Virus inactivation in low ozone exposure water reuse applications

In drinking water applications, an ozone exposure (Ct) based framework has been historically used to validate ozone disinfection. However, significant viral inactivation can be achieved with little to no measurable ozone exposure. Additionally, ozone exposure depends on multiple water quality variables as well as the calculation/ozone measurement method used. In this study, we evaluated alternative ozone monitoring frameworks as well as the impact of water quality variables on ozone decay kinetics and virus/coliform inactivation. Here we show that both change in UV₂₅₄ absorbance and applied O₃:TOC were well correlated with viral inactivation and these frameworks were resilient to changes in water quality. Both increasing temperature (12-30 ⁰C) and pH (5.5-8.4) was shown to significantly increase the ozone decay rate and decreased the resulting ozone exposure by as much as ∼90% in the case of pH. However, due to the increased reaction rate of ozone with viruses at elevated temperature and pH, there was only a minor impact (∼20% in the case of pH) in overall disinfection performance for a given O₃:TOC. These frameworks were also considered for variable source water with TOC (5-11 mg/L) and TSS (1.2-5.8 mg/L). Change in UV₂₅₄ absorbance or applied ozone dose (mg/L) were the strongest indicators of disinfection performance for source waters of variable TOC, however site-specific testing may be needed to apply this framework. Challenge testing with influent nitrite indicated that ozone disinfection performance is significantly impacted (>50% reduction in inactivation) in the presence of nitrite thus enforcing the importance of accounting for this value in the applied ozone dose. Multi-point ozone dissolution was investigated as an alternative ozone application method that may present a benefit with respect to overall disinfection performance especially if nitrite was present. Developing and validating these alternative monitoring frameworks and ozone application methods is imperative in water reuse applications where unnecessary elevated ozone exposure may lead to harmful byproduct formation.