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Which filter is better: sand, DE, or cartridge?

The purpose of the pool’s filter is to keep the water clean and clear by removing dirt, algae, particles, and other stuff that makes the water cloudy.  Diatomaceous Earth (DE) takes out much smaller particles much more quickly than any other filter.  Take a look at a bottle of filter sand and a bottle of diatomaceous earth.  The different is apparent.  On this, there is no question, end of discussion.

DE removes particles down to three microns, all the time.  A micron is very small – a millionth of a meter.  Cartridge filters work down to 40 microns only when they are working correctly.  Sand filters work around 100 microns right after they are backwashed, and get down to about 75 microns when they are dirty.  Shocking, which is a chemical oxidation done to pools weekly to monthly, burns off organics (which is mostly what is that small in a pool) up to two microns.

So over time, pool water will build up anything above two microns, and below 3, 40, or 75 microns, depending on your filter.  Then what happens?  If you have a sand or cartridge filter, some day, perhaps in a few years, there’s enough of this material in the water to give it a light haze or cloud.  Then you drain the pool, borrow a DE filter for a few days, or start using a clarifier or filter aid on a regular basis.  So why doesn’t everyone just use a DE filter?  Because they are more expensive.

Cartridge filters clean the water better than sand, and are fairly inexpensive to buy.  Supposedly, they are better for the environment, but that’s sort of a “Paper vs. Plastic” argument.  If you use tap water to wash them off, they put fewer chemicals into the environment.  Then, someday, you put the old cartridge into the trash and it goes into the landfill for 1000 years!

In my experience, cartridge filters are easily overwhelmed by an algae attack.  When they are old and soft, they allow dirt to go right past the cartridge and back to the pool.  Most of the owners of cartridge filters that we know (mostly on spas and small pools) are constantly fighting cloudy water.  When you only have a few hundred gallons of water, draining and starting over is not a big deal.  When you have 20,000 gallons of water, it is!

A sand filter is a very simple apparatus – it’s basically an empty bucket filled with sand.  As water passes down through the sand, dirt is caught in the spaces between the pieces of sand.  When the sand is full of dirt, you flush it out by running the pool water backwards through the sand, and out onto the ground.  This operation – backwashing – is done roughly once a week, and uses about 50-100 gallons of water. Obviously, how much and how often is variable and depends on usage, etc.  Sand can be reused for a number of years, but it’s actually cheaper and easier to throw it away in fall and replace it in spring (maybe $10 to $30 per year, depending on filter size).  Sand filters are very easy to operate, except when you’re changing the sand at the end of the year.

A DE filter is internally much more complex, though you rarely actually open it up.  Diatomaceous Earth is a white powder found in huge beds in the ground on the west coast.  It is made of the fossilized bodies of diatoms – microscopic animals that lived in the Pacific Ocean millions of years ago.  It is used most commonly by nurseries as mulch, in restaurants and food service to filter cooking oil and clean grills, and on large commercial pools and hot tubs where absolute water clarity is very important.

You will probably backwash your DE filter as often as a sand filter.  Two differences:  it will probably only use half as much water, and you will then replace the flushed-out DE by adding a few scoops of it into the through-wall skimmer.  DE is very inexpensive – about $15 a summer should do it.

The real differences?  Sand filters are less expensive to purchase, though slightly more expensive to operate.  They will generally keep the pool water clear, though probably not as sparkling as the water cleaned by DE.  If you should develop a major cloudiness or algae problem, the sand filter will probably not develop a major clogging problem.  In the same situation, the DE filter would clog up two or three times a day for two or three days – until the pool was clear.  Of course, that’s because the DE filter was clearing up, in two or three days, what the sand filter was going to take a week or two to accomplish.

The new “permanent media” cartridge filters will probably keep the water cleaner than sand – with no backwashing ever.  However, all water research suggests that they should still be chemically cleaned twice a year even if they are not yet overloaded with dirt.

Here’s the problem: Just because you can hold a year’s worth of dirt, algae, bacteria, skin cells, bugs, and other gross stuff in your filter does not mean that you should want to.  Even through you don’t see all that stuff; you’re swimming in water that runs right through it all three times a day.  You can end up growing slime, infections, and smells in your pool water that came from your filter cartridge.  You can fix that with more pool chemicals, but why have to?

The big cartridges don’t have to be replaced all the time like the small cartridges, but that doesn’t mean that they last forever.  The first four years seem cheap enough, but a $150 bill in year five can be a shock!