# Exam Study Guide: Backwash Rate

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When operators are faced with licensing exams, they sometimes struggle with test anxiety because they don’t fully understand how to properly study and prepare for exam questions. Here we take a look at a sample water treatment licensing exam question, how to solve the problem, and the correct answer.

Sample Question:

What is the backwash rate in gallons per minute (gpm) for a filter that is 30 feet by 25 feet and 6 feet deep with a backwash water rise rate of 6 inches in 30 seconds?

Answer: 30 feet x 25 feet x 0.5 feet x 30 seconds x 2 x 7.48 gal/ft3 = 5,610 gpm

Sometimes water treatment plants do not have a means via flowmeter to determine backwash rates. These rates need to have a velocity high enough to separate the media so trapped particles that work their way into the filter can be lifted and removed during a backwash. Typically, the backwash rate should be a minimum of three times its filter rate.

However, without a meter attached to a backwash pump, physical measurements need to be taken from time to time to establish the backwash flow rate. The backwash filter rise is when the filter level has been dropped low enough to expose the troughs, and then the pump is engaged and a measurement is taken from the water level over a period of time that can be easily calculated.

For best results, take two measurements while the pump is running, and then average the rise rate. This way the actual time needed to charge the backwash line is taken into consideration. To do this, an operator using proper personal protective equipment to prevent him or her from falling into the filter, needs to be quick with their measurements.