Wastewater Operator Wastewater Math and Calculations Questions and Answers

Question 1

A treatment plant's influent flow of 4.2 MGD has an average Biochemical Oxygen Demand (BOD) concentration of 210 mg/L. How many pounds of BOD enter the plant each day?

Accepted Answer

7,356 lbs/day

Answer

The 'pounds formula' is used to calculate the pounds of a substance in wastewater. The formula is: Pounds/day = Flow (MGD) × Concentration (mg/L) × 8.34 lbs/gal. Calculation: 4.2 MGD × 210 mg/L × 8.34 lbs/gal = 7,355.88 lbs/day, which is rounded to 7,356 lbs/day.

Question 2

An activated sludge system has the following characteristics: Influent flow to the aeration basin is 1.5 MGD, primary effluent BOD is 150 mg/L, the aeration tank volume is 0.5 MG, and the Mixed Liquor Suspended Solids (MLSS) is 2,800 mg/L. What is the Food to Microorganism (F/M) ratio?

Accepted Answer

0.16

Answer

The F/M ratio is calculated by dividing the pounds of incoming food (BOD) by the pounds of microorganisms (MLSS) under aeration.
1. Calculate Food (BOD) in lbs/day: 1.5 MGD × 150 mg/L × 8.34 = 1,876.5 lbs/day.
2. Calculate Microorganisms (MLSS) in lbs: 0.5 MG × 2,800 mg/L × 8.34 = 11,676 lbs.
3. Calculate F/M Ratio: 1,876.5 lbs/day ÷ 11,676 lbs = 0.16.

Question 3

A circular clarifier is 60 feet in diameter and 12 feet deep. If the influent flow rate is 2.5 MGD, what is the detention time in hours?

Accepted Answer

2.4 hours

Answer

Detention time is calculated by dividing the tank volume by the flow rate.
1. Calculate Tank Volume in cubic feet (cu ft): Volume = π × r² × depth = 3.14 × (30 ft)² × 12 ft = 33,912 cu ft.
2. Convert Volume to gallons (gal): 33,912 cu ft × 7.48 gal/cu ft = 253,662 gal.
3. Calculate Detention Time in days: 253,662 gal ÷ 2,500,000 gal/day = 0.101 days.
4. Convert Detention Time to hours: 0.101 days × 24 hrs/day = 2.43 hours.

Question 4

An operator performs a 30-minute settleability test on a mixed liquor sample, and the settled solids occupy a volume of 320 mL/L. The MLSS concentration is determined to be 2,500 mg/L. What is the Sludge Volume Index (SVI) in mL/g?

Accepted Answer

128 mL/g

Answer

The Sludge Volume Index (SVI) formula is: SVI (mL/g) = (Settled Sludge Volume, mL/L × 1,000 mg/g) ÷ MLSS (mg/L). Calculation: (320 mL/L × 1,000 mg/g) ÷ 2,500 mg/L = 128 mL/g.

Question 5

A pump is required to move 500 gallons per minute (gpm) against a total head of 45 feet. What is the water horsepower (WHP) required?

Accepted Answer

5.7 WHP

Answer

Water Horsepower (WHP) is calculated using the formula: WHP = (Flow, gpm × Total Head, ft) ÷ 3,960. Calculation: (500 gpm × 45 ft) ÷ 3,960 = 5.68 WHP, which rounds to 5.7 WHP.

Wastewater Operator Certification Practice Test

Wastewater Operator Certification Practice Test

Wastewater Operator Wastewater Math and Calculations Questions and Answers