Wanna know the difference between barbarism and civilization? Plumbing.
Specifically, a system for cleaning the waste out of your activities (whatever they are). That means, at a minimum, removing sewage. Ideally the sewage is treated to recapture anything useful and ensure whatever is dumped is safe for the (your) environment.
- Pre-treatment: obvious stuff like tires and mobsters in cement are screened out.
- Primary: let the sewage sit so that things separate by density.
- Secondary: remove the dissolved and suspended biological matter with microbes (might have to then remove the microbes).
- Tertiary: anything else that might be necessary to render the fluid safe for dumping/use.
People are nasty, however, pretty much everything in their sewage can be treated with some sort of natural system.
If you thought people were nasty, they don't hold a candle to industry. The Primary process is the same for removing really dense things and really not-dense things. Depending on what's in the wastewater, it can required several cycles through each process. For example, neutralizing acids and bases often creates a precipitate that has to go back to the Primary stage for removal.
This is a whole 'nother ball game. Getting water up to the level where people an just drink it straight without fear of biological or chemical side-effects takes real work.