Exam Study Guide: Influent Flow Rate

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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.

Example question:

As an operator, you are responsible for ensuring you have enough treated water available for pumping into the distribution system and enough water to maintain ongoing operations. During your 12-hour shift, you need to backwash eight of the 20 filters at the facility. Each filter is backwashed for 12 minutes at a total rate of 9,400 gpm each. You need to add this volume to your chemical treatment rate in addition to the facility's effluent pumping rate, which is 4,800 gpm. 

What is your influent flow rate in mgd?

A)  78.2 mgd
B)  It doesn’t matter
C)  7.8144 mgd
D)  6.2896 mgd

Many operators overlook production water in addition to effluent pumping rates when establishing influent rate so that treatment is consistent.

To do this, we first need to determine how much water is needed for backwash: 9,400 gal/min X 12 min X 8 filters = 902,400 gallons. Because this water is taken from the same source as the effluent pumps, if we do not add this volume into our treatment rate for the day, we lower the water level in the clear well, which will have a direct impact on providing adequate disinfection. So, 4,800 gal/min X 1,440 min/day = 6.912 mgd flow rate leaving the plant plus .9024 mgd for backwashing equals 7.8144 mgd. 

The correct answer is C) 7.8144 mgd.


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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