What is UVT transmittance?

UV transmittance, or UVT, is a ratio of the amount of ultraviolet light (usually at 254 nm because of its germicidal effect) that passes through a sample of water to be treated to the amount of light that passes through a sample of pure water. The measurement is expressed as a percentage, % UVT.

The treatment of water with ultraviolet light is an extremely fast physical process.

In fact, ultraviolet light mutates and/or degrades the DNA of micro-organisms passing near the lamp.

Once irradiated, they are no longer able to function properly and reproduce. To improve the efficiency of UVC disinfection systems, filters are usually installed upstream of the reactor.

In this section, we will detail:

Why is transmittance the most important parameter in UV disinfection?

The effectiveness of a UV disinfection system is determined by the dose that the system is able to deliver to the target microorganisms in the water. The effective UV dose depends mainly on:

  • the combined effects of UV light intensity
  • the length of exposure
  • UVT

Transmittance varies over time and from site to site, as it is linked to the amount of :

  • organic matter
  • colloidal solids
  • other materials present in water that absorb and disperse UV light as it passes through the water column

In a UV disinfection system, if the amount of UVT in the water is low, the UV light cannot penetrate the water as effectively. This lack reduces the power of the dose. This is the reason why it is very important to monitor the UVT and make sure that the levels are kept above the minimum set by the equipment so that proper disinfection can take place.

How to measure transmittance?

Special equipment is needed. Typically, a sample is sent for analysis to accurately determine the VWU value.

The measurement is performed as follows:

  • ultraviolet light (generally at 254 nm) passes through a quartz sleeve
  • then passes through 10 mm of water from the sample
  • The intensity of the attenuated light is measured with a sensor and compared to a pure water sample
  • The instrument then shows the UVT value in percent.

What are the average WVU values per application?

Application Transmittance
Freshwater pool > 95%
Seawater / salt water 85%
Freshwater fish farming 90%
Wastewater 60%
More applications Please ask us!
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