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 can be properly
maintained with any of the filter systems available: Sand, Cartridge, or
Diatomaceous Earth (DE). Here is a brief description of each type:
Sand – The bullet proof 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 “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. 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.
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 start
at 100 sqft and the majority of the cartridge filters
sold are larger than 300 sqft. So they don’t clog up
as quickly and therefore you touch them less frequently. 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 5
or more years. In both cases, 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.
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 60-70 sqft 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. DE filters
run at higher pressures than cartridge filters and as such can lead to some
inefficiency and flow loss.