Maintaining clear, clean, and safe water is a top priority for aquatic facilities, but it often requires more than just chlorine. While chlorine remains the primary disinfectant for most pools and spas, it has limitations — especially when it comes to removing chloramines, controlling organic contaminants, and managing high bather loads. That’s where secondary disinfection systems like UV (ultraviolet), ozone, and advanced oxidation processes (AOPs) come into play.
These technologies are not replacements for chlorine or bromine but are supplemental systems designed to improve water quality, reduce chloramine formation, and enhance overall sanitation. In fact, for certain high-risk venues — such as spray pads and indoor public pools — the CDC’s Model Aquatic Health Code (MAHC) now mandates secondary disinfection.
Understanding the Limitations of Chlorine Alone
Chlorine is effective at killing most pathogens and oxidizing organic matter, but it doesn’t do it instantly — and it’s not equally effective against all contaminants. One of the biggest issues in heavily used pools is the buildup of chloramines, the combined chlorine compounds that form when chlorine reacts with nitrogen-based waste (sweat, urine, etc.). Chloramines:
• Cause eye and skin irritation.
• Contribute to the strong “chlorine smell.”
• Are far less effective as sanitizers.
• Are difficult to remove without high-dose (breakpoint) chlorination or supplemental oxidation.
Secondary disinfection systems target and destroy these chloramines and other difficult-to-remove contaminants, helping to keep water clear, healthy, and inviting.
UV Systems (Ultraviolet Light)
UV disinfection systems expose water to high-intensity ultraviolet light, typically at a wavelength of 254 nanometers (UV-C). As water passes through a UV chamber, the light damages the DNA or RNA of pathogens, rendering them incapable of reproduction.
Benefits:
• Destroys chloramines, particularly the volatile trichloramine responsible for odor and irritation.
• Inactivates chlorine-resistant pathogens such as Cryptosporidium and Giardia.
• Reduces combined chlorine levels and improves air quality, especially in indoor pools.
• Does not alter pool chemistry. Limitations:
• UV does not provide residual disinfection — it only treats water passing through the chamber.
• Requires regular cleaning and lamp replacement to maintain effectiveness.
• Does not oxidize organic contaminants on its own.
Best Uses:
• Indoor pools, where air quality issues from chloramines are common.
• Splash pads and interactive fountains, where Cryptosporidium risk is elevated.
• Competitive swimming facilities, therapy pools, or others with high bather loads.
The MAHC requires UV or ozone in systems where chlorine-resistant pathogens are a high concern — especially spray grounds with recirculated water.
Ozone Systems
Ozone (O3) is a powerful oxidizer generated by passing oxygen through a corona discharge unit or UV light. The resulting ozone gas is injected into the water, where it reacts with organic contaminants and microorganisms.
Benefits:
• Stronger and faster oxidizer than chlorine.
• Breaks down chloramines, body oils, and other organics.
• Inactivates protozoa like Cryptosporidium.
• Improves water clarity and reduces chemical demand.
Limitations:
• Like UV, ozone does not provide a disinfectant residual.
• Ozone is unstable and must be generated on-site.
• Ozone off-gassing must be controlled — systems must include degassing and destruct units.
• More complex and costly to install and maintain than UV. Disinfect
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Best Uses:
• High-end spas, hotel pools, and health clubs where water clarity is paramount.
• Pools with low turnover rates, where oxidizer buildup is a concern.
• Any application where organic load is high (e.g., therapy or batherheavy pools).
Ozone is excellent at oxidizing organic material before it reacts with chlorine, which helps reduce the formation of combined chlorine and disinfection byproducts.
Advanced Oxidation Processes (AOPs)
AOPs combine ozone, UV, and/ or hydrogen peroxide to generate hydroxyl radicals (·OH) — the most reactive oxidizing agents used in water treatment. These radicals rapidly destroy a wide variety of contaminants, including bacteria, viruses, chloramines, and trace organics.
Benefits:
• Extremely fast and complete oxidation.
• Eliminates chloramines and other disinfection byproducts (DBPs).
• Improves water clarity and reduces odor.
• Offers broad-spectrum inactivation of pathogens, including Crypto.
Limitations:
• Higher cost and complexity than UV or ozone alone.
• Requires tight control of chemical dosing and monitoring.
• Energy consumption can be significant depending on the setup.
Best Uses:
• High-use aquatic facilities seeking the gold standard in water quality.
• Luxury resorts, water parks, and therapy centers prioritizing clarity and comfort.
• Facilities with challenging source water or high levels of contaminants.
AOPs are emerging as the most comprehensive secondary disinfection option, especially in advanced pool systems that demand superior water quality.
Choosing A Secondary Disinfection System
When deciding which system is right for your pools and spas, consider the following: For most public aquatic facilities, UV is a solid first step — particularly for indoor air quality and MAHC compliance.
Ozone is better suited for water clarity and oxidation where space and budget allow. AOPs are ideal where maximum disinfection and oxidation are needed, and cost is less of a constraint.
Secondary disinfection systems are not luxury add-ons — they are increasingly considered best practice, especially for high-use, indoor, or public venues.
By addressing the weaknesses of chlorine alone — particularly its inability to rapidly destroy chloramines and certain pathogens — technologies like UV, ozone, and AOP can significantly enhance water quality and bather safety.
For service pros and facility managers, understanding the strengths and limitations of each technology helps you recommend and maintain the right system.
Whether you're targeting clearer water, healthier air, or simply fewer complaints, secondary disinfection is a modern solution to a longstanding challenge in pool and spa care.
