Chlorine Contact Tank: CT Value & Pathogen Inactivation

Chlorine Dose

Free chlorine C 2.0 mg/L

Flow Rate

Q 60 m³/min

Baffling Condition

Target Pathogen

Live Readouts

Theoretical HRT (T)

50.0 min

T₁₀ (10% breakthrough)

25.0 min

CT achieved

50 mg·min/L

CT required

100 mg·min/L

Log inactivation at T₁₀

1.4-log

Regulatory compliance

FAIL

How it works

A chlorine contact tank must hold water long enough, at a high enough chlorine concentration, to inactivate pathogens before the water leaves the plant. Disinfection follows the Chick–Watson law:

N/N0 = exp(-k · C · t)
log10 inactivation = k · C · t / ln(10)

where C is the free-chlorine concentration (mg/L), t is contact time (min) and k is a pathogen-specific rate constant. Regulators don't credit the average detention time V/Q — baffle walls create dead zones and short-circuiting, so only T₁₀ (the time by which 10% of a tracer has already broken through) counts:

Baffling factor BF = T10 / T   (T = V/Q, theoretical HRT)
CT value = C · T10   (mg·min/L)

Dose & Flow — set the free-chlorine concentration and the plant flow rate; a higher Q shortens the theoretical detention time T = V/Q for this fixed-volume tank.
Baffling condition — the five classes (Unbaffled 0.1 … Perfect 0.95) are the standard USEPA baffling-factor ratings used to convert theoretical HRT into the credited T₁₀. Poor baffling sends a large fraction of flow through the serpentine on a short-cut path (visualised as the faster, redder particles skipping most of the channel).
Target pathogen — Giardia cysts need roughly a 3-log (99.9%) reduction, viruses roughly 4-log (99.99%); each has its own approximate rate constant and required CT, illustrative of USEPA Surface Water Treatment Rule CT tables at ~10°C, pH 7.
Particles fade from red to green as their individual elapsed contact time × dose accumulates enough log-inactivation — short-circuited particles exit the tank still under-treated.

Under-treated (low log-inactivation) Fully inactivated Baffle wall

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