SME
Environmental Engineering · Problem type
CT for disinfection
Use this for “Free chlorine at 1.2 mg/L is held for a t₁₀ of 25 min. What is CT, and does it meet a required CT of 24 mg·min/L?”
Chart
24 mg·min/L
This is an example. Type your own values to start a new problem.
CT (mg·min/L)
=
C (mg/L)
×
t₁₀ (min)
IR
=
CT (mg·min/L)
CTreq (mg·min/L)
Formulas
CT
=
C
×
t₁₀
30 = 1.2 × 25
IR
=
CT
CTreq
1.25
=
30
24
Assumptions
•
C is the free chlorine residual at the contact tank’s outlet, held through the contact time.
•
t₁₀ is the time the first 10% of the water takes to pass the tank, from a tracer test.
•
CTreq is read from the regulator’s tables for the log inactivation, temperature and pH.
Step-by-step
WE KNOW
C = 1.2 mg/L, t₁₀ = 25 min, CTreq = 24 mg·min/L
FIND
CT achieved (CT), inactivation ratio (IR)
Step 1 · Find CT achieved (CT)
Use CT = C × t₁₀
The credit grows with both the dose the microbes meet and how long they meet it.
CT = 1.2 × 25
CT = 30 mg·min/L
Step 2 · Find inactivation ratio (IR)
Use IR = CT ÷ CTreq
Compare the CT achieved with the CT required: 1 or more meets the target.
IR
=
30
24
IR = 1.25
Check
Put the numbers back in. Both sides should match.
30 = 1.2 × 25 ✓
1.25
=
30
24
✓
Topic check
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