# Exam Study Guide: Sludge Detention Time

### Interested in Filtration?

Filtration

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