The basic idea is that you can have something like a can of 7-up, it looks clean, but would kill a fish dead. It's got loads of dissolved sugar, and if you dumped it into a river, microbes would go nots on the sugar, sucking out all the oxygen as they grew, and causing a boatload of other problems.

But you can't filter dissolved sugar. I mean you can but it requires a highly specialized filter that would imeaditley foul is wastewater. And we're not just talking dissolved sugar, there's a whole range of everything in there.

So we do something very simple: we let the 7up grow moldy. 7 up would go through a coffee filter, but mold wouldnt. We let the mold suck up the sugar, then seperate that.

In reality it's a bit different but that's the idea. We grow a ton of microbes, let them suck everything out, then move them to another tank where they all stick together and settle to the bottom. The result is that the liquid at the top of the tank is clear, and free of anything dissolved as the bugs ate it all. (how we then get rid of all of that bug mass is whole other thing but basically we squeeze all the water out we can and it becomes dirt).

The problem with stuff like pfoas or any other micropollutant is that the bugs aren't that interested. And they're at such a low concentration it's incredibly hard to get such a small amount out of millions of gallons every day.

Wastewater treatment was meant to mimic natural ecology so that when it was discharged we didn't overwhelm the environment. If you poop in the woods, it's probably not a problem. If everyone poops in the same area of the woods... The woods are going to look different and not in a good way. We solved that problem, but not the one of what to do when the poop is "radioactive".

One last note: what I described above is secondary settling (letting the bugs settle and taking the liquid off the top). There ARE filters to seperate the bugs from the liquid, but they are no where near selective enough for specific compounds. Again, think coffee strainer.