However, like any sanitation technology, UV isn’t an end-all solution — its   effects are rapid but limited, and it often requires help from other types of   sanitizers.

Here, we talk with professional chemists about the exact nature of UV, the   means by which it sanitizes, and the most effective ways of implementing it.   A fuller, more detailed understanding of these principles will inform   decisions about where and how to apply UV for maximum effectiveness.

What is UV?
In the simplest terms, UV is a kind of light — to be more precise, it’s a   specific range of electromagnetic radiation wavelengths, most of which lie   outside the range visible by humans. While we can generally see light whose   wavelength falls between 390 and 750 nanometers (nm), UV’s wavelengths are   between 100 and 400 nm.

As with most artificial light, UV is produced by a bulb designed to generate   radiation in a specific range of wavelengths. “There are medium pressure and   low pressure UV lamps, and each one produces a different range of UV   wavelengths,” says Ellen Meyer, the Charleston, Tenn.-based tech service   manager for Lonza.

Shorter wavelengths indicate higher energy, and the UV range is subdivided   along a sort of energy scale, progressing from UVA (315 to 400 nm) to UVB   (280 to 315 nm) to UVC (200 to 280 nm) to high-energy radiation known as   vacuum UV (100 to 200 nm). Most UV radiation is at least somewhat effective   at killing microorganisms and breaking down organic compounds, but the   high-energy radiation of vacuum UV is the most powerful — and thus, the most   deadly to microbes. Still, even lower-energy UV is effective at deactivating   many organisms that resist the disinfection effects of chlorine.

Like other kinds of light, UV travels fast and doesn’t hang around — a   property that has its ups and downs. On the positive side, this means UV   reaches its entire target area almost instantly, and destroys the organic   contaminants it touches in a matter of seconds. The downside is, UV can’t   diffuse throughout the water the way, say, chlorine can — so its   effectiveness is limited to the path along which it’s projected, and it can’t   maintain a sanitizer residual in the pool.

Thus, some kind of additional sanitizer is a must in pools using UV. “Even in   Europe, where UV and UV-generated ozone are commonplace, additional chlorine   is required by health authorities,” says Corinne Lehr, assistant professor at   the California Polytechnic State University Department of Chemistry and   Biochemistry in San Luis Obispo, Calif.

Another significant concern in UV-sanitized pools is the replacement and   disposal of bulbs. “The bulbs have to be replaced every six to nine months,   and that costs $750 to $1500 every time,” says Jeff Jones, the Dallas-based   North American sales director of the residential pool division at Del Ozone.   In addition, Jones points out that the bulbs contain the chemical mercury,   whose disposal many cities regulate closely.

What does UV do?
Just as the sun’s UV rays can cause damage to our skin if we don’t wear   sunscreen, artificially generated UV can cause severe disruptions to the biochemistry   of microorganisms in pool water.

A predominant explanation for UV’s damage to microbes hinges on the damage   this radiation causes to DNA — the self-replicating molecule necessary for   life to reproduce. “Once an organism’s DNA is sufficiently damaged, that   organism can’t reproduce anymore,” Meyer explains. “So even if you were to   swallow some of the organism, it won’t be able to grow and reproduce and   cause an infection — you’ve pretty much disarmed it.”

But UV’s effects may also play more immediate roles in microbe destruction.   “Similarly to how UV gives us sunburns, it can cause physical damage to   microbes,” Lehr says. “But that damage can be even more serious to them — it   can kill them.”

The degree to which UV inactivates or kills an organism depends on multiple   factors, including the wavelength of the UV and the biological makeup of the   organism. The chart above details some kill and inactivation figures for   several common microorganisms under UVA radiation.

Along these same lines, UV can be used to destroy other organic contaminants,   like the chloramines that form when chlorine reacts with bather wastes in the   water. The only problem is, the molecular pieces of some chloramines stick   around in the water, and may re-form into their original compounds if they’re   not filtered out quickly enough — as can be seen in the charts to the right.

In the first chart, levels of monochloramine (NH₂Cl) and nitrogen   trichloride (NCl₃) are both lower after UV than before UV, but the   level of dichloramine (NHCl₂) can sometimes be slightly higher. In   the second chart, it’s equally clear that even after a dose of UV, some of   the chloramines have re-formed, and their levels have actually risen. “So you   actually see more of certain disinfection byproducts with UV than you do   without it,” Meyer says.

How can UV be used effectively?
UV performs its work most powerfully when it’s used as a supplementary   disinfection system, supplementing other sanitation products such as   chlorine.

Some microorganisms, such as the notorious cryptosporidium, are highly   resistant to chlorine, because they produce hard shells known as cysts.   Others, such as black algae, produce slimy coats called biofilms, which also   can be tricky for chlorine to penetrate.

However, UV is often highly effective at breaking down these defenses. By   combining UV with chlorine, it’s possible to cut through the organisms’   protective layers and attack their vulnerable bodies and DNA — then oxidize   and destroy the remaining organic contaminants. This “one-two punch” can keep   even a large public pool free from infectious and otherwise annoying   invaders.

It’s also important to note than UV rays only attack organic matter in areas   they can directly reach — so if corners of the pool are “in shadow,” so to   speak, other disinfection methods will be necessary for keeping those areas   clear of algae and bacterial growth.

To ensure that the UV lamp is as effective as possible, it pays to examine   the shape of the pool carefully, and place the lamp in a location where its   rays will reach as many surfaces as possible. Another alternative is to   install an inline UV system, which bathes a closed chamber in UV light as   water continually flows through it.

Finally, it’s worth mentioning that UV won’t have an effect against metal   stains or phosphates — these issues will need to be treated with other   chemical solutions, such as sequestrants and phosphate removers. However, if   levels of these chemicals are kept within acceptable ranges, UV will prove a   powerful ally against any unwanted microbes that try to move in.

The bottom line is, there’s no end-all system for treating a pool — no   perfect solution that kills every organism in the water, produces no   disinfection byproducts, and operates in a way that’s completely safe and green.   Still, the highest priority is to keep swimmers free from infection and   irritation — even if the best way to do that is different for every pool. A   working understanding of how UV disinfection can fit into an overall   sanitation scheme will inform much clearer decisions about how best to apply   this technology.