20-25% of the water consumed for evaporative cooling becomes contaminated waste water. It is used to clean out the residue left behind by the 75-80% of the consumed water that is evaporated.

The contaminants consist of dissolved solids and minerals that were in the incoming water, plus additives such as anti-scalers, corrosion inhibiters, and antibacterials.

How much additive gets in there? Is it more or less contaminated than normal sewage? A reasonable amount of treatment is part of the water bill.

The excess minerals don't worry me much.