How Do Sand Pool Filters Work?

Coupled with proper chemical balance of your swimming pool water, pool filters are the first line of defense against all sorts of contaminants, debris, dirt and dust. Based on the type of pool and the size, you can choose sand, cartridge or a DE filters which will keep your water crystal clean and enjoyable for swimmers. If you want to understand the difference between cartridge and sand filters, check this article where we compare them.

In this article we are going to address only one topic: how does a sand filter work?

Related article: Everything You Need to Know about Sand Filters

How do I know if my pool filter is working?

It’s quite simple: if your pool water is clean and clear (just take a look) and if the return jets seem to be letting in water with sufficient pressure (simply put you palm in front of them and check), your filter system is working well.

If your pool water is cloudy or murky and the return jets are low in pressure, there’s something wrong either with the pool water chemical balance or with the filter.

Maintaining balanced water and having a pump running water through a well operating and regularly maintained filter system is a must for every pool or spa.

Filters are important for eliminating windblown dirt and debris, dead skin cells, sweat, saliva and a lot of other contaminants.

How do sand filters work?

Sand pool filters are integral and important part included into a pool’s plumbing network. They are a type of water filter that removes impurities from the pool by means of using sand as a physical filtering barrier. They capture both large and small particles and are a pool’s most important line of defense against water contaminants.

Sand pool filters work in conjunction with a pool’s motor and pumps. The pump is the main part of a pool’s circulation system. It does the heavy duty work as it is the pump that pulls water from the pool through the skimmer and main drain. It then pushes the water towards the filter, and then pumps the newly filtered water back towards the pool through the main returns.

As water is pumped through the circulation system, it passes through the sand filter which traps dirt and debris inside and creates clean and filtered water. This newly filtered water is then sent out to the other parts of the pool including the heater and chlorinator and through an outlet pipe back into the pool.

This runs in a circular mode working continuously as long as the system is on.

The filtration process starts when water is drawn from the pool and skimmer and then sent out to the pump and motor. After reaching the pump, the water passes by the “strainer pot basket” which is the first filter. This basket removes most debris, which is large enough to be caught in it. The pre-filtered water is then sent along to the pump housing.

The water is then sent through the motor and on towards the main filtering process in the sand pool cartridge filter. Any remaining debris and contaminants are then removed from the water.

The filtered water is now ready to be sent back to the pool’s heater which changes the water’s temperature to your specifications and sends the water to the chlorinator where chlorine is added. The water is then returned to the pool itself via return jets.


Importance of Sand Pool Filters for your swimming experience

You simply cannot have a swimming pool without a pool filter and expect the water to be clean and safe for swimming. Pool filters free the water from dirt that you can see and particles as well as contaminants that are too small to be visible to the naked eye.

These can include disease-causing pathogens, microorganisms from infected swimmers, sweat, urine, saliva, fecal matter, food particles, and tiny insects. Pool filters also prevent the growth of bacteria and algae.

Dirty pool water can cause you to acquire RWIs (recreational water illnesses). These include diarrhea, gastrointestinal disorders as well as ear, skin, eye, respiratory irritations and wound infections. 

What is a sand pool filter and how does it work?

Sand filters are one of the oldest and most used filters used in residential and commercial swimming pools. Today’s models are the much improved and advanced versions of the older and larger sand and gravel filters which were first used.

They are more convenient and cost-effective, but require 40% more water because they must be cleaned, or in pool terms, backwashed quite often.

Sand filters can filter down to 20 microns and can last you for a long time, as long as you change the sand and maintain the filter on regular basis.

If you are interested in buying one, we reviewed 8 best pool filters in 2018 for you.

How does a sand filter work for a pool?

Sand filters can be used in most types of pools, especially above ground and saltwater pools. You can determine if the filter is working properly if the water pressure coming into the pool is strong, the pressure of the gauge is normal, and the pool water is clear.

The usual parts of a sand filter include:

  • Corrosion proof hollow tank. This is the main body and holds all the parts and the sand.
  • Specially graded # 20 silica sand, which is the filter media that goes into the tank.
  • Integral top diffuser. This allows for even distribution of water.
  • Slotted lateral pipes. These extend out in a fanned out fashion and are meant for a balanced flow of sand.
  • Laterals. These hold back sand to allow filtered water to pass through.
  • Drain plug. For easy draining during backwash, for instance.
  • Corrosion proof base which supports the tank.
  • Multiport valve. This is found on top of the tank with an o-ring and clamp band that secures the valve on to the filter. This valve allows between 3 and 6 functions for the filter. The primary mode is filter mode, while other modes include backwash, rinse, waste, close (for winterizing), re-circulate etc.

The filter media that goes into the tank is sand (usually filling up to 75% of the tank). The sand used is not your typical sand found in any beach and is instead the #20 silica sand (which is 0.45-0.55mm in size) and a standard in the sand pool filters. It is specially graded to trap particles in the 20-40 micron range and up.

As you can see, sand filters generally have very few parts, which makes them simple to use and maintain than other types of filters.

Step by step sand filtering process is simple and goes as follows:

  1. The filtration process begins when the water from the pool enters through the filter’s inlet pipe which leads to the tank’s water distribution head inside.
  2. As the inside of the tank is filled with water from the pool, the water passes through the filtering sand and the contaminants that are large enough to be caught are filtered out by the jagged sand edges. The trapped particles will remain in the tank until the system is backwashed.
  3. The filtered water is then pushed back into the pool.

Advantages of sand pool filters:

  • Low maintenance compared with cartridge and DE filters
  • Inexpensive and easy to replace
  • Effectively traps particles between 20-100 microns
  • Alternative filter media can be used to improve filter efficiency

A few notable disadvantages:

  • Manual backwashing every week, or so leads to thousands of gallons of water being wasted each year.
  • The smaller particles can pass back into the pool. Sand filters are the least in terms of filtering ability. They filter down to a particle size of 20-40 microns.

Maintenance of your filter

The specially graded sand has particles that are ground into .45 to .55m in diameter. This size is fine enough that water is able to pass through and rough enough that debris gets trapped. Check out our guide on how to find the right size filter for your pool.

As more water and debris are pushed through, the filter’s pressure increases. As the pressure increases, the more a sand filter plugs up and the finer the particles that are captured so it is somewhat okay to let the filter plug up a little, so there’s no need for excessive backwashing.

The normal operating pressure for most residential tanks is between 10 and 20 Psi but largely depends on your system’s pump size and resistance. When the pressure rate increases 8 or 10 psi above the starting pressure, the filter will now need to be backwashed.

To backwash, the power to the pump needs to be OFF and the valve reversed to BACKWASH. After this, turn the pump back on. The water will then flow in reverse and send the dirty water out the port on the valve which is connected with a backwash hose with a clamp, and out to where your backwash hose leads to the drain port. Backwashing is usually done weekly or so.

Sand should be replaced every five to seven years but this also depends on how often the pool is used and how well it is maintained, as well as the weather, wind and water properties. The sand eventually locks together with all the water running back and forth, thus rendering a less effective filtration and needs to be replaced.

Here’s a short video explaining the parts and functioning of a sand pool filter:

Any type of filter you choose is going to be a compromise between the filter’s efficiency and difficulty in terms of filter maintenanceYou also need to take into account factors such as the swimming pool filter price and how it matches your budget, the size of your pool, environmental factors, your lifestyle, and the time you have for maintaining your filter system.

Sand filters are the easiest to maintain but they also offer the lowest filtering ability limited by the sand as means of filtration of particles up to a certain size. Those of you who are in favor of easier maintenance will opt for sand filters. They are also easy enough to use and contain a few basic parts.

As for other types of filters, DE filters are the most powerful and offer the best filtering capabilities but require much more maintenance. Cartridge filters are the middle ground: they are the greenest alternative as they do not require backwashing so they save a large amount of water.

Also, DE and cartridge filters work best only with smaller and spa type pools so if you have a large pool you will most probably go for a quality sand filter.

Aleks Stanic

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