Theory and System
System components: power source, ballast, lamps, reactor.
UV light, in a range of 250 to 270 nm, may be used for disinfection purposes

Contact and Intensity
Low pressure mercury lamps, operate at a temperature of 95 deg F to 122 deg F for maximum efficiency; about 85 percent of the light is monochromatic at 253.7 nm.

Lamps are usually 2.5 feet to 5 feet in length, and about 0.6 to 0.8 inches in diameter.

Medium pressure mercury lamps, with higher operating higher temperatures, use more electrical power to operate; are 15 to 20 times the disinfecting power of low pressure lamps due to the much higher light intensity that is given off. Medium pressure of lamps cost 4 to 5 times the cost of low pressure lamps; (as expected, the price decreases as the quantity purchased increases.)

Lamps may be submerged (contact type) by placing in quartz sleeves (tubes) to prevent adverse cooling effects on the lamps, or may be outside (non-contact type) the water being disinfected, transmitting light into the liquid.

Replace ballasts 10 - 15 years; quartz sleeves 5 - 8 years.

Disinfection usually occurs in "disinfection channels," as "detention tanks" are not required, therefore requires less space when compared to chemical disinfection methods. Level/flow control is applied utilizing gates or weirs to maintain appropriate water levels over and around the lamps in the contactor.

Lamps may be placed vertically or horizontally, as long as there are no dead zones or short circuiting occurring. All water MUST be exposed to the same light intensity for the same amount of contact time for maximum effectiveness and process control.

UV is now stated to be cost effective for wastewater filtered effluents when compared to disinfection chemicals and their contact basins, (and even your "basic chlorine" when leak scrubbers and the new fire code regulations are factored in.)

Water hardness (calcium, magnesium, etc.) affects UV performance with precipitation/scaling of carbonates on the quartz sleeves.

Disinfection Action
UV light penetrates the cell wall, thought originally to kill the cells outright, but further research shows major action may be in that it destroys the duplicative nature of DNA and therefore prevents reproduction.
Water being disinfected must be clear, free from turbidity, for maximum effectiveness.
Must therefore reduce particulate and colloidal materials. Suspended solids absorb UV radiation and subsequently shields embedded microbes from disinfection action.

Contact time with the UV light, and the intensity of the light dosage are very important operational parameters. Light has limited ability to disinfect in the water at distances other than those close to the lamps. Lamps are placed in close proximity to one another, or are placed to create turbulence around them with a thin film of water passing among the lamps for maximum exposure to the UV light. Some systems place the lamps horizontal to the direction flow for lower head loss through the contactor.

Among the further advantages are: No chemical residual to be removed. After wastewater treatment, no subsequent beneficial nor adverse environmental impacts are noted in the research at this time.

References:
Wastewater Engineering, 3rd Edition; Metcalf & Eddy c 1991 McGraw Hill, Inc

Geo. Clifford White, "Handbook of Chlorination & Alternative Disinfectants" 4th Edition; c 1999, John Whiley & Sons, Inc.

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