Between the rising costs of electricity and growing consumer demand for “green” water treatment solutions, ultraviolet (UV) radiation is becoming an increasingly popular tool for disinfecting pools and spas.
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
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
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
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
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
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
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.