Alternative Sanitizers + Chlorine: The Quest For Synergy

Eric Herman Headshot
photo of a pool using an alternative sanitizer

The effort to avoid or reduce the use of chlorine to sanitize pools continues to make steady progress by combining water treatment technologies. To help make sense of a basic set of key issues, Senior Editor Eric Herman, with the help of a number of key resources, offers this brief survey of the science and sense driving today’s spectrum of treatment options.

Can the whole really be greater than the sum of the parts?

When it comes to the synergies made possible by combining pool water treatment technologies, the answer appears to be, in some cases, a resounding yes — but only if those doing the combining know what they’re doing.

Achieving those synergies is anything but simple. According to proponents of ozone and UV technologies, two of the most popular options in the so-called “alternative sanitizer” category, getting the most bang for the buck requires understanding how the two technologies work both individually and in tandem. Ultimately, that means tailoring the system selection based on a gamut of conditions, including water volume, turnover, flow rates, anticipated bather loads and various environmental factors.

There simply are no one-size-fits-all solutions.

To help sort out the key issues, here’s a primer on some of the assumptions and basic tenants of combined water treatment approaches.


If there’s one unifying goal across the spectrum of alternative treatment systems, it’s the ever-increasing desire to reduce chlorine or bromine levels.

Aside from the near universal distaste for chlorine in the general public, there is the more specific concern regarding the presence of chloramines. These disinfection byproducts (DBPs), including monochloramines, dichloramines and trichloramines, generate the familiar and dreaded chlorine smell and other problems such as eye irritation and respiratory problems in indoor aquatic facilities. Chloramines can damage construction materials, ruin bathing suits and cause asthma.

(Other byproducts targeted in drinking water standards include trihalomethanes, halocacetic acids and chloroform, further stoking the demand to reduce chlorine levels.)

By using a smaller amount of chlorine in a pool, along with a secondary sanitizer, fewer chloramines are produced and some of the ones that are produced can be destroyed through exposure to ozone or UV.

Otherwise, managing chloramine levels requires shocking the water, which in turn means adding more chemicals in the form of super-chlorination, potassium monopersulfate, sodium percarbonate, hydrogen peroxide, etc.

Also, super-chlorination is only effective at oxidizing inorganic chloramines. Organic chloramines are unaffected by chlorine shock treatments; they just sit there in the pool and continue building up over time. Thus, the common wisdom is that it’s better to prevent their formation in the first place.

That desire to limit DBP production via reducing chlorine levels must be balanced against the need to sanitize the water and kill organisms such as legionella, giardia, pseudomonas and the most menacing of all, cryptosporidium, a potentially deadly organism that’s proven highly resistant to chlorination.

Added to that, there is growing concern and evidence that many types of bacteria and viruses are becoming more chlorine-resistant as they evolve. This growing resistance to chlorination is one of the reasons, according to public health officials, that RWIs have been steadily on the rise in commercial aquatic facilities in recent years.

Yet another concern involves the increased likelihood of potentially harmful organic contaminants that can find their way into the water in the form of insecticides and fertilizer, pharmaceutical byproducts and increasingly polluted rainwater.

The good news is that research confirms the presence of ozone and/or UV systems offer germicidal benefits that enable the control of pathogens while also enabling the reduction of chlorine consumption.

For all of the drawbacks and desire to reduce chlorine, however, there remains the need to have some form of residual sanitizer to prevent bather-to-bather exposure. In other words, there is no completely stand-alone halogen replacement. Therefore, reducing halogen levels, at least for now, requires combining treatment technologies.


For decades now, ozone has remained one of, if not the, most popular of the alternative sanitizers. So much so that ozone manufacturers and proponents consider it a primary sanitizing technology for pool and spa applications.

In brief, ozone, also known as triatomic oxygen, is a molecular allotrope of oxygen with three oxygen atoms (O3, compared to the two atoms in atmospheric oxygen, O2). It exists naturally as a gas and is highly toxic.

When ozone comes in contact with an oxidizable substance, an oxidation reaction takes place. The stuff being oxidized can include a wide range of organic and inorganic substances commonly found in pools, including microorganisms (bacteria, viruses, spores, plankton, protozoa), disinfection byproducts like chloramines, chloramine precursors, as well as algae, oils, sweat and urine. When the unstable ozone molecule encounters one of these, the third oxygen atom detaches and breaks down the substance.

In effect, it destroys as it oxidizes. The only ozone byproduct is O2, meaning it has no ancillary impact on water balance.

There is no debate that ozone is an extremely powerful oxidizer and germicide. The only stronger oxidizers known to science are hydroxyl radicals (discussed below) or elemental fluorine, which has no application in pool and spa water treatment. As a germicide, ozone is equally impressive. The disinfecting capability of one ppm of ozone dissolved in water is equivalent to 10 to 15,000 times an equal concentration of free available chlorine, depending on variables including pH, temperature, and on the specific microorganisms it’s killing.

That remarkable power results in an extremely well documented list of ozone benefits, including elimination of organic and inorganic contaminants, killing harmful pathogens (including chlorine-resistant cryptosporidium), improved water clarity, destruction of biofilm a.k.a. surface slime, chloramine control, and reduction of chlorine by 50 to 75 percent, a factor that can vary widely depending on a number of key variables.

Ozone proponents further point to the fact that for all its oxidizing and sanitizing power, it reacts very slowly with free-available chlorine (FAC), meaning while it oxidizes chloramines, it has little effect on FAC levels.

Overall, that makes ozone a well-suited companion with chlorine treatments, which is great news for proponents of saltwater chlorination systems. Dissolved salt has no impact on ozone generation or its sanitizing and oxidizing action, and ozone in turn has no impact on the electrolytic process of chlorine generation.

In addition, ozone proponents note that when combined with saltwater chlorination, the result is a dual automatic system that reduces maintenance and the need to transport and store chlorine. Such a system also treats water evenly over time with reduced peaks and valleys in chemical levels, while at the same time being able to respond quickly to sudden increases in bather loads when used with ORP controllers.

And by reducing the required chlorine residual level, the presence of ozone also extends the service life of the electrolytic cells used in saltwater chlorination, which are expensive.

For all of those profound benefits, ozone does not, however, provide a lasting chemical residual in water, which is why a low-level halogen residual remains necessary to prevent bather-to-bather contamination. Critics of ozone point out that it is a highly toxic gas, and when sizing ozone units and configuring the circulation system, care must be taken to avoid ozone off-gassing.


Ozone isn’t the only alternative treatment technology that has gained widespread acceptance in both commercial and residential settings. Ultraviolet light also offers an impressive set of benefits that fit well with traditional chlorination.

Although relatively new to the pool and spa industry, UV systems have enjoyed decades of effective use across a range of applications including public water and wastewater treatment, biological systems such as ponds and streams containing fish and aquatic plants, industrial and manufacturing processes and even the treatment of bilge water on ocean-going ships.

UV adds no chemicals to the water, making it a wonderfully non-invasive adjunct to ozone or chlorine treatment, and therefore has zero impact on water balance, including pH. (UV is effective in whatever pH the water exhibits.) The system works by simply passing water at a prescribed flow rate past a UV lamp, which delivers light at specific intensities needed to stop the proliferation of pathogens and reduce the presence of chloramines.

Proponents are quick to point out that UV does not kill microorganisms per se, but essentially scrambles their genetic codes, rendering them unable to reproduce, including crypto. When pathogens cannot reproduce, the risk of infection is dramatically reduced. Its photolytic action also breaks up chloramines and retards their formation.

It’s important to know that not all UV light is created equal; different points on the light spectrum achieve different things. UV’s germicidal action takes place at 254 nanometers, depending on the UV dose. The destruction of chloramines takes place across different wavelengths.

In general there are two different types of UV lamps: a low-pressure, high-output lamp that only emits monochromatic UV rays at the all-important 254 nm wavelength; and a medium-pressure lamp that emits polychromatic UV rays between 200-600 nm. The broader frequency range is required to impact all three chloramines types at the same time, as well as pathogens.

UV manufacturers and proponents point to data indicating that a properly sized system, particularly in relation to flow rate — the most important parameter in system selection — will reduce chlorine consumption by at least 50 percent, and in many situations even more. Secondly, several studies indicate that air quality in indoor aquatic facilities dramatically improves when using a UV system, largely due to the reduction in chloramines.

However, flow rate is crucial because when water flows past the UV lamp too quickly, it does little to disrupt pathogens or chloramines. If, on the other hand, water flows too slowly, it can strip away a halogen residual. UV systems are designed differently to accommodate varying levels of turbidity. In other words, a unit designed for wastewater treatment or treating pond and stream water is very different from one designed for a commercial or residential pool.

UV systems are also well suited for use with saltwater chlorination systems for many of the same reasons. In particular, by reducing the demand on the chlorination system, the service life of the electrolytic cells are greatly extended.

Note: It’s important to remember that UV treatment systems differ completely from ozone systems that use a UV lamp to generate very low concentrations of ozone, a point that has led to some level of confusion.


In recent years, proponents of both UV and ozone systems, supported by volumes of research, point to a synergy that further enhances the effectiveness of both systems and further reduces the amount of halogen required.

That synergy comes in the form of what is known as the Advanced Oxidation Process (AOP). The science behind AOP is extraordinarily complex and, in fact, remains the subject of ongoing research.

The simple explanation is that when ozone is added upstream of the UV unit, small amounts of hydroxyl radical (OH) species are produced. Hydroxyl radicals are extremely short-lived, just a fraction of a second, but they are also extremely powerful in terms of oxidation. In fact, hydroxyl radicals are the only compounds more powerful than ozone used in water treatment.

AOP processes are so powerful they can fully oxidize all forms of organic contaminants, including microorganisms, human waste, dangerous chemicals like pharmaceutical waste and petrochemicals, fungus, algae, pesticides and other toxic elements. They also oxidize non-organic materials such as dissolved metals (iron, manganese, etc.) found in potable water, enabling their removal by filtration.

Although a relatively new concept to the pool and spa industry, AOP systems, such as those combining UV and hydrogen peroxide or ozone and hydrogen peroxide, have become mainstays in municipal and wastewater treatment along with a host of industrial and clinical applications.

Comments or thoughts on this article? Please e-mail [email protected].

Model Mandate

At this writing, the Centers for Disease Control is nearing completion of its much anticipated Model Aquatic Health Code, a massive document that addresses the “two pillar” approach to water treatment. With publication of the MAHC expected this summer, coupled with growing demand for treatment options that reduce reliance on halogen sanitizers, understanding system synergy will become increasingly important for those in the pool and spa industry.


The statements made in the accompanying text are broadly supported by a number of sources. For the purpose this discussion, author Eric Herman relied heavily on interviews with and reports written and presented by

  • Beth Hamil, vice president of corporate compliance with Del Ozone; and
  • Jeff Boynton, director for Delta UV.

Other resources include:

  • Buxton, G. V., Greenstock, C. L., Helman, W. P., and Ross, A. B. (1988). “Critical-Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen-Atoms and Hydroxyl Radicals (⋅OH/⋅O-) in Aqueous-Solution.” Journal of Physical and Chemical Reference Data”
  • Feng, Y., Smith, D. W., and Bolton, J. R. (2010). “A Potential New Method for Determination of the Fluence (UV Dose) Delivered in UV Reactors Involving the Photodegradation of Free Chlorine.” Water Environ.
  • Feng, Y., Smith, D. W., and Bolton, J. R. (2007). “Photolysis of aqueous free chlorine species (NOCI and OCI-) with 254 nm ultraviolet light. “Journal of Environmental Engineering and Science”
  • Watts, M. J., and Linden, K. G. (2007). “Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water.” Water Res.
  • Donofrio, R. (2013) Laboratory Validation of an Ozone Device for Recreational Water Treatment. Journal of Water and Health; Published by IWA Publishing in collaboration with the World Health Organization (WHO)
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