UV technology and ozone

by Sally Bouorm | April 1, 2010 10:27 am

[1]

By Zach Hansen and Karen Rigsby

Supplemental systems can go a long way towards improving the water quality of a given pool or spa. Understanding how these systems work and how to use them effectively are vital to any diligent pool or spa operator. While these systems will not replace a registered sanitizer, they can be very effective tools, ones that should be used to their full advantage.

Ultraviolet (UV) technology

UV disinfection is used extensively and increasingly throughout the world in a vast array of water treatment applications. Most notable for its use in the drinking water industry, UV is a particularly effective supplemental sanitation system when used on pools and spas. The technology utilizes specifically targeted UV radiation to inactivate microbial contaminants and reduce certain disinfection byproducts such as chloramines, allowing for a safer, more comfortable swimming environment. Its effectiveness as a proven treatment against chlorine-resistant parasites, such as Cryptosporidium and Giardia, has received the attention of health departments across North America, where there is a growing debate as to whether such systems, or others with similar disinfection profiles, should be required at public facilities. As a result, UV technology will likely become more prevalent on commercial and residential pools for years to come.

How it works

A natural emission of the sun, UV light is further broken down into bands of decreasing wavelength. UVA and UVB garner the most notoriety, as these bands reach the Earth’s surface and are responsible for chronic skin ailments, such as accelerated aging and cancer. Other bands—namely, UVC and vacuum UV—also exist, but are filtered by the Earth’s upper atmosphere. UV disinfection systems use radiation within the UVC band that exhibits a wavelength range from 200 to 280 nanometres (nm), which is why this band is more commonly referred to as ‘germicidal UV.’ UV energy within this range inactivates micro-organisms by damaging their DNA/RNA, thereby rendering them unable to replicate. If a micro-organism cannot replicate, it cannot cause infection.

The effect of UVC radiation on chloramines is another positive benefit. Trichloramine is a particularly strong absorber of UV energy, whereas free chlorine tends to be a relatively poor absorber of UVC. Thus, free chlorine sanitizer shows minimal photodegradation when exposed to UVC. However, the ensuing effect on trichloramine is much greater. Data developed from a study published by Jing Li and Ernest R. Blatchley in Environmental Science & Technology (‘UV Photodegradation of Inorganic Chloramines’) show significant degradation of trichloramine when exposed to UVC. This is noteworthy, as trichloramine is the main chloramine component present in the air directly above the pool surface, and has been has been identified as an irritant of mucous membranes and the upper respiratory system.

Determining dosages

Achieving a sufficient dosage is critical when applying UV technology to recreational water systems. Dosage is essentially the amount of UV energy to which a contaminant is exposed; different micro-organisms require different dosage levels for neutralization. Dosage is a function of time and is measured in mJ (milijoules/cm²). Subsequently, the amount of contact time between light and water within a vessel determines the UV dose. Therefore, the rate at which water flows through the vessel (measured in gallons per minute [gpm]) affects the applied dosage to the micro-organism—the contaminant is exposed to more energy the longer it spends in the vessel. Alternatively, if the flow rate exceeds recommended levels, the applied dosage will decrease (UV devices are sized according to flow rate, not gallonage, as with other pool equipment).

[2]Recent notable Crytposporidium outbreaks throughout the U.S. underscore the importance of UV technology, given its effectiveness at inactivating chlorine-resistant parasites. Results from a study carried out under the NSF International-USEPA Environmental Technology Verification Program show a nearly four-log inactivation of Cryptosporidium oocysts at UV dosages as low as 19 mJ/cm². In addition to the aforementioned study, a second analysis was conducted to verify the degree of Cryptosporidium oocyst inactivation when exposed to low dosages of UV radiation. Data from Clancy et. al. show a five-log inactivation of the oocysts at 33 mJ/cm², which equate to a 99.999 per cent reduction of infection capacity.

UV is a one-pass treatment application, which means the system needs to be sized to achieve the target dosage for inactivation during a single pass through the unit. Dosage increases as flow rate is reduced, so system design plays a crucial role in unit selection. It is important to note that due to general recirculation patterns within a pool, it takes approximately four turnovers to expose the entire pool volume to the UV light within a 24-hour time period, based on a six-hour turnover rate.

UV technology has been proven to be an effective and systematic approach to recreational water treatment. The ability to inactivate microbiological contaminants, especially those resistant to primary sanitation programs, makes a UV system an attractive option when addressing concerns about overall public health. Additional efficacy against undesirable chlorinated disinfection byproducts only enhances its value.

Ozone

When used systematically, ozone has also proven effective in recreational water treatment. Its effectiveness against chlorine-resistant organisms and ability to remove chlorinated disinfection byproducts makes it an attractive choice for supplemental sanitation and oxidation. It is commonly used in wastewater and drinking water treatment applications, as well as disinfection in commercial aquariums. Much like UV, when used as a supplemental sanitizer for recreational water, it is effective against chlorine-resistant organisms such as Cryptosporidium and Giardia.

Ozone is comprised of three oxygen atoms (O₃) as illustrated in Figure 1. It is produced onsite through the use of UV energy or, more commonly, corona discharge (CD).   Corona discharge is a method whereby oxygen is exposed to electrical energy causing the oxygen molecules (O₂) to split. The single oxygen atoms then combine with other oxygen molecules as follows:

O₂ + energy → O + O

O   +   O₂   →   O₃

How it works

Ozone is a gas that is dissolved in water using a venturi injector. Once in the water, ozone decomposes to form free radicals (HO₂ and OH). These species are highly reactive and can oxidize impurities such as metal salts and organic matter. It is generally believed micro-organisms are destroyed through a process called cell lysis (oxidation of the cell protoplasm resulting in disintegration of the cell wall). In addition, ozone has been found to be beneficial for the reduction of chloramines, as it reacts directly with monochloramine.

It is important to note that only the water passing through the ozone contact tank is exposed to the disinfectant. Ozone does not hold a residual in a pool; therefore, the only contaminants destroyed are those passing through the tank. In reality, it will take days (depending on flow rate) for all the water to pass through the contact tank. Therefore, ozone must be used for supplemental sanitation only; an appropriate residual of a registered sanitizer must be maintained in the pool at all times.

Ozone can be used in conjunction with bromine systems, as it will regenerate hypobromous acid from the inactive bromide ion as follows:

Br – +   O₃ + H+   →   HOBr + O₂

There are a number of ozone manufacturers and installers. Basic installation is very similar—all systems require an ozone generator, injector and air management system. Operators should follow manufacturer’s guidelines for installation, operation and maintenance.

When ozone is used for indoor installations, air monitoring is required, as ozone is considered a ground level air pollutant. The U.S. Occupational Safety and Health Administration (OSHA) has set permissible exposure limits for air contaminants. One such measure of exposure is the time weighted average (TWA). OSHA has set ozone’s TWA at 0.1 parts per million (ppm) over a period of eight hours. Specifically, the ozone concentration in air shall not exceed the eight-hour TWA in any eight-hour work shift of a 40-hour workweek.

A final word

In conclusion, both UV and ozone can be effective at improving water quality through both the inactivation of microbes and the destruction of chloramines. However, they are not a replacement for use of a registered sanitizer. A sanitizer residual must be maintained at all times to help prevent transmission of disease among swimmers in the water. These supplemental systems, while helpful, are just that—supplemental.

References

Clancy, J.L., Z. Bukhari, T.M. Hargy, J.R. Bolton, B. Dussert, and M.M. Marshall. Comparison of medium- and low-pressure UV light for Cryptosporidium inactivation. JAWWA, 29:97-104, 2000.

Geo. Clifford White, Handbook of Chlorination and Alternative Disinfectants, 1999, Chapter 13.

 

Zach Hansen, who obtained his bachelor of science degree in chemical engineering from Auburn University, he three years of product-development experience in the pool and spa industry with BioLab Inc., A Chemtura Company.

 

 

 

Karen Rigsby works as a technical services senior specialist for BioLab Inc. She has been involved with the recreational side of water treatment since 2001, focusing on education, problem resolution and new product development. Rigsby is a member of the Association of Pool & Spa Professionals (APSP) Recreational Water Quality Committee and a National Swimming Pool Foundation (NSPF) certified instructor.

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