I had a game in which there was temple for a local god. One of the practices of the temple was to make sure that the river that flowed through the town was clean. They managed this by doing what the Romans did, with public bathrooms. All of the plumbing comes to a spot at the temple where they have clerics casting purify food and drink on the sewage.
The description of the purify food and drink spell says:
All nonmagical food and drink within a 5-foot-radius sphere centered on a point of your choice within range is purified and rendered free of poison and disease.
Regardless of why they're using this, would Purify Food and Drink (by RAW) actually clear the sewage from the water? Or would their efforts be in vain and it would still be nasty sewage water, unfit for drinking? (Note: They have as many clerics as is necessary.)