Supplemental sanitizer (ozone, UV, AOP)
Also called: secondary sanitizer, ozone, UV, AOP, advanced oxidation.
A supplemental sanitizer is an ozone, UV or AOP unit on the equipment pad that treats water only as it passes through. It leaves no residual in the pool, so a chlorine or bromine residual is still required. It reduces chlorine demand; it does not replace chlorine.
Why it matters
A pool needs a residual: sanitizer that stays in the water, ready for whatever the next swimmer brings in. A device on the pad cannot do that. Ozone is a strong oxidizer that then off-gasses. UV inactivates what flows past the lamp, including chlorine-resistant Cryptosporidium, and destroys a little free chlorine on the way, so chlorine is dosed downstream of it. AOP combines UV and ozone and has the same limit. The CDC's Model Aquatic Health Code classifies UV and ozone as secondary or supplemental, never primary. What they buy you is lower chlorine demand and fewer chloramines.