We have used treated industrial wastewater on several projects in the past. Many industrial wastewaters are easily "cleaned" up to make suitable cooling tower makeup water. Another good source is RO reject, especially if the supply water to the RO is softened. POTW effluent is also a potential source but will require additional biological control measures.
When the heat removal demand is sufficiently large (100 MW and above), some cities may have treated effluent available from the POTW, even piped into the locale of the plant. Sometimes this water may already have been spoken for to the extent allowed by the city permit (requiring them to continue to release X amount of water per day to streams, or rivers. Alternative sourcing really depends on site location, availability of water from the start, permitting, and a sundry of other legal and environmental issues which could come into play.
Depending upon the water source. For instance, in Aerospace industry we recycle the treat process wastewater back to the same rinses tanks at 1 megohm water quality. However, in the semiconductor industry with more sensitive manufacturing processes, process engineers would not like to see recycled wastewater. In these cases, we recycle the treated wastewater to their cooling tower.