… my understanding is that the sewer system, after many stages, feeds into the drinking water supply. One of those “you don’t want to see how sausages are made” things 
Everything involving water eventually feeds into everything else involving water. The point is, with the right intermediate stages, there’s no problem.
There’s a big difference between a modern, lined landfill, and the old unlined dumps that are now banned in all 50 states. Modern landfills have an impermeable clay or geomembrane bottom liner, a leachate collection system, a leachate treatment system, and a top liner after the landfill is closed. Properly designed and maintained, nothing in a modern landfill should adversely impact the surrounding soil and water.
just seen on feature story on local tv newscast.
a plastic in ground toilet bowl. the trick is teaching your dog to use it, Lassie would be able to. not really. you empty your pooper scooper into it and flush.
I always scoop the poop and send it out with my weekly trash. The town I live in did a similar campaign a few years back because dog poop does get into sewers, water supply and our rivers. I never really thought about it since I always scooped and threw it out with the trash. I have a small white plastic bucket I line with bags and it has a snap on lid so I can scoop it a few times a week into the bucket and not attract flies. It stays contained in the landfill and out of our water.
Well, the system is designed for human waste, actually.
But dog or cat poop is pretty similar, and it’s a minuscule percentage of what the sewage treatment plant deals with, so it usually is no problem. (It’s probably a smaller percentage than the burnt or spoiled food that is tossed down the toilet, for example.)
What do you mean untreated?
The washing it into larger bodies of water dilutes it significantly. That’s the first step taken by any city sewage treatment system, too. And then it settles to the bottom of the lake (like the settling ponds in a sewage treatment system) and is filtered through the sand & rocks until it reaches the underground aquifer (like the filtration steps of a sewage treatment plant). Everything the modern sewage treatment plant does is modeled after the actions that happen in nature.
Trust me on this one–the bacteria and protozoans in the activated sludge don’t care whose poop it is that they’re eating. 
Wastewater that ends up in a nearby water body is not considered to have been treated, which of course is why we build those sewage treatment plants instead of just letting sewage flow into the nearest water body (which is what used to be done). In fact, waste washed into water bodies tends to cause problems and can spread disease, because typical urban and suburban densities can easily overwhelm the natural cleaning capacity of the water body.