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Advanced Oxidation Processes – What Are They And Why Do They Matter?

For the past few years, the aquatics industry has adopted a new concept known as advanced oxidation processes (AOPs). The term, which was first coined in 1987 by research scientist William Glaze, refers to a set of chemical reactions that produce a chemical species known as hydroxyl radicals (OH) or sometimes incorrectly referred to as nascent oxygen.

Hydroxyl radicals are one of the most powerful oxidizers known to science, capable of oxidizing almost any organic compound. The catch is that in water they exist in solution for only a tiny fraction of a second, so short-lived, in fact, there is no way to test for them. To a large extent, their very existence requires what amounts to a theoretical assumption.

Hydroxyl radicals occur naturally in the atmosphere and have long been considered “nature’s scrub brush” because of the way they clean the air we breathe. In water chemistry, AOPs are artificially achieved by providing a primary oxidative resource, either ozone, hydrogen peroxide or dissolved oxygen, combined with an energy resource, typically UV light or in some cases, a catalyst such as titanium dioxide. Once the hydroxyl radical is formed, it reacts almost instantaneously with any organic compound it comes in contact with, which are “mineralized” to non-organic compounds such as water, carbon dioxide and various salts.

Although scientists have been studying these complex chemical reactions for decades, and especially so since the term was coined in 1987, AOP technology was slowly adapted in wastewater treatment by only a handful of companies in the pool and spa industry. Today, it is recognized by most chemistry experts and is one of the main reasons that in the Model Aquatic Health Code (MAHC) the CDC recommends a “secondary” pillar of sanitization using ozone and UV in order to produce supplemental AOPs in commercial pools and spas.

While the hard science is established in terms of many of the chemical reactions of how hydroxyl radicals are formed and the way they mineralize organic compounds, there remains a number of aspects of AOPs that are still unknown, most of which are based on a purely subjective analysis of water quality.

Feeling is Believing

I’ve been working with AOPs for about a decade now. My approach is to ozonate water, and then treat it with UV light, which in turn transforms the ozone into hydroxyl radicals. This has a dual benefit. First, by treating the ozonated water with UV light, the ozone is destroyed and does not enter the pool, which prevents injury and damage that can result from high levels of dissolved ozone, and which can remain in solution for 18 minutes. Secondly, by forming hydroxyl radicals, the water is cleansed of nearly all the organic compounds present, which reduces demand on the low levels of chlorine we use. It also eliminates any disinfection by-products and obliterates any nutrient source for pathogens and algae.

As I’ve mentioned in other discussions, I’ve done a great deal of personal experimentation with these processes in my own spa, which I use for my daily soaking and hydrotherapy ritual. Through these ongoing observations, I’ve learned that this combination of ozone and UV and the resulting AOPs provide a level of water quality that is nothing short of phenomenal. It has a silky quality and level of impeccable cleanliness I’ve never experienced any other way.

In fact, I’ll go so far to say that the sensation associated with soaking in this water is the kind of experience that can only be described in highly subjective, almost spiritual terms. It is water that in my experience becomes remarkably rejuvenating and healing; one that reaches the highest possible level of hydrotherapy.

As I’ve played with the process by comparing the results with the UV system on and off, I’ve run into some surprisingly consistent results. When the UV is not running, the water quality changes. While it’s still clean and clear, it takes on an entirely different characteristic. It has a bite or sharpness to it, sort of like the crisp flavor of Coca Cola. You can feel it in your eyes and on your skin.

I assume this is due to the ozone that is not being turned into hydroxyl radicals. In practical applications, we can measure the oxidative reduction potential (ORP) generated when the UV is running or not. Because it changes the ozone into the incredibly short-lived hydroxyl radical, the difference is reflected in a drop in ORP. That’s where things get somewhat confusing for a lot of people, because it’s counterintuitive to think of a drop in ORP as a good thing, but in this case, it means that the ozone is being transformed into the highly reactive hydroxyl radicals that are oxidizing the organics in the water. Because hydroxyl radicals exist only for a few thousandths of a second, they are not measured as part of ORP.

A Noticeable Difference

When I’ve run my informal experiments with my kids and a handful of my clients, it’s amazing how attuned they become to the difference in water quality. When the UV system is off, I immediately get comments about how the water just doesn’t feel quite as nice. When it’s turned back on, those comments immediately change from complaints to nothing but high praise for the way the water makes them feel. The only difference is the presence or lack of the hydroxyl radicals. It’s come to the point that my kids can jump in and immediately tell whether the UV system is on or off. They’ll yell, “Dad, UV’s off!” And, they’ve been right every single time.

How exactly it is that hydroxyl radicals make such a huge difference in the feel of the water remains unknown. We do know quite a lot about the chemical reactions it drives when it’s formed and then subsequently makes such blindingly quick work of organic compounds, but why exactly it so dramatically impacts the aesthetic experience, we do not now, nor may ever fully understand.

To my way of thinking, we should be comfortable with these mysteries because there is no question about the water quality AOPs support. As the technology becomes more widespread in public water treatment, industrial and laboratory applications, as well as in pools and spas, it’s very likely that we’ll find out more and more about the amazing advantages of AOPs.

For now, I contend we really shouldn’t need any more proof to know that this dynamic set of chemical processes will be a part of artisan water treatment from this point forward. Those who adapt AOPs will be on the cutting edge of providing superior water experiences to their clients, and those who don’t will likely fall far behind in those efforts. When we consider the future of aquatics, it is obvious that the story will be written with the letters, A, O, and P.

the Water Geek


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