UV pre-treatment of the water is sometimes necessary. In fact, UV disinfection only works if the water is clear. If the water is turbid or contains suspended matter, the UV light cannot effectively reach the microorganisms. To prevent particles from blocking the rays, a pre-filter must be used.
What is UV pre-treatment?
UV disinfection is an effective way to fight microbiological contamination of water. However, it is required to provide an appropriate amount of UV light in order to disinfect the water. For this, the UV transmission must be optimal, hence the need for UV pre-treatment. The reason for low UV light is the presence of organic molecules or minerals in the water, which absorb or scatter UV light, thus protecting the pathogens.
For most UV systems, at least 95% UV transmittance is required.
Why is it important?
To guarantee optimal efficiency, the water must respect the following parameters:
- Turbidity < 1NTU (Turbidity Unit): the content of suspended particles in water, usually invisible to the naked eye, 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 usually not soluble in water, and prevent effective disinfection by scattering UV light.
- Color = transparency: Colored water is synonymous with turbidity and indicates the presence of organic matter or suspended solids that may prevent adequate UV transmission. Usually unwanted in drinking water, color must be removed before the water reaches the UV system to ensure effective disinfection.
- Iron total < 0,2 mg/L : It gathers on the quartz sleeve and contributes to its fouling, preventing UV light from penetrating the water.
- Manganese < 0,05 mg/L : same problem as with iron.
- Hardness < 120 ppm : It is an indicator of a high mineral content in the 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 by a process called “water softening”. Water hardness contributes to scale formation and also prevents the UVC light from penetrating the water properly.
How long is the pre-filter upstream of a UVC reactor?
Filters should normally operate for 6 to 12 months, depending on water quality.
In areas where the water contains a large amount of sediment (dirt/sand/mud), a double or triple filter with one or two filters upstream of the UV lamp is recommended.
How to choose a prefilter?
There are several parameters to consider when performing UV pretreatment, including:
- the hardness of the water which must be properly removed
- Next, you need to check the pressure in your system. Pre-filters create a back pressure (pressure drop) on the incoming water flow, thus reducing the water flow reaching the UV
- Finally, it is generally recommended to prefilter at 5 microns.
In any case, it is preferable to do a water analysis with the parameters listed above and to use the filters recommended by the manufacturer to ensure the proper functioning of the entire system.
What quality of wastewater for reuse in irrigation?
In the case of waste water, for example for watering golf courses, it is necessary to obtain at least 60% UV transmittance. The treated water at the outlet of the treatment plant must be of class A, i.e. respect the following parameters
- Suspended matter < 15mg/L
- DBO5 < 25 mg/L
- Iron total < 0,2 mg/L
- DCO < 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.