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The Reality of Pool Chemical Treatment

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For at least 40 years, many pool service companies have been successfully maintaining their 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 weekly treatment programs.

Also, pool service companies provide a great service by keeping hazardous chemicals out of the hands of pool owners and their children.

One reason that pools can be kept properly sanitized with weekly visits is due to the use of various feed or auto-systems which usually use either bleach, Cal hypo, Salt, Trichlor, or bromine for sanitizing.  And of course, the use of Ozone and UV systems also keep pool water safe without daily treatments.

The use of cyanuric acid, which slows down chlorine loss, also helps to provide the ability for less frequent chemical treatments.  In fact, there are no reports or studies that suggest that professional once-per-week treatments automatically result in unsafe, cloudy, or algae pool water conditions.

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, 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 completely 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 not true.

It is well known that CYA reduces the amount of “active” hypochlorous acid (HOCl) in the water.  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.

In 1974, a report written by J. E. O’Brien indicated that when the pH rises from 7.5 to 7.8, 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 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 2.2 ppm.  Obviously, that isn’t very much additional chlorine in a 15,000-gallon residential pool.  Only about 3.2 ounces of pool bleach.

Let’s also understand that if the chlorine is at 3 ppm in the above example with a pH of 8.2, it means that there is 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 pool water can be better 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 level a little higher results in a higher reservoir of OCl- ready to convert into the more active HOCl.

Finally, studies have shown that a pH of 8.0 to 8.4 alone does not cause eye and skin irritation. So service techs can stop fretting about a pH rising above 7.8 between visits. In fact, many pool service companies have been successfully maintaining the pH from 7.8 to 8.2 for many years.

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Comment by John on January 4, 2017 at 3:32pm

Thanks Kim, that was pretty much my understanding of the CYA un-bonding topic, not as clear cut as others made it out to be as still the largest portion of the chlorine remains bound, glad I don't use CYA just a lower level of chlorine injected more frequently through more points around the pool. Works for me!

Comment by Kim Skinner on January 4, 2017 at 12:11pm

When pool water contains CYA, the amount of HOCl (from the total chlorine) content is very small, around 1%, as compared to 23% at pH 8.0 with no CYA.   

Some algae (and bacteria) are not effectively eliminated by very low amounts of HOCl, and therefore, require higher amounts of chlorine to kill faster than it multiplies.  It is not that the HOCl is not being released or formed. 

Comment by John on January 3, 2017 at 5:08pm

Thanks Kim,  good to be reminded on what occurs, that reminds again of a conversation with Richard (Chem geek) on the CYA/HOCL/OCL/pH and although I don't doubt it, how it actually applies has always given me cause for concern. Richard sent me his notes and mentioned HOCL will un-bond from CYA in under 8 seconds but if that is actually occurring at all levels why do some have green pools that need shocking if for example the CYA is 60ppm and the chlorine is 2.4ppm.  Is there a ratio at which HOCL won't un-bond?

Comment by Kim Skinner on January 2, 2017 at 7:22pm

As you know, when pool water does not contain any CYA, the sodium hypochlorite added will become HOCl and OCl.  At 7.5, both the HOCl and OCl will be at 50%.  At pH 8.0, the HOCl will be 23% and the OCL will be 77%.  That is known as the equilibrium percentage of HOCl/OCl based on pH.

As the more active HOCl is being used up or consumed, the OCl is immediately converting over to HOCl to re-establish the same equilibrium percentage based on the current pH.  I hesitate to bring this issue up, but that process may cause the pH to lower slightly.  But that can be ignored for simplicity sake.

When pool water contains CYA and sodium hypochlorite is added, the amount of HOCl and OCl in the water is much lower.  But the principle is the same.  As HOCl is used up, more HOCl is immediately produced by the OCl converting over to HOCl.  And of course, in this case with CYA, as HOCl is consumed, more chlorine is released from CYA molecules to again re-establish the HOCl/OCl content, and based on the CYA/HOCl/OCl/pH equilibrium.  And that continues to happen until all of the chlorine is used up.

No matter what the pH is, the process of converting OCl to HOCl will occur, and do so immediately.  A high pH will not stop or slow down that process. 

Comment by John on January 2, 2017 at 3:13pm

Many thanks Kim, another of your really interesting pool topics. On the previous alkalinity one, that tally's with my own work with vinyl liner pools that are rock solid pH wise at 7.4-7.6 using alkalinity of around 40ppm but not using pucks. One of my customers vinyl pool s has his water naturally around 25ppm alkalinity and also very little in the way of calcium hardness 40-60ppm and he uses pucks with no problem. The pH doesn't drop rapidly as one may expect from the old reports, neither does it yo-yo, the buffering effects of the CYA also help to stabilise the pH. Not forgetting borates to.

If you could take a bit of time to explain how at a higher pH of say 8.0-8.2 sodium hypochlorite will change from OCL to HOCL I am all ears as i struggle to understand sometimes.

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