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Chlorination and CT: California D2 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 well pumps 2,350 GPM around the clock. How many lbs/day of chlorine are needed for a dose of 3.8 mg/L?

  1. A) 4.47 lbs/day
  2. B) 107 lbs/day
  3. C) 12.9 lbs/day
  4. D) 166 lbs/day
Show the answer and worked solution

Answer: B) 107 lbs/day

1. Flow = 2,350 ÷ 694.4 = 3.384 MGD [Sheet p.1, Units and Conversion Factors, row 29: "1 MGD = 694.4 GPM"]. 2. Gas (lbs) = MG × mg/L × 8.34 [Sheet p.1, Chlorination: "Gas (lbs) = MG x mg/L x 8.34"] = 3.384 × 3.8 × 8.34 = 107.2 lbs/day. 3. Answer: 107 lbs/day (rounded to 3 significant figures).

Source: CA Exam Conversion Sheet (Revised 4/30/2026), p.1, Units and Conversion Factors, row 29: "1 MGD = 694.4 GPM"; p.1, Chlorination: "Gas (lbs) = MG x mg/L x 8.34"

Example 2. A 14-inch well casing holds a 260-ft column of water. How many pounds of 65% HTH are needed to dose the water in the casing at 25 mg/L?

  1. A) 1.00 lbs
  2. B) 0.667 lbs
  3. C) 2.67 lbs
  4. D) 1.33 lbs
Show the answer and worked solution

Answer: B) 0.667 lbs

1. Volume = .785 × D² × Height × 7.48 [Sheet p.1, Area and Volume: "Cylinder Volume (Gal) = .785 x D2 x Height or Length (Ft) x 7.48"], with D = 14 in ÷ 12 [12 in = 1 ft]. 2. = .785 × (14 ÷ 12)² × 260 × 7.48 = 2,078 gal. 3. MG = 2,078 ÷ 1,000,000 = 0.002078 MG [Sheet p.1, Unit Definition: "MG Million Gallons" (1 MG = 1,000,000 gal)]. 4. HTH Solid (lbs) = MG × mg/L × 8.34 ÷ (Strength/100) [Sheet p.1, Chlorination: "HTH Solid (lbs) = MG x mg/L x 8.34 / (Strength (%)/100)"] = 0.002078 × 25 × 8.34 ÷ 0.65 = 0.6666 lbs. 5. Answer: 0.667 lbs (rounded to 3 significant figures).

Source: CA Exam Conversion Sheet (Revised 4/30/2026), p.1, Area and Volume: "Cylinder Volume (Gal) = .785 x D2 x Height or Length (Ft) x 7.48"; p.1, Unit Definition: "MG Million Gallons" (1 MG = 1,000,000 gal); p.1, Chlorination: "HTH Solid (lbs) = MG x mg/L x 8.34 / (Strength (%)/100)"

Example 3. A system treats 15.1 MGD with a chlorine dose of 1.5 mg/L. How many pounds of chlorine gas are fed per day?

  1. A) 291 lbs/day
  2. B) 22.7 lbs/day
  3. C) 189 lbs/day
  4. D) 169 lbs/day
Show the answer and worked solution

Answer: C) 189 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. = 15.1 × 1.5 × 8.34 = 188.901 lbs/day. 3. Answer: 189 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"

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