The quality requirements for cooling water used in a cooling tower are specified by the cooling tower manufacturers. They tend to use their own specification and not International standards. They also tend to ask for the composition of your available water and design the cooling tower for that particular quality of water, ignoring again the International standards. By the way, I have never heard of an International standard or the like to specify the minimum requirements for the cooling tower water, perhaps someone would know better then me...
cheers,
gr2vessels