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Please note: a sand filter does not filter dissolved salts from the water. A sand filter can only filter solid particles from the water, like algae and organic material. Salts that do not dissolve in water, like salts that appear in fertilizers, can be filtered from the water by a sand filter, provided that the particles are large enough.

How does sand filter water? A sand filter works as follows. The unfiltered water is distributed in the top of the sand filter and will slowly sink through the filter bed. This filter bed consists of sand, gravel or another filter medium. The small pores ensure that the dirt particles are not able to pass through and retain the dirt particles. 

When a certain quantity of water has been filtered or when the sand has reached a certain degree of pollution, the sand filter will automatically switch to backwashing. During the backwashing the pores enlarge upon which the dirt is washed from sand again. Subsequently, this dirt is discharged, together with the backwash water. As a result, the sand filter will not get blocked, retaining its optimal purification qualities.

The diameter of the sand is predominantly dependent on the size of the sand filter, the desired flow and the desired filtration. In most case, for the filtering process, sand with a diameter of 0.8 mm to 1.25 mm is used. This may however vary per installation. For example, a UV installation has a lower flow and sand of 0.4 mm to 0.8 mm will be used to filter more restrictively. 

When the sand has a small diameter, the resistance will be increased, resulting in less flow. For this small diameter less capacity is required for backwashing, because the sand is lighter and will thus easily wash with the dirt. Also, in case of a smaller sand grain diameter, you should take into account that more frequent backwashing should take place because more dirt is being captured than when using a larger diameter.When a certain quantity of water has been filtered or when the sand has reached a certain degree of pollution, the sand filter will automatically switch to backwashing. During the backwashing the pores enlarge upon which the dirt is washed from sand again. Subsequently, this dirt is discharged, together with the backwash water. As a result, the sand filter will not get blocked, retaining its optimal purification qualities.

Check the filling of the sand filter periodically. If the filling clumps together or gets rubbed off, or if the sand filter requires frequent backwashing, then replace the filling. Also ensure that you maintain the sand filter well.

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