5.5 CT Values, Contact Time & Log Inactivation Compliance

DISINFECTION is the single heaviest objective across the Missouri exam family — 126 items across the eleven published test descriptions, more than any other topic. This section covers the compliance arithmetic that turns a residual into a demonstrated kill.

What CT Is

$\text{CT} = C \times T$

where C is the disinfectant residual in mg/L measured at the end of the contact zone, and T is the contact time in minutes. The product has units of mg-min/L.

The insight behind CT is that inactivation depends on both concentration and exposure, and they trade off. A residual of 2.0 mg/L for 30 minutes (CT = 60) delivers roughly the same inactivation as 1.0 mg/L for 60 minutes (CT = 60). This gives operators a real degree of freedom: a plant that cannot raise its residual without exceeding a disinfection byproduct limit can instead gain contact time by baffling a clearwell.

Compliance is expressed as a ratio:

$\frac{\text{CT}_{\text{achieved}}}{\text{CT}_{\text{required}}} \geq 1.0$

The required value comes from published tables indexed by pathogen, disinfectant, temperature, pH and the target log inactivation.

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Contact Time Is Not Detention Time

This is where most CT errors originate. Theoretical detention time assumes plug flow — every drop spends exactly the same time in the basin:

$\text{TDT} = \frac{\text{Volume}}{\text{Flow}}$

Real basins short-circuit. Water finds a fast path from inlet to outlet, and some of it leaves far sooner than the average. Regulations therefore use T10 — the time at which 10 percent of the water has passed through, meaning 90 percent was held at least that long.

$T_{10} = \text{TDT} \times \text{Baffling factor}$

Pipelines are the operator's friend. A transmission main from clearwell to the first customer is essentially plug flow with a baffling factor of 1.0, and many small systems earn most of their CT there.

Worked example. A clearwell holds 180,000 gallons and the plant runs at 1.8 MGD. The basin has a perforated inlet baffle and an outlet weir with intra-basin baffles, so the baffling factor is 0.7. Free chlorine residual leaving the clearwell is 1.1 mg/L.

$\text{TDT} = \frac{180{,}000 \text{ gal}}{1{,}800{,}000 \text{ gal/day}} = 0.100 \text{ day} = 144 \text{ minutes}$

$T_{10} = 144 \times 0.7 = 100.8 \text{ minutes}$

$\text{CT}_{\text{achieved}} = 1.1 \times 100.8 = 111 \text{ mg-min/L}$

If the required CT for 0.5-log Giardia inactivation at that day's temperature and pH is 46 mg-min/L, the ratio is $111 / 46 = 2.4$, comfortably compliant.

Note what happens if the plant increases production to 3.6 MGD: TDT halves to 72 minutes, T10 falls to 50.4 minutes, and CT drops to 55 mg-min/L. Contact time is inversely proportional to flow, so the highest-flow hour of the day is the compliance-critical condition.

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What Drives Required CT

In Missouri, cold winter water at elevated pH is the controlling condition. A plant that comfortably meets CT in August at 25 °C can fail in January at 2 °C with the same residual and the same flow, because required CT may have quadrupled while achieved CT did not change at all. Operators who softened water to pH 9.5 and never revisited disinfection are a classic failure case.

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Log Inactivation and the Overall Requirement

Log inactivation is a logarithmic expression of how much of the organism population is destroyed:

The Surface Water Treatment Rule requires an overall 3-log (99.9%) removal or inactivation of Giardia lamblia and 4-log (99.99%) of viruses. Conventional filtration is credited with 2.5-log Giardia and 2.0-log virus removal, so disinfection must supply the remainder: 0.5-log Giardia and 2.0-log virus. Direct filtration receives less credit and therefore demands more from disinfection.

$\text{Log inactivation achieved} = 3.0 \times \frac{\text{CT}_{\text{achieved}}}{\text{CT}_{\text{required for 3-log}}}$

Cryptosporidium is the exception that shapes modern practice. Its oocysts are so resistant to free chlorine that achieving meaningful inactivation with chlorine is impractical at any realistic CT. The Long Term 2 Enhanced Surface Water Treatment Rule therefore relies on filtration performance, ultraviolet light and ozone for Cryptosporidium credit. UV is extraordinarily effective against Cryptosporidium and Giardia at low doses but provides no residual and essentially no virus credit at those doses, which is why UV plants still chlorinate.

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Operational Practice

• Measure the CT residual at the end of the contact zone, not at the point of application.
• Record the peak hourly flow, the lowest residual and the actual temperature and pH each day; compliance is assessed at the worst condition, not the daily average.
• Recalculate the baffling factor after any clearwell modification. Adding baffles is often the cheapest possible compliance improvement.
• If the clearwell is drawn down for maintenance, the effective volume falls and CT falls with it — plan disinfection around tank outages.