Overheard in an exchange about the possible use of cooling tower blowdown water for irrigation:

Blowdown water for irrigation it depends! Blowdown typically contains high concentrations of minerals precipitated out of the cooling evaporation process.

Well maintained cooling towers may provide suitable water (lower Total Dissolved Solids TDS) but incoming municipal water will affect quality as well. Of course secondary filtration to remove the TDS is an option but at a cost.

Best bet is to collect blow down with other sources such as rainwater, condensate and greywater to dilute the TDS.

This article is useful.