Depends on the amount of treatment steps it undergoes.

Standard procedure is aimed at just removing solid debris and organic matter, to return clarified and chemically balanced water to nature, with no excess nutrients that could feed algae in water streams.

From that point forward, it is just a question of how far the treatment can be taken.

For reuse for cleaning, washing, etc? Maybe it just gets a minute dosage of sodium hypocloride.

Highly sensible areas, like beaches or lakes? UV treatment, maybe followed by micro filtering. Extreme scenarios? Reverse osmosis.

If the protocols in place are strong, it's safe.