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The key point to remember is that just like water purification for drinking water, wastewater is usually regulated; that is, to dump wastewater back into the environment, concentrations of numerous compounds have to be below the regulated limit. This helps to protect lakes, other water bodies, the things that live in them, and those that drink from them.
Alright, so the water comes out the tap, you do your thing, and if it doesn't get poured into or onto something, it usually goes down the drain. A series of pipes in your house convey this wastewater to a septic tank, somewhere no one can see, or to a municipal wastewater treatment plant.
The simplest case is probably a septic tank in which, I believe, the waste is retained in a tank filled with bacteria. The bacteria come and multiply from your waste as long as oxygen and food (substrate) is available; there could also be anoxic or anaerobic species if there is no oxygen (I’m still confused with anoxic and anaerobic). This also gives solids a chance to settle out of the water to the bottom. After the bacteria have had their way and a significant portion of the waste has been broken down, this "treated" water is dispersed into the environment - that's what I remember hearing. Everything that settles to the bottom of the tank has to be cleaned out regularly.
The more complicated case is that of a municipal plant. It's more complicated in that the scale is much larger (hundreds, if not thousands of homes, instead of just one), and the process is more engineered. However, the basic principal is the same: use the "free" naturally occurring bacteria and use them to process the waste. If my memory serves me right, the first process of a treatment plant is simple gravity settling (i.e. let the water sit, remove whatever settles). Afterwards, the waste that doesn't settle is sent through a series of bio-reactors to remove organics, nitrogen, phosphorous, and whatever bacteria are capable of using for food and respiration.
Here are the cliff’s notes on bacteria: they eat, breath, and reproduce. To do these three things, they need an electron acceptor, electron donor, carbon and energy source, and possibly something else I can't remember - just like people.
And just like a septic tank, the bacteria have their way and then the cleaner water goes through additional treatment processes similar to water purification (i.e. potable water). This can include more gravity settling, sand or membrane filtration, an anaerobic bio-reactor, and disinfection using an agent like chlorine. If you're wondering, the idea behind anaerobic reactors is that the aerobic bacteria have had its chance, so why not give the anaerobic bacteria a shot? Also, different bacteria species eat and breathe different things; hence the aerobic/anaerobic divide. After all is said and done, the water is usually released back into the environment.
Now, think about all of these pumps, energy and material inputs, waste outputs, and the overall environmental footprint. The more water used, the more that these plants have to process, and the more of these plants that we will need.
“That's that, Mattress Man.”