Effective Or Inadequate?

A number of dangerous contaminants and unsightly debris, such as fine sand, silt, algae, and bacteria, are well below 20 microns in size. The majority of these pathogens are between 2 and 20 microns.

For example, the single most-focused recreational water pathogens focused on by the NSPF and the CDC are Cryptosporidium and Giardia, which according to Duke University’s Cruising Chemistry educational module, are 3 to 4 microns and 5 to 6 microns in diameter, respectively.

E.coli, arguably the most generally known and feared pathogen found in pools, along with that which causes Legionaire’s Disease (the more prevalent one with fatal consequences), is estimated to be 2 microns in length according to an article published by the American Ground Water Trust in the American Well Owner (2002, Number 2).

While some of the most common pool contaminants–such as viruses–are below 2 microns, the majority can be dealt with by a filter that has porosity smaller than 20 microns, such as D.E. filters or most high-end robotic cleaners and some handheld battery-powered pool and spa vacuums. But no regulation mandates their use.

Sanitation And Oxidation

Sanitation and oxidation will kill most pathogens, but any remnants still must be filtered out of the water. Here also lies a major misconception, namely that sanitizing a pool by adding disinfecting chemicals will remove the remnants of the killed pathogens.

Overlooked is the fact that circulating any chemical throughout an entire pool is a slow process, taking several hours in many instances. The code requirement that a pool-cleaning system must have a vacuum available, and the ones that require brushing, imply this will be sufficient to rid the pool of dirt, debris, and pathogens.

The accepted idea is that the debris on the walls, steps, floors, and everywhere else in a pool that static particles adhere to will be brushed into the water and, as it settles on the bottom, will be vacuumed into the filter system and removed.

The problem with this notion is that the only particles that will be removed are those larger than the porosity of the filter. At best, in sand filters, the debris goes in one end and out the other, and right back into the pool–continuing to expose bathers.

There are maximum-velocity requirements for two primary reasons:


  • Creating a greater suction, which increases the risk of bather entrapment
  • Preserving the life of the PVC piping system.

Turnover Rates

Probably the greatest misconception perpetuated involves turnover rates, with the Gage and Bidwell Laws of Dilution being inaccurately paraphrased. This writer has only been able to find one such law and only references it, and he continues to seek the original report.

It’s up to you to make sure your pool water is healthy. © Can Stock Photo Inc. / cteconsulting

In 1926, at the 56th Annual Meeting of the American Public Health Association (APHA), Stephen DeM. Gage, Chairman of the Public Health Engineering Section, in a report by the Joint Committee on Bath Places of the APHA, wrote:

“It can readily be demonstrated by computation and by experiment that 7 turnovers are required to effect a removal of 99.9 percent of the dirt present in the water of the pool when recirculation was started. At the end of the first turnover the purification will be about 63 percent, after two turnovers about 86 percent, at the end of three turnovers about 95 percent, after four turnovers about 98 percent, after five turnovers 99.3 percent, and after six turnovers 99.7 percent. To accomplish a purification of 99.99 percent, 10 turnovers will be required.”

The raw data and any detailed report do not appear to have been preserved, or at least are not readily available, but those who refer to them play fast and loose with these numbers.

The National Parks and Recreation Association’s Certified Aquatic Facility Handbook states that the first turnover will filter less than half the water, and even includes that as a question on its certification examination. There are no data to even imply modern filter systems have improved the numbers, nor any claim they have.

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