Time is critical when it comes to emptying tanks, basins or lift stations. Heavy rain or snow, flooding, water main breaks and other overflow events can create a need to know how long it will require emptying, maintaining a certain level or decreasing a level of a tank, basin or lift station. Let’s calculate a few of these types of problems.

- If two 520 gpm pumps are used how, long will it take in hours to empty a rectangular tank 80 feet long by 40 feet wide and 18 feet deep? The pumps are 85 percent efficient.

First calculate the volume of the tank and convert to gallons.

Volume = length x width x height

Volume = 80 ft x 40 ft x 18 ft = 57,600 ft^{3}

= 57,600 ft^{3} x 7.48

= 430,848 gallons

Divide the number of gallons by the pump rate to find the time. Convert to hours.

Time = Total gallons

Pump rate

= 430,848 gal_

(1,040 x .85) gal/min

= 430,848 gal

884 gal/min

= 487.39 min

= 487.39 min

60 min/hr

= 8.12 hr

2. A 45,000 gallon tank receives 320,000 gpd flow. A 225 gpm pump is attached to the tank, but it is broken. How long do they have to repair or replace the pump before the tank will overflow? Assume the tank is empty now.

First find the average flow per hour. Then find the number of hours till the overflow occurs.

Hourly flow = Total gallons

Hrs in a day

Hourly flow

= 320,0000 gal

24 hrs

= 13,333.33 gal per hour

Hours till overflow = Tank size

Hourly flow

Hours till overflow

=45,000gal/hr

13,333.33 gal/hr

= 3.38 hrs or 3 hrs 23 minutes

3. The wet well at a lift station receives a flow of 235 gpm. The wet well has a diameter of 25 feet. How many minutes will it take to raise the water level eight feet in the wet well?

First calculate the volume of water it will take to fill eight feet of the wet well. Then determine based on the gallons per minute how long it will take to rise to a level of eight feet.

Volume = .785 x diameter x diameter x height of water level.

Volume = .785 x 25ft x 25 ft x 8 ft = 3,925ft^{3}

3,925 ft^{3} x 7.48 = 29,359.00 gal

Minutes to raise to eight feet = Total gallons

Flow rate

Minutes to rise to eight feet

= 29,359.00 gal

235 gal/min

= 125 min

All of the above problems can be helpful at some point or another if time is a factor in a repair, emergency situation, or other instance where it is necessary to know how much time you have until an overflow or certain level is reached. So contrary to the belief of some, there is a use for math for situations in the operations of treatment systems.

If you would like to try some practice problems, check out Test Your Knowledge – Calculating Time to Pump Out A Tank.

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