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.

Trojan UVSwift Reactor open view
Lamp 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 following photo.

Lamp sleeve

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 this photo.

Lamp solarization

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 lamps.

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.