Every treatment plant has to discharge to the surface somewhere; and generally it is to a creek or river, though sometimes it is into a lake or the ocean. There are treatment plants that effluent directly into the Trinity River in the DFW area for instance. Some plants recycle water by piping it to golf courses and such, but not all of the effluent can go back recycled due to the overwhelming volume.
Most plants are measured by million gallons of water per day treated/discharged (MGD). Roughly speaking 10,000 people will generate One MGD of sewage. Most large cities have multiple sewage treatment locations that treat sewage from different areas of the city. Denton's Pecan Creek waste water treatment plant (WWTP) discharges 15 million gallons per day for instance. The Dallas south side WWTP releases 50 MGD into the Trinity. Fort Worth's Village Creek WWTP releases 166 MGD into the Trinity. No doubt there are more than just these two along the Trinity.
Additionally, storm water run-off from cities also drains into nearby waters. I doubt that there is any way to calculate how much water drains from streets and ditches in the DFW area into the Trinity River. Storm water can be particularly nasty since it is never treated at all. It picks up everything, every pollutant, every drop of oil spilled, every heavy metal, every pesticide, every herbicide, every illegally disposed of chemical, and carries it straight into the river. And it's not just when it rains that it drains - there's a zillion gallons of water being sprayed on lawns that runs down the curb to the drain system every day. Find the outfall pipe and you'll see a steady stream of water even during drought.
Personally I wouldn't put my big toe in the Trinity - but that's just me. Thousands of people play in the Trinity every year with no ill effects at all, right?
Smaller towns/cities generally have poorer process and control of the waste water process than large cities due to limited small town budgets. Bigger budgets mean better controls and processes, but they also mean larger volumes, so mistakes are bigger.
The questions are; how well is it treated, how far away is the release point, how many creeks, rivers, tributaries, and lakes between you and that outfall pipe. You can get on the internet and do a search for city/town WWTPs and see where they are located and how much sewage they treat and discharge. Finding the storm drain outfalls is more difficult. If you look at a map of your area, you can bet that all of the WWTPs and storm drains are located on the downhill side of the town/city. So maybe the rule of thumb is to fish up stream from population centers, depending on your personal tolerance level. Mine is low, because I am too familiar with the WWTP problems.
Dilution is a very good thing. Dilution is King.