Understanding the Pounds Formula in Water Treatment
The Pounds Formula (also known as the "Davidson Pie Chart" equation) is the single most critical formula in water treatment and wastewater certification exams. It calculates the weight in pounds of pure chemical or pollutant added or removed per day.
Why is 8.34 lbs/gallon used?
One gallon of fresh water at standard temperature weighs approximately 8.34 pounds. One milligram per liter (1 mg/L) is equivalent to one part per million (1 ppm). Therefore:
Real Exam Practice Question #1
Q: A water treatment plant treats an average flow of 4,500,000 gallons per day. The operator wishes to maintain a chlorine dosage of 1.8 mg/L using 12.5% sodium hypochlorite solution. How many pounds per day of liquid hypochlorite must be fed?
Step 1: Convert Flow from GPD to MGD → 4,500,000 ÷ 1,000,000 = 4.5 MGD
Step 2: Calculate 100% Pure Chlorine Required → 4.5 × 8.34 × 1.8 = 67.554 lbs/day
Step 3: Adjust for 12.5% Solution Strength → 67.554 ÷ 0.125 = 540.43 lbs/day of 12.5% Hypo