Kim – The Reality of Pool Chemical Treatment

For 50 years, it has been common for pool service companies to successfully maintain residential pools on a once-a-week visitation basis. Empirical evidence has shown that pool water can be kept safe and properly sanitized without harmful bacteria and algae developing with such weekly treatment programs.

Also, these pool service companies provide a great service by keeping hazardous chemicals out of the hands of children (and even some pool owners!).

One reason that pools can be kept properly sanitized with weekly visits is due to the use of various chemical feed or auto-systems which usually use either bleach, cal hypo, salt, trichlor, or bromine for sanitizing.  And of course, the use of ozone or UV systems also keeps pool water safe without daily treatments.

The use of cyanuric acid, which slows down chlorine loss, provides the ability for less frequent chemical treatments.  In fact, once-per-week treatments to properly stabilized residential pools by thousands of professional and experienced service techs for over 50 years has shown that providing safe, balanced, and algae-free pool water is achievable.

Another important aspect of maintaining a good chemical and sanitizing program for residential pools is maintaining the pH within a proper range.  One thing that worries some service techs is when pH drifts higher in the days between service visits.

In that regard, it is unfortunate that some service techs and others in our industry seem to believe that a pH of 8.0 to 8.4 makes chlorine in pool water ineffective and unable to properly sanitize and kill bacteria and algae. It is also incorrectly taught that eye and skin irritation develops at these pH levels.  Both of those concepts are false.

It is well known that CYA significantly reduces the percentage of “active” hypochlorous acid (HOCl) and less active hypochlorous ion (OCl) in the water, converting these to bound, or stabilized chlorine in solution.  But what is not well-known is that when the pH rises slightly in pools containing CYA, the amount of HOCl almost remains the same, and therefore has nearly the same sanitizing efficacy and bacteria killing power.

A 1974 report written by J. E. O’Brien* indicated that when the pH rises from 7.5 to 7.8 in stabilized water, the reduction in HOCl only drops by about 10%, and when the pH rises from 7.8 to 8.2, the reduction in HOCl only drops by 10% also.

But is that the end of the story?  What about that lower amount of active chlorine? Is the water unsafe? While a 10% lower amount of HOCl wouldn’t make a lot of difference in sanitizing, the lower amount of HOCl can be easily compensated for and overcome by simply maintaining the chlorine 10% higher.

For example, pool water with 2.5 ppm of chlorine, with 50 ppm of CYA, and at a pH of 7.8 is considered acceptable by industry standards.  All one needs to do to obtain the same killing power and chlorine efficacy when the pH is maintained at 8.2, is to raise the chlorine level by 10% or to 2.75 ppm.  Obviously, that isn’t very much additional chlorine in a 15,000-gallon residential pool.  Only about 3.2 ounces of pool bleach.

If the chlorine is at 3 ppm in the above example with a pH of 8.2, it means that there is actually more sanitizing (killing) efficacy than having the chlorine at 2 ppm with a pH of 7.5, or even at a pH of 7.2.  This example shows that stabilized pool water can be properly sanitized at a pH of 8.2 (due to higher chlorine levels), and a higher pH does not automatically render chlorine incapable of killing bacteria and algae.

Plus, maintaining the chlorine levels a little higher will result in a higher reservoir of OCl ready to convert into the more active HOCl as needed.

Finally, studies have shown that a pH of 8.0 to 8.4 alone does not cause or increase eye or skin irritation. A pH rising above 7.8 between visits is common in all chlorinated pools, and especially in salt pools. In fact, many pool service companies have been successfully maintaining the pH from 7.8 to 8.2 for many decades.

Addendum:

Pool water with a negative LSI will not typically stay that way (negative and aggressive) permanently or for very long. And that is because aggressive water, after a period of time of slightly etching a plaster surface, will cause the water to become non-aggressive (neutral) and balanced when the calcium, alkalinity, and/or pH level increases. Therefore, in practice, trying to keep pool water aggressive (at a -0.4) is difficult (as well as hard on the plaster!) because the water will re-balance itself repeatedly.

* J. E. O’Brien, J. C. Morris & J. N. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate”, Ch. 14 in Alan J. Rubin’s “Chemistry of Water Supply, Treatment, and Distribution”, Ann Arbor:  Ann Arbor Science Publishers, Inc., 1974, ISBN 0-250-40036-7, pp. 333–358.

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