Jar testing for coagulation produced an optimum coagulant dosage of 42.8 mg/L at a plant treating 2.0 MGD; with alum costing $200 per ton. What is the chemical cost for a 30-day period?

• A. $1,500
• B. $1,944
• C. $3,600
• D. $2,100

Answer: (B)

When you test jar dosing, the cost comes from turning the chosen coagulant dosage into a daily mass of chemical and then multiplying by the price per mass. Start by converting the plant flow to a daily volume in liters, apply the jar-test dosage to get the chemical mass per day, convert that mass to tons, and finally multiply by cost per ton and the number of days.

The plant treats 2.0 million gallons per day, which is 2,000,000 × 3.78541 ≈ 7,570,820 L/day. At 42.8 mg of alum per liter, the daily alum usage is 7,570,820 × 42.8 ≈ 324,031,096 mg/day, which is 324.031 kg/day (about 0.324 metric tons per day).

Cost per day is 0.324 tons/day × $200/ton ≈ $64.81/day. Over 30 days, the cost is 64.81 × 30 ≈ $1,944. So the chemical cost for a 30-day period is about $1,944.