# Exam Study Guide: Operational 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 study and prepare for exam questions. Take a look at this sample water treatment licensing question along with an explanation of the correct answer.

Sample question:

You operate an 18 mgd facility that has six available secondary clarifiers. Your settling tests determine you need three hours settling time in your secondary system. Each clarifier is 18 feet deep with a diameter of 69 feet. How many clarifiers need to be in service to meet the treatment requirement?

A. 2
B. 3
C. 4
D. 5

To calculate this take the flow of 18 mgd and convert it to three hours of flow: 18,000,000 /8  (8 x 3 = 24 hours). Then, take that answer and convert it from gallons to cubic feet, which equals 300,802.1 cubic feet. Now, take your clarifier dimensions: 69 x 69 x .785 x 18 = 44848.6 cubic feet. Next, take your required three hours of the daily flow 300,802.1 cubic feet/44,848.6 cubic feet per clarifier and this equals 4.47 or 4.5. This means that five of the six clarifiers are required to meet the three-hour settling time requirement.

So, the correct answer is D, 5 clarifiers.