Pool Filter Overview

pool filterA swimming pool filter plays in important role in maintaining a clean and healthy swimming environment. After all, that big hole- filled with water- is subjected to all kinds of environmental attack. On the one hand, we have all sorts of biological debris that can entire the pool water- from falling tree leaves, dirt, pollen, plant debris, and even dead bugs and rodents. We also have all sorts of nasty and potentially deadly pathogens such as Shigella, Giardia, Cryptosporidium and Legionella.  (There was a Legionnaire’s Disease outbreak in Philadelphia that killed 25 people.) Bottom line: We need to keep our pool water clean and healthy and this is why pool filtration- in conjunction with proper sanitation becomes so important.

The primary job of the pool filter is to trap small particles that make their way into the water and keep them isolated. This prevents their entry back into the pool where they would recontaminate our water. How the process works depends on which type of filter we are utilizing.

In the pool industry we have three basic types of filters:

  • Sand Filters;
  • DE (diatomaceous earth); and
  • Cartridge.

Pool Filters with Sand Filtration

A sand filter is a large vessel usually round or oval shaped averaging about 24” circumference. There is an access port on top in which the sand media is introduced to the filter. Normally offered in three sizes to accommodate smaller or larger pools, an average residential sand filter will contain between 350-900 pounds of sand. Depending on the model the filter will have either a side-mounted valve or top-mounted valve, which contains our plumbing ports. The ports bring in water from the pool pump and return the water back.

As the incoming water flows into the filter and through the bed of sand, particles from dirt and debris are trapped in the top several inches of sand. The clean water continues on through the bottom of sand and out of the lateral plastic slotted arms located at the bottom of filter. This prevents the sand from passing, but allows the water to move back to the pool.

Also located on our filter is a pressure gauge to indicate how much the trapped particles are increasing our pressure in the filter- which translates to decreased water flow. This valve also performs the method of cleaning the filter (which is known as “back-washing”). When the pressure gauge reads 10 PSI (pounds per square inch) over our clean filter pressure, it is time to back-wash the sand bed to rid it of the trapped debris. This is easily performed by first turning off the pool pump if running and then noting the position marked on our backwash valve and depending on type, either turning, or sliding to the back wash position. Then we turn the pump back on. Now, since our valve has reversed the incoming and outgoing ports, we are bringing water up through bottom of filter pushing the trapped debris out the top of sand bed and out a hose or drainage pie to an area in yard where the water can run off. Usually we let the backwash cycle go for approx. one minute before turning off the pump and re orienting the valve back to the filtration position. This process is repeated as necessary (usually monthly) when our pressure gauge again raises 10 PSI above the starting pressure.

Of our three filter types, sand filters do the poorest job of filtration. They filter down to a 25 micron size particle. They do an adequate enough job as long as the pool water sanitation is kept in close check, but tend to show their weak points in pools that are already having issues with algae or the regular care is lacking.

Diatomaceous Earth (DE) Filters

The next in line for our filter discussion is the DE or diatomaceous earth filter. OK, so you’re asking yourself, “What the heck is diatomaceous”?

Good question. Besides being hard to spell, the word diatomaceous comes from the source word diatoms. OK, so you just said to yourself, “sorry I asked”. Right?

It’s simple. Diatoms are the prehistoric fossilized remains of hard shelled algae. There are large deposits of diatomaceous earth (DE) in various places around Europe and North America. It’s kind of funny that we are using the remains of old algae in order to trap algae and other small particles in our filter, but that’s what we’re doing.

Inside the DE filter tank there is a set of curved grids that are coved in porous fabric material. These grids are arranged in a semi- circular assembly so that they each overlap each other creating a 1” space between each grid. The DE powder is added to the pool skimmer with the pump running. As the DE enters the filter it coats the surface of the fabric grid material thus that the grid is really just a support structure for the DE. Each different size filter will require a different amount of DE to use as per the manufacturer’s instructions.

The pool water enters the filter and particles and debris are trapped on the surface of the DE powder as the water flows through the tank and back out the return plumbing to pool. Just as in our sand filter, the DE filter has a pressure gauge as well as a backwash valve mounted on it. And just as in our sand filter, when the pressure reads 10PSI over our starting pressure, it is time to clean the filter via back-washing. We follow the same back-washing guidelines as the sand filter except when finished, we need to add some more DEfilter cartridge to replace the amount that was flushed out. Usually a few coffee can size scoops will do. Unlike sand filters however, DE filters can actually be opened up so that the grid assembly can be fully removed and hosed down for a thorough cleaning. This is always our preference as even though there is some work involved in disassembling the filter, the benefits of a more complete cleaning are preferable.

DE filters do an excellent job of filtration and can filter particles down to a 2-5 micron size. That’s small enough to even remove all the little “floaties” from in front of the pool light at night! They can quickly clear a pool that might have gotten a little out of hand much better than a sand filter.

Cartridge Pool Filters

This brings us to our last pool filter type, the cartridge filter. Cartridge filters look basically the same as the DE filter with the exception that there is no back wash valve and only a pressure gauge on top of filter. That is because the design of the cartridge filter totally eliminates any needed back washing.

Inside the filter tank there are four cylindrical pleated elements that look very similar to a cars air filter only much larger.