zedprime posted:

If you pick the right geologic features, simply putting it into the aquifer is doing 90% of the treating. Physical filtration does most of what you need and you can get a lot of that from porous rock and/or aggregate strata. So all that remains after pumping it out is something like an activated charcoal filter and a microorganism treatment.

Not sure how prevalent its become but some of the water jurisdictions in California were treating sewer water to be near as drinkable as you can get, but its otherwise discouraged to circulate that straight back into the drinking water loop. So they would pump it into a a low residence time aquifer and pump out "fresh water" out the other end and really the aquifer did the last bit of polishing needed to turn the poop water fully clean and the drinking treatment was easier than dealing with dregs of the Colorado the farmers leave the cities.