Chlorine Contact Tank & Disinfection CT Calculator

Determine EPA Surface Water Treatment Rule (SWTR) CT compliance, T10 effective contact detention time, baffling factors, and log-inactivation of pathogens.

Contact Basin & Plant Flow

Peak Hourly Flow Rate (MGD):

Basin Liquid Volume (Gallons):

EPA Baffling Classification (T10 / T):

Water Quality & Disinfectant

Effluent Free Chlorine (mg/L):

Water Temperature (°C):

Water pH:

Target Pathogen:

EPA Disinfection CT Evaluation

Achieved CT Value

-- mg·min/L

-- compliance

T10 Effective Time

-- min

-- min nominal T

Required EPA CT

-- mg·min/L

-- ratio

Achieved Log Inactivation: -- Log

Short-Circuiting Lost Volume: --% dead space

Min Safe Chlorine Residual: -- mg/L

EPA Surface Water Treatment Rule requires using T10 (time for 10% of water to pass through basin) rather than theoretical retention time T to protect against short-circuiting.

Frequently Asked Questions

What is the "CT" disinfection concept in water treatment?

CT is the mathematical product of the disinfectant residual concentration (C in mg/L) and the effective contact time (T in minutes): CT = C × T. The EPA Surface Water Treatment Rule (SWTR) establishes minimum CT requirements to guarantee verified log-kill of waterborne pathogens.

Why does the EPA mandate using T10 instead of hydraulic retention time?

Unbaffled rectangular or circular tanks suffer from intense hydraulic short-circuiting where incoming water rushes straight from inlet to outlet along channel flow paths. T10 is the time it takes for the first 10% of water molecules to pass through the basin, representing the minimum disinfection exposure experienced by the fastest-moving water.

Why does higher pH make chlorine disinfection significantly less effective?

When chlorine dissolves in water, it forms hypochlorous acid (HOCl) and hypochlorite ion (OCl-). HOCl is roughly 80 to 100 times more potent a germicide than OCl-. At pH 6.5, over 90% exists as potent HOCl; but at pH 8.0, only 20% remains as HOCl, requiring up to triple the CT contact time to achieve identical pathogen kill.