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Source: adapted from Manual of Individual Water Supply Systems,
EPA-430/9-74-007, U.S. Environmental Protection Agency

This method uses an ultraviolet light tube sealed in a quartz sleeve inside a stainless steel cylinder, which is connected to the water supply line. The untreated water from the source comes in at one end of the cylinder, passes around the ultraviolet tube, and exits through the opposite end into the safe water line. Ultraviolet light emanating from the tube kills the bacteria. Unlike chlorination, the ultraviolet light process does not leave a residual disinfectant in the water and the possibility exists for water to be recontaminated. For this reason the ultraviolet light unit should be installed at the end of all underground piping.

There is no simple test to determine whether the ultraviolet light is operating effectively. Ultraviolet rays must reach bacteria to kill them. To ensure proper operation, the ultraviolet light bulb should be replaced when it weakens. If the manufacturer does not supply an ultraviolet intensity indicator on the unit, the bulb should be replaced once a year. Some units are equipped with a photocell that indicates whether the light is on or off. This does not provide sufficient protection against light failure, because the light may be intense enough to indicate that it is operating, yet ultraviolet light output could be diminished to the point of ineff ectiveness.