You can’t swim in aerated water and I’m sure there was aeration in progress nearby. The solids and FOG (fats, oils, and grease) present their own hazards too. I’ve been dunked a time or three myself. To say it’s unpleasant is an understatement. I fell into a wet well once, and as if bouncing off an iron pipe and smacking the surface of the grease cap weren’t bad enough, getting back up through the grease cap with two broken ribs, a concussion and the resulting disorientation were even worse. That day I decided that fall harnesses were my best friend. Sometimes a grease cap can be thick enough to walk on.

The plant that I run now produces potable quality water. It’s considered non potable because of the source and because the quality simply isn’t as reliable as something like the Sparta facility or whatever system they’d be using in SF. I’m running a low tech system that only processes a few hundred gallons per day. With a bad weather upset I could end up discharging some pretty gross stuff, then I have to get the health department involved, environmental impact studies, get in some trouble, get fines levied against my company and it could have a big negative impact on nearby communities if it goes uncorrected. For obvious reasons, I try to avoid that. My old plant processed 3 million GPD, and I think the Sparta plant is designed for a 10 million GPD max capacity and I don’t believe it’s ever had to run at max capacity. It was built with the prospect of expansion in population. Large plants are MUCH more labor and energy intensive, but also more reliable. In very simple terms, the much larger volume acts as a bit of a shock absorber when something happens. Like if some jerk decides to dump a bucket of bleach or car wash soap in one of my manholes, I’ll have to take my plant offline, have it pumped out, go get some activated sludge from a friend at another plant and start from scratch. If that happened at the Sparta, they wouldn’t even notice it.

But the whole point here is, water is recyclable, especially if we make sure our waste is “clean”, as in chemical free, trash free. Only thing that should ever go down a sewer line besides water, is the four P’s: poop, pee, puke and paper. In a closed system such as maybe a space station, the biggest obstacle I’d see right away is the equipment. You’d need a way to dewater and disinfect the waste. I’d personally advocate a dewatering press and UV system, but doing that in zero or micro G would require some extensive modifications. The most economical way with the tech we have now is to resupply fresh water and vent the waste. Someone else mentioned there’s plenty of ice floating around out there. I wonder how space ice would taste?

Our usual solution to clean anything is to dilute it. IE add more water to it so the parts per million of the dirty thing drops. If you're trying to do anything else the processes are a LOT more energy/resource intensive.

It’s about scale and expense more than anything. The more you need to clean, the more difficult and expensive it is. The source quality will make a difference too. I treat mostly ground water, it’s about the easiest and cheapest. Most expensive part is the electricity to run the pumps. Then there’s desalinization, reverse osmosis, surface water treatment too.