Quantifying drought-driven temperature impacts on ozone disinfection credit and bromate control†

Climate change and drought can lead to unprecedented changes in surface water temperature requiring utilities to examine their ozone system's disinfection capability while minimizing bromate production. This pilot-scale study investigated temperature (15–30 °C) as a single/isolated variable affecting ozone operating performance (demand, decay rate, exposure (CT)) and the ability to achieve a Cryptosporidium log reduction value (LRV) of 0.5–1.5 logs, as defined by the United States Environmental Protection Agency (USEPA). When dosing 3.0 mg L⁻¹ of ozone into a surface water with 2.5 mg L⁻¹ of total organic carbon, an increase in temperature from 15 °C to 30 °C increased ozone demand in the dissolution zone from 1.0 mg L⁻¹ to 1.6 mg L⁻¹ (60%) and ozone decay rate from 0.07 min⁻¹ to 0.27 min⁻¹ (385%). Despite more rapid demand/decay, the required ozone dose to achieve an LRV of 1.5 logs remained at 2.4–2.8 mg L⁻¹ due to the reduction in USEPA's CT requirement at higher temperatures (9.35 mg min L⁻¹ at 15 °C vs. 2.31 mg min L⁻¹ at 30 °C). Bromate formation exceeded the USEPA maximum contaminant level of 10 μg L⁻¹ when ozone was dosed to achieve LRV > 0.5 log at all temperature conditions. Chlorine–ammonium pretreatment (0.5 mg L⁻¹ Cl₂, 0.1–0.5 mg L⁻¹ NH₄⁺-N) lowered bromate formation to <5 μg L⁻¹ under ambient (80 μg L⁻¹) and elevated (120 μg L⁻¹) bromide concentrations at all temperatures. These results were applied to evaluate a full-scale ozone system designed to achieve an LRV of 1.5 logs if drought increases temperature from 13 °C to 26 °C. The study systematically examined the role of temperature on ozone system performance, which can assist utilities planning for future drought-driven changes.