Chlorine CT Disinfection
Free-chlorine log inactivation for Giardia cysts and viruses, per EPA Surface Water Treatment Rule (SWTR) Guidance Manual. Computes achieved CT, required CT for target log credit, and inactivation ratio.
Defaults: 1.5 mg/L free chlorine, 30-min T10 contact time, 10°C, pH 7.5 — typical surface-water plant winter conditions targeting 3-log Giardia.
EPA SWTR CT tables — what's coded here
This calculator uses the EPA's free-chlorine CT tables for 3-log Giardia inactivation interpolated by free chlorine, temperature, and pH. The tables are from the SWTR Guidance Manual (1991 / amended 1999, 40 CFR §141.74). For viruses, EPA provides separate (smaller) CT tables — viruses are easier to kill with chlorine than Giardia.
Interpolation here is linear in temperature and free chlorine (approximating EPA's approach). pH dependency is captured through the dominant chemistry: hypochlorous acid (HOCl) is the active disinfectant, and HOCl/OCl⁻ equilibrium shifts toward less-active hypochlorite at high pH. CT requirements roughly double from pH 7 to pH 9.
T10, not theoretical detention time
The contact time used in CT must be T10 — the time at which 10% of a tracer pulse has exited the chamber. T10 is always less than theoretical V/Q because of short-circuiting. The ratio T10/HRT depends on baffling:
- Excellent baffling (perforated wall, serpentine): T10/HRT ≈ 0.7
- Good baffling (intra-tank baffles, multiple basins): 0.5
- Average (single inlet/outlet baffle): 0.3
- Poor (no baffling, plug rectangular): 0.1
Tracer studies (with NaCl or fluoride) measure T10 directly. Without a tracer study, EPA recommends conservative T10/HRT factors above.
Why 3-log Giardia, 4-log virus
The SWTR requires 3-log overall removal/inactivation of Giardia cysts and 4-log of viruses through the entire treatment train. Chlorination contributes part of this; conventional clarification + filtration is credited 2.5-log Giardia and 2-log virus removal, leaving disinfection responsible for the residual 0.5-log Giardia and 2-log virus credit.
Free vs. combined chlorine
This calculator is for free chlorine only. Combined chlorine (chloramines) has 100× lower disinfection power — the CT required for combined chlorine is much higher and is dominated by NH₂Cl decay kinetics rather than HOCl chemistry. Use combined-chlorine CT tables (separate, in the EPA Guidance Manual) for chloraminated water.
Reference: USEPA (1991, amended 1999). Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources. CT tables in 40 CFR Part 141 Subpart H.