By Omar Legrini, Ph.D., and Gaspar Lesznik

Summary: Swimming pool waters are exposed to high organic contamination from bathers. While these waters may be properly filtered, gradual accumulation of pollutants occurs in filters leading to substantial bacterial growth requiring increased pool water backwashing and equipment servicing. Field studies and installations of advanced oxidation processes—combining ozone and UV in swimming pools—show this is highly efficient for oxidative degradation of chloramines and organic and chloro-organic materials without generation of hazardous by-products both in the water and atmosphere. This article discusses this sanitation method.

In swimming pool water treatment, widespread use of chlorine is predominantly due to two benefits—its effectiveness as protection against cross infection between bathers and its ease of application. However, it’s well known chlorine disinfection in water treatment leads to formation of a variety of chlorinated by-products, the most common of which are trihalomethantes (THMs) and chloramines. Strongly volatile, these disinfection by-products (DBPs) can escape to the pool’s immediate atmosphere, leading to increased risk of exposure of the pool occupants to potentially noxious substances by inhalation.

Established pool treatments
Due to increasing concern about the effectiveness and risk to human health of chlorination as a sole treatment procedure for swimming pools, alternative methods such as ozone and ultraviolet (UV) light have been utilized and become well established as effective water treatment in swimming pools in North America, Europe and Asia. Ozone, for instance, has been used for 30 years in German pools. These technologies—because they don’t maintain a disinfection residual—do not eliminate the need for chemical disinfection but do reduce the amount required for proper sanitation.

Ozone is known as the strongest available disinfectant as well as an effective oxidizing agent for removing many organic pollutants from water. A primary advantage of using ozone is the oxidant itself (O is converted to water (H2O) and oxygen (O2), leaving no undesirable taste or odor in the treated water.

Ozone CT values—CT = concentration x contact time—applied to swimming pools have evolved from potable water treatment practices and to inactivate more resistant organisms sometimes found in pools. Examples of these are: poliovirus, cocksackivirus, enteroviruses, Naegleria fowleri acanthamoeba, Giardia lamblia cysts and Cryptosporidium parvum oocysts. Ozone also gives additional security against bacteria such as streptococcus and pseudomonas and against skin complaints such as “swimming pool granuloma,” which are small bumps or lesions caused by Mycobacterium marinum.

Acting as an oxidizing agent, ozone reduces organic pollutants in pool water, hence limiting nutrients for growth and establishment of bacterial colonies and improving overall water quality. As such, ozonation also improves pool water clarity and thus UV transmission.

Although ozone and UV are used separately for pool water treatment applications with varying degrees of success, extensive field trials and installations show ozone and UV, in a sequential arrangement, are far more effective to control high rates of contamination from both bacteria and organic substances.

UV irradiation
The ability of UV light to kill microorganisms in water is well proven. However, UV energies required to destroy difficult-to-oxidize organic contaminants in water are much higher. In this context, given the UV intensities presently applied in swimming pool applications and very short irradiation time in the reactor, oxidation won’t take place or won’t be nearly as effective without ozone as a complement. The rate of chemical oxidation of the contaminant is limited by the rate of formation of hydroxyl radicals.

Hydroxyl radicals
Advanced oxidation processes involve the generation of hydroxyl radicals, the most powerful oxidizing species after fluorine (see Table 1), in relatively high, steady-state concentrations in order to affect dissolved and/or dispersed organic contaminants with better efficiency. This is because in the natural decay of dissolved ozone to oxygen, hydroxyl radicals (OH) are formed when one of the oxygen atoms is released and combines with a hydrogen ion. But UV’s effect on ozone is to speed up degradation creating a higher concentration of hydroxyl radicals that, while they last for only a fraction of a second, augment the disinfection/oxidation power of the ozone itself.

Ozone/UV process
With the higher oxidation potential of generated hydroxyl radicals with respect to ozone, the ozone/UV process provides a significant enhancement of oxidative degradation rate through which a wide range of organic and inorganic compounds are attacked. The ozone/UV process is one of the most advanced water treatment methods available for effective oxidation and destruction of toxic and refractory or “inert” organic matter.

Common with other hydroxyl radical generating degradation processes, ozone/UV oxidizes a wide range of organic compounds including partially halogenated (i.e., chlorinated) and unsaturated halogenated hydrocarbons and humic substances. The rate of hydroxyl radical reaction is typically 106-to-109 times faster than the corresponding attack with molecular ozone (see Table 2).

There have been extensive studies into treatment of water and wastewater containing low and high concentrations of organic pollutants by this process. The efficiency of the process has been proven on a pilot and technical scale with destruction of toxic or refractory organic pollutants from the parts per million (ppm) or parts per billion (ppb) range to acceptable or non-detectable limits without generation of hazardous by-products

Advanced oxidation process
The process of combining ozone with UV first involves installation of a bypass from the return line to the pool, downstream of the filters as shown in Figures 1 and 2. Chlorinated water is first pumped through a venturi injector, which draws the necessary vacuum through an air-fed corona discharge ozone generator and efficiently mixes the ozone created with the water flow. One or more contact vessels with sequential UV reactors allow oxidation and disinfection processses to be carried out. As a result, residual ozone is photochemically destroyed before the water leaves the contact vessel(s).

The process air, together with any residual ozone, is vented from the contact vessel(s) through activated carbon to destroy remaining ozone. A chemical disinfectant, most commonly chlorine, would still be required to prevent recontamination of the water—albeit with substantially lower dosage required.

Field testing
Experiments were carried out in a local swimming pool over a four-month period, investigating different pool water treatment methods including conventional chlorination, ozonation/chlorination and an advanced oxidation process in combination with low level chlorination. Water samples were collected at different points of the pool and, in the case of ozonation and advanced oxidation treatment methods, samples also were taken from before and after the purifying device.

For consistency within this article, these processes will be named as follows:

  • Chlorination (ozone off, UV off)—chlorination
  • Ozone/chlorine (UV off)—ozonation
  • Ozone/UV/chlorine—advanced oxidation

Bacteriological analysis
Samples were analyzed for coliforms, E. coli, heterotrophic bacteria capable of growth at 37°C and 25°C.

Samples were collected in sterile 300 milliliter (ml) bottled with the caps and necks covered with foil. Each sample bottled contained 0.3 ml of a 1.8 percent solution of sodium thiosulphate to neutralize chlorine present in the swimming pool water. The method used was to collect one sample from each of the four sides of the pool. The samples were analyzed immediately on return to the laboratory (within one hour after collection).

Bacteriological, turbidity results
An abbreviated set of results showing mean values and standard deviations is shown in Table 3.

All samples collected gave low turbidity readings. Values varied between 0.5 and 1.5 nephelometric turbidity units (NTUs). These readings were close to the sensitivity of the instrument used. No coliform organisms or E. coli were detected in any of the 100-ml sample volumes analyzed. Even at the presumptive stage of the test using Minerals Modified Glutamate Medium, all bottles were negative so that no confirmatory tests were required with any sample. Recovery of bacteria from some samples was obtained using the plate count method but, in general, the numbers recovered were low.

On the basis of the bacteriological results obtained, it can be concluded all swimming pool samples examined in this study were of satisfactory quality. No coliform bacteria including E. coli were detected in any of the 100-ml sub-samples tested. Colony counts recorded at the normal reading times of two days at 37°C and three days at 25°C were very low for all samples. Turbidity readings were also low, although precise accuracy of measurements was difficult to achieve over the low range of values found.

UV transmission results
UV transmission results at the 253.7 nanometer (nm) wavelength shown in Figure 3 indicate better optical water qualities can be achieved by both ozone and advanced oxidation treatment processes.

During chlorine treatment, the pool water optical quality in terms of UV transmission at 253.7 nm was relatively poor. Average UV transmission values at 90-to-93 percent maximum were measured. An average value of 97 percent was obtained by both ozonation and advanced oxidation. Results shown in Figure 4 clearly indicate higher UV transmission values were achieved by advanced oxidation, leading to better overall pool water clarity.

Combined chlorine results
Chlorine treated pool—Figure 5 shows monitoring of free and combined chlorine during the three-week trial period. Relatively high combined chlorine levels, as high as 0.6 ppm, were sometimes measured. The average combined chlorine measured over the whole test period was 0.48 ppm. This is still relatively high, as can be expected from a chlorinated pool. This problem should be of great concern to both swimmers and pool managers.

Ozone treated pool—Results of treatment of the pool water with ozone and chlorine are shown in Figure 6. Lower combined chlorine was achieved during the two-week trial period and an average value of 0.25 ppm was obtained. However, we sometimes measured higher levels of combined chlorine (up to 0.4 ppm). Ozone treatment reduced the combined chlorine with far higher efficiency than chlorine treatment. However, results indicate reducing and maintaining combined chlorine to a low and satisfactory level was not reliably achieved. Enhanced destruction and control of combined chlorine was therefore required.

Advanced oxidation treated pool—Typical results of the combination process using ozone and UV light in a sequential arrangement are presented in Figure 7. As shown, the process efficiently controlled the combined chlorine in the pool to below a concentration level of 0.2 ppm. It’s clear that, alone among the treatment methods investigated, the advanced oxidation process reliably kept organic contaminants at very low and safe levels—to the benefit of pool occupants and the environment.

Additionally, it’s worth noting excellent bacteriological results were consistently maintained while the free chlorine residual was as low as 0.6 ppm. Further comparative tests involving chlorination, ozonation and advanced oxidation were conducted in the same day in order to keep all pool parameters constant and also to eliminate any effect of free chlorine dosage and water chemical quality variations. For each process, combined chlorine at the treatment device exit was measured immediately after sampling. A typical plot for the oxidative degradation of combined chlorine is presented in Figure 8.

An obvious and significant improvement of combined chlorine reduction is observed with advanced oxidation, where the oxidation rate is greatly enhanced due to participation of radical species, especially hydroxyl radicals. This result is of particular importance because of the ability of the process to keep the combined chlorine concentration level below 0.2 ppm.

Air quality improvement
Pool management and staff were asked for their subjective impressions of any changes in air quality during the ozonation and advanced oxidation trial periods. There was general agreement amongst them that there was a marked reduction of “chlorine smell” and resulting discomfort. During both periods, pool supervisors talked of “less stinging to the eyes” when working around the pool. Management was enthusiastic about the obvious overall improvements to the pool hall air quality. All of these improvements can be attributed to reductions in dissolved and volatilized chloramines and chloro-organic compounds such as chloroform in the pool water and in the pool hall air.

Best choice for pools?
It’s fair to say no one process can treat all types of water. Each situation requires careful consideration to develop an effective treatment process. In the context of swimming pools, ozone has long been known to be the strongest disinfectant and chemical-oxidizing agent available for water treatment. However, as shown in Figure 1 and 2, hydroxyl radicals generated by the combination of ozone and UV light have higher oxidation potential and reaction rate constants than ozone leading to significantly faster oxidative degradation of almost all organic pollutants. A basic comparison of the relative effectiveness of different treatment methods for pool water purification is given in Table 4.

The results of experiments on the treatment of pool water by advanced oxidation clearly indicate a synergistic effect exists between UV and ozone in oxidizing and removing organic contaminants. This effect may be due to generation of highly reactive radicals during decomposition of dissolved ozone in pool water by UV light. The generated free hydroxyl radicals significantly enhance decomposition of organic pollutants, leading to a notable improvement of water clarity and UV transmission. This powerful reaction is short-lived and can only occur in a photoreactor so no harm is caused to pool occupants.

Furthermore, because water directly beneath the filter doesn’t contain a sufficiently high level of chlorine there’s a significant risk of bacterial growth in this area. With the advanced oxidation process, because ozone is photochemically destroyed, these disadvantages are virtually eliminated and no carbon filter is required. This has a major bearing on both capital costs and space requirements.

  1. The comparative experimental study demonstrates higher efficiency of a combined ozone and UV process over other processes. It provides the following beneficial effects in swimming pools: excellent bacteriological control, significant reduction of chloramines, excellent water clarity and quality, no risk of accumulation of DBPs, odor control, reduced skin and eye irritation, improvement in pool hall air quality, no adverse effects on swimmers and staff, and a proven ability to safely lower the free chlorine residual.


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About the authors
Dr. Omar Legrini received his graduate degree in chemical engineering in 1984 from the Algerian Petroleum Institute and his doctorate in 1988 from the École Nationale Supérieure de L’Industrie Chimique (ENSIC) in Nancy, France. Legrini is applications manager with Ozonia Triogen Ltd., Glasglow, Scotland, where he developed the UVAZONE advanced oxidation process discussed here. He can be reached at +44 141 810-4861 or +44 141 810-5561 (fax).

Gaspar Lesznik earned his college degree in civil engineering from the University of Dayton, Ohio, in 1975. He has over 22 years of experience in the application of ozone technology and is sales and marketing manager for Ozonia North America in Elmwood, Park, N.J. Lesznik can be reached at (201) 794-3100, (201) 794-3398 (fax) or email: [email protected]


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