There is a lot of confusion about various filters, and many different opinions.
Here are a few facts to consider.
The first is that a pool with quality media can properly be maintained with any of the filter systems available; Sand, Cartridge, or Diatomaceous Earth (DE). It is important to understand that if your filter media in not properly maintained, it does not matter what type of filter you have. Clean DE, clean grids, clean cartridges and fresh sand are essential to having crystal clean water. Also, proper water turnover rate is necessary to have desired results.
When talking about filtering water we begin with microns. There are 25,400 microns in one inch. 1/64 of an inch of an inch wide equals 615 microns. The human eye can see down as low as 10 microns. Including human hair (40 to 100), household dust, and plant/mold spores (10 to 30). You need a microscope to see less than 10 microns. The degree to which water is filtered is measured in microns. Sand typically 50 microns. Cartridge around 20 microns. DE as low as 5 microns.
Sizing your filter. When installing a new filter, the size of filter should be taken in to account. Do not undersize your filter area, this will lead to less efficiency and more maintenance. See pool professional for sizing guidelines.
Here is a brief description of each type:
Sand – The yesterday filter.
Water is pushed through a bed of filter sand and removed through a set of lateral tubes at the bottom. The filter area of a sand filter is equal to the area of the filter itself. For example, a 24” filter will have 3.14 sq. ft. of filter area. Only the top 1” of sand is actually used to filter the water. The principle behind this filter is that water is pushed through the filter sand, somewhat like an espresso machine. Dirty water goes in the top and clean water exits out the bottom. As the filter sand becomes plugged with debris from the pool, the pressure increases on the filter and the water flow drops. In order to clean the filter, you just run it in reverse and dump the waste water; this is referred to as “backwashing” the filter. Once the filter is backwashed, you move to the rinse mode and that repacks the sand and then back to filter. This has to be done manually every few weeks. From a hydraulics standpoint, a backwash valve is typically the most inefficient piece of equipment you can add to a swimming pool system. The sand should be replaced every 3 to 5 years. In terms of particle size filtered out, sand is the lease effective method filtering, approx. 40-50 microns – it can allow smaller particles to pass back into the pool. Common problem after backwashing. PRO: Don't have to teardown once/twice per year. CON: Least efficient filter, lots of backwash and sand must be changed every 3 to 5 years.
Cartridge – The economic low maintenance filter.
This one is easy to understand. Water passes though a filter material and the filter captures the debris. This is just like the water filters used under your sink. Cartridges have much more available area to filter than sand. Most spas and fountains start at 100 sq. ft. and the majority of the cartridge filters sold are 320 sq. ft. to 520 sq. ft. So they don’t clog up as quickly and therefore you touch them less frequently, no backwashing. There are two types of cartridge filters in general. In the first case, there are filters elements that are inexpensive to replace and as such, they don’t tend to last as long. Then there are other filters that have very expensive elements and these last 3 or more years. In both cases, cartridge filters are designed to run at lower pressure than sand. This puts less backpressure on the pump and hence you get more flow and turnover for an equivalent pump size. Generally these filters have to be cleaned once or twice a season by simply high pressure hosing, so you don’t touch them as often. In terms of particle size filtered out, cartridge is somewhere around 20 microns and rates between sand and DE. PRO: No backwashing. CON: Must replace cartridge elements every 3 years.
DE – The water polisher.
Diatomaceous earth is mined and is the fossilized exoskeletons of tiny diatoms. They are used to coat “grids” in the filter housing and act as tiny sieves to remove debris. They are very small and as such can filter out particles as small as 5 microns. Diatom filter area are sized between sand and cartridge – around 48-60 sq. ft. are most common. Once the filter pressure rises, the filter is backwashed just like a sand filter and then “recharged” with more DE powder. Typically it is poured in a slurry into the skimmer and it then coats the filter grids. Generally the filter grids have to be cleaned once or twice a year. DE filters run at higher pressures than cartridge filters. DE is clearly on top when it comes to filtering the swimming pool water. PRO: Brilliant, clear water. CON: DE filter media is messy.
Tropical Pool and Spa offers Pentair Pool Products including: Clean and Clear Plus Cartridges Filters and Series 4000 Fiberglass DE Filters.