When I was teaching filtration at technical schools and seminars throughout the country, I kept hearing the same question: “Which type of filter is best?” Attendees were not asking which manufacturer made the best filter, but rather which of the three types — DE, sand, and cartridge — was most effective
To answer that question, I got involved in some in-depth testing
of all of the pool and spa filters that were available at the time.
The tests were commissioned in the 1980s by a manufacturer that was
preparing to introduce a new line of filters.
During the testing process, we discovered some of the reasons
that pool and spa professionals gave for using their filter of
choice. Here are a few:
“This is the filter we have always
“We like the salesperson from that
“Backyards are getting smaller, and this filter takes
up less room.”
“If we buy 100 of these filters, we get a free trip to
“This filter is very easy to install and
“I don’t know why we use this
Some of these answers may sound silly, but if you think about
them, they are good reasons. For example, if you have used one type
of filter for a long time, it’s probably because it does the
job for you. Or, you may like the way the salesperson takes care of
you and any problems that arise. And, of course, a filter that is
easy to install and maintain saves you money and is good for your
But if you don’t know why you are using a certain filter,
maybe you should ask yourself that question. Experimenting with
different filter brands can be beneficial. For example, during the
testing process, we found that filters of the same type from
different manufacturers performed differently due to design
variations. This means that Filter A might need to be cleaned more
often than Filter B, even though they are the same size, have the
same flow rate (gallons per minute), and have the same dirt-loading
It is also beneficial to explore the different types of filters.
Each of the filtration types will work in most applications.
However, there are some applications where one type of filter will
work better than the others.
The choice of which filter to use is affected by a variety of
factors. For example, the sand filter may be a good choice for
desert areas because it is easy to backwash. However, in drought
areas of the country, the use of sand filtration is discouraged
because of the water wasted during the backwashing process. If the
pump is delivering 60 gpm, and the filter requires 5 minutes of
backwashing every month, that is 3,600 gallons of water a year. On
top of that, the chemicals, and sometimes even the heat, need to be
The same problem is associated with DE filters, which also
require backwashing. However, DE filtration can be used in areas
where water is scarce if the filter system contains a separation
tank. This is a capture tank with a bag inside that catches the DE
and the dirt, allowing clean water to return to the swimming pool
or spa during backwashing. The trade-off is that the cost to add
the separation tank is probably more than the cost of the filter.
Also, the equipment pad will need to be enlarged to accommodate the
tank and the extra plumbing.
Where water is at a premium, one successful sales approach is to
recommend a very large cartridge filter for the swimming pool. This
filter is big enough that it only needs to be cleaned once or twice
during a swimming season. This saves water and also makes the pool
easier to maintain.
A smaller filter may be just the right size and fit the
pocketbook, but the trade-off is that the filtration quality will
not be as good. A smaller filter also will become an ongoing
maintenance problem, which means it will need to be replaced much
sooner than a larger one. Some of the new “mega”
cartridge filters may go as long as 6 or 7 years before the filter
elements need to be replaced.
The best fit
Some filtration types work better for specific applications than
others. For example, DE filtration works best for pools that
require a high degree of clarity, since this type of filter
can remove very small particles. In fact, by the time you are ready
to clean the filter, it is pulling out particles that are close to
1 micron in size.
The sand filter works best for commercial pools because the
backwashing process can be automated. This means that it can be
programmed to shut itself down, reverse all of the valves, backwash
the filter, and then reverse the valves again and go back into
operation — all without any human interaction.
For hot tubs and spas that contain smaller water volumes (e.g.,
400 gallons), the cartridge filter is particularly effective
because it is so large. A DE filter would work in a hot tub or a
spa, but the problem is that you are dealing with hot water that
contains a lot of body oils and suntan lotions that end up coating
the filter. The cartridge filter is larger, so it can handle a lot
more of this type of coating before it needs to be cleaned. A sand
filter used in a spa at a resort, for instance, would need to be
backwashed almost daily.
When deciding on the best filter for your project, you need to
consider several factors. Make sure the filter fits your budget. It
should also be manufactured by a company that stands by its
products and has a good salesperson that takes care of your
company. Remember: It is a good idea to experiment with different
types and brands of filters. There is always a new idea that may
become the next best filter design.
The optimum flow rate
To maximize the effectiveness of a filter, make sure that the
water flow is at the optimum level [see graphic]. The most
important finding from the tests we performed was the effect that
high-velocity water flow had on the quality of filtration and the
cycle time, which is the time it takes for a filter to accumulate
enough debris to require cleaning.
High velocities of water flow will cause filters to short-cycle
and be harder to clean. “High velocity” means exceeding
a certain speed of water flow through the piping as measured in
feet per second (fps). This includes the piping inside the filter.
A measurement of 10 fps is considered high velocity. This is
approximately 70 gpm through a 1-inch PVC pipe (a modern 1-hp pump)
and 100 gpm for a 2-inch PVC pipe (a 1.5-hp or higher pump).
Some manufacturers once claimed that the higher the velocity,
the better the filtration — but this is the opposite of what
really happens in a filter. But, if you take a filter and move very
high-velocity water flow through it, you can practically pass a
bowling ball through the filter. If you slow it down, you are
actually pulling microbes and bacteria out of the water. The
problem with velocity is that it just drives the particles right
through the filter.
Here are the negative effects of high-velocity water
flow through the different types of filters:
High-rate sand filters. High-velocity water flow will drive the
dirt particles deeper into the sand bed than the filter was
designed for, requiring a longer backwash cycle to release the dirt
particles so they can exit the filter to waste. In some cases,
high-velocity flows can drive the dirt completely through the sand
bed and back into the pool or spa. This is called channeling. When
this happens, there is little or no filtration taking place.
High-velocity flows will wear down the rough sand particles to the
shape of smooth marbles. At that point, the sand will need to be
Diatomaceous earth filters (DE). High-velocity flows can drive
the dirt particles into and through the fabric covering the filter
grid. The weave of the cloth becomes clogged and cannot be
backwashed out to waste. The clogged weave reduces the amount of
filtering area, causing short filter cycles. Also, this makes the
filter difficult to clean. You will need to take the filter apart
and soak and clean each grid manually. This condition may require
the replacement of the filter grids as well.
Cartridge filters. The cartridge filter has a more difficult
time handling excessive flows than the other two types of filters.
When this filter was introduced in the 1960s, there was great
excitement. With a lot of filter area in a small tank, the industry
saw many benefits to using it. Unfortunately, this was also a time
when high-horsepower pumps were being aggressively sold. The
high-velocity water flows just drove the dirt particles into the
filter cloth and, in most cases, completely through the filter. It
was so bad that many of the health departments banned the use of
cartridge filters for commercial pools and spas.
These bans are no longer in place, thanks to larger cartridge
filters with improved plumbing. When cartridge filters first came
out, the water flow rate was fast because the piping was smaller (1
inch). Now the plumbing is up to around 2 inches — or in the
case of one filter, there is hardly any plumbing in it at all. This
means we can now move higher flows through cartridge filters than
previously. Thanks to these advancements — and the
convenience of easy maintenance — the cartridge filter has
become the predominantly used filter in the pool and spa
For best results, keep your water flow rate as low as possible.
Variable frequency drive pumps are a new energy-efficient way of
pushing water through the system. With the rising costs of
electricity, these pumps are very popular today, and provide the
unique advantage of allowing you to control water flow with the
push of a button.Fred Hare is an aquatic consultant based in Roseville, Calif.
He retired from Pentair Pool Products in 2002 after 44 years in the