Learn how UV systems are validated?
How are UV Systems Validated?
It is important to be sure that a particular UV disinfection system actually achieves the log removal it is supposed to. Validating the performance of each and every UV system in situ is impractical, therefore another system is necessary.
Over the years, there have been many systems developed. The one that has come to be accepted by the international community as the most appropriate is that which verifies system performance by “first principles” – the so called “biodosimetric” approach. This validation system uses actual pathogens to test the log reduction achieved by a given UV system in the following steps.
1. The pathogen of interest is cultured under controlled and reproducible laboratory conditions.
2. The pathogens are exposed to UV light under controlled laboratory conditions.
3. The culture is then exposed to a known UV intensity, of known wavelength, for a fixed period of time, thereby delivering a known UV dose to a known area of the presentation plate – hence dose and intensity are measured per cm2 (area) rather than cm3 (volume). The apparatus used to perform this test is called a Collimated Beam apparatus.
4. The exposed area of the plate is re-cultured to quantify the survival of the pathogen.
5. This procedure is replicated many times at systematically increasing doses in order to build a Dose Response Curve. This curve enables the log survival (and by inference, log reduction) for the pathogen of interest to be determined for any given UV dose. This entire procedure is then replicated at every UVT level across the required UVT range.
6. After the various Dose Response Curves have been constructed in the laboratory, these then need to be applied to test an actual UV disinfection system in order that it might be validated. The pathogens used to test the UV system are cultured (albeit in much higher volumes) under exactly the same conditions as used in the laboratory.
7. A sample of the water is taken at the inlet to and exit from the UV system and re-cultured to determine how many pathogens have survived.
8. The observed log survival of the pathogen is then compared to the pathogen’s Dose Response Curve (see Step 5) and the actual UV dose delivered read off from the curve. This dose is termed the Reduction Equivalent Dose – RED.
Graham Smith. Fluidquip Australia Pty Ltd.