Chlorine dose, demand, and residual

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 Dose = demand + residual. The residual is what is LEFT after the demand is satisfied, so you always feed for both. Once you have the dose, the feed rate is the pounds formula in MGD. The classic miss is dosing only to the residual you want to see.

1. The chlorine demand is 2.2 mg/L and you want a 0.8 mg/L residual. What dose must you apply?

   • A 1.4 mg/L
   • B 2.2 mg/L
   • C 0.8 mg/L
   • D 3.0 mg/L
   Show the worked solution

   Correct answer: D. 3.0 mg/L

   Dose = demand + residual = 2.2 + 0.8 = 3.0 mg/L. Dosing only to the 0.8 residual ignores the 2.2 the water consumes and never holds a residual; 1.4 subtracts instead of adds.

   Source: Public relationship: chlorine dose = chlorine demand + chlorine residual

2. You apply a 5.0 mg/L dose and measure a 0.7 mg/L residual afterward. What was the chlorine demand?

   • A 0.7 mg/L
   • B 3.5 mg/L
   • C 5.7 mg/L
   • D 4.3 mg/L
   Show the worked solution

   Correct answer: D. 4.3 mg/L

   Demand = dose - residual = 5.0 - 0.7 = 4.3 mg/L. Adding instead of subtracting gives 5.7. The demand is the part consumed before a stable residual holds.

   Source: Public relationship: chlorine demand = dose - residual

3. A 4.0 MGD plant has a 1.5 mg/L demand and wants a 0.5 mg/L residual. What is the chlorine feed rate?

   • A 16.7 lb/day
   • B 667 lb/day
   • C 50.0 lb/day
   • D 66.7 lb/day
   Show the worked solution

   Correct answer: D. 66.7 lb/day

   Dose = 1.5 + 0.5 = 2.0 mg/L. Feed = 4.0 x 2.0 x 8.34 = 66.7 lb/day. Feeding only to the 1.5 demand gives 50; feeding only to the 0.5 residual gives 16.7.

   Source: Public relationship: dose = demand + residual, then lb/day = MGD x dose x 8.34

4. A 1.0 MGD plant has a heavy 3.8 mg/L demand and wants a 1.2 mg/L residual. Feed rate in pounds per day?

   • A 10.0 lb/day
   • B 50.0 lb/day
   • C 31.7 lb/day
   • D 41.7 lb/day
   Show the worked solution

   Correct answer: D. 41.7 lb/day

   Dose = 3.8 + 1.2 = 5.0 mg/L. Feed = 1.0 x 5.0 x 8.34 = 41.7 lb/day. Dosing only to demand gives 31.7; only to residual gives 10.0.

   Source: Public relationship: dose = demand + residual, then lb/day = MGD x dose x 8.34

5. You dose 3.0 mg/L of chlorine and 30 minutes later the free chlorine residual reads 0.0 mg/L. What does this tell you?

   • A The residual is fine and no action is needed
   • B The water has no chlorine demand
   • C The dose was too high and should be cut
   • D The chlorine demand is at least 3.0 mg/L, so a higher dose is needed to hold a residual
   Show the worked solution

   Correct answer: D. The chlorine demand is at least 3.0 mg/L, so a higher dose is needed to hold a residual

   A zero residual after dosing 3.0 mg/L means the water consumed all of it: the demand is at least 3.0 mg/L. To hold a residual you must raise the dose above the demand, since dose = demand + residual.

   Source: Public relationship: a zero residual means demand >= dose applied