UV pretreatment of the water is sometimes necessary. This is because UV disinfection only works if the water is clear. If the water is cloudy or contains suspended solids, UV light cannot effectively reach the microorganisms. To prevent particles from blocking the rays, a pre-filter must be used.

What is UV pretreatment?

UV disinfection is an effective way to combat microbiological contamination of water. However, it is necessary to provide an appropriate amount of UV light in order to disinfect the water. To achieve this, UV transmission must be optimal, hence the need for UV pretreatment. If it is low, it is because of the presence of organic molecules or minerals in the water, which absorb or scatter UV light, thus protecting pathogens.

For most UV systems, at least 95% UV transmittance must be achieved.

Why is it important?

To ensure optimal efficiency, the water must meet the following parameters:

  • 1NTU Turbidity < (Turbidity Unit): The content of suspended particles in water, which are usually invisible to the naked eye, and which can absorb or scatter UV light, preventing it from reaching pathogens. Turbidity can cause taste and odor problems.
  • Suspended matter < 10mg/L : Visible or invisible to the naked eye, they are small solid particles. They are generally not soluble in water, and prevent effective disinfection by scattering UV light.
  • Colour = transparency : Coloured water is synonymous with turbidity and indicates the presence of organic matter or suspended solids, which can prevent adequate UV transmission. Generally undesirable in drinking water, the color must be removed before the water reaches the UV system to ensure effective disinfection.
  • Total < Iron 0.2 mg/L : It accumulates on the quartz sheath and contributes to its fouling, preventing UV light from entering the water.
  • Manganese < 0,05 mg/L : same problem as with iron.
  • Hardness < 120 ppm : This is an indicator of a high mineral content in water (Ca++ and Mg++). This is very common in groundwater, as water seeping from the surface through the rock dissolves these minerals and carries them with it. These minerals are removed through a process called “water softening.” The hardness of the water contributes to the formation of scale and also prevents UVC light from penetrating the water properly.

How long does the pre-filter last upstream of a UVC reactor?

Filters should normally operate for 6 to 12 months, depending on the quality of the water.

In areas where the water contains a large amount of sediment (dirt/sand/mud), it is recommended to use a double or triple filter, with one or two filters upstream of the UV lamp.

How to choose a prefilter?

Several parameters must be taken into account when carrying out a UV pretreatment, including:

  • the hardness of the water that must be properly removed
  • Next, you need to check the pressure of your network. Pre-filters create back pressure (pressure drop) on the incoming water flow, reducing the flow of water reaching UV
  • Finally, it is generally recommended to prefilter at 5 microns.

In any case, it is best to do a water analysis with the parameters listed above and use the filters recommended by the manufacturer to ensure the proper functioning of the complete installation.

What quality of wastewater for reuse in irrigation?

In the case of wastewater, for the watering of golf courses for example, it is necessary to obtain at least 60% UV transmittance. The water treated at the outlet of the treatment plant must be class A, i.e. comply with the following parameters:

  • Suspended matter < 15mg/L
  • BOD5 < 25 mg/L
  • Iron total < 0,2 mg/L
  • < COD 60 mg/L

With these values, it is possible to achieve 3 log reduction at 30 mJ on suitable devices, and 4 log at 70 mJ.

FAQ

Why is pre-treatment essential before UV water disinfection?

Pretreatment is crucial to ensure the effectiveness of UV disinfection. It helps remove suspended particles, sediments, and other impurities that can block or attenuate UV rays, reducing their ability to inactivate microorganisms. Pre-treated water ensures better UV transmission and more effective disinfection.

Recommended filters include:

  • Sediment filters : Remove solid particles like sand, rust, and debris.

  • Activated carbon filters : reduce chlorine, volatile organic compounds and improve the taste and smell of water.

These filters ensure clear water, thus optimizing the effectiveness of the UV treatment.

Signs indicating the need for pretreatment include:

  • High turbidity : cloudy water or visible presence of particles.

  • Presence of chlorine : Chlorine smell or taste, which can damage UV lamps.

  • Water from untreated sources : wells, rivers or rainwater.

In these cases, pre-treatment is recommended to ensure effective UV disinfection.

Yes, rainwater can contain suspended particles, organic contaminants and microorganisms. Pretreatment, including sediment and activated carbon filters, is essential to remove these impurities and ensure the effectiveness of the UV treatment.

Poorly pre-filtered water can contain particles and contaminants that accumulate on the quartz sheath protecting the UV lamp, reducing UV transmission and disinfection effectiveness. This can cause premature wear of the lamp and require more frequent replacements.

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