Want water that doesn’t reek of chlorine or sting your eyes and skin? Supplementary disinfection with a UV-C lamp or an ozone generator can cut chlorine consumption by roughly a third to a half and significantly reduce the formation of unpleasant chloramines. But here’s the crucial point: neither technology fully replaces residual disinfection — you still need a small residual level of chlorine or bromine in the water. In this guide we explain why, and how to combine both methods.
Why bother with supplementary disinfection at all?
Conventional chlorine disinfection works reliably, but it has one side effect. When free chlorine reacts with the organic matter we bring into the water — sweat, urine, cosmetics, dead skin cells — it forms chloramines (combined chlorine). It’s the chloramines, not the chlorine itself, that cause the typical “pool” smell, stinging eyes, and irritated airways.
Paradoxically, then, a strong chlorine smell doesn’t mean there’s too much chlorine in the water — it means chloramines have built up and the water needs treatment. Supplementary disinfection targets exactly this: it breaks down organic matter and chloramines before they can accumulate, so you need less chlorine and the water feels noticeably nicer.
The benefit isn’t just comfort — it’s practical too. Less chlorine means less chemistry to keep buying, more stable readings, and less strain on the skin of sensitive swimmers, children, and allergy sufferers. In hot tubs, where the ratio of bathers to water volume is extremely high, this effect is even more pronounced than in a large pool.
How does a UV-C lamp work?
UV-C is short-wave ultraviolet radiation with a wavelength of roughly 254 nm. Water flows through a sealed stainless-steel or plastic chamber housing a germicidal lamp. Microorganisms — bacteria, viruses, algae, and protozoa — pass by the lamp, and the radiation damages their DNA and RNA enough that they can no longer reproduce. Water leaves the chamber essentially free of live pathogens.
The big advantage of UV-C is that it handles organisms that are resistant to chlorine. A textbook example is the protozoa Cryptosporidium and Giardia: according to the US Centers for Disease Control and Prevention (CDC), their cysts are exceptionally resistant to normal chlorine concentrations and chlorination alone won’t reliably kill them, whereas UV-C inactivates them. That’s exactly why hygiene standards for public pools recommend UV systems as a second disinfection stage.
UV-C, however, only acts the instant water passes through the chamber. It has no residual effect at all — once water leaves the lamp, it’s completely unprotected against any new contamination introduced by bathers. This is the key reason UV can never be the sole disinfection method (more on that below).
How does an ozone generator work?
Ozone (O₃) is a very powerful oxidizing agent — more reactive than chlorine. An ozone generator produces it on site from ambient oxygen and injects it into the water, where it rapidly oxidizes bacteria, viruses, organic contaminants, and chloramines. It then breaks back down into oxygen, leaving no lasting residue or by-products like chlorine does.
Ozone is produced in two ways. UV ozone generators create ozone using a special UV lamp — they’re cheaper and simpler, but have lower output. CD generators (corona discharge) create ozone via an electrical discharge, produce significantly more ozone, and are used where higher output is needed. In hot tubs, an ozone generator is essentially standard today — a small CD or UV module is often built directly into the tub’s equipment pack.
Like the UV lamp, ozone’s drawback is that it acts mainly within the equipment loop, and its residual effect in the tub itself is very short-lived. Ozone must also be fully broken down before the water returns to bathers — breathing ozone gas is harmful. Quality systems therefore include a degassing and mixing chamber that ensures the ozone is consumed in the water and never reaches the air.
UV vs. ozone vs. combination: comparison
Both technologies aim at the same goal — less chlorine and cleaner water — but they differ in principle, cost, and where they’re best suited. UV is purely physical (radiation); ozone is chemical (oxidation). The table below gives a simplified comparison for a home pool and hot tub. Prices are indicative and vary by output and brand.
| Parameter | UV-C lamp | Ozone generator (CD/UV) | UV + ozone combination |
|---|---|---|---|
| Principle | Physical — germicidal UV radiation | Chemical — oxidation by ozone | Ozone oxidizes, UV breaks down residual ozone |
| Effectiveness against chlorine-resistant pathogens | High (incl. Cryptosporidium, Giardia) | High (strong oxidation) | Highest |
| Breakdown of chloramines and organics | Moderate | High | Highest |
| Residual effect in the water | None | Very short | Very short |
| Purchase price (home segment) | approx. €160–600 | approx. €200–800 | approx. €600–1,400 |
| Replacement / servicing | Lamp after approx. 8,000–13,000 hours (once a year) | UV cell / CD module after approx. 2–5 years | Both, depending on type |
| Running costs | Low (lamp power draw + replacement) | Low to moderate | Moderate |
| Typical use | Pool, public wellness facilities | Hot tub, smaller pool | Heavy-use settings, sensitive users |
In practice, hot tubs most often come with ozone (usually built into the tub), while for family pools with filtration a UV-C lamp fitted after the filter is the popular choice. Combining both makes the most sense where usage is heavy or water-quality requirements are unusually strict.
Why do you still need residual chlorine?
This is the most important part of the whole article. Both the UV lamp and the ozone generator work only within the equipment loop — inside the chamber the water flows through. They kill whatever passes through them, but they have no ability whatsoever to protect the water in the tub or pool itself. The moment a new bather, fresh sweat, or bacteria enters the water, there’s nothing there to neutralize it instantly until the water cycles through the filtration again.
Residual chlorine (or bromine), by contrast, stays dissolved in the water and works continuously, in every litre, at every moment. That’s exactly why it’s called “residual” disinfection — it’s the safety cushion between filtration passes. Without it, the water between filtration cycles would have no protection at all.
The practical upshot: UV and ozone let you reduce your chlorine level, typically from the usual 1–3 mg/l down to the lower end of the recommended range, but not switch it off entirely. The goal of supplementary disinfection isn’t a chlorine-free pool, but a pool with less chlorine and no chloramines. Anyone promising completely chlorine-free operation is either being misleading or offering a system that doesn’t meet the minimum hygiene requirements. We cover exactly how to maintain hot tub chemistry in a separate guide, Hot Tub Chemistry Step by Step.
Installation and sizing
Both the UV lamp and the ozone generator are always fitted after the filter, on the return side, just before the water goes back into the pool. The logic is simple: the filter first removes coarse debris and cloudiness so the water entering the UV chamber is clear. Cloudy water absorbs UV radiation, and the lamp’s effectiveness drops dramatically — UV needs clear water to work.
Sizing is based on flow rate (m³/h) and water volume. The UV lamp must be powerful enough for your flow rate, otherwise water passes through the chamber too quickly and doesn’t receive an adequate dose of radiation. The ozone generator is chosen based on the tub’s volume and bather load. In both cases, an undersized unit is wasted money — it’s worth having the output confirmed against your equipment’s actual parameters.
Installation requires an electrical connection and work on the plumbing, so if you’re not confident, leave it to a professional. In hot tubs, ozone is usually already built in and only the module needs replacing. Don’t skip checking the actual chlorine level after installation — it’s exactly how you’ll know how far you can safely reduce dosing.
Who benefits most from supplementary disinfection?
The biggest benefit goes to households with sensitive skin and young children, where every bit of irritation and chlorine smell matters. Fewer chloramines mean more pleasant bathing for allergy and asthma sufferers, and for anyone whose skin dries out from conventionally chlorinated water.
The second major group is heavy use — hot tubs in frequent operation, pools with many bathers, rentals, and apartment buildings. The more organic matter that gets into the water, the more work UV and ozone do, and the greater the chlorine savings. If you’re also weighing your overall water treatment approach, you’ll find a comparison with another popular system in Salt Water vs. Chlorine.
Frequently asked questions
Can a UV lamp or ozone let me stop chlorinating entirely?
No. Both technologies act only at the moment water passes through the equipment and have no residual effect in the tub. You must still maintain a small amount of chlorine or bromine in the water as ongoing protection between filtration cycles. UV and ozone do let you reduce chlorine significantly, typically down to the lower end of the recommended range.
How much does chlorine consumption actually drop?
Depending on bather load and settings, roughly a third to a half. The biggest savings tend to occur in hot tubs and pools with high bather numbers, where the most chloramines form. You’ll only know the exact figure by measuring chlorine levels after installation and gradually fine-tuning your dosing.
How often does the UV lamp need replacing?
A germicidal lamp typically lasts 8,000 to 13,000 operating hours, which for seasonal use works out to roughly one replacement a year. The lamp may keep glowing beyond that, but its germicidal output declines, so don’t delay replacement past the manufacturer’s recommendation. The lamp’s quartz sleeve also needs occasional cleaning to remove limescale.
Is ozone in the water dangerous?
Dissolved ozone breaks back down into oxygen quickly and, at the concentrations typical of a home system, isn’t a problem while bathing. What’s dangerous is breathing gaseous ozone, which is why quality systems include a degassing chamber that ensures it breaks down before the water returns to the tub. With a certified unit installed according to the instructions, this is a safe technology.
Do UV and ozone also work with salt water?
Yes. Both the UV lamp and the ozone generator work regardless of whether you dose chlorine manually or generate it via salt electrolysis. Combined with a salt pool, they continue to reduce the required chlorine level and limit chloramines — you just need to choose a UV chamber resistant to salt water.
Supplementary disinfection is one of the simplest ways to boost bathing comfort and cut back on chemistry without compromising on hygiene. If you’d like to choose a UV lamp, an ozone generator, or the right chemistry to fine-tune them, browse our water treatment range and pick a solution to match your water’s volume and load.
