LAMP AND SLEEVE:
UV light is generated by an electrical current in a mercury
vapor. The wavelength of the UV light emitted is dependent on the type
of lamp used.
Lamps & Sleeves:
The lamp consists of a quartz tube filled with inert gas such as argon,
and a small quantity of mercury. The following photos show examples
of UV lamps.
UVSwift Reactor open view
ports close-up view
Lamps are all fused quartz that pass 85-90% of 253.7 nm energy (Malley
et al., 2001). UV lamp radiation varies with temperature, voltage, and
lamp age as well as other factors (Arora et al., 2001). The lamp is
protected by a lamp sleeve to thermally and electrically insulate the
lamp from pressure of the flowing water. A lamp sleeve is shown in the
The lamp sleeve is generally made of quartz, but Teflon has been used
in some systems (Malley et al., 2001). The quartz sleeve also helps
the lamps to operate at optimal temperature. Temperature changes up
or down may reduce UV radiation when the lamp temperature deviates from
the optimum (Cotton et al., 2001). There are different types of lamps,
low pressure, low pressure high output, and medium pressure.
Lamp irradiance decreases over time. Reasons for this decrease are
solarization (effect of UV light on the lamp and sleeve that causes
lamp to become more opaque over time), electrode failure, and plating
of mercury on lamp wall. An example of lamp solarization is shown in
The occurrence of these may reduce output by 30% after 7,000 hours
of use (Jesky et al., 2001).
Low Pressure Lamps:
Low pressure lamp systems emit nearly monochromatic light at 253.7
nm. This has 85% of the germicidal effectiveness as the ideal wavelength
of 260 – 265 nm. These lamps are best used where low intensity
UV treatment is economically practical such as low flow systems of 2
– 100 gpm (Jesky et al., 2001). These lamps are about 35 –
40% efficient in converting electricity to UV energy. Lamp life is approximately
7,500 to 8,000 hours but can be shorter with frequent stopping and starting,
high currents, and voltage fluctuations (Malley et al., 2001). Low pressure
lamps typically have twice the life as medium pressure lamps (Cotton
et al., 2001). Manufacturers recommend lamp replacement once a year.
Low Pressure High Output Lamps:
Low pressure high output lamps emit monochromatic emissions at a wavelength
of 254 nm. The advantage of using a low pressure high output lamp is
that it provides a higher germicidal UV output than low pressure lamps.
This means a UV disinfection system will use fewer lamps than the traditional
low pressure disinfection system.
Medium Pressure Lamps:
Medium pressure lamps emit a broader spectrum of UV than low pressure
lamps. Medium pressure systems can be used in plants with higher flow
rates (Jesky et al., 2001). An advantage of these lamps is higher intensity
output. This means that one high pressure lamp may be able to replace
6 – 16 low pressure lamps resulting in lower costs. A concern
is that medium pressure lamps are more prone to fouling because of higher
lamp temperatures (Cotton et al., 2001). Lamp life is approximately
2,000 – 5,000 hours (Malley et al., 2001).
Low pressure lamps are more common, especially in older systems, but
low pressure high output and medium pressure lamps are growing in use
(Malley et al., 2001, Cotton et al., 2001).
The table below summarizes general properties of low and medium pressure
Mercury Vapor Lamp Characteristics (USEPA, 2003)
Lamps require adequate time to warm up before flow begins, typically
2 – 5 minutes. Safety is always a major concern. UV systems are
typically sealed but UV light and heat can cause burns and skin and
eye damage (Malley et al, 2001). Periodic cleaning of UV reactors (i.e.
lamp sleeves) is required because components can become fouled.