In an article written by Ray Denkewicz for Water Quality Magazine, Ray discusses germicidal UV, ozone disinfection, and a combination of the two. This article is reproduced with the author’s permission.
Combining UV & ozone for increased disinfection & oxidation efficacy
Ultraviolet (UV) and ozone are clearly established as viable secondary disinfection methods to chlorine for combating recreational waterborne illnesses (RWIs). As recommended by the Centers for Disease Control and Prevention (CDC) in its recently issued Model Aquatic Health Code, UV and ozone help inactivate Cryptosporidium, the primary causal agent of RWIs.
Chlorine, unfortunately, is not effective against Cryptosporidium—it is not the end-all, be-all sanitizer, but neither is UV or ozone, which is why, when they are used as a complement to chlorine, greater microbial efficacy is achieved.
It is logical, then, to ask, “If UV and ozone are potent in their own rights, then might the combination of the two be even better?”
Microbial inactivation studies employing a combination of UV and ozone have been reported for decades in water treatment. These studies have been made almost exclusively in non-swimming pool applications, so less notice may have been taken of them within the pool and spa industry. In these research investigations, which span multiple water treatment applications, researchers observed a synergistic effect when UV and ozone were used in combination for disinfection and oxidation purposes. That is to say, the observed results were greater than the expected contributions of their parts.
The synergistic action, as observed by many investigators, has been attributed to the formation of hydroxyl radicals when UV light interacts with ozone. The use of hydroxyl radicals in water treatment is commonly referred to in scientific literature as advanced oxidation.
The potent nature of hydroxyl radicals results in what is effectively a one-two-three punch when UV and ozone are used together.
Synergy of UV & Ozone
A landmark study published in 2006 by Magbanua, et al., clearly and systematically delineated the synergistic effect of a UV/ozone combination against E. coli.
At first, the researchers studied the efficacy of UV and ozone against E. coli in separate, independent tests. In doing so, the team was able to establish the requisite dosages of each disinfectant to affect a specific level of microbial reduction. In a second series of tests, the team utilized UV and ozone together to determine if the resulting microbial reduction was simply the sum of the contributions from each disinfectant.
The results were nothing short of astounding (see Figure 1). It was discovered that the individual UV and ozone doses required to destroy E. coli to the same extent (e.g., 99.9%) could be reduced by factors of 18 and four, respectively, when the two disinfectants were used together.
In essence, there was a synergistic effect on the microbial reduction of E. coli when UV and ozone were paired in a dual disinfection strategy. According to Magbanua, et al., the synergy associated with UV/ozone water treatment is attributed to the presence of supplementary hydroxyl radicals. In pure water, ozone reacts with hydroxide ions to form hydroxyl ions via a complicated pathway.
The combined use of UV and ozone promotes the formation of additional hydroxyl radicals by photolysis of ozone through a hydrogen peroxide pathway (see Figure 2).
Aqueous ozone absorbs UV radiation at wavelengths of 200 to 310 nm, and, in turn, decomposes to form hydrogen peroxide. Hydrogen peroxide then further reacts with UV to produce hydroxyl radicals. The reaction pathway in Figure 2 also suggests that hydrogen peroxide, when added to a UV system, would provide a similar benefit in the production of hydroxyl radicals. For this reason, hydrogen peroxide/UV systems and hydrogen peroxide/ozone/UV systems are in use for many water treatment applications where oxidation is a primary objective.
Hydroxyl radicals are extremely fast-reacting, potent, non-selective chemical species. In fact, their oxidation power is recognized as being far more potent than chlorine gas, hypochlorous acid or ozone (see Table 1).
Furthermore, the reactivity of hydroxyl radicals has long been recognized as extremely fast—in some instances as much as 1 million times faster-acting than ozone for bond breaking via chemical oxidation. For these reasons, the inactivation rate of waterborne pathogens is much greater due to the additional oxidizing power provided by the supplementary hydroxyl radicals.
The Trio of UV, Ozone & Hydroxyl Radicals
Scientific literature is replete with studies on the use of UV and ozone together for water treatment. This dual technology approach, commonly referred to as advanced oxidation, has the capability of achieving oxidation and disinfection. Table 2 provides a list of just some of the reported research on advanced oxidation in water treatment and the applications in which it has been employed. It is clear that the breadth of the applications for the trio of UV, ozone and hydroxyl radicals within water treatment is vast, owing to the potency of the combination for disinfection and oxidation.
The Trio as Disinfectants
In addition to the research by Magbanua, et al., the combined effects of UV, ozone and hydroxyl radicals as disinfectants were demonstrated in work performed at the University of Arizona’s Water Quality Center under a grant sponsored by the U.S. Department of Homeland Security.
In that research, the disinfection performance of UV and ozone against adenovirus and Naegleria fowleri was shown to be remarkably improved when paired together (see Figure 3).
Importantly, this study established the synergistic effect of UV and ozone against viruses and amoebas, augmenting prior results against bacteria and parasites. Taken together, the potency of a UV/ozone combination as a disinfection approach seems unparalleled.
The Trio as Oxidizers
The benefits of pairing UV with ozone do not stop with disinfection performance. While UV has virtually no oxidizing ability, the resulting hydroxyl radicals created from UV and ozone are tremendous oxidizers. As is the case with disinfection, the literature is replete with studies that reflect the superior oxidation performance of hydroxyl radicals formed from UV and ozone.
In Hayward Industries’ laboratories, it has been established that hydroxyl radicals are highly effective at reducing chlorine and combined chlorine (i.e., chloramines) by 50% or more in swimming pools, as well as oxidizing urea, a major component of sweat and urine (see Figure 4).
These findings were expected, owing to the potency of hydroxyl radicals. The implications of these findings are significant for swimming pool and spa applications, because urea and chloramines limit chlorine’s effectiveness and affect bather comfort.
Furthermore, chloramines can volatilize, creating an unpleasant “fishy” or “chlorine” odor, and wreak havoc on indoor materials due to their corrosive nature.
The Trio as a Water Treatment Strategy
It should come as no surprise that a pairing of UV and ozone is gaining attention as a strategy for a variety of water treatment applications, although as recently as 1999, the U.S. Environmental Protection Agency had not discussed the UV/ozone combination in its comprehensive Alternative Disinfectants and Oxidants Guidance Manual.
The synergy that is provided with respect to both oxidation and disinfection is invaluable in most instances. Unfortunately, neither UV nor ozone, nor the resulting hydroxyl radicals, provide a lasting disinfection residual. Therefore, in some applications, such as pools and spas, chlorine is still a necessary part of the overall treatment strategy.
As discussed in my article, “UV & Ozone for Secondary Disinfection” in the May 2015 issue of WQP, the manner in which chlorine is complemented by UV and ozone makes their use together a logical and prudent water treatment approach for the highest level of water sanitization.
To this end, CDC’s recommendation to use either UV or ozone as a secondary disinfectant to chlorine for swimming pools and spas is a wise approach; however, utilizing both UV and ozone provides an even more robust treatment regimen, not only for pools and spas but for many water treatment applications.
Ray Denkewicz is the global manager for sanitation and chemical automation for Hayward Pool Products in Elizabeth, New Jersey.