You have a rapid sand filter that is 12' wide x 12' long x 6' deep. During a backwash event, it takes 90 seconds to drain 6". What is the filter rate in gallons per minute per square foot?

To find the filter rate in gallons per minute per square foot, you need to calculate the flow rate during the backwash and relate it to the filter area.

First, calculate the volume of water drained during the backwash. The filter dimensions are 12 feet wide, 12 feet long, and 6 feet deep. The area of the filter is:

Area = Width x Length = 12 ft x 12 ft = 144 square feet.

During the backwash, it takes 90 seconds to drain 6 inches of water. Convert 6 inches to feet:

6 inches = 0.5 feet.

Now calculate the volume of water drained in cubic feet:

Volume = Area x Height = 144 sq ft x 0.5 ft = 72 cubic feet.

Next, convert the volume from cubic feet to gallons, knowing that 1 cubic foot = 7.48 gallons:

72 cubic feet x 7.48 gallons/cubic foot = 538.56 gallons.

To find the flow rate in gallons per minute, convert the time from seconds to minutes. Since there are 60 seconds in a minute:

90 seconds = 1.5 minutes.

Now calculate the flow rate:

Flow Rate (gpm)