Where does the waste go, and how is it treated? How efficient is the processing?
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I can answer the centralized question in a bit more detail.
We currently separate our collected water into two categories – grey water and black water. Grey water would be “used” water that isn’t biologically dangerous, things like water from sinks, washing machines, roof drains, etc. In newer construction you might see this water getting reused in various ways, for example you can run grey water into garden irrigation or use it to flush toilets in lieu of fresh, “domestic” water. Black water is your poopy stuff that needs separate treatment. Newer construction systems divert grey water from the black water system for reasons that’ll be clear below.
You typically process black water in two ways – assuming you’re in something like a city with a central processing plant they first process it to remove the non-biological solids, the stuff you’re not supposed to be flushing in the first place, like plastic bags, condoms, tampons, cigarette butts, etc. Most blackwater treatment plants tie into storm water drains so all that street trash goes here too. So the non-poop flushed stuff gets filtered out and the poop water goes to treatment pools. They intentionally add microorganisms and oxygen to the pools to literally ferment the poop, this breaks down the material physically but also “consumes” the nutrients in it so that you don’t have a problems with the blackwater growing harmful microbes or harming the environment because that’s the next place it goes.
They typically remove as much of the remaining biological solids as they can and either throw in a dump site or maybe even process it for fertilizer or other chemical processes. The remaining liquid is in now theory basically grey water from above and you can use it for irrigation or just pump it out into the ocean/lake/river, etc.
Example 1 – The city of Philadelphia has two rivers on either side, it gets it fresh water from the left riiver, people poop it up, it gets processed and then dumped into the right side river. You need to do this carefully because the city of Camden in New Jersey get’s its fresh water from the right side river too, you need to make sure you’re not dumping your grey water directly into the line of Camden’s fresh water intake.
Problem 1 – Since the storm water system is part of the sewage system, even modest rain falls can cause tons of water to flood the treatment plants and they need to just drain them en masse into the output. This does cause massive ecological damage because the poop water can provide nutrients that cause microbial blooms down stream. I hate to say it, but when it rains in Philly, Camden literally drinks our poop water, and that’s why domestic water processing is really important too.
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