UV-tubes

The bactericidal effect of concentrated ultraviolet (UV) light has been known over many years and is used in many areas and set-ups. The first use of UV light for drinking water disinfection has been reported to be in France in 1910. With the recent development of the UV tube, using locally available components, this technology is now a viable household water treatment method (CAWST 2009). Facing the challenge that disinfection by-products (DBPs) are formed during chlorination of water containing high organic contents, simple physical instead of chemical disinfection methods, such as UV tubes have gained in interest (U.S. EPA 1999).

A typical UV system provides a flow path surrounding a UV lamp, structured to provide close proximity of the water flow along the length of the UV lamp all by preventing direct contact.

The UV tube is basically the same as a commercial fluorescent bulb, except that it lacks the phosphor coating and the glass exterior is replaced by fused quartz. This means the bulb emits mostly UV light. UV light is generally defined to be wavelength of electromagnetic radiation shorter than 400 nm and is further divided into UV-A (315-400nm), UV-B (280-315nm) and UV-C (200-280 nm). UVA and UVB are responsible for sun tanning and sunburning. UVB is partly filtered out by the atmosphere and only a few percent reach the earth surface. This is good, because UVB light can be directly absorbed by DNA, where it would induce cellular damage. UVC also penetrates cells and damage the DNA, but it is almost entirely filtered out by the ozone layer. The UV tubes used for disinfection do contain the whole spectrum of UV light, including the UVB and UVC. The light, when it reaches microbial cells leads to damage of the genetic material (DNA), rendering them unable to replicate.

The bulbs are suspended inside a larger tube in a covered channel. Water enters at one end and flows through the outlet at the other end. While the water flows through the tube, the UV light emitted from the bulb inactivates the microorganisms. The inactivation is directly related to continuous UV dose and depends on the intensity of the UV light and the duration of UV exposition. The height of the outlet determines the depth of the water in the tube and thus the hydraulic retention time (HRT, CAWAST 2009). In theory, the hydraulic retention time is equal to the exposure time of the microorganisms and should be designed, depending on the bulb intensity, to achieve a sufficiently long contact time for all microorganisms to be inactivated. For the calculations of the HRT, the intensity of the bulb may be adjusted according to the water depth of the farthest point away from the light source as the UV light is attenuated as it penetrates the water.

UV tubes are effective in inactivating most pathogens, including bacteria, viruses, and cyst forming protozoa such as cryptosporidium (CAWST 2009). Yet, the effectiveness depends strongly on the UV dose.

Potential treatment capacity of UV tubes. Source: CAWST (2009)

UV tubes are not effective for all pollution other than microbial (chemical and physical pollution) and there is now residual disinfection effect during storage of the treated water. The efficiency of UV light for bacterial inactivation is lowered by the presence of organic matter, iron, sulphites, and nitrites. Turbidity can also interfere with UV radiation, protect microorganisms from inactivation and serve as a food source for re-growth after the disinfection step. Therefore, it is common for UV tubes to incorporate a pre-filtration step, such as a settling unit.

Operation and maintenance of UV tubes is generally low, provided that there is a more or less continuous source of electricity to run the bulb. Once the system is installed, the user needs to make sure that the theoretical flow rate of the water and the corresponding HRT are respected and that the bulb is working correctly. If the bulb becomes dirty, it needs to be taken out and cleaned to maintain the light emission intensity. The systems have generally a long life spam, but bulbs should be replaced at least once every 12 months (CAWST 2009). During any maintenance activity, however it is important that operators or users protect their skin and eyes from the UV radiation as it is also damaging for human cells.