4.3 Stabilizer, UV, Ozone, and Other Oxidizers

Once you can name HOCl and dose breakpoint chlorine, two more oxidation questions show up on every commercial deck: Should this water have stabilizer? and What do UV, ozone, salt cells, and non-chlorine shock actually replace? Domain 1E (sanitation and oxidation) on the 2025 AFO candidate handbook is bigger than the chlorine-product table. Independent teaching uses CDC/MAHC public-health language as a voluntary model, plus ordinary operator practice. Follow the AHJ. The public 2025 handbook PDF does not print a national AFO cyanuric-acid operating table you can memorize as "the NRPA number." Do not invent one.

Cyanuric acid — outdoor sunscreen for chlorine

Cyanuric acid (CYA), often sold as conditioner or stabilizer, is a chlorine stabilizer. In sunlight, ultraviolet light photolyzes (breaks down) free chlorine. Unstabilized outdoor water can lose a large share of its residual on a bright afternoon even if the feeder is honest. CYA forms a reversible complex with chlorine. That reservoir reduces how fast sunlight destroys the residual. That is the only operational reason to want CYA in a swimming pool.

The cost of that sunscreen is slower disinfection. Bound chlorine is not as available as HOCl in unstabilized water. MAHC glossary language even reminds you that a DPD free-chlorine reading in the presence of CYA includes cyanurate-bound available chlorine, not a pure HOCl assay. Operators still use DPD — it is the commercial method — but they should not pretend 2.0 ppm DPD-FC at 80 ppm CYA kills like 2.0 ppm DPD-FC at 0 ppm CYA.

Typical outdoor working talk versus model-code ceilings

Commercial outdoor conversation often sits near 30–50 ppm CYA: enough to matter against sun, not so much that kill collapses. That band is operator discussion, not a number to fake as an unpublished AFO handbook table.

Public-health model language you may meet at inspection (2024 MAHC, 5th ed.):

If your state prints 40 ppm, 80 ppm, or "no stabilizer indoors," that print wins. High CYA slows kill — that sentence is the chemistry, whether or not the inspector uses MAHC numbers.

Over-stabilization and "chlorine lock"

Operators call extreme CYA chlorine lock: the kit still shows free chlorine, algae and cloud persist, and dumping more trichlor makes it worse. Chemists dislike "lock" as a metaphor because the chlorine is not in a sealed box; HOCl is buffered down by the cyanurate system. Either vocabulary points to the same fix: stop adding CYA, raise the unstabilized residual only as the label/AHJ allow, and dilute.

CYA is not removed by breakpoint chlorination, by MPS, by ozone, or by hoping. The cyanurate ring stays until you replace water (drain-and-fill, splash-out plus makeup, or a dilution plan). Some facilities use reverse osmosis; that is a specialty process, not the exam's default button. If trichlor has been the only sanitizer all summer, CYA walked up with every tablet. Switching to cal-hypo or liquid stops the climb; it does not erase the 80 ppm already dissolved.

Indoor water: generally avoid CYA

Indoor natatoriums are not under overhead UV the way an uncovered outdoor basin is. Stabilizer's benefit is near zero. Its harm — slower kill, confusing DPD-FC, CYA that never leaves except by dilution — remains. Indoor programs generally avoid CYA and therefore avoid dichlor and trichlor as daily sanitizer (section 4.1). 2024 MAHC §5.7.3.1.3.1 goes further for spas and therapy pools on new construction, substantial alteration, or disinfection-equipment replacement: do not use CYA or stabilized chlorine products there. Even where an older indoor pool still "has always used tabs," professional practice is to treat that as a problem to retire, not a model.

Test CYA on the schedule the AHJ wants (MAHC-style language includes monthly at venues that use it, more often when you feed stabilized chlorine, and a check about 24 hours after you add conditioner). Log it. Indoor logs that never mention CYA should match indoor product choice: unstabilized.

Trap: "we'll just add dichlor until the chlorine holds overnight"

Overnight hold indoors is usually a feeder, demand, and combined-chlorine story, not a missing sunscreen. Outdoor, if chlorine vanishes by 2 p.m. in July with CYA at 0–10 ppm, stabilizer is in the conversation. Outdoor, if chlorine vanishes with CYA already 90 ppm, you have over-stabilized plus maybe a dirty filter, not a need for more dichlor.

Secondary and supplemental systems — what they do and do not replace

A residual sanitizer in the pool water is still required by most public codes even when the pump room looks like a science fair. Secondary systems can reduce chloramines, improve crypto control in a chamber, or generate the chlorine you would otherwise buy. They are not a permission slip to run zero free chlorine in the basin.

UV

Medium-pressure UV is common indoors specifically to break chloramines in the recirculation stream and to add a non-chemical barrier against chlorine-tolerant protozoa in that stream. Water has to reach the lamp. A toddler-filled zero-depth entry does not get UV until that water is drawn through the system. If the UV is off for a new lamp, the only sanitizer in the water is whatever residual you still maintain. Size, NSF/ANSI 50 listing, and validation live with the engineer; the operator's exam point is supplemental, not substitute.

Ozone

Ozone oxidizes organics and some combined chlorine in contact. It is generated on site (corona discharge or UV-ozone) and must be contained. A pool that "has ozone" and a DPD-FC of 0.0 ppm is not a code-compliant public pool in ordinary jurisdictions. Ozone also does not add CYA. It does not replace breakpoint math when CC is already 1.1 ppm in the basin — it may help the next week's average if the system is running.

Ionization

Copper/silver ionizers are marketed hard to facility boards. Ask the AHJ before anyone skips chlorine. Staining, ineffective residual in feces incidents, and "the ions will handle it" are classic failure modes. If the code requires chlorine or bromine, ionization is at most additional.

Salt-water / on-site hypochlorite generation

A salt system is a chlorine factory. Dissolved sodium chloride is split at the cell; hypochlorite enters the water. Guests say "salt pool" and mean "no chlorine." Teach the opposite: it is a chlorine pool with a brine residual, typically rising pH, hydrogen at the cell that must vent, and scaling on plates. Outdoors you may still use CYA. Indoors you still generally should not. Combined chlorine still forms from bather nitrogen. The cell does not replace DPD tests, does not replace breakpoint when CC is high, and does not make trichlor-in-the-feeder a good indoor idea.

Potassium monopersulfate — non-chlorine oxidizer

Potassium monopersulfate (MPS, potassium peroxymonosulfate), often sold as "non-chlorine shock," is an oxidizer that does not add free chlorine. Facilities use it to oxidize some organics and to reduce combined-chlorine load without the huge temporary FC spike of a chlorine breakpoint. That can look attractive indoors when you cannot wait for 33 gallons of hypo to come back down.

Do not overclaim:

• MPS is not the AFO handbook CC × 10 chlorine calculation. If the exam gives 1.1 ppm CC and asks for gallons of liquid chlorine, the answer is still the chlorine-chart path (33 gallons in the 360,000-gal example), not a mystery bag count from a consumer label.
• MPS does not replace a residual sanitizer. You still need FC (or bromine) in the water.
• MPS can interfere with DPD, often reading as extra chlorine (a false high) for a period after addition. Some kits include an MPS-neutralizing reagent. Follow that product's literature for wait times and for whether your DPD number is trustworthy. Do not invent a universal "wait 24 hours" rule as if NRPA published it.
• MPS will not strip CYA.

If the label's use directions and your AHJ allow MPS as a supplemental oxidizer, it is a tool. If someone treats it as a complete disinfection program, it is a violation waiting for an inspector.

ORP versus ppm

ppm (mg/L) is concentration: how much free chlorine the DPD method reports in the sample. It is an inventory number. It is what most codes write in the rulebook.

ORP (oxidation-reduction potential) is an electrical reading in millivolts (mV). A probe in the recirculation line estimates how oxidizing the water is. Automatic controllers commonly feed chlorine (and sometimes acid/CO2) to hold an ORP set point. ORP is not a second name for ppm. The same 2.0 ppm FC can print different millivolts when pH, CYA, combined chlorine, temperature, probe coating, or organic load change. High CYA in particular can suppress ORP even when DPD-FC looks comfortable — another reason indoor CYA is unwelcome.

Operator rules that keep controllers honest:

• Calibrate and clean probes on a maintenance schedule (chemical-system PM is a later chapter). A coated ORP probe that reads "fine" while DPD says 0.3 ppm is how venues fail inspections with the automation lights still green.
• Verify with DPD (and phenol red for pH) on the code's wet-test cadence. Log both. When they disagree, believe a valid wet test until the probe is proven.
• Do not convert 750 mV into "750 ppm." Do not skip combined-chlorine testing because ORP is high; chloramines and HOCl do not look the same to a kit, and odor is still a water-and-air problem.

Controllers are excellent at feeding every few minutes. They are not a legal substitute for the DPD residual the AHJ asked to see on paper.

Putting the sanitation chapter together

• Choose an unstabilized product indoors; use CYA on purpose outdoors and test it.
• Keep free chlorine as HOCl by holding pH (previous chapter) and by destroying combined chlorine with breakpoint math (CC × 10, PSF, handbook bases).
• Add UV, ozone, salt generation, or MPS only as helpers that still leave a residual sanitizer in the water.
• Let the controller ride ORP, and let DPD ride along as the truth serum.

Sunlight, nitrogen, and automation all try to talk you out of that residual. The operator's job is to keep it anyway, with chemistry you can explain to an inspector without inventing unpublished AFO tables.