I'd like to know which order of adjectives is correct in the following sentence. If both are OK, what's the difference?
Finally, the wastewater flows into the last tanks, where the chemical chlorine is added to kill the remaining bacteria. After the bacteria are destroyed, the chlorine is eliminated from the water, and the treated clean water is discharged into a river or the ocean.
From the context of the sample paragraph, 'treated' and 'clean' are equal adjectives. Either order of their appearance is hence fine. A comma, however, is needed between them.
'After the bacteria are destroyed, the chlorine is eliminated from the water, and the treated, clean water is discharged into a river or the ocean.'
Edit to answer @Smart Humanism's comments
Coordinate adjectives should be separated with commas.
Exceptions are rare. I found only one mention, in Thoughtco, which states Amy Einsohn's observation that "[t]he convention of placing a comma between coordinate adjectives seems to be fading", for various reasons.
All other sites I have checked say commas should be used.
As Micah has said English does have some conventions for ordering of adjectives.
In this case, to my ears, both orders do seem valid, the choice could be on the basis of the emphasis of the sentence. Here I think the focus is that
clean water is discharged
so I think that the author has chosen correctly in grouping clean and water.
