How a Pool Filter Works: Your Fantastic One-Stop Resource

Swimming pools are normally built to be a source of fun, relaxation, and joy. There’s nothing like wading through cool waters especially during the summer when temperatures are high. We all get that excitement of jumping in especially when we see that a pool has sparkling blue water, clear enough that you can see through to the bottom.

On the other hand, most of us have probably gone through an experience wherein we couldn’t wait to swim, bask in the sun, and sip on something cold – only to be greeted by an unpleasant sight of murky pool water.

To avoid such mood dampening experiences, pool filters are built into pools – commercial or residential.

We all know of the existence of pool filters and just how important they are in keeping pools free from things such as dead skin cells, windblown dirt and debris, saliva, and a host of other things.

But at the same time, not a lot of us know much else about pool filters. Here we are going to help you get a better understanding of them – whether you simply want to know how they work or you’re looking to purchase one and want to know your options. We will answer questions you may have that include:

types of pool filter
  • The different types of pool filters
  • What kind of pool or kind of water each type works best for
  • Parts of each filter type and their purpose
  • How each filter type works
  • Best features of each filter type
  • Downsides of each filter type
  • General idea of how much each filter type would cost

But before we proceed to covering those areas, let us first give you a background on pool filters.

Pool Filters

Pool filters are equipments attached into a pool’s plumbing network. They are a type of water filter that removes impurities from the pool by means of a physical barrier. They capture both large and small particles and are a pool’s first barrier against water contaminants.

There are 3 common swimming pool filter types namely: sand filters, cartridge filters, and DE filters. Each type has its own pros and cons, but when used properly, any of the three will do an excellent job of keeping your pool clear and free of all but the smallest particles.

We will be discussing more about these further ahead. But before we get into the details of the individual types, let's spend a minute looking at how a pool filter works in general and what the 3 types have in common.

How They Work in General

They essentially 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 lifting as it is what pulls water from the pool through the skimmer and main drain, pushes the water towards the filter, and then pushes the newly filtered water back out towards the pool through the main returns.

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As water is pumped through the circulation system, it passes through the filter trapping dirt and debris inside and creating 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 fashion working continuously as long as the system is on.

Green Living Ideas provides us with a basic step by step process on how it works alongside a pool pump.

  • 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 initial filter. This basket removes most debris, which are large enough to be caught, from the water. The pre filtered water is then sent along to the pump housing.
  • The water is then sent though the motor and on towards the main filtering process in the pool’s cartridge filters. Any remaining debris and contaminants are now removed from the water.
  • The newly filtered water is now ready to be sent to the pool’s heater which changes the water’s temperature to your specifications.
  • After this, the water proceeds onto the chlorinator where chlorine is added.
  • The water is then returned to the pool itself.

Importance of Pool Filters

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

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

Who would want to swim in a pool full of those? Not only is it gross, it can also affect your health.

types of pool filters

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A clean pool (top) and a dirty pool (bottom)

Dirty pool water can cause you to acquire RWIs (recreational water illnesses). RWIs include diarrhea, gastrointestinal, ear, skin, eye, respiratory, neurologic, and wound infections, among others. Environmental Science and Technology’s journal also reports that chlorine produces trichloramine and cyanogens –two chemicals that are dangerous to human health – when mixed with sweat and urine.

You can imagine that those can take the fun out of swimming. As we previously mentioned, our pool should be a source of fun, relaxation, and joy. Not a source of diseases and problems. Which is why, filters are a universal and essential part of any swimming pool.

3 Different Filter Types (Sand, Cartridge, DE)

1. SAND FILTERS

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Sand filters are one of the oldest filters around. Today’s models are basically the improved versions of the older and larger sand and gravel filters back in the day. They are more convenient and cost effective, but require 40% more water because they must be cleaned often, or in pool terms, backwashed.

Sand filters can filter down to 20 microns.

Sand filters can last you forever, but the aspect of the sand sitting inside of the filter tends to be a concern for some people.

What Type of Pool?

Sand filters can be used in most types of pools, especially above ground and saltwater pools.

Parts

Typical parts of a sand filter include:

  • Corrosion proof hollow tank. This is the main body and holds the parts and the sand.
  • Specially graded sand. The 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.
  • Corrosion proof base. This 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 around 3 to 6 functions for the filter. The primary mode is filter mode, while another one is backwash mode.

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 a kind specific to the pool industry. It is specially graded to trap particles in the 20-100 micron range.

Sand filters generally have very few parts which makes them simpler than other types of filters.

How it Works

According to Hub Pages, during sand filtration water is sent to the filter media (specially graded sand) via a diffuser and exits through the bottom of the filter and back onto the pool in a newly cleansed state.

Step by step process:

1. The filtration process begins when dirty water from the pool enters through the filter’s inlet pipe which then 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 specialized sand and the contaminants that are large enough to be caught are filtered out. These will remain in the tank until the system is backwashed.

3. The filtered water is then pushed back into the pool.

Pros

Pros of getting a sand filter for your pool include:

  • 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

Cons

Cons of getting a sand filter for your pool include:

  • Manual backwashing every few weeks. Backwashing leads to thousands of gallons of water being wasted each year.
  • 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.

Cost

Cheapest and minimal maintenance cost.

The average price of an in ground sand filter is around $650. This does not include installation costs, which varies depending on the pool.

Installation can run from around $248 to $1,135.

The average price of an above ground sand filter is around $250. Since the installation of an above ground filter requires only hooking up some hoses, it is possible to do it yourself.

According to Pool Products, the estimated cost of ownership for repairs over a 10 year period would be at around $250. That is assuming that you go through at least two repairs and two sand changes.

Hayward sand 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.

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.

swimming pool filter price

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The normal operating pressure for most residential tanks is between 10 to 20 Psi but largely depends on your system’s pump size and resistance. When the pressure rate increases 7 or 8 psi above the starting pressure, the filter will now need to be backwashed or cleaned.

To backwash, power to the pump will have to be cut and the valve reversed. After this, turn the pump back on. The water will then flow in reverse and send the dirty water out another port on the valve which is connected with a backwash hose with a clamp, and out to where your backwash hose leads. Backwashing is usually done every few weeks.

Sand should be replaced every three to five years. Also largely depending on how often the pool is used and how well it is maintained. This must be done because the sand eventually locks together with all the water running back and forth, thus rendering a less effective filtration.

You can determine if the filter is working if the water pressure coming into the pool is strong, the pressure of the gauge is normal, and the pool water is clear, among other things.

Click here to watch a video explanation of how sand filters work. And click here to watch a video of sand filter cleaning.

2. CARTRIDGE FILTERS

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Cartridge filters are one of the most popular choices when it comes to pool filters because they can be cleaned without backwashing, which saves water and time and energy for pool owners.

Cartridge filters can filter down to as low as 10 microns.

Paper type cartridges are the filter media used. They wear out over time and have to be replaced every few years.

What Type of Pool?

Spa type pools and smaller pools. They are less ideal for larger pools.

Parts

Commonly replaced and sold parts include the tank lid O-ring, gauge, and cartridges. Cartridge filters do not have a multiport valve, one of the reasons why of all filter types, cartridge filters often need the least amount of repairs.

Typical parts of a sand filter include:

  • Corrosion free tank. The filter’s main body that holds the parts.
  • Single locking knob. This securely fastens the filter head to the filter tank.
  • Filter head. This is for easy access to cartridge element. This can also be rotated to effectively position pressure gauge and manual air relief valve.
  • Automatic air relief. This gets rid of trapped air during filtration.
  • Single element cartridge. This is the filter’s media and is usually engineered from high quality polyester with gasketed moulded end caps.
  • Precision engineered extruded core. For additional strength and superior flow.
  • Elevated filtered water collector and debris sump. This prevents heavy debris from passing into the pool when the cartridge is taken out for cleaning or whatever purpose.
  • Socket connections. For plumbing versatility.
  • Filter drain. For faster draining.

Hayward provides us with a diagram of their cartridge filter parts.

How it Works

The operation of a cartridge filter is similar to that of a DE filter except there is no DE powder involved.

Step by step process:

1. Water flows into a large tank.

2. Inside this tank is a pleated paper or polyester element that takes up nearly the whole interior of the tank from top to bottom. This is the media (cartridge) that strains out impurities.

​3. These impurities are left behind on the outside of the media.

​4. The clean water is collected in the center of the media and piped back into the swimming pool.

Pros

Pros of getting a cartridge filter for your pool include:

  • These are the most environmentally friendly option.
  • Require a lot less water to clean.
  • They take up less room
  • Easier to install. Less plumbing because no backwash piping is required

Cons

Cons of getting a cartridge filter for your pool include:

  • Lacking a backwash cycle like the other filter types, you have to disassemble the filter every time the cartridge requires cleaning.
  • Can get dirty fairly quickly

Cost

Mid-ranged price and maintenance cost.

According to Pool Products, the estimated cost of ownership for repairs over a 10 year period is at around $500. This is assuming you go through two filter replacements and other small parts.

new pool filter

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Hayward cartridge filter

Unlike sand and DE filters, cartridge filters do not require the backwashing method to be cleaned.

But they also require periodic cleaning to remove loose debris from the filter media. Depending on how often a pool is used, the media has to be cleaned either: once a month, once a week, or more often, by removing the top of the pool filter tank and lifting the media out of the tank. It is then sprayed down with a hose to rinse off all debris that has been collected.

Once or twice each swimming season, the media will have to be soaked in a filter cleaning solution to remove more stubborn and stuck debris and oils.

Overall, the filter can last many years depending on how it is maintained and cleaned.

Click here to watch a video on how to operate and clean a cartridge filter.

3. DE (DIATOMACEOUS EARTH) FILTERS

DE is short for diatomaceous earth. DE is a sedimentary rock composed of chemically inert, fossilized (or skeletal) remains of billions of microscopic algae-like organisms, called diatoms.

DE filters are often cited as the most effective of all pool filter types, offering the clearest pool water possible. They can trap particles as low as 3 to 5 microns. This is well below 35 microns which is what the naked eye can spot.

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They are more difficult to maintain than both the sand and cartridge filter types.

In a DE filter, water from the pool passes through filter grids coated with DE powder.

What Type of Pool?

DE pool filters are best for residential pools.

Parts

Typical parts of a DE filter include:

  • Corrosion free filter tank. The filter’s main body which houses other parts.
  • DE powder. This is the filter media that goes into the tank.
  • Automatic air relief. This purges out any trapped air during filtration.
  • Integral lift handles and uniform low profile tank base. This is built in for simple and fast removal of grid nest.
  • High impact grid elements. For up flow filtration and top-down backwashing efficiency.
  • Bolted center flange clamp. This fastens the tank top and bottom together. It also increases accessibility of internal components without disrupting piping or connections.
  • Union locknuts. This makes disassembling and reassembling of filter piping faster and easier.
  • Bulkhead fittings. This increases strength and heat resistance.
  • Inlet diffuser elbow. Ensures flow of incoming unfiltered water and DE powder is distributed evenly to all filter elements.
  • Integral drain. This makes flushing of tank easier, faster, and cleaner.
  • Valve and plumbing options. This allows for a more customized control.

Hayward provides us with a diagram of their DE filter parts.

How it Works

DE powder is what actually does the filtering. Without a DE powder coating, the filter grids inside of the filter tank, which holds the powder, can easily clog up in under an hour.

DE filters can have either a push-pull valve (also known as slide valves) or a multiport valve.

Common replacement parts include E-clips, O-rings, and gauge. Over time, filter grids and fingers may also need to be replaced. DE filters are also prone to more repairs done because they contain more parts than other filter types.

Step by step process:

1. The DE filter works by first putting in Diatomaceous Earth powder, which contains microscopic skeletons of diatoms, a type of algae, to the pool filter.

2. The powder, in a sponge like effect, absorbs the dirt and debris particles from the water that goes through the filter.

3. The dirt and debris are sifted out using the skimmer box.

4. Filtered water is pumped back into the pool.

Pros

Pros of getting a DE filter for your pool include:

  • D.E. offers the finest level of filtration available delivering the clearest water possible.
  • Require a lot less water to clean.
  • They take up less room
  • Easier to install. Less plumbing because no backwash piping is required

Cons

Cons of getting a DE filter for your pool include:

  • Loss of water during backwashing
  • Needs to be disassembled once or twice a year to clean further

Cost

Most expensive and high maintenance cost.

Initial and installation cost is at around $490 to $1,500

According to Pool Products, the estimated cost of ownership for repairs over a 10 year period is at around $500. This is assuming you go through two repairs, plus cost of DE powder, and filter grid replacements.

Cost of pool pump and filter are not one and the same regardless of the filter type. Pool pumps and installation can start at around $2,000.

Monitoring the pressure gauge of your pool filter tank is important if you have a DE swimming pool filter system. This is because an increase in pressure signifies a large amount of dirt in the system signalling time for backwashing.

A general rule of thumb is to backwash when the gauge reads 8-10 pounds above the clean starting pressure. Backwashing involves moving a valve so that the water will flow through the filter backwards, flushing out the dirt. Same as backwashing in the context of sand filters.

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A disadvantage of the DE filter is that each time you backwash the dirt out of the pool filter tank, the DE powder also flows out. This piles on to the overall cost of maintaining a DE filter tank as you need to add fresh DE powder into the filter after each time you backwash.

Also, there are two basic types of DE filters: the vertical grid and the spin type.

Pool and Spa explains the vertical grid type,

The grids in a vertical grid type filter are assembled vertically on a manifold. These grids are secured to the manifold through a holding wheel and rod screws are retained into the base of the tank to secure the assembly. Water goes into the tank at the bottom and flows up and around the outside of the grids. The water then flows down the stem of each grid, towards the hollow manifold, and out of the filter.

Meanwhile, the spin type is said to already be obsolete but can still be found in older pool systems. The grids are wheel shaped and lined up horizontally resembling a box of donuts. They work the same way as a vertical grid filter. The difference is in cleaning them which is done by turning a crank to spin the grids.

Click here to hear a pool expert talking about DE filters. And click here to watch how to backwash a DE filter.

General Pool Filter Tips

No matter what type of filter you use it is important to note that its effectiveness will also be greatly affected by how you take care of it.

Tips include:

  • During springtime, o-rings should be lubed
  • Teflon tape should be used on threaded fittings
  • At the end of each swimming season, your filter medias should go through thorough cleaning
  • Using shut off valves and unions between pool equipment pieces

Taking care of your pool filter can make you spend less on repairs and you’ll have a clear pool much longer.

Conclusion

Overall, choosing which type of filter can be a tough decision. But this is made easier by going through proper research. And since this is not something that you can skip out when building a pool, taking the time to look at your options is a must.

The fundamental differences between the types of filters would be that:

Sand filters are the easiest to maintain but they also offer the lowest filtering ability. So those who care more for easier maintenance can opt to go with sand filters. They are also easy enough to use and contain just a few parts.

DE filters are the most powerful and offer the best filtering capabilities but require much more maintenance. So those looking for the clearest possible water and do not mind that extra level of maintenance can opt to go with DE filters.

Cartridge filters are kind of the middle ground. They are the greenest alternative as they do not require backwashing thus saving a large amount of water.

The water clarity they offer is almost up there with DE filters but not quite. And they work best with smaller and spa type pools. So those who are looking for the most environmental friendly option and do not need a filter for a large pool can opt to go with cartridge filters. They are also fairly easy to use.

Other factors to consider

Essentially, any type of filter you choose is going to be a compromise of the filters efficiency and difficulty in terms of maintenance. But it really isn’t that simple. You also have 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 such.

When it comes to expense, DE filters would have to be the most costly overall, taking into account initial price and maintenance. While, sand filters are usually the cheapest.

When it comes to size, cartridge filters are the most compact and save the most space.

Nowadays, all types have been improved to better meet your pool filtering needs. Some have even combined the best qualities of each filter type, like combining the quality of a DE filter with the convenience of a cartridge filter. Hopefully, we have given you enough information to both understand how different types of pool filters work and which one you think will work best for you.

BONUS VIDEO!