How to calculate absorption coefficient for dirty water in a UV application?

UV disinfection systems are used to kill bacteria in water. UV Lamps emit radiation that has germicidal effect. In this industry, water transmittance is used to describle the quality of water. There are two definitions for absorption coefficient used by (a) chemists and (b) physicist. Which one to choose?
FLUENT's input for absorption coefficient is what the physicist use. It is called Napierian absorption coefficient and is the negative of natural logarithm of water transmittance values based on 10 mm optical length.

So, if water transmittance values are measured as 90%, absorption coefficient will be,
a=-ln(0.9)*100=10.5 (1/m) where 100 is a conversion factor from 1/cm to 1/m.

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