Monochloramine Formation and Decay in the Presence of ${\mathrm{H}}_2{\mathrm{O}}_2$ after $\mathrm{UV}/{\mathrm{H}}_2{\mathrm{O}}_2$ Advanced Oxidation

When treating drinking water using $\mathrm{UV}/{\mathrm{H}}_2{\mathrm{O}}_2$ advanced oxidation, the ${\mathrm{H}}_2{\mathrm{O}}_2$ residual is usually quenched to allow for downstream chlorine stability. In some utilities, monochloramine (${\mathrm{NH}}_2\mathrm{Cl}$) is used for secondary disinfection, and ${\mathrm{H}}_2{\mathrm{O}}_2$ may not need to be quenched because ${\mathrm{NH}}_2\mathrm{Cl}$ and ${\mathrm{H}}_2{\mathrm{O}}_2$ can coexist for some time. However, when ammonia and chlorine are applied to form ${\mathrm{NH}}_2\mathrm{Cl}$ in the ${\mathrm{H}}_2{\mathrm{O}}_2$-containing water, ${\mathrm{H}}_2{\mathrm{O}}_2$ will compete with ammonia to react with the applied chlorine, compromising the ${\mathrm{NH}}_2\mathrm{Cl}$ formation efficiency. This research combined theory and experiments to evaluate ${\mathrm{NH}}_2\mathrm{Cl}$ formation efficiency in the presence of ${\mathrm{H}}_2{\mathrm{O}}_2$ in Lake Ontario water and, after ${\mathrm{NH}}_2\mathrm{Cl}$ formation, its subsequent decay at different temperatures and pH. The results demonstrated that at many typical temperatures and pH, the presence of ${\mathrm{H}}_2{\mathrm{O}}_2$ does not significantly impair the formation of ${\mathrm{NH}}_2\mathrm{Cl}$. Furthermore, while ${\mathrm{H}}_2{\mathrm{O}}_2$ accelerates ${\mathrm{NH}}_2\mathrm{Cl}$ decay, the results suggest that under specific conditions, such as short to medium residence times (e.g., less than 48 h), it may be possible for ${\mathrm{NH}}_2\mathrm{Cl}$ to coexist with as much as $5\mathrm{mg}/\mathrm{L}$ ${\mathrm{H}}_2{\mathrm{O}}_2$ without compromising secondary disinfection.