When retrieving information about UV systems for water disinfection I frequently encounter minimum intensity values.
e.g. here: Basics of UV disinfection, page 23
All those I find are given in a unit I do not understand. Frequent value for processing tap water is: $$ 40 \frac{mJ}{cm^2} $$ However a value given in $$ \frac{mJ}{cm^2} $$ is perfectly understandable for a dose of radiation hitting a surface like skin exposed to sunlight or similar. For treatment of a fluid I had expected $$ \frac{J}{m^3} $$ or something in that direction, because to damage all organisms present in a certain volume there has to be a minimum dose of radiation as far as I think. Of course, given germicidal lamps commonly are submerged in the medium they should treat, they have an envelope area surrounding them, but still there needs to be some relation to the distance the light has to travel through the fluid and the speed of the fluid passing the light sources. Can someone "enlighten" me how the values are derived and how to use them?