All About Filters

A properly maintained pool incorporates both filtration and sanitation. We sanitize to kill bacteria, viruses, and algae, and we filter to remove particles suspended in the water. While clear water is not a guarantee that the water is safe to swim in, cloudy water is definitely a sign that something needs attention.

Your filter is a critical part of your pool system, and should be selected with care and properly maintained. In this issue, we’ll provide some information to help you understand the various options and advantages and disadvantages of each.


Cartridge filters are usually what you will get with pre-packaged pools or spas. They work by trapping particles in a pleated, non-woven fabric. Cartridge filters are readily available and easy to operate. They occupy very little space and do not require a backwash outlet. However, they are less than ideal for a dog pool.

The cartridges require frequent cleaning or replacement. Cleaning usually involves a hose, lots of water, and a fair bit of time. An automated cartridge cleaner such as the Blaster ( is a good investment. It’s also a good idea to have extra cartridges available: one in the filter, one being cleaned, and one ready to go.

Cartridges are my least favorite choice. The amount of hair and dirt that dogs bring in causes them to clog quickly – you may be changing/cleaning them daily. They can remove particles from about 30 to 100 microns in size. This is the least effective of all filtering options, so you will not have optimal water clarity.

If you do use cartridges, try to choose something with as much square footage as possible (I’d recommend at least 200). The more square footage you have, the less often you’ll have to clean or replace cartridges.


Sand filters, which use several cubic feet of sand as a filter medium, have much lower maintenance requirements than cartridges. Their effectiveness actually improves as particles accumulate (up to a point). Sand filters can go several weeks between cleanings, and they are the best filter for handling large amounts of hair. Sand filters will remove particles as small as 20 microns.

Sand filters are cleaned by backwashing, which involves running the pool water backwards through the sand and out as waste. Very little or no sand is lost in the process. Backwashing also provides an opportunity to do a partial water change – a good practice no matter what type of filter you use.

Approximately every six months the sand should be washed with a special cleaning solution to remove oils that may accumulate. The sand itself should not require replacement for 4 years or more. A properly operating sand filter will NOT put sand into the pool.

Sand filters take up more space than cartridge canisters, and you must be able to run a waste hose from the filter to a drain or the outdoors.


Zeolite (sold under the brand name Zeobrite, is used in sand filters instead of “generic” sand. It is a natural, granular mineral similar in appearance to sand, but it has filtration abilities that are far superior. Zeolite can filter particles as small as 3 to 5 microns, which rivals DE. The cleaning and maintenance requirements are even better than sand – less backwashing is required, although you should still wash out the oils every six months.

Zeolite is also capable of boosting the effectiveness of you sanitation system because it can actually remove ammonia from the water.

A sand filter filled with Zeolite is my choice for filtration.

DE (Diatomaceous Earth)

Diatomaceous Earth is a fine white powder that looks similar to talcum powder. It is actually the skeletons of billions of tiny sea creatures. DE filters generally provide optimum water clarity, as they will filter down to 3 microns.

DE filters utilize a fabric covered grid. The DE powder is added to the water, usually by pouring it into the skimmer, and it then becomes trapped on the fabric.

DE filters require more maintenance than sand or zeolite. They require regular backwashing, but unlike sand, the DE powder is lost during the process and must be replaced. Most municipalities prohibit washing DE down the sanitary or sewer drains, as it will clog the filters in the treatment plants. You may have to add a recovery tank to meet building codes.

Because it is so fine, DE filters clog quickly. When clogged, they become “channeled”, meaning the water will bypass the DE and even though it is running through the canister it is not being filtered. Most DE filters have a “bump” function, which is like shaking the powder to redistribute it evenly.

DE filters should not be used with biguanide sanitizers such as Baquacil or SoftSwim, since they cause the powder to cake.

If you choose a DE filter, I recommend using it as a secondary filter after a cartridge filter. This will keep the larger particles out of the DE grid, and reduce the need for backwashing or bumping.

How Big?

Well, that question will fill a whole issue, so we’ll cover that in a future Splash. In short, however, bigger is better when it comes to filters, so don’t scrimp here.

Happy Swimming!

Note from ACWT President Cindy Horsfall: I value and understand everything that Karen says here in her article on filters. She’s got great advice and knowledge. However, I feel compelled to say here that I do love my cartridge filter and I wouldn’t replace it. I have a 150 sq ft cartridge filter for my 8×20 swim spa and I hose it out every 2 weeks and replace it (they cost about $80) every month. They are the easiest solution for me and my lifestyle.

Got questions about water quality, chemistry, or equipment?
Email Karen Hunter at:

The ACWT Pool GuruThe ACWT Pool Guru:
Karen Hunter
22609 102nd Ave SE
Woodinville, WA 98077
425 487-3078

Posted in ACWT, Pool Basics.

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