There's a lot of confusion about various filters, and many different opinions. Here are a few facts to consider:
The first is that a pool can be properly maintained with any of the filter systems available: Filter Cartridge, Sand or Diatomaceous Earth Swimming Pool Filter Systems. Below is a brief description of each type.
Filter Cartridge Filtration
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 start at 50 Sq.ft and the majority of the cartridge filters sold are larger than 100 Sq.ft. Cartridge Filters are designed to run at lower pressure than sand. This puts less back-pressure 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 hosing them off, so you don’t touch them as often. In terms of particle size filtered out, cartridge is somewhere between sand and DE.
The older style 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 sqft 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 “back washing” the filter. Once the filter is back washed, 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. Should the sand ever become really dirty, it is easily and inexpensively replaced. In terms of particle size filtered out, sand is the lease effective method – it can allow smaller particles to pass back into the pool.
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 60-70 sq ft are most common. Once the filter pressure rises, the filter is back washed 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. DE filters run at higher pressures than cartridge filters and as such can lead to some inefficiency and flow loss.