To create germicidal power with UV 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

The calculation of the dose of a UV system depends on three variables:

Exposure time (determined by the flow rate)
Radiation power of the UV lamp (power per unit of surface area)
UV transmittance in the water

The duration (T in seconds) is determined by the flow rate (Q in m³/s) and the reactor volume (V in m³): T = V / Q

The power corresponds to intensity × voltage × the UV lamp, divided by the surface area (in Watt/m²).
However, it should be noted that UV lamps lose some of their efficiency over time. Therefore, a system must be sized using the end-of-life power.

The dose is the product of the intensity emitted by the lamps and the exposure time to the radiation:
Dose = UV intensity (Watt/m²) × seconds = Joule/m².

For example, in order to meet the microbiological criteria for drinking water treatment, the UV dose must generally be 300 Joule/m² = 30 mJoule/cm² = 30 mWatt/cm².

Transmittance describes the ability of a substance to let UV light pass through, measured over 1 cm of liquid. It is usually expressed as a percentage (0% for an opaque material). When UV light irradiates the water, the water absorbs part of the radiation. The disinfecting effect decreases as the radiation moves away from the source.

To properly size a UV disinfection reactor, it is therefore necessary to:

Know the application (to determine the correct UV dose)
Identify the flow rate (to calculate the exposure time in the reactor)
Have information on the transmittance of your water

Fortunately, 1H2O3 has already done all these calculations for you! You can size your UV installation online for various applications with this form.

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