There are some instances of overt failure-to-treat; but most historical wastewater infrastructure (and some current) is biased heavily toward some flocculation and settling to get the solids, along with the appropriate conditions to encourage decomposition of biological material, and a shift in the microbial balance toward the non-pathogenic decomposers and away from parasites and pathogens.
Unless you are willing to get out the checkbook, in a serious way, “treatment plant” doesn’t imply some sort of broad-spectrum filtration, unless specially fitted for chemical plant wastewater or the like. It’s mostly about solving the old ‘drinking your own shit makes you sick’ problem. That’s a major part of the concern about ‘unconventional’ waste (certain plastics, pharmaceuticals and metabolites, heavy metals and persistent toxins, that sort of thing) that largely survive conventional processing unchanged.
