Performic Acid Disinfection at Capital Region Water: Including Stormwater Events

The Capital Region Water (CRW) Advanced Wastewater Treatment Facility (AWTF) sits along the banks of the Susquehanna River in Harrisburg, the capital of Pennsylvania. The facility treats 21 MGD, with a service population of 125,000. CRW is one of the largest potential point sources of water contamination in the Chesapeake Bay watershed. The city's collection system includes approximately 60% combined sewers, many built in the 19th century. Wet weather can push peak flows to 80 MGD. The CRW process includes grit removal, primary clarifiers, biological treatment, post-anoxic tanks, final clarifiers, and disinfection contact tanks (Fig 1). Recent capital investments at the AWTF have focused on nutrient removal and energy recovery; however, the disinfection process is nearly identical to the one installed at initial commissioning in 1959. Currently, chlorinators dissolve gaseous chlorine into an aqueous solution, which is then added to treated effluent. Increased chlorine costs, the risk of storing large volumes onsite, and concern regarding chlorine's disinfection byproducts (DBPs) challenge the future use of chlorine at CRW. Implementing increased wet weather capacity also increases the demand on the disinfection process. For these reasons, CRW is evaluating alternative disinfection technologies. PFA is a wastewater disinfection technology that has been in use in Europe for over a decade. Currently, PFA is used in nine European countries for disinfection at 18 municipal wastewater treatment plants (WWTPs) and at 20 paper manufacturing facilities. Studies were performed prior to PFA implementation at Paris and Biarritz (France), and Venice (Italy). These case studies, addressing efficacy and European discharge requirements, were published in 2021 (7). PFA is a peracid made from hydrogen peroxide and formic acid; it is similar to peracetic acid (PAA). Compared to PAA, PFA has a shorter carbon chain that makes it unstable for storage, so PFA must be made onsite. PFA reacts rapidly with a higher redox potential compared to most disinfectants (9). Post disinfection, residual PFA degrades into carbon dioxide and water. Studies have shown that PFA has a low potential for toxic DBPs formation (2, 8). To comply with regulations for selling PFA in North America (NA), lab scale efficacy tests were performed. Good results were obtained using doses from 0.25 to 1.0 mg/L with contact times of 5 to 20 minutes (3, 4). Later PFA efficacy was tested at pilot scale in QC, Canada and OR, USA using secondary WW. PFA reduced required indicator bacteria below their permit limits with doses of 1.0 -1.5 mg/L using 15 - 30 minute retention times (5, 6). No toxic effects were documented in these studies (4, 6). The purpose of this research is: (1) Evaluate if PFA reduces fecal coliform (FC) below CRW's permit limit in secondary WW, (2) Evaluate disinfection during stormwater events, (3) Compare methods for FC enumeration Initial lab studies showed good efficacy for PFA at 0.5 mg/L (Fig 2). The pilot design system was similar to a previous pilot run (6). The pilot located at CRW (Fig 3) included the PFA generator, contact basins, precursor chemicals, and automatic samplers (Fig 4). The online probes included pH, TSS, and residual PFA. The pilot flow rate was 34 gpm of WW, and the effluent was collected from port #6 with a retention time of 34 minutes. To mimic storm events, the pilot at CRW included the testing of blended mixtures of secondary and primary WW (Fig 5). FC was maintained below the required limit using (Fig 6): (1) 0.5 mg/L PFA with 100% secondary WW, (2) 1.5 mg/L PFA with 90% secondary and 10% primary WW, (3) 2.3 mg/L PFA with 80% secondary and 20% primary WW. The effect of WW blended with PFA doses is shown in Fig 7. A higher proportion of primary WW increases the demand for PFA. PFA didn't affect other parameters tested in composite samples (Table 1). Two methods for FC enumeration, IDEXX Colilert-18 and IDEXX Tecta system, were used during the pilot. Tecta was tested to simplify FC enumeration during the pilot. This is a semi-automatic system where a sample is tested, without dilution, and an email report is sent upon test completion. The instrument follows FC growth by identification of the beta-galactosidase activity signal. Tecta system is an EPA approved method for drinking water but not currently approved for WW. Comparable results between the Tecta system and the Colilert method have been published for FC enumeration (1). During the pilot at CRW, samples were collected and tested using both methods. Comparable results were obtained (Fig 8) after IDEXX technical support provided additional calibration of the Tecta unit. In conclusion, the pilot study demonstrated that very low doses of PFA disinfected and met CRW FC limits. Low doses have a positive impact on disinfection costs. Compliant disinfection was achievable with a mixture of primary and secondary WW using elevated doses of PFA. This simulates stormwater events when primary and secondary WW are mixed. The IDEXX Tecta system produced comparable results to the IDEXX Colilert-18 in this study and it decreased the time, cost and complexity of the FC testing process. The result of this study and the known environmentally preferable characteristics of PFA support its use as an alternate disinfectant. These benefits are most important during storm events when higher levels of organics react with chlorine increasing DBPs.