CT disinfection

Problem set: 5 worked problems

Each problem is original, authored from the underlying non-copyrightable relationship, with the full worked solution and the exact slip that produces each wrong answer. Work the problem before opening the solution. For a timed, randomized run with a per-topic score, use the free practice test.

Watch for CT = residual concentration x contact time, but the contact time is the T10, not the full detention time. T10 = detention time x the basin's baffling factor. Skipping the baffling factor overstates CT and can hide a compliance failure. Compliance is the CT achieved divided by the required CT, which must be at least 1.

1. A free chlorine residual of 1.0 mg/L contacts the water for a T10 of 40 minutes. What CT is achieved?

   • A 41 mg/L-min
   • B 400 mg/L-min
   • C 20 mg/L-min
   • D 40 mg/L-min
   Show the worked solution

   Correct answer: D. 40 mg/L-min

   CT = C x T = 1.0 x 40 = 40 mg/L-min. CT is a simple product once you have the residual and the T10 contact time.

   Source: Public relationship: CT = concentration (mg/L) x contact time (min)

2. A contact basin has a 60-minute detention time and a baffling factor of 0.7. What is the T10 contact time?

   • A 86 minutes
   • B 30 minutes
   • C 60 minutes
   • D 42 minutes
   Show the worked solution

   Correct answer: D. 42 minutes

   T10 = detention time x baffling factor = 60 x 0.7 = 42 minutes. The baffling factor is below 1 because short-circuiting means not all water gets the full detention time. Dividing by 0.7 instead (giving 86) is a common inversion.

   Source: Public relationship: T10 = detention time x baffling factor

3. Residual 0.5 mg/L, detention time 100 minutes, baffling factor 0.3. What CT is achieved?

   • A 50 mg/L-min
   • B 5 mg/L-min
   • C 30 mg/L-min
   • D 15 mg/L-min
   Show the worked solution

   Correct answer: D. 15 mg/L-min

   First T10 = 100 x 0.3 = 30 minutes. Then CT = 0.5 x 30 = 15 mg/L-min. Using the full 100-minute detention time (giving 50) ignores the baffling factor and overstates disinfection.

   Source: Public relationship: T10 = detention x baffling factor, then CT = C x T10

4. A plant achieves a CT of 24 mg/L-min. The required CT from the EPA table is 18 mg/L-min. Is the plant compliant, and by what ratio?

   • A Compliant, ratio 6
   • B Not compliant, ratio 432
   • C Not compliant, ratio 0.75
   • D Compliant, ratio 1.33
   Show the worked solution

   Correct answer: D. Compliant, ratio 1.33

   The CT ratio is CT achieved / CT required = 24 / 18 = 1.33. A ratio of 1.0 or more means the required inactivation was met, so the plant is compliant. Inverting the ratio gives 0.75 and the wrong conclusion.

   Source: Public relationship: CT compliance ratio = CT achieved / CT required, >= 1 is compliant

5. An operator adds baffling walls to a contact tank, raising its baffling factor from 0.3 to 0.5 at the same flow and residual. What happens to the CT achieved?

   • A It falls, because baffling reduces contact
   • B It is unchanged, because only residual affects CT
   • C It falls, because the tank now holds less water
   • D It rises, because a higher baffling factor lengthens the T10 contact time
   Show the worked solution

   Correct answer: D. It rises, because a higher baffling factor lengthens the T10 contact time

   T10 = detention x baffling factor, so raising the baffling factor from 0.3 to 0.5 lengthens the effective contact time and raises CT proportionally. Better baffling reduces short-circuiting, which is why plants add baffle walls to gain CT credit.

   Source: Public relationship: CT rises with the baffling factor through T10