In the past 40 to 50 years, regenerative diatomaceous earth filters have become the norm for maximum filtration efficacy, replacing the older style of open-top vacuum DE filters. It has been generally accepted that DE has the ability to remove smaller particles than any other type of pool filtration media.
A well-engineered, quality regenerative DE filter has the
capability of efficiently removing particles down to 2 to 3 microns
in size. Removing any particles larger than 3 microns from the pool
water achieves maximum water quality. Typically, other media filter
down to only 15 to 20 microns.
DE’s filtration efficacy retains a major portion of any
cryptosporidium protozoa that enter the filter.
Crypto is listed as an elongated pathogen, approximately 5
microns in diameter by 7 microns in length. At the 3 micron
retention of DE filters, we might be able to produce results better
than other approaches. In many installations, these filter systems
are already in place and wouldn’t require any new capital
So why aren’t more facilities using DE? Ever since I entered
the pool industry 25 years ago, I have been hearing talk of banning
DE because it is a “carcinogen.” Of course, DE has
never been banned. Let’s quickly examine the facts:
• DE is a Class 1 carcinogen — but only when airborne!
Once it has been wetted, it is considered inert. To put this into
perspective, other common Class 1 carcinogens are beach sand,
filter sand, plaster dust and sawdust, according to the
International Association for Research on Cancer.
Because DE is only a Class 1 carcinogen when airborne, modern
regenerative DE filter systems use a vacuuming process to charge
the filter with DE. That totally eliminates any airborne DE. An
agent of a major DE and perlite producers/distributor recently said
you are exposed to more Class 1 carcinogen from blowing sand at the
beach than you are when you put DE in a modern regenerative DE
• A nationwide docket search failed to find a single current
case of litigation related to the use of DE in pool applications.
In almost 70 years of using DE in aquatics, there is no record of
lawsuits for death or serious injury.
DE even has a Food & Drug Administration CFR 21 rating as an
“incidental food additive.” DE is such a common media
for filtering food products such as soda pop, drinking water, beer
and the like that there’s always the possibility of some
miniscule particles of DE ending up in the final product. That has
been studied by the FDA and deemed safe according to its
We are primarily interested in incorporating equipment that
provides the utmost filtration efficacy, so we’re always
testing new media. Our interest stems from the desire to use media
with the best performance. It is also slightly driven by the
“bad press” that DE still frequently receives.
In the past, we have tested cellulose and perlite and compared
those results with the performance of DE. Several years ago we
published a “white paper” laying out the results. In
those tests, DE far outperformed those two.
Recently, articles have been stating that some tests show perlite
performed as good as, or sometimes better than, DE. Those articles
prompted us to revisit our testing to see what could create this
apparent disparity. One possible factor is that the perlite product
we had tested had not yet been approved by the National Sanitation
Foundation. The perlite was subsequently reformulated to pass NSF
testing. The reformulation was, obviously, to develop an improved
Some suppositions for the different test results were verified upon
closer examination of the contradictory testing methods. Those four
disparities were as follows.
• We used a “perfect filter” test vessel. This
allowed us to eliminate any unwanted vibrational flow noise
signatures associated with inefficient filter design. In this way,
we were able to accurately measure “pure” media
performance only. The contradictory testing was done with
hotel/motel grade filters (not regenerative filters).
• Loading Rates for DE used in the contradictory tests were
less than manufacturer’s recommendations. We tested both
media with an identical 0.125-inch “tortuous path.”
Tortuous path is the distance the water being filtered has to
travel through the media to reach the hollow inner portion of the
filter element or septum.
• Contradictory tests compared perlite with permeability of
approximately 1.5 Darcy (fine) vs. DE of 3 Darcy permeability
• Our new tests were performed using both media at 1.5 Darcy
permeability, so that we could have an
Our results were used for a paper presented at the World Aquatic
The perlite at 1.5 Darcy used in this test performed better than
the 3 Darcy perlite we had used in our earlier tests. However,
retention numbers for our new tests showed a 4 Log (99.99 percent)
vs. 2 Log (99 percent) retention of 5 micron sized particles
(crypto size). That’s 100 times better efficacy for
DE over perlite.
To put that into more understandable (less scientific) terms,
consider that a single diarrheal accident can potentially contain
millions of crypto oocysts. If a million of these
pathogens are present in the pool water passing through a DE
filter, less than 100 will make it through and back out to the
By comparison, if a million crypto oocysts are present in
the pool water passing through a filter using perlite as its media,
more than 10,000 of those infectious pathogens would be returned to
the pool. Because as few as 10 crypto oocysts are needed
to infect a susceptible individual, it is vitally important that as
many as possible be removed in the filtration process. The level of
chlorine maintained in the pool is insufficient for crypto
inactivation. It could take at least two weeks for crypto
inactivation at 1 ppm chlorine residual.
We tested flat plate elements (used in vacuum DE Filters) and
tubular elements (used in regenerative DE filters). The results
presented here are for tubular elements because they are used in
regenerative DE filters, which are currently the preferred DE
filtration product. Many years ago, Dr. Frank M. Tiller (“the
father of modern filtration theory”) proved that filtering
with tubular filter elements created a longer “tortuous
path” than a flat plate septum. Our results from tests of
both types of elements reconfirmed his findings.
From the results of our tests using 1-, 3- and 5-micron-sized
microspheres, we have formed a reasonable expectation of the
ability of DE to quite effectively remove crypto from
filtered pool water (full pass). Please keep in mind that this was
a test of the filtration efficacy of media only, under ideal
conditions. Your decision as an owner or designer must take into
account the capabilities of the filtration product into which you
put the media. Turbidity performance tests at system start-up are
an excellent way to determine what your system will be able to