This contact time calculator works out how much contact time is needed to adequately inactivate giardia and viruses, for a range of disinfectants. These calculations can be useful in determining compliance with the Surface Water Treatment Rule.
The contact time needed is determined from the disinfectant, the logs of inactivation required, the temperature, and — for free chlorine against giardia — the pH and disinfectant residual. It uses the methods and tables in EPA manual 815-R-99-013, “Disinfection Profiling and Benchmarking Guidance Manual”, dated August 1999.
How a result is reached
The tables are published on a coarse grid, so a reading between grid points is interpolated — across temperature alone for most disinfectants, and across temperature, residual and pH together for free chlorine against giardia. Conditions outside the published range are refused rather than extrapolated, because past the edge of the tables there is no measurement to extend.
For free chlorine against giardia the manual also publishes a regression fitted to the same surface. It is shown alongside the table reading for comparison. Where the two disagree, the table is the authority: the rule is written against the tables, and the regression is a curve drawn through them afterwards.