olmedo,
The cleaner the water you are using, the longer you can circulate it in your coling tower. The dirtier, more of it has to be blown-down to be acceptable for a particular cooling tower you purchased, more make-up has to be pumped in the system, to not mention the hasless of disposing off the contaminated water. Also, the local chemicals you have to use will cost you more. In other words, you trade the less treated water for additional costs related to the use of untreated water. Obviously, you are the only one to judge it to suit your budget and the plant circumstances. You should consult the cooling tower fabricator for his recommandations also.
gr2vessels