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How does a sand filter work?

Sand filter
Written by Joshua Reijnen | Last update: 17-08-2022

A sand filter, or sand water filter, makes use of sand to filter water. The water to be purified slowly sinks through the sand of the sand filter, leaving the dirt particles from the water behind in the fine pores of the sand. This filter technology is especially suitable for pre-filtration of disinfection equipment, preparation for micro- and ultra-filtration, recirculation systems, rinse water processes, irrigation systems, basin water filtration and ground water cleaning.

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. 
  • Sand filters for water

    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. 

Sand filter backwash

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.

How does a sand filter work

Diameter of the sand

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.

Checking and maintenance of the sand filter

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.

Questions about sand filters as a filter technology

Do you have a question about the working of a sand filter, how does sand filter water, or would you like customized advice about this filter technology? Then contact water specialist Joshua, or ask your question via below form. We will then contact you as soon as possible, even within 24 hours on working days. 

Joshua ReijnenAbout Joshua Reijnen

Joshua Reijnen is a product specialist in the area of water technology. In 2015 he started studying water management, while working at Royal Brinkman as a trainee. Meanwhile, he has been supporting growers on a daily basis since 2017, by advising them about different kinds of water-related issues. "A satisfied customer and being able to really make a contribution; that is what I like most about my job."

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