How Does a Pool Sand Filter Work?

There are three types of swimming pool filters: Diatomaceous Earth (DE) Filters, Cartridge Filters and Sand Filters.  Sand filters are commonly used on the East Coast, for above ground swimming pools and on commercial swimming pools.

How Do Sand Filters Work?

Sand filters utilize a tank, also referred to as a housing or canister, which holds the media of sand.  This media is what your swimming pool water passes through in order to filter out the “bad stuff” in the swimming pool water.  This can be dirt, debris and even some bacteria-causing waste.  You don’t want to just back up your truck to your local park and load ‘er up.  Most sand filters require “pool sand”, which is traditionally #20 silica sand.  This sand has grains that fall into the 45-55 millimeter range.  Any sand labeled as “Pool Sand” or “Filter Sand” should be the right stuff.  You can find this at your local pool store or even some hardware stores.

As your pump pushes the swimming pool water through the various pieces of equipment, the filter is the most crucial stop.  Whatever type of filter you have you always want to make sure it is in good working order.  This means cleaning or backwashing as needed, replacing any broken parts, and making sure there are no leaks.

The way a sand filter functions is as the water moves from the top of the filter to the bottom of the tank, using the high pressure from your swimming pool pump, the water trickles down.  Since sand has rough, or sharp, edges which allows the grain to snag particles like dirt & debris.  It also can catch body waste material.  Once the particles have passed through the sand filter, the clean water is pumped back into the swimming pool.

How Effective are Sand Filters?

Of the three types of filters mentioned above, a sand filter would be considered the “worst” at filtrating.  What I mean by this is the ability to filter out different sizes, which are measured in microns.  A cartridge filter can filter out 5-20 microns.  A DE filter filters to the smallest particle size of 2-3 microns.  A sand filter boasts 20-30 microns.  Just to put things into perspective, a human hair is approximately 50 microns (on average).  

Believe it or not, as your sand filter becomes dirtier, it actually becomes more effective.  The debris that is caught helps catch even smaller particles trying to sneak back into your pool.  Sand filters are the least expensive to buy and maintain.  They require full cleanings and/or backwashing much less frequently than any other type of filter.  Like any filter, when the pressure exceeds 8-10 PSI above your clean starting filter, it needs to be at least backwashed.

How Often Do I Need to Change the Sand in a Sand Filter?

As dirt and debris starts to build up in your sand filter, you will notice the pressure on your gauge on the top of your filter start to increase.  By backwashing the filter, you will alleviate some of this excess build-up.

Sadly, even if you backwash regularly, you’ll eventually need to change out the sand and replace it with new sand.  This of course depends on usage of the pool and the quantity of constant debris your particular pool receives. A National average is 3-5 years. Why would you need to change the sand at all?  Excellent question!  The reason is that those jagged edges of the sand that we rely on to grab our junk out of the pool start to become smooth and therefore are less effective.

When Should You Choose a Sand Filter?

As I mentioned above, they are usually the most affordable.  They are also the most compact.  Sand filters are easy to operate, use and maintain.  Even backwashing them is next to effortless.  The media lasts longer when compared to a cartridge or DE filter.  The sand itself is relatively inexpensive. (In California, less than $20 a 50 pound bag).

The downside?  We already talked about the micron filtration ability of sand filters, so your swimming pool water will never appear as clear as ,say, a Diatomaceous Earth filter.  The entire filter needs to be replaced anywhere between 5-8 years.  Again, this depends on you: your water chemistry, how well and often you net your pool, and how high you let the pressure get.  Invest in a good pool brush and net to not only keep your pool clean and inviting, but also to extend the life of your filter.  You can get a groovy package here. Since the sand rests on what are known as “laterals” at the bottom of the filter, increased pressure for too long can break these, causing costly repairs or replacement.  Also, when we backwash, we lose pool water and chemicals.  This can cost us money to get certain fields back to the acceptable range.  Not to mention the cost of the water.

So, in simple terms, if the pros outweigh the cons, a sand filter may be the right choice for you!  See you poolside!


This article explains how you can backwash your pool filter efficiently. Want to know how to clean the bottom of your pool? Read more here.