Using UV Light to Kill Bacteria in Water

Use of a UV unit
Are you wondering – how does UV light kill bacteria in water?

Research has shown that under ideal conditions, a UV unit can provide a 99% reduction of all bacteria in water and surfaces. However, even with this performance, ultraviolet disinfection has the following limitations:

“Point” Disinfection

UV units, including handheld UV light, only eliminate bacteria at one point in a watering system and do not offer any residual germicidal effect downstream. If just one bacterium passes though unharmed, then 100% destruction of bacteria cannot be guaranteed. There is nothing to prevent it from attaching to downstream piping surfaces and proliferating.

Cells Not Removed

Bacteria cells are not eliminated in a UV unit but are converted into pyrogens. The eliminated microorganisms and any other contaminants in the water are a food source for bacteria that do persist downstream of the UV unit.

Due to these limitations, the piping in a watering system treated by UV disinfection will require to be periodically sanitized and cleaned with a chemical disinfectant.

Maintenance requirements for UV units

Lamp Replacement

UV lamps similar to those used in handheld UV light sterilization do not burn out as normal fluorescent lamps do. Nevertheless, the UV lamps will solarize, reducing their intensity to around 60% of a new lamp after about one 12 to 18 months of continuous use. When lamps are new, they will generate a relatively high dosage level. When the dosage drops to the minimum dosage required to kill bacteria effectively, lamps should be replaced. Lamp life will be reduced significantly if the lamp is turned on and off more often than once every eight hours.

Monitoring Performance

UV light sterilization
Water should be sampled and tested for bacteria frequently and before and after the UV unit to test its performance. Moreover, the water should also be sampled in all rooms since bacteria accumulation can occur downstream of the UV unit.


As water passes through the UV unit, debris, minerals, and other material in the water will deposit out and onto the quartz or Teflon sleeve. That limits the penetration of UV rays through the sleeve and into the water. The glass around the lamp must be cleaned repeatedly to maintain high clarity. Cleaning frequency is based on the water quality and will be minimal with RO, distilled, or deionized water.

Monitoring UV Dosage

UV light intensity meters indicate the penetration of UV light through the glass sleeve, and the water is available. Low intensity means the UV dose is too low to give adequate disinfection. The meter will indicate when cleaning or lamp replacement is needed.

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