About

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.

The main result is the rounded value, read straight from the tables with no interpolation. Each condition is rounded to the published value that needs more contact time: the next lower temperature, and the next higher residual and pH.

The interpolated value is shown beneath it. 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 has a formula, called the Regression Formula, that fits a curve to the values in the tables. It is shown alongside the table readings for comparison. Where they disagree, the tables are the authority: the tables are included in the Regulations, and the Regression Formula is a curve drawn through them afterwards.

Every table the calculator reads is on the EPA tables page, and works offline. The manual itself is on the EPA's site: EPA 815-R-99-013 (PDF).