Log Reduction Explained
So what does Log Reduction mean?
Log Reduction (as in Logarithmic Reduction) refers to a predictable percentage that a given pathogen will be inactivated by a disinfection method. A 1 log reduction will see the pathogen of interest reduced by 90% from the influent level. A 2 log reduction will see a 99% reduction, 3 log by 99.9%, and so on.
Does it take the same amount of UV light to achieve the same log reduction for different pathogens?
No. The UV Dose requirements to achieve a certain log reduction are different for all pathogens. For example as you can see in the video above, to achieve a 4 log reduction for E. Coli you would need a UV dose of 12.8 mJ/cm2 whereas for Salmonella you would need a UV dose of 7 mJ/cm2.
Does it take the same marginal increase in UV Dose to get to a higher log reduction level of inactivation?
No. The curve that reflects a given pathogens response to a UV dose is rarely linear. So it will take a higher amount of energy to increase from one log reduction level to another. One common mistake that is made is take to the UV dose for 1 log activation and multiply it to calculate the higher level.
Most importantly, why does this matter?
When installing and designing a UV system its extremely important to know the target pathogens as well as the needed level of inactivation for a given process. Knowing these two elements will help to ensure that the UV system is delivering the proper UV dose.
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