Disinfection is a first$-$order process. Accordingly, the US EPA developed a $$simplified$$ method for determining the amount of disinfection to require for drinking water treatment. The CT data are based on disinfection as conducted in a batch reactor. A CT of $100$ mg$/($L min$)$ that provides for a $99\mathrm{.}9\%,3-$log$,$ reduction of Giardia really represents the combination of concentration and time in a batch reactor required to achieve a $99\mathrm{.}9\%$ reduction in viable Giardia cysts$.$ The US EPA has added a safety factor of $1$ log to all CT values $($meaning a CT that is for $99\mathrm{.}9\%,3-$log$,$ reduction in reality should provide a $99\mathrm{.}99\%,4-$log$,$ reduction$).$