Just beyond the visible light spectrum there is a band of electromagnetic radiation that we commonly refer to as ultraviolet (UV) light. When ultraviolet radiation is absorbed by the cells of microorganisms it damages the genetic material in such a way that the organisms are no longer able to grow or reproduce, thus ultimately killing them. This ability of UV radiation to disinfect water has been understood for almost a century, but technological difficulties and high energy cost prevented widespread use of UV systems for disinfection. Today, however, with growing concern about the safety aspects of handling chlorine and the possible health effects of chlorination byproducts, UV disinfection is gaining in popularity. Technological advances are being made and several manufactures now produce UV disinfection systems for water and wastewater applications. As operating experience with installed systems increases, UV disinfection may become a practical alternative to the use of chlorination in wastewater treatment plants.
The usual source of the UV radiation for disinfection systems is from low pressure mercury vapor UV lamps which have been made into multi-lamp assemblies, as shown below. A quartz sleeve protects each lamp and each have watertight electrical connections. The lamp assemblies are mounted in a rack (or racks) and these racks are immersed in the flowing water. The racks may be mounted either within an enclosed vessel or in an open channel. Most of the UV installations are of the open channel configuration.
When UV lamps are installed in open channels, they are typically placed either horizontal and parallel to the flow or vertical and perpendicular to the flow. In the horizontal and parallel-to-flow open channel lamp configuration, the lamps are arranged into horizontal modules of evenly spaced lamps. The number of lamps per module establishes the water depth in the channel. For example, 16 lamps could be stacked 3 inches apart to provide disinfection for water flowing through a 48-inch deep open channel.
The light from a UV lamp can cause serious burns to your eyes and skin. ALWAYS take precautions to protect them. NEVER look into the uncovered parts of the UV chamber without proper protective glasses. Do not plug a UV unit into an electrical outlet or switch a unit on without having the UV lamps properly secured in the UV water chamber and the box closed. UV lamps contain mercury vapor, a hazardous substance that will be released if a lamp is broken. Handle UV lamps with care and be prepared with the proper equipment to clean up any spills.
The operation of ultraviolet disinfection systems requires very little operator attention. To prevent short-circuiting and ensure that all microorganisms receive sufficient exposure to the UV radiation, the water level over the lamps must be maintained at the appropriate level. Weirs or automatic control gates can control water levels in channels.
Lamp output declines with use so the operator must monitor the output intensity and replace lamps that no longer meet design standards, as well as any lamps that simply burn out. Lamp intensity monitors can be installed to assist the operator in monitoring the level of light output. Lamp failure indicators connected to the main UV control panel will alert the operator when a lamp burns out and requires replacement.
Care must be taken not to exceed the maximum design turbidity levels and flow velocities when using this type of equipment. Suspended particles will shield microorganisms from the UV light and thus protect it from its destructive effects. Flows should be somewhat turbulent to ensure complete exposure of all organisms to the UV light, but flow velocity must be controlled so that the wastewater is exposed to UV radiation long enough for the desired level of disinfection to occur.
Since ultraviolet rays leave no chemical residual like chlorine does, bacteriological tests must be made frequently to ensure that adequate disinfection is being achieved by the ultraviolet system. In addition, the lack of residual disinfectant means that no protection is provided against recontamination after the treated water has left the disinfection facility. When the treated water is exposed to visible light, the microorganisms can be reactivated. Microorganisms that have not been killed have the ability to heal themselves when exposed to sunlight. The solution to this problem is to design UV systems with a high efficiency for killing microorganisms.