• Image
    DE vs. PerliteThis chart shows the efficacy of DE filtration compared with perlite for microspheres ranging from 1 to 5 microns. These microspheres are meant to approximate crypto oocysts in size.Conditions: Perfect filter — Tubular | Particle retention test results Filter aid comparison — 1.5 Darcy permeability Results averaged from 10 filter runs, 20 samples per run @ 1.5 gpm/sq. ft.

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 outlays.

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 filter.

• 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 requirements.

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 perlite media.

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 (coarse).

• Our new tests were performed using both media at 1.5 Darcy permeability, so that we could have an “apples-to-apples” comparison.

Our results were used for a paper presented at the World Aquatic Health Conference.

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 pool.

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 achieve.