Exam Study Guide: Sludge Detention Time

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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:

As an operator, you must ensure your solids waste is disposed of properly and that treatment is complete before removal for dewatering.

Your facility uses a gravity thickener that is 28 ft in diameter and has a sludge blanket depth of 4.5 feet.  If the sludge is pumped from the bottom of the unit at a rate of 20 gpm, what is the sludge detention time (in hours) in the thickener?

A)  13.5 hours
B)  17.3 hours
C)  14.1 hours
D)  12.8 hours

This is a mathematical problem that is solved by dividing the volume of the thickener by the flow and converting the answer to hours. Because you are determining the diameter of a circle, .785 (pi divided by 4) is used in the equation along with 7.48, which is the number of gallons in a cubic foot. 

28 feet X 28 feet X .785 X 4.5 feet X 7.48 gal/cubic foot divided by 20 gpm X 60min/hr  = 17.26 or 17.3 hours

The correct answer is B, 17.3 hours

About the author
Mike Smith is program coordinator and lead faculty of the Water Quality Management program at Red Rocks Community College in Lakewood, Colo. He has been in charge of the multi-faceted training program since 1996.


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