UV Filter System Math

[Over59]

I am putting in a 3 Stage Filteration System. 3 GPM with a 2GPM pump.
It will use excess solar power or hook up.
I will put water (Muni or Campground not surface) from the 50 gallon tank
and put it to the 25. From their the common connection brings it back to the 50Gal. Any engineer know the equation for this. How long does it need to run for 99.9% treatment. I think I could treat the problem as one 75 gal tank put still don't know how long to run it.


Thanks.