Filtration Facts  

Effective filtration is a cornerstone of pool water maintenance. We see pools everyday where the water looks almost invisible it’s so clear. These are pools that are getting filtered correctly. There are a great many others though where the water is slightly to very cloudy.  If you think your water looks good, just check it out at night with the underwater light on. That cloudiness you see emanating from the light is dirt in suspension in the water. CHEMICALS CAN'T FIX DIRT. But proper filtration can.

There are 3 types of filters in common use.

  1. Sand Filters
  2. Cartridge Filters
  3. Diatomaceous Earth Type Filters


Backwash or clean your filter when the pressure on the gauge increases by 10psi over  the clean pressure.   Most filters operate at about 12 to 15psi when clean. If you clean your filter based on the increase on the pressure gauge, you get a few benefits.  Filters of all types filter better the dirtier they get.  The dirt collected becomes a filtering medium and helps the cleaning process.  When the pressure gets too high however, the water flow can become restricted going through the filter.  At this point it's good to backwash or clean out the filter.   If you clean your filter following these recommendations you may find yourself doing less work than if you stick to backwashing once a week and that's more time you have for something enjoyable. 

Sand Filter

Recommended hours of filtration = 12 or more.

The most common and easiest to maintain is a sand filter. This works by running water through a sand bed that is several inches thick. A sand filter can filter particles of dirt as small as 40 microns in diameter. A micron is 1 millionth of a meter. If you have good eyesight you can just see a particle that is about 40 microns in size. A human hair is approximately 75 microns in diameter. Any dirt smaller than 40 microns passes through a sand filter and remains in the pool water. 

A lot of 40 micron particles that you can’t see doesn’t seem like it could cause a cloudy water problem until you get billions of them together in your pool. Then the water looks cloudy and the dirt just runs right through your filter and back into the pool.  For a quick and easy demonstration, put your pool light on at night and notice the gray cone illuminated by the light.  That is unfiltered dirt.

Solution? Drain and refill the pool or hire a pool service with a portable Diatomaceous earth filter. This is usually a cart with a pump, motor and DE filter mounted. They just put the inlet hose in one side of the pool and the outlet in another and let it run until the water’s clear. 

Sand is not a permanent medium. It  becomes contaminated with suntan oil and oils from the human body as well as dirt. Sand should be changed every 4 to 6 years (if you don’t know how long it’s been since the last change or you moved into your house without knowing how long and you have cloudy water, change it NOW!). Be sure to buy your sand at a pool store or a store that carries sand meant for sand filters. Beach sand or sand from the local plant nursery does not have the squared edges required for filtration and won’t work.

Cartridge Filters

Recommended hours of filtration = 10 or more.

Cartridge filters are pleated paper cartridges that look like the air filter or oil filter in your car. They can remove particles from your water that are as small as 10 microns in diameter. This is a quarter the size that sand filters can remove. A cartridge still allows all particles that are smaller than 10 microns to pass through. A cartridge filter captures much of the dirt that passes right through a sand filter. Cartridges should be replaced after a year and a half of use. 

Some of the problems with cartridge filters can include a cartridge that develops a rip or tear in its body will allow water to pass through unfiltered. Water will take the path of least resistance and pour through an opening in the cartridge. Also if the seal at the top or bottom of the cartridge is damaged or is not properly sealed, the water will get through unfiltered. Some solutions would be to apply Magic Lube or Aqua Lube (common pool sealing products available at your pool store) to the seals, or replace the cartridge. 

You might have a spare cartridge on hand to swap with the one that’s giving you trouble, that way you don’t lose any filtration. Some people use diatomaceous earth with their cartridges.  The DE powder clings to the cartridge and the dirt clings to the DE.  This makes the cartridge easier to clean off and you get cleaner water in the bargain.


Diatomaceous Earth Filters


Recommended hours of filtration = 9 or more.  

I’ve saved the best till last. DE (as it’s commonly known) filters can filter particles as fine as .1 micron. This is ten times better than a cartridge filter and 40 times better than sand. Wineries use DE filtration to remove particles from wine. Diatomaceous earth is a fine white powder, which is made up of the exoskeletons of marine plankton. Most of the Diatomaceous earth used in this country comes from ancient sea beds around what is now Los Angeles California. 

A diatomaceous earth filter has several PVC plastic grids inside which are covered with Polypropylene mesh. 

The mesh is fine enough to hold back the diatomaceous earth, and the earth allows the water to pass through while stopping the dirt. Particles that are smaller than 1-micron pass through and remain in the pool water. Some problems with DE filters include, if one of the grids inside the filter develop a rip or tear then the water passes right through. This is easy to diagnose since the DE powder also passes through and makes a grayish powdery mess on the bottom of the pool. Your pool sweep stirs it up during the day and makes the water look cloudy.  If you turn your pump off it will settle back down to the bottom. You'll need to replace the grid. Over time the mesh becomes brittle and it’s very easy to put a finger through and cause a tear.

Also if a grid isn’t seated properly where it plugs into the base of the filter housing you could have a leak allowing unfiltered water to pass through.

The polypropylene mesh that covers the grids is so fine that it can screen out the calcium that’s in the water. When you take the filter apart and look at the grids look for grayish white patches that indicate calcium. If significant portion of the grids is covered with calcium, you need to take the grids to your favorite pool store for an acid bath. This will remove the calcium and allow the water to flow more freely through your filter. It’s recommended that you acid wash your filter grids annually.  


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