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Chlorine’s True Role In Maintaining Water Quality

For more than 100 years, chlorine has been the primary chemical used to treat swimming pool water and public drinking water. It remains in widespread use in spite of the fact that it has stiff competition from so-called alternative sanitizers.

For all of its water-treatment supremacy, chlorine is poorly misunderstood. To a large extent, it has been maligned, even demonized, and that’s really too bad. The reason for the negativity surrounding chlorine is, I believe, due to the way it’s been misused and over used.

The Good and the Bad

The ills associated with chlorine are familiar and have almost become a mantra for those who seek to eliminate it altogether. It’s harsh on skin, eyes, hair, and bathing suits; it smells; it’s highly corrosive; it forms disinfection byproducts; and it can be rendered almost useless at high pH and in pools with elevated cyanuric acid levels.

Truth be told, most of those issues—particularly the smell as well as skin and eye irritation—are the result of disinfection byproducts such as chloramines, which form when free available chlorine oxidizes compounds containing ammonia and nitrogen. The fact that it’s pH sensitive is only a problem in pools with improper water balance. Plus, its complex relationship with cyanuric acid, a UV stabilizer, only becomes an issue if you let the level climb too high.

Nonetheless, the dark side of chlorine has given rise to an ethos that says less is more and none is even better.

Still, there are reasons that chlorine has stuck around in spite of the negative perception. It’s both an effective oxidizer and sanitizer, it stays in solution, and it can be stabilized in sunlight. No other chemical I know combines all those upsides.

A Layer of Protection

The problems associated with chlorine occur when it’s used as a solo act. When used by itself, the recommended 1-to-3 parts per million (ppm) in a residential pool and 3-to-5 ppm in a commercial pool are, indeed, problematic. First of all, even when you’re using a feeder and/or an oxidation reduction potential (ORP) controller, the level of free available chlorine, hypochlorous acid, will fluctuate due to chlorine demand courtesy of bathers and other environmental factors. At those residual levels, there’s a lot of chlorine in the water that will combine with organic compounds and form the unwanted by-products.

That all points to a simple remedy: keep the residuals down. That’s one of the many reasons our preferred treatment method involves ozone, UV, and chlorine. In our SRK HydroZone 3® system, UV and ozone handle the lion’s share of sanitizing and oxidizing, respectively. We maintain a small 0.6 ppm chlorine level to kill any bacteria that develops in the pool itself before the water has had a chance to circulate through the UV and ozone systems.

In other words, the chlorine plays a limited role yet offers a very necessary layer of protection. 

The Many Flavors of Chlorine

Prior to developing our system, I was like many, many other service technicians, driving around with a huge amount of liquid bleach in my truck, which is expensive and to an extent, somewhat hazardous. I was constantly testing and adding chlorine to keep up with the demand, which was almost impossible in high-use pools. When the combined chlorine, i.e., by-products, developed or the pool had an algae bloom, I’d turn to either chlorine or non-chlorine shock, which requires adding huge amounts of chemicals to reach what chemists call “breakpoint chlorination.”

Using chlorine as the one and only sanitizer was like living on the proverbial hamster wheel. It felt like a futile pain in the backside.

Chlorine is sold in many forms and over the years, I have tried them all. And all that experience taught me that each type of chlorine has different characteristics and using each correctly means selecting the right type for the application at hand. To sum up, you have to understand the specific characteristics of the chlorine you’re using. Some types dissolve very slowly and others more quickly. Some perform better in hot water than others.

These days, I really like using cal-hypo tablets because they’re very slow dissolving, which means it’s easy to control the residual level. It also adds calcium, which in many areas is a plus because of low levels of calcium in the tap water.

Overall, I’ve found that when you use chlorine in a sparing way, where it is a layer of protection, the problems associated with it essentially vanish. Using less also saves money, which contributes to the return on investment for the other systems we use.

That’s why I believe the objective shouldn’t be to eliminate chlorine, but instead to use it in the best possible way.

the Water Geek


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A blog about all things water, written by SRK's founder Steve Kenny.