Comparative Evaluation of Chlorine, Performic Acid, and Peracetic Acid Disinfection Kinetics Using Fixed‑Dose and iCT‑Paced Control

1. Introduction Wastewater disinfection is a critical barrier for protecting public health, yet its performance is strongly influenced by disinfectant chemistry, wastewater matrix characteristics, and operational variability. Sodium hypochlorite remains the most common disinfectant in North America, but increasing interest in alternatives such as performic acid (PFA) and peracetic acid (PAA) has emerged due to concerns about chlorinated by products, evolving regulatory expectations, and the need for more resilient disinfection strategies. Despite this growing interest, comparative data evaluating how these disinfectants differ in their disinfection kinetics, CT requirements, and reactivity under real secondary effluent conditions remain limited. The objective of this study was to generate a comprehensive comparison of sodium hypochlorite, PFA, and PAA by characterizing their pathogen inactivation performance, disinfectant specific CT needs, and kinetic behavior, including demand and decay. A second objective was to evaluate how differences in disinfectant reactivity influence the consistency of disinfection performance when applied under variable wastewater conditions. By establishing clear kinetic benchmarks and examining their implications for operational control, the study aims to provide utilities with data driven guidance for selecting and optimizing disinfectants in modern wastewater treatment applications. 2. Methods This study used a combined bench scale and onsite approach to characterize disinfectant kinetics under both controlled and real world conditions. Bench scale testing with field collected wastewater was used to determine disinfectant specific CT values for reducing fecal coliform and enterococci, and to quantify inactivation kinetics, demand, and decay under controlled dosages. Onsite evaluation was carried out in Mississauga, Ontario, with a 32 L slip stream batch reactor operated in fixed dose mode (3 mg/L) and adaptive iCT paced dosing mode.

• Fixed dose operation (3 mg/L), which held the applied dose constant and allowed observation of natural variability in demand, decay, and residual behavior.

• iCT paced operation, which continuously adjusted disinfectant dosing in real time to meet the CT target, compensating for changes in water quality, demand, and decay

Residual disinfectant was measured every 2.5 minutes to calculate instantaneous demand and first order decay. Pathogen samples and residual profiles were collected across sequential batch cycles to assess how disinfectant reactivity such as demand, decay, and CT, translated into actual disinfection performance. Together, this integrated approach enabled a comprehensive comparison of disinfectant behavior across both conventional fixed dose control and real time iCT paced operation. 3. Results and Discussion Bench scale testing was first conducted to determine the disinfectant specific CT values required to reduce both fecal coliform and enterococci to 100 cfu/100 mL, ensuring each disinfectant was evaluated against the same performance target. These tests established clear CT targets (8.7 mg·min/L for PFA, 25 mg·min/L for sodium hypochlorite, and 60 mg·min/L for PAA), which served as the basis for subsequent onsite evaluation for iCT-paced operation. Fixed dose operation and iCT paced operation were carried out onsite using the slip stream system which is a small scale, controlled side stream setup that diverts a portion of wastewater to evaluate disinfectant kinetics and performance without affecting full scale plant operations and is operated in a sequential batch mode. Under fixed dose operation (3 mg/L), the disinfectants exhibited distinct kinetic behavior. Average instantaneous demand followed the order PAA > NaOCl > PFA, with values of approximately 0.98 mg/L, 0.85 mg/L, and 0.64 mg/L, respectively. In contrast, decay rates exhibited the opposite trend: PFA decayed fastest (0.660 1/hr), sodium hypochlorite decayed at a moderate rate (0.131 1/hr), and PAA decayed most slowly (0.058 1/hr). The application of iCT paced dosing markedly improved operational consistency. Because iCT control adjusts the applied dose based on real time residual trends, it compensated for fluctuations in water quality that otherwise caused variability under fixed dosing. As a result, the system-maintained target CT levels with significantly greater stability by adjusting dosage based on real time demand and decay value and alignment with intended microbial reduction. This demonstrates a key operational insight: a disinfectant's apparent performance depends not only on its intrinsic kinetics but also on the dosing strategy's ability to manage variations in demand and decay. Overall, the combined bench scale and onsite results show that PFA offers the most rapid inactivation at the lowest CT, sodium hypochlorite remains the most stable and operationally reliable, and PAA provides moderate effectiveness with relatively high CT requirements. More importantly, iCT paced dosing enhances CT delivery accuracy and produces more consistent pathogen inactivation, positioning it as a powerful tool for both research and full scale operational control. The findings underscore the importance of accounting for kinetic behavior such as demand, decay, and reactivity in selecting disinfectants and designing dosing strategies for resilient, adaptive wastewater disinfection.