Disinfection CT (USEPA SWTR)

ScopeCT achieved (C * t10) compared to USEPA SWTR Guidance Manual required CT for 3-log Giardia inactivation by chlorine residual, temperature and pH. Bundled free-chlorine table covers 0.4 to 3.0 mg/L; state primacy agency table governs final compliance.

Formula or tableCT_achieved (mg-min/L) = chlorine_mg_l * t10_minutes. CT_required for 3-log Giardia is interpolated linearly in chlorine residual (<=0.4 to 3.0 mg/L), temperature (0.5-25 C) and pH (6.0-9.0) over the SWTR free-chlorine table. CT for 4-log virus is interpolated in temperature from the free-chlorine virus table (pH 6-9).

Source and editionUSEPA Disinfection Profiling and Benchmarking Technical Guidance Manual EPA 815-R-20-003 (2020), Tables B-1 and B-2, reproducing the SWTR Guidance Manual CT tables (public domain).

Free accessepa.gov/dwreginfo/surface-water-treatment-rules.

What governsEstimate. Operator of record and primacy agency govern.

API field nameschlorine_mg_l, t10_minutes, temperature_C, pH, CT_achieved, CT_required_3log_Giardia, pass_3log_giardia, pass_4log_virus

• Lookup table free-chlorine 3-log Giardia CT, 14 residuals (<=0.4-3.0 mg/L) x 6 temperatures (0.5-25 C) x 7 pH (6.0-9.0)USEPA EPA 815-R-20-003 Table B-1
• t10 contact time input is the tracer-derived t10, not the theoretical detention timeSWTR Guidance Manual procedure
• Virus credit 4-log virus CT 12 / 8 / 6 / 4 / 3 / 2 at 0.5 / 5 / 10 / 15 / 20 / 25 C, linear in temperatureUSEPA EPA 815-R-20-003 Table B-2
• Chlorine residual residuals below 0.4 mg/L read the <=0.4 row; above 3.0 mg/L the 3.0 row is used with a warningUSEPA EPA 815-R-20-003 Table B-1