Disinfection, Part 1: Developments in Ultraviolet Disinfection
Water treatment systems are designed to address public
concerns such as taste, odor and hardness. The average person's greatest
concern is microbiological safety; he wants to know that there isn't anything
in his water that can make him ill. This concern explains why one of the
fastest growing technologies in the water treatment industry is ultraviolet
In an age of increased concern for personal health and
protection of the environment, UV is an attractive alternative to chemical
disinfectants. Chemical disinfectants often produce byproducts that can be
harmful and may alter the taste of the water. UV disinfection adds no
chemicals, and it does not produce byproducts. Additional benefits include easy
installation, low maintenance, minimal space requirements and whole-house
There are several recent advances that make UV systems even
more appealing today. One is a timer that monitors how long the system has been
operating--this feature warns the user when his lamp is nearing the end of its
12-month life so he does not have to try to remember if it was one, two or
three years ago that he bought his last lamp.
More efficient lamps are another recent development. These
have allowed some manufacturers to produce systems that are half the size of
conventional systems--an important consideration given that most systems are
installed vertically in the basement.
UV Intensity Monitors
There is no better way to verify the real-time performance
of a UV system than with a UV intensity monitor. The monitor is much like an
"eye" looking into the reaction chamber. It measures how much light
is getting through the water and often is calibrated to set off an alarm when
sufficient UV light is not detected. On some systems, such a condition also can
trigger a shut-off valve.
Whether it is changing water conditions, lamp fouling or
lamp aging, a monitor assures that an adequate dose is delivered at all times.
UV monitors have been available for many years, however some
older models tended to drift over time. Some manufacturers are now able to
offer sensors that are much more resistant to the degradation caused by
exposure to the UV light.
Pathogens traveling through a UV reaction chamber may follow
a number of different paths. Some paths are longer, some are shorter, and some
paths flow at faster velocities than others. The result is that the amount of
UV light to which a pathogen is exposed can vary. To maximize the efficiency of
a reactor, all the paths should expose pathogens to the same amount of UV light.
Only by using sophisticated computational fluid dynamic (CFD) modeling can
designers optimize a reactor. CFD modeling is the same process used to optimize
car and airplane designs for air flow.
Figure 1 illustrates an efficient CFD-modeled reactor. Each
colored strand represents the path of an individual particle through the
reactor. The color changes from blue to red as the retention time increases.
For all its strengths, CFD is only a design tool and
performance tests must still validate the effectiveness of each system. The
best assurance of effectiveness is certification by a third-party testing body
such as NSF International.
NSF Standard 55
NSF International is an independent accreditor of water
treatment systems. The protocol for validation of residential UV systems is NSF
Standard 55. The standard has two parts.
A is for UV equipment that is certified to be used for treatment of
microbiologically unsafe water that meets all other drinking water criteria.
The dose requirement is 40 mJ/cm2 for a municipality. Until very recently,
there were not UV systems with Class A certification.
B is for UV equipment that is used to provide supplemental treatment of treated
and disinfected public drinking water. The dose requirement is 16 mJ/cm2.
Regulators generally are interested in Class A certification
because of the health effect claims that can be made by products receiving this
certification. This includes 99.99 percent inactivation of Rotavirus. Whether
viral, bacteriological or protozoan (such as Cryptosporidium parvum and Giardia
or beaver fever) Class A certification assures that dangerous microorganisms
Systems that are certified and listed by NSF are the only
systems that have been tested and have passed the Standard 55 Class A protocol.
Do not be misled by statements that indicate the product "meets Standard
55 requirements." If a product is certified it will be listed on the NSF
website (www.nsf.org/certified) under "Drinking Water Treatment Units,"
"Reduction Claims for UV Microbiological Water Treatment Systems" and
"Disinfection Performance, Class A."
UV disinfection for residential use is growing rapidly in
popularity. POE systems provide chemical-free disinfection, easy installation,
minimal maintenance and proven protection against waterborne illnesses. There
have been many advances in UV technology for the residential market. Water
chamber and lamp efficiency, automatic timers, UV intensity monitors and NSF
certification are some of the key features to consider when selecting the
appropriate product for you and your customers.