The Science Of Supplemental Disinfection Systems
As pool operators turn their attention to supplemental disinfection, there is a lesser understood, yet powerful alternative to UV that’s making a big splash--ozone sanitation. Why? And what should you know?
Chlorine has been the standard water-treatment method since 1961 when the United States Public Health Service published its model ordinance governing public pool-water sanitation. Today, as scientists learn more about the limitations and potential hazards of chlorine, and as the number of Recreational Water Illness (RWI) outbreaks increase, talk of supplemental disinfection is gaining more than momentum--it’s earning acceptance.
Supplemental disinfection is used in commercial pools to mediate the inherent weaknesses of chlorine, providing a safer water experience for swimmers and pool employees.
The two key technologies that form the mainstay of the discussion around supplemental disinfection are UV and ozone. UV has been well understood and recommended by pool designers, whereas ozone, a rapidly advancing technology, has been largely ignored, despite compelling reasons to choose ozone over UV.
Chloramines And RWIs
Chlorine is long known to react with organic material in pool water to create chloramines. Chloramines cause foul odors, irritation of the skin, eyes and ears, swimmers asthma, and can damage indoor-air handling systems (HVAC). Since the discovery in the 1930s, the unending challenge for pool operators has been to balance antimicrobial efficacy (keeping the water sanitized) and swimmer safety (eliminating chloramines).
Increased attention to the weaknesses of chlorine-only disinfection has resulted not only from these harmful chloramines but also from an increasing number of RWIs caused by chlorine-resistant microorganisms. RWIs have become more common over the past two decades by repeated outbreaks of illness caused by Pseudomonas aeruginosa, E. coli, Staphylococcus aureus, Giardia and Cryptosporidium parvum (Crypto), to name a few.
Technically speaking, chlorine should be able to handle common RWIs, with the exception of chlorine-resistant Crypto. However, as repeated RWI outbreaks have shown, conventional chlorine treatments alone will not do the job.
Chloramines and RWIs are the key reasons for supplemental disinfection coming to the forefront of aquatic water sanitation.
The Model Aquatic Health Code
The less-than-ideal efficacy of chlorine has inspired the Centers for Disease Control to form a Model Aquatic Health Code (MAHC). Soon to be published, it will highlight the importance of supplemental disinfection systems (SDS). When the present draft is adopted, the MAHC will list ozone and UV as accepted technologies for commercial pool SDS.
Several states have already mandated the use of SDSs for their high-risk swimming venues, including interactive spraygrounds. For example, Pennsylvania includes both UV and ozone in its 2008 Operational & Biological Contamination Protocol Recommendations for aquatic facilities. It is likely that additional states will soon adopt the MAHC for public aquatic facilities. With this in mind, operators and officials are wise to consider ozone sanitation.
UV Or Ozone
UV achieved earlier adoption among pool designers for its simplicity. Ozone, on the other hand, has been used to sanitize municipal water for the past 100 years, is widely used as life support for aquatic mammals and fish in commercial aquariums, and is the primary supplemental sanitation technology for the hot-tub industry. Despite all of this, ozone has yet to earn its place in supplemental disinfection for commercial pools.
A major reason for ozone’s lack of acceptance in the commercial pool world is its perceived complexity and the common misperceptions about what ozone is, how it works, and how it compares to UV.
Ozone is an efficacious antimicrobial oxidizer. When used in conjunction with low-dose chlorine, the pool water quality and clarity are improved. These water-quality improvements result from ozone’s potent and rapid oxidation of organic and inorganic compounds.
Ozone is cost-effective, uses little energy, is safe for pool patrons, and causes no structural degradation to the pool, the facility or HVAC system.
How An Ozone System Works
The expensive, cumbersome ozone systems of the past have given way to smaller, modern systems that provide public pools a viable water-quality solution at an affordable cost. Many are skid-mounted with flange-to-flange hook-ups for easy installation as a partial wall-mount or floor-mount.
An ozone disinfection system consists of two basic components:
• The ozone-generation system, which manufactures ozone gas
• The ozone-management system, which dissolves the gas into the water while preventing off-gas of undissolved ozone.
The ozone-management system maintains appropriate ozone levels in the water regulated with an oxidation reduction potential (ORP) controller/monitor. These components operate in unison, and are installed in an existing public pool as a side-stream to the pool’s main filtration system. Ozone is introduced to the water after filtration (and heating), and before the chlorine feeder. The side-stream flow is normally 15 to 25 percent of the main flow, depending on the size and type of pool. The system output is increased as the water quantity or organic loading increases.
When sized and applied in compliance with the MAHC, the broad spectrum oxidation of an ozone system offers significant benefits:
• It controls the chloramine levels in the water to 0.2 PPM or less.
• It potentially eliminates “shocking” for chloramine reduction.
• It offers significantly more oxidation than free available chlorine (FAC) alone.
• It produces a minimum 3 log (99.9 percent) kill of Cryptosporidium parvum and other RWI pathogens in a side-stream applied single-pass.
• It provides micro-flocculation to aid filtration and water clarity.
• It reduces the amount of chlorine usage to maintain an FAC residual.
• It is not intended to be used with bromine because ozone oxidizes “spent” bromine (bromide) back to useful bromine, which depletes the ozone before it can oxidize the organic contaminants in the water.
Comparing Ozone To UV
There remains some confusion about the difference between an ozone system and a UV system. Some of this confusion stems from UV-generated ozone that was commonly used in residential pools and spas before compact CD ozone generators took over in the 1990s. Still, UV-generated ozone has never been used in public pools because the systems cannot make enough ozone to benefit the pool water.
Ozone is a natural oxidizer. Ozone gas dissolves in water to kill microorganisms, destroy organics that create chloramines, and break down existing chloramines by oxidation. This oxidation happens immediately at the ozone gas-injection point, and continues as the side-stream remixes with the main return. A small residual (~0.1 PPM) of dissolved ozone then enters the pool, providing further oxidation of contaminants. There are no consumables in an ozone system.
UV light inactivates microorganisms and breaks down chloramines with light energy. This happens only while the water is in the UV chamber, and as long as the water has no turbidity. Once the flow leaves the chamber, no further process occurs. UV provides no oxidation except in trace amounts as a result of the formation of a limited number of hydroxyl free radicals.
Ozone and UV technologies are comparatively priced at approximately $30,000 for a 100,000-gallon pool. Ozone systems have a larger footprint than UV. Operational costs of each system vary with the local price of electricity. However, the minimal maintenance and reduced chemical requirements of an ozone system can create cost savings, resulting in a system payback of three to six years on a 100,000-gallon pool, varying by bather load. UV lamps must be replaced every three to 12 months, and must be figured into the maintenance costs for these systems.
The safety and health of our public recreational waters is of great concern to aquatic-facility operators and officials. When deciding the appropriate supplemental disinfection system for your commercial or public-pool operations, it will be worth the effort to compare ozone and UV.