a recent international conference on wastewater treatment technology,
Ben F. Kalisvaart (senior adviser on UV technology at Berson UV-techniek,
The Netherlands) raised the question: “What is best? Open-channel
or closed-channel UV disinfection of wastewater?”.In his paper he
described why more and more operators of wastewater treatment facilities
are now opting for closed-channel UV disinfection systems instead
of older, open-channel systems.The reasons for this move are due
partly to the drawbacks of open-channel systems, as well as the
advantages of closed-channel systems.
The disadvantages of open-channel disinfection include the danger
of personnel being exposed to UV light; the growth of algae in the
open channels and the difficulty of cleaning the lamps, which have
to be either cleaned manually – a laborious procedure – or physically
lifted and moved to an acid bath.
Also, the hydraulic movement of liquids through open channels
is not particularly turbulent, so some sections of the wastewater
may not pass close enough to the UV lamps to receive the minimum
required UV dose.
Closed-channel UV systems on the other hand present no hazard
to staff as the UV lamps are enclosed in a stainless steel chamber;
this also eliminates the problem of algal growth. Each lamp is fitted
with a mechanical wiper on its protective quartz sleeve which keeps
it clean. Periodic chemical cleaning, if required, is simple and
can be done 'in-line' without removing the lamps. In addition, lamp
change-over is easy and can be done in minutes.
The hydraulic design of closed-channel systems also
means the movement of wastewater through the treatment chamber is
more turbulent than in open-channels, ensuring all the wastewater
receives the minimum required UV dose by passing close to the lamps.
Along with closed-channel UV systems, many engineers are also opting
for medium pressure, 'polychromatic', UV lamps. They produce UV
over a broad wavelength and have been shown in independent tests
to cause permanent inactivation of both pathogenic and non-pathogenic
micro-organisms such as E.coli and Cryptosporidium parvum (1, 2,
Low pressure 'monochromatic' lamps, on the other hand, produce
a single peak of UV output.
It has been shown that many micro-organisms are able to repair
themselves after exposure to UV from these low pressure lamps, especially
if they are subsequently exposed to sunlight – as is often the case
in wastewater treatment facilities.
In addition, only a few medium pressure lamps do the
same job as many low pressure lamps – this makes medium pressure
systems much easier to operate, monitor and maintain.
Because of these factors, low pressure UV lamp technology
should be avoided in wastewater applications.
Cleaning of Quartz Sleeves
A major factor to consider with UV wastewater treatment plant is
fouling of the protective quartz sleeves surrounding the UV lamps.
Suspended solids and minerals in the wastewater attach themselves
to the sleeves and must be removed at regular intervals to ensure
maximum UV output. This is something that happens to both low and
medium pressure UV lamps, and in both open-channel and closed-channel
There are two main ways to control fouling: mechanical
cleaning of the sleeves (with O-rings or brushes) or chemical cleaning
Even when mechanical cleaning is used, the sleeves
will still need to be chemically cleaned from time to time. As explained
above, with open-channel UV systems the UV lamps must be physically
lifted from the channel and transferred to an external chemical
With closed-channel UV systems, cleaning agents are
simply added to the UV chamber and cleaning takes place internally.
To assist this process, Berson has developed the UltraWipe®,
a combined mechanical and chemical cleaning system. When the UV
system is on, automatic wipers move up and down the quartz sleeves,
removing any deposits. At the same time, a small volume of low concentration
acid is applied directly to the sleeves..
This 'direct dosing' means significantly less chemicals
are required to keep the sleeves clean, than with conventional chemical
dosing. The chemicals used are not harmful, in any way, either to
the environment, or to the wastewater plant's pipeline infrastructure
Closed-channel UV wastewater treatment systems are increasing in
popularity with operators of wastewater treatment plant. There are
many reasons why these systems are now taking over from older, open-channel
Firstly, closed-channel systems are safer.
Secondly, cleaning the UV lamps' protective quartz sleeves is
straightforward – either mechanically or chemically – without having
to remove the lamps.
Thirdly, the hydraulic design of closed-channel systems ensures
most of the wastewater receives the minimum required UV dose.
Finally, closed-channel systems, in conjunction with
medium pressure UV lamps, ensure that micro-organisms are permanently
deactivated and cannot repair themselves.
1. Zimmer, J. L., Slawson, R. M. & Huck, P.M.
Potential repair of Escherichia coli DNA following exposure to UV
radiation from both medium- and low-pressure UV sources used in
drinking water treatment. Applied & Environmental Microbiology,
Vol. 68 (2002), No. 7, 3293-3299.
2. Oguma, K., Katayama, H. & Ohgaki, S.
Photo-reactivation of Escherichia coli after Low- and Medium-Pressure
UV Disinfection Determined by an Endonuclease Sensitive Site Assay.
Applied & Environmental Microbiology, Vol. 68 (2002), No. 12,
3. Zimmer, J. L., Slawson, R. M. & Huck, P.M.
Inactivation and potential repair of Cryptosporidium parvum following
low- and medium-pressure ultraviolet