PFA. Kemira KemConnectâ„¢ DEX is an advanced technology designed to meet future sustainability requirements for disinfection. KemConnect DEX is a treatment process that combines hydrogen peroxide and formic acid onsite to form PFA. Peracetic acid (PAA), like PFA, is another peracid that has been used in WW disinfection for 40 years. PAA has a longer carbon chain that makes it stable to be stored onsite as a liquid product. In comparison, PFA, which has been used in Europe safely and effectively for over a decade, has a greater reactivity and is, therefore, made on-site using the DEX unit. The DEX unit includes a built-in reaction vessel, dosing pumps, process control box, safety flushing system, cooling/heating, and an uninterruptible power supply (Figure 1). Disinfection occurs within minutes, and any residual PFA degrades into CO2 and H2O within an hour and has low potential for disinfection by-product formation (Karpova et al., 2013). A full-scale implementation of KemConnect DEX at an Italian wastewater treatment plant (WWTP) showed that PFA is an effective disinfectant, ensuring the reduction of bacteria to below established limits (Regazzo et al., 2013). Kemira is seeking product registrations to sell PFA in North America. As part of the registration process, efficacy tests were conducted using PFA on WW from two Canadian WWTPs. The data showed greater than a 3-log reduction for indicator bacteria using a 1 mg/L PFA dose and 5 minutes contact-time on secondary effluent (Porat et al., 2019). PFA efficacy was tested on samples from three WWTPs in the US. Results demonstrated that low PFA concentrations (0.25-0.5 mg/L) reduced E. coli to below required counts, and PFA degrades to below toxic levels in a short time (Porat et al., 2021). In addition, PFA toxicity data against Pimephales promelas and Ceriodaphnia dubia, model organisms for toxicity in the US, showed higher toxicity for C. dubia. The PFA LC10 value for C. dubia tested in WW was 0.30 mg/L. This could become the PFA discharge limit value when no dilution is available (Porat et al., 2021).
OBJECTIVES
It is expected that US and Canadian regulatory authorities will require PFA residual monitoring to ensure dosing meets microbial reduction targets and to confirm that water quality limits are being achieved. PFA measurement methods should be rugged, accurate, fast, and operator-friendly to monitor residual PFA levels efficiently and effectively. This study aims to validate methods for detecting PFA, manual or online applications, and to test an online PFA detection method in a KemConnect DEX pilot.
METHODOLOGY and RESULTS
PAA (CH3COOOH) is chemically similar to PFA (HCOOOH). Currently, PAA is more widely used than PFA, and commercial methods exist to analyze PAA. The Reflectoquant PAA test with the RQflex® 20 (Merck KGaA, Darmstadt, Germany) is designed to determine PAA concentrations. The detection range for the PAA test is 1 to 22.5 mg/L PAA. Due to the difference in the molecular weights of PFA and PAA, a factor of 0.82 is used to convert the PAA value into PFA concentration. To validate the Reflectoquant method for PFA, fresh PFA solution was prepared, and the concentration was determined first by the two-step titration method (Gehr et al., 2009). PFA measurement was linear (r2=0.98) throughout the detection range (0.8 – 18.5 mg/l); the accuracy (recovery 85 – 116 %) and repeatability (RSD <9.2%) were within acceptable parameters. A PAA method using N-N-diethyl–p-phenylenediamine (DPD) method is now published (4500-PAA PERACETIC ACID). A Joint Task Group has been established to validate this method for PFA measurement. Results of our testing include calibration curves for PFA and PAA that showed slopes with a ratio of 0.82 (3.2/3.9), in agreement with the molecular weight conversion factor mentioned above. Also, accuracy (recovery 102%), repeatability (RSD <4.6%), and method detection limit (MDL=0.03 mg/L PFA) were comparable to Standard Methods PAA single operator data signaling that the DPD method is applicable for PFA. Two online devices (Device 1 and Device 2) provided a good correlation with the manual method CHEMetrics K-7913 kit (Figure 2). Both online devices were set up in the lab, and both devices were fed from the same tank containing secondary effluent with PFA. KemConnect DEX technology with online Device 1 was tested in a pilot run at a Canadian WWTP, where their WW was previously tested (Porat et al. 2019, Figure 3). The WW to be treated came from the clarifiers and was sent to a contact tank using a centrifugal pump. The treated water then passed by overflowing into a retention tank (tote) before being returned to the beginning of the plant's treatment process. The DEX unit was installed inside a container (Figure 3), and it generated 10.4±0.7% PFA. The WW was treated with 1.1 mg/L PFA for 15 minutes retention time in the contact tank. After treatment, the residual PFA in the WW was monitored by the online Device 1, connected to the KemConnect system. Kemira's KemConnect system provides live online results to customers. During the pilot, the system generated daily reports (Figure 4). Under current conditions, the fecal coliform was maintained (7 out of 9 times tested) below the required limit (200 CFU/100 mL), and the residual PFA was below 0.52 mg/L PFA (PFA LC50 for Daphnia magna, Porat et al. 2021). In addition, no significant difference between PFA treated and untreated WW were found during the pilot run when tested for pH, COD, TSS, alkalinity, ammonia.
CONCLUSIONS
Two manual methods are applicable for the detection of PFA. CHEMetrics K-7913 kit for lower concentrations and Reflectoquant PAA test if the concentrations are higher. The KemConnect DEX system provides disinfection by reducing bacteria to below-required limits, then the active disinfectant, PFA, degrades to below toxic limits. The KemConnect DEX system offers online monitoring of PFA production and residual PFA.
REGULATORY NOTE
The pesticide technology described herein, is not available for sale or distribution in the United States or Canada until the applicable registration is approved by the respective agency.