Sand filters remove contaminants from water in a variety of applications; you can find them in municipal water treatment plants as well as in home swimming pools or greywater treatment systems. Sand filters may use slow or rapid filtration -- the finest material is layered on top of coarser materials in a slow sand filter, while coarser material is layered on top of finer materials in a rapid sand filter so the water flows through faster. (References 2 & 3) For either type of filter, knowing the volumetric flow of water through the filter is crucial to ensure the filter can handle the volume of water that needs to be treated.

- Skill level:
- Moderate

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### Things you need

- Tape measure
- Calculator

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## Instructions

- 1
Measure the length and width of the filter in feet if the filter is square or rectangular. Measure the diameter if it is round. Only measure the area covered by filter material; do not include filter walls in your measurement.

- 2
Calculate the surface area of the filter material. For a square or rectangular filter, multiply the width and length together to obtain the surface area in square feet. For a round filter, divide the diameter by two, square the result, and multiply by pi (3.14).

- 3
Estimate the water's velocity through the filter. If your calculations are for a municipal or commercial water treatment plant, get an estimate of the velocity from a plant employee. Otherwise, estimate 0.015 to 0.15 gallons per minute per square foot for a slow sand filter and 2 to 3 gallons per minute per square foot for a rapid sand filter, according to Virginia Community Colleges. Use the size of the filter material to help you estimate; coarser sand results in a faster velocity.

- 4
Multiply the surface area of the filter by the velocity of the water through the filter to obtain the volumetric flow rate in gallons per minute. For example, for a surface area of 100 square feet and a velocity of 0.1 gallons per minute per square foot, (100ft^2)*(0.1 gpm/ft^2) = 10 gallons per minute.

#### Tips and warnings

- If you have access to the filter, test the accuracy of your calculation by running a known volume of water through the filter and timing how long it takes to filter completely; dividing the volume by the time gives the flow rate. Refine your velocity estimate by dividing the flow rate resulting from this test by the filter surface area to obtain a more accurate velocity.