What is UVT transmittance?

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

Treating 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. In order to improve the effectiveness of UVC disinfection systems, filters are usually installed upstream of the reactor.

In this section, we will detail:

Sizing your UV reactor

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 present in the water. The effective dose of UV depends mainly on:

  • Combined effects of UV light intensity
  • the length of exposure
  • of the UVTs

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

  • organic matter
  • colloidal solids
  • other materials in the water that absorb and scatter 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 why it is very important to monitor UVTs and ensure that their 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 value of UVT.

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 using a sensor, and compared to that of a sample of pure water
  • The instrument then indicates the value of the UVTs as a percentage.

What are the average UVT values per application?

Application Transmittance
Freshwater> pool 95%
Seawater/Salt Water85%
Freshwater fish farming90%
Waste water60%
Other applications Ask us!

FAQ

Does UV purification remove nitrates, pesticides or heavy metals?

No, UV purification only acts on living microorganisms such as bacteria, viruses and protozoa. It has no effect on chemical pollutants such as nitrates, pesticides or heavy metals. To remove these substances, it is necessary to add a complementary treatment, such as an activated carbon filter or a reverse osmosis system. UV is therefore a microbiological barrier, but not a complete solution on its own.

Not always. If the water is clear and already filtered, UV treatment may be enough to make it drinkable by eliminating pathogenic germs. On the other hand, for water from a well, a river or a recovery system, a prior treatment is essential to remove particles, chemical substances or odours. UV radiation does not address these aspects. It is therefore often recommended to combine several purification steps to obtain truly drinkable water.

Most UV lamps used for water purification have a lifespan of around 9,000 to 10,000 hours, which is a year of continuous use. Even if the lamp remains on after this time, its effectiveness decreases sharply. This means that it emits less UV rays, making the treatment less reliable. It is therefore essential to replace it every year to ensure optimal disinfection.

The pre-filter plays an essential role because UV is only effective if the water is clear enough. In the presence of mud, sand, iron or other particles, UV rays can be absorbed or deflected, reducing their ability to kill germs. A good pre-filter (at 5 or 20 microns) therefore maximizes the effectiveness of the UV system while protecting the lamp and its quartz sleeve from clogging.

Yes, it’s even a frequent application. UV disinfection is very useful for securing spring or well water, which is often contaminated with coliform bacteria or other microorganisms. However, a water analysis is recommended before installation, to check for turbidity, iron or manganese. If necessary, a suitable pre-treatment will be added to ensure clear water, compatible with effective UV disinfection.

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