system
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human
ELI5: how is sewer water treated? what's the process it goes through?

is it done mechanically or by people?
gpt
It depends a lot of where the treatment is done, but it’s generally mechanical, not people. 

Some places don’t treat at all, they just dilute it by dumping raw sewage straight into the ocean or a river. Lest you think this is just an undeveloped country thing, there are major Western cities that still do this. 

Assuming some level of treatment, it goes in roughly this order:
-Mechanical screening: removes trash, baby wipes, condoms, plastic bags, etc. Basically anything large that won’t break down. 
-Settling: sewage goes to a big pond where it slow rises and everything heavier than water sinks to the bottom and is collected by an auger or rake. The settled solids can be land filled or sent to a thing called a digester for further bacterial breakdown into, basically, dirt. 
-Filtration: run it through sand packs or some other high-surface medium to encourage bacteria to eat whatever’s left in the water, and remove most remaining particles. 
-Aeration: inject lots of air (bubbles, waterfall, etc.) to encourage bacterial growth to break down any leftover organic material. 

At this point it’s not safe to drink but it’s basically clean and you can dump it to a river/lake/ocean without much issue, or use it for irrigation. 

If you want to take it all the way to potable (safe to drink) you need to do one last step to remove any inorganic chemicals or stuff that didn’t break down. That can be distillation, reverse osmosis, ultrafiltration, etc. This is expensive so typically only happens in places with a fresh water shortage and cheap energy.