1
$\begingroup$

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?

Improve this question
$\endgroup$

2 Answers 2

Reset to default
1
$\begingroup$

The formula shown is: UV Dose = UV Intensity x Retention Time

Intensity is power per surface area ($W/cm^2$) and retention time is time (seconds). In other words how long the medium is exposed multiplied by the intensity of the UV it is exposed to.

And energy (Joules) is power (Watts) * time (seconds).

Improve this answer
$\endgroup$
1
  • $\begingroup$ That's true, but that doesn't take into account how deep the layer of water is, which the UV light has to pass. Won't it be absorbed with increasing distance to the light source? And isn't it typical that radiation density is a function of distance from the light source (tubular)? $\endgroup$
    – Ariser
    Jun 2, 2022 at 18:11
1
$\begingroup$

Re: surface are vs volume -- the absorbance will depend on the liquid you are sterilizing (turbidity, the absorbance spectrum, thickness, the extinction coefficient of the chemicals in your liquid etc). The manufacturer has no information on what you are sterilizing, and thus can only provide the irradiation intensity before it hits your sample (power per surface area -- same units as used in lasers, LEDs etc). It is assumed that the air between the source and your sample does not absorb.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.