AOP Systems - Spa Sanitization Systems

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Since 1990, HydroTher Hot Tubs have been the #1 choice of architects, consultants, designers and facility operators for commercial aquatic applications.

As spa disinfection technology evolves, and spa maintenance for complex systems becomes more and more challenging, spa owners and spa professionals continue to seek solutions for chlorine-resistant pathogens as well as the reduction of chemical use. This edition of Tech Notes examines Advanced Oxidation Process systems, also known as AOP systems, and their use in modern spa maintenance. 


AOP systems are classified as a disinfecting device or equipment that treats pool and spa water through generation of hydroxyl radicals3 . Disinfection efficacy (inactivation of pathogens) for AOP systems may be validated by the applicable disinfection testing requirements of NSF-504 inactivation of pathogens; however, a specific test for hydroxyl radical concentrations has yet to be developed for swimming pool/ spa applications. Federal law, AOP systems are required to be manufactured in an EPA-registered establishment; however, similar to electrolytic chlorinators, they are not EPA-registered products.5 AOP systems may be NSF-50 certified as combination systems, either as supplemental disinfection (validated for inactivation of Enterococcus faecium and Pseudomonas aeruginosa), secondary disinfection (additional validation for inactivation of Cryptosporidium parvum), and/ or reduction of chloramines and chlorine use per the requirements of NSF-50 for water conditioning devices.


By injecting Ozone (O3 ) into the water and exposing it to UV light, AOP systems create hydroxyl radicals in the pool water. Hydroxyl radicals are unstable hydrogen-oxygen molecules (OH-) that are highly reactive. These molecules combine with and break down organic materials in the pool water (bacteria, viruses, oils, sweat, etc.).6


AOP systems do not provide a sanitizer residual; therefore they must be used in conjunction with an EPA-registered sanitizer, or chlorine generating device. Minimum sanitizer residual levels should be maintained in accordance with EPA and local health department regulations, as well as EPA-registered sanitizer label directions.7,8,9 


AOP systems are typically plumbed in-line as the last piece of equipment on the main pool return. When an electrolytic chlorine generator is used, the chlorine generator should be plumbed after an AOP system to prevent the accumulation of trapped hydrogen gas. Electrical installation should be done in accordance with all national, state, and local regulations, and a GFCI must be used when required by local code.


Follow the manufacturer’s operating instructions for use. The operating instructions typically include:

1. Start-Up

2. Water Balancing

3. System Maintenance

4. Winterizing

5. Equipment Head Loss (pressure drop)


1. Cryptosporidium parvum inactivation: AOP systems that are NSF 50-certified as Secondary disinfectants have been tested to confirm a minimum 3-log inactivation of Cryptosporidium parvum.

2. Reduced chemical use: AOP systems provide additional disinfection and breakdown of chloramines, which should reduce the amount of chemicals needed to maintain proper water chemistry. Validation of reduced chlorine consumption may be confirmed by testing per the NSF-50 requirements for Water Conditioning Devices.


As with all other spa equipment, installation and operation should be done by qualified professionals. Always follow the manufacturer’s instructions, as well as federal, state and local regulations. UV light is harmful to eyes and exposed skin. When operating and maintaining UV light sources, care should be taken to prevent exposure to eyes and skin.


Always follow the manufacturer’s instructions for compatibility to other pool and spa treatment products and equipment. 

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