Exam Study Guide: Organic Loading Math

Exam Study Guide: Organic Loading Math

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When faced with a licensing exam, don't struggle with test anxiety! Use the question below and others from our Exam Study Guide series to properly study and prepare for exam questions. Take a look at this sample wastewater treatment licensing question along with an explanation of the correct answer.

Sample question:

What is the organic loading on a lagoon/pond that measures 850 feet long and 650 feet wide, and is 7 feet deep? Flow into the lagoon/pond for the day was 380,000 gallons. The lab reports the influent TSS was 265 mg/L and the influent BOD was 220 mg/L.

A) 117.8 lbs BOD/day/acre

B) 84.3 lbs BOD/day/acre

C) 54.9 lbs BOD/day/acre

D) 24.8 lbs BOD/day/acre


To figure out this problem, you must find the relevant numbers to input into your formula. You need the dimensions of the pond (850 feet x 650 feet), the flow (.38 mgd), and the influent BOD (220 mg/L).

To remember the formula, simply look at the answer choices. The units tell you how to build the equation. Pounds of BOD per day per acre or pounds BOD divided by acres. Using the pounds formula to find pounds of BOD  — 8.34 (constant) x .38 mgd (flow) x 220 mg/L (BOD concentration) — your answer should be 697 pounds BOD/day. You can then calculate the area of the lagoon/pond in acres: (850 feet x 650 feet) divided by 43,560 (square feet in an acre). Your answer should be 12.7 acres.

Divide the pounds of BOD/day (697) by the acres in your pond (12.7). This should give you an answer of 54.9 pounds BOD/day/acre. So, the answer is C.

About the author
Ricky Lallish is the Water Pollution Control program director at the Environmental Resources Training Center of Southern Illinois University Edwardsville. He provides training for entry level operators in the wastewater field and operators throughout the state looking to further their education.


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