To generate germicidal power with light, a specific UVC radiation must be created. Afterwards, it must be placed in a suitable reactor in order to apply the UVC doses necessary to destroy the pathogenic microorganisms.

How to create UV light?

UV is a spectrum of light located just below the visible range for the human eye. The UV-C spectrum (185 – 280 nm) is known as the germicidal spectrum because it is the area with the maximum germicidal wavelength (260 nm). UV reactors for water treatment include at least one UV lamp that usually contains argon and a small amount of mercury, sometimes in the form of an alloy: amalgam.

UV lamps also contain filaments that, in the presence of electricity, generate an electric charge that causes the mercury inside the lamp to vaporize. This evaporated mercury generates electric arcs that generate ultraviolet radiation at 254 nm, very close to the optimum at 260 nm. Germicidal lamps still emit some visible light because of the other spectra emitted by mercury. All germicidal lamps require an external ballast to regulate the current flowing through them.

UVC LED

Another interesting technology that is not yet available in industry is the creation of UVC radiation by UV LEDs. UV LED semiconductors emit narrow radiation between the positive and negative poles when energized. UV LEDs are definitely the future of UV disinfection, with higher durability, no mercury and low power consumption.

Excimer lamps

A third mercury-free technology is similarly prospective. A modulated electrical circuit is applied to a quartz glass body filled with Xe gas (e.g. several hundred kHz; several kV high voltage). Thanks to an activated Xe2 molecule, and depending on the type of rare gas and halogen used, several types of quasi-monochromatic radiations can be obtained. The KrCl* excimer lamp is the most important for disinfection, radiating at 222 nm and XeBr* with a radiation of 282 nm. This technology does not require a warm-up time. However, those Excimer lamps suffer from a low UVC efficiency of ~8. Another disadvantage so far is the high investment costs for the lamps and the electricity consumed.

How to design a UV system?

The UV dose is the design parameter of a UV installation. Each microorganism needs a specific level of UV energy to disrupt its DNA. This energy level is called “UV dose”. The dose is therefore a measure of the biological effect of UV radiation. The measurement is expressed in mWs/cm² and/or mJ/cm².

The higher it is, the larger the chamber that contains the UV lamp and the exposure of the water to it. The UV disinfection systems available on the market can therefore meet the different needs in terms of UV dose, which depend on the microorganisms to be eliminated.

Is the UV light exposed directly to the water?

Yes, UV rays are exposed to water. However, the UV lamp never gets in direct contact with water. It is placed inside a quartz sleeve in the center of the reactor. The lamp is arranged in a way that the water can pass through a flow chamber and the emitted UV rays are absorbed by the flow.

UV dose calculation

Le calcul de la dose d’un système UV dépend de trois variables :

  • Durée d’exposition (déterminé avec le débit d’écoulement)
  • Puissance du rayonnement de la lampe (puissance par unité de surface)
  • Transmittance UV dans l’eau

La durée (T en secondes) est déterminée par le débit traversant (Q en m3/s), et le volume du réacteur (V en m3): T = V / Q

La puissance correspond à l’intensité x tension x de la lampe UV divisé par la surface (en Watt/m²). Attention cependant, les ampoules UV perdent un peu de leur efficacité avec le temps, il est nécessaire de dimensionner un système en utilisant sa puissance en fin de vie.

La dose, c’est le produit de l’intensité émise par les lampes avec la durée d’exposition au rayonnement : Dose = intensité UV (Watt/m²) x seconde = Joule/m².

Par exemple, pour respecter les critères microbiologiques de potabilisation des eaux, la dose UV doit généralement être de 300 Joule/m² = 30 mJoule/cm² = 30 mWatts/cm².

La transmittance est la capacité d’une substance à laisser passer la lumière UV, mesurée sur 1 cm de liquide. La transmittance s’exprime couramment en pourcentages (0 % pour un matériau opaque). Lorsque la lumière irradie l’eau, l’eau absorbe une partie du rayonnement. L’effet désinfectant diminue à mesure que le rayonnement s’éloigne de la source.

Pour pouvoir dimensionner correctement un réacteur de désinfection UV, il faut donc :

  • Connaitre l’application (pour déterminer la dose correcte d’UV)
  • Connaitre le débit (permettant de déterminer la durée d’exposition dans le réacteur)
  • Connaitre la transmittance de votre eau

Heureusement, 1H2O3 a déjà fait tous ces calculs pour vous ! Vous pouvez dimensionner en ligne votre installation UV pour plusieurs types d’application avec ce formulaire 🙂

Influencing factors of the UV radiation dose

Several factors can affect the water disinfection process, and thus the ability of UV radiation to deactivate and render harmless the microorganisms in the water.

Influencing factors Effects
UV transmittance May affect the design requirements of the system
Iron/Manganese UV absorption
Water hardness Possible formation of scale on the protection tube
Turbidity (suspended particulate matter content) Provides a barrier to protect microorganisms from radiation
Tannins (natural organic substances) Can affect UV transmission
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