I heard that our local water district was giving virtual Zoom tours of their water purification plant. Curious, I signed up. It turned out to be an hour of a couple PR guys explaining what the water district does (basically, they're responsible for ensuring water supply, wholesale distribution to local water companies, wetland environment maintenance, and flood control) and describing the plant, which was illustrated with a lot of still photos of large rooms with heavy water pipes in them and some outdoor tanks, plus a couple embedded videos outlining processes. The basics of the purification process are outlined here.
What interested me was the context in which this process operates. The plant is attached, limpet-like, to a much larger wastewater treatment plant shared by two of the district's largest cities. It siphons off a relatively small quantity of water that's already been cleansed enough for dumping into the Bay, and runs it through this treatment, ending up with what, the PR guys said, is five times purer, in terms of lack of contaminants, than our potable faucet water. But that's still not good enough to use the purified water for household use: they didn't say why, but I'm guessing that regulations are sensitive to the "ick" factor of processed wastewater.
So instead, they mix it back in with more of its own source water from the city plant, tamping the mixture down to a contaminant level that's legally clean enough for agricultural use.
Future plans, however, are to add yet another purification step that will allow them to inject the purified water into groundwater supplies, which will add yet another layer of cleansing before it's drawn out from wells. (That will require building a pipeline 15 miles uphill from the plant to the percolation ponds.) Delicate business, isn't it?