How Does a Sand Filter Work

In Blog, Maintenance Tips by David Jerkins

Filtration is a necessary and important step of any water circulation system but not many understand how it filters the water beyond the basic understanding that the water passes through sand. In this short series, we’ll delve into how different filtration systems function. Up first is the most popular method of filtration currently, sand filters.

There are several different sand filter types, with different ports. This article will focus on the sand filtration method used in commercial aquatics, with two ports on the filter.

The dirty water flows into the top port – known as a bulkhead – and through a diffuser (possibly more than one diffuser). The diffuser will distribute the water, keeping the sand bed level at the top. The water will then pass through. Anything larger than 20-40 microns will become trapped in the sand. This cleans the water. At the bottom of the tank, are “laterals” which are usually PVC pipes (sometimes glass) spread out in a pattern at the bottom of the tank to “catch” the water. The holes in this pipe are many, and smaller than a grain of sand. The water enters these pipes, moves to the center of the filter and then moves up through a central pipe that exits through the bottom bulkhead. The now filtered water continues through the system for treatment.

A typical sand filter is filled with a media, that is the material that is filtering the water. As the name implies, it is usually #20 silica sand but many prefer to use a different media. The advantages and disadvantages of using different media types are outside the scope of this article, but note that the writer believes that most of them are just marketing fluff and sand is as effective as glass or other sand alternative. Often, the bottom of the filter is filled with pea gravel. This is a preference, and ultimately depends on the manufacturer’s guidelines. If the manual states that it must be used, use it of course. If it states it can be used, it is optional. The reasons you might want to use gravel are:

  • On some larger commercial sand filters, it is necessary to support the laterals to the weight of the very large laterals.
  • If a lateral cracks or breaks and you have gravel packed in around them, you might not get sand in the pool (See: Why is There Sand in my Pool? – TMI Sustainable Aquatics (
  • Increases the flow rate around the laterals. May prevent clogging of laterals

As the water is filtered, the resulting filth accumulates in the sand. This will need to be cleaned out. Every commercial sand filter should have two pressure gauges. One measuring the pressure before the filter and one measuring after. The gauge that is after the filter is the one inspected to determine if the filter needs to be cleaned. If you have a brand new, or a very clean filter, that pressure should be roughly 10-20 PSI and is considered to the the clean filter pressure. 10-15 PSI higher than your clean filter pressure is considered the dirty filter pressure and indicates the filter must be cleaned. If a quick and dirty rule of thumb is needed, if the pressure is 30 PSI or higher, the filer must be cleaned.

Cleaning the filter is called “backwashing” and is when the flow of water is reversed through the filter to remove the debris. That means that the water will go up through the sand and through the diffuser. Imagine it like this: The impurities (filth) are removed from the water when it “falls” through the sand. By sending the water back through reversed, we are putting the impurities back into the water and then sending that water to waste. How to do this varies depending on a variety of factors and exact instructions cannot be given here. This includes: Pump type, VFD, valve type, number and position of valves, sand filter make and model, and other possible factors. In the end, the backwashed water is send to a waste drain.

The pressure gauge before the filter is used to ensure you have enough pressure to send the water through the filter. If you see 0-10 PSI on this gauge it usually means that there is a clog or obstruction before the filter. The most common cause is the strainer pot needing to be cleaned.

The difference between the two gauges is called a “pressure differential” and is important to learning more about the filters. If there is very little to no difference, it could mean that the filter is “channeling” where the water is creating a channel through the sand and passing through without being fully filtered. This is uncommon and the other causes should be investigated first. If the pressure is very high, you may not be meeting turnover rate requirements and this should be also be investigated.

In an odd little paradox, a sand filter is more effective when it has not been cleaned recently. The filth filters more filth, as it were. To this end, it is not recommended to backwash too often as it could reduce the effectiveness of the filter and leads to unnecessary water loss. Instead, inspecting the gauges will tell you when the filter needs to be backwashed.