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Chlorination and CT: California D3 water distribution operator math

Chlorine feed, demand and residual, hypochlorite, well and main disinfection, CT, chloramines.

Below are 3 worked examples. Each answer is computed with the formulas and constants on the CA Exam Conversion Sheet (revised 4/30/2026), and the solution names the line it uses.

Formulas from the sheet for this topic

Example 1. A system treats 18.3 MGD with a chlorine dose of 0.8 mg/L. How many pounds of chlorine gas are fed per day?

  1. A) 122 lbs/day
  2. B) 14.6 lbs/day
  3. C) 110 lbs/day
  4. D) 188 lbs/day
Show the answer and worked solution

Answer: A) 122 lbs/day

1. Gas (lbs) = MG × mg/L × 8.34 [Sheet p.1, Chlorination: "Gas (lbs) = MG x mg/L x 8.34"]; with MGD it gives lbs/day. 2. = 18.3 × 0.8 × 8.34 = 122.1 lbs/day. 3. Answer: 122 lbs/day (rounded to 3 significant figures).

Source: CA Exam Conversion Sheet (Revised 4/30/2026), p.1, Chlorination: "Gas (lbs) = MG x mg/L x 8.34"

Example 2. A clearwell has a contact time (T10) of 110 minutes and a free chlorine residual of 0.7 mg/L. The required CT from the regulatory table is 33 mg·min/L. What is the inactivation ratio?

  1. A) 3.33
  2. B) 0.300
  3. C) 2.33
  4. D) 0.429
Show the answer and worked solution

Answer: C) 2.33

1. C × T Calculated = T10 × Chlorine Residual [Sheet p.2, C x T Calculations: "C x T Calculated = T10 Value, min x Chlorine Residual (mg/L)"] = 110 × 0.7 = 77 mg·min/L. 2. Inactivation Ratio = Actual C × T ÷ Table "E" C × T [Sheet p.2, C x T Calculations: "Inactivation Ratio = Actual C x T / Table "E" C x T"] = 77 ÷ 33 = 2.333. 3. A ratio of 1.0 or more means the required inactivation is being met. 4. Answer: 2.33 (rounded to 3 significant figures).

Source: CA Exam Conversion Sheet (Revised 4/30/2026), p.2, C x T Calculations: "C x T Calculated = T10 Value, min x Chlorine Residual (mg/L)"; p.2, C x T Calculations: "Inactivation Ratio = Actual C x T / Table "E" C x T"

Example 3. Water leaving a contact tank has a free chlorine residual of 1.05 mg/L after a contact time (T10) of 10 minutes. What is the CT value?

  1. A) 5.25 mg·min/L
  2. B) 10.5 mg·min/L
  3. C) 0.175 mg·min/L
  4. D) 9.52 mg·min/L
Show the answer and worked solution

Answer: B) 10.5 mg·min/L

1. C × T = Chlorine Residual × Time [Sheet p.2, C x T Calculations: "C x T = Chlorine Residual (mg/L) x Time (min)"] = 1.05 × 10 = 10.5 mg·min/L. 2. Answer: 10.5 mg·min/L.

Source: CA Exam Conversion Sheet (Revised 4/30/2026), p.2, C x T Calculations: "C x T = Chlorine Residual (mg/L) x Time (min)"

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