Wastewater treatment is not achieved through a series of chemical treatments. Instead, it’s a managed natural process in which the effluent from the sewer system is filtered, aerated and then broken down by wild microbes.
Technically it doesn’t need to be aerated, but anaerobic bacteria, which don’t need oxygen, produce much stinkier byproducts during this breakdown process, so most decent-sized urban wastewater treatment plants do aerate in order to spare the neighbors. Very simple rural wastewater plants may just pump the wastewater into a lagoon—an outdoor pond, usually with a liner of some kind—and just let whatever grows in there do so.
As the wastewater is breaking down, it separates into a layer of muck (sludge) and a layer of cleaner water. Large wastewater treatment plants have a series of stages with clearer and cleaner water emerging from each one. Small plants may have a series of lagoons or just one. The more stuff you have in your wastewater that’s not poop (say maybe you accept discharge from a local factory, or there’s a restaurant district with a lot of food waste in their greywater), the longer and harder it is to treat.
Discharge from wastewater treatment plants to natural water bodies is heavily regulated and monitored for quality. In the US, it’s regulated by the EPA, and they take it seriously. The finishing step involves testing your discharge to make sure it’s within your approved discharge limits. This is the only step where a chemical treatment is commonly added: a little chlorine, to kill off the last of your microbes. But some plants use constructed wetlands or sand filters instead. If they do apply chlorine, they also have to take it back out before releasing the water, so that they don’t upset the ecosystem the water will be released to.
(If you have wastewater that’s mostly human waste and a correctly sized plant, you shouldn’t technically need this step. The microbes should be slowly precipitating out of the water along with the sludge. But things like high volume, cold temperatures, and complex effluent can make those benchmarks hard to hit without a finishing step.)
MORE COOL STUFF ABOUT WASTEWATER:
DID YOU KNOW? Potassium, an important component in fertilizer, is actually mined out of the earth? Did you know potassium deposits are running low? DID YOU KNOW POTASSIUM IS A WASTEWATER BYPRODUCT!?
DID YOU KNOW? Some large plants can trap and clean methane from their wastewater and use it for power?
DID YOU KNOW? The precipitated sludge can be further treated and used for fertilizer? I particularly liked the plant that was using it to fertilize fast-growing trees for the paper trade.
DID YOU KNOW? Many wastewater plant operators have a protective—if sometimes frustrated—relationship with their microbes, which they call “the bugs”, and include not just bacteria but also other microorganisms like algae and daphnia. The bugs are the workhorses of the wastewater plant: if their ecosystem becomes imbalanced, everyone’s job gets harder. I doubt they’d appreciate this, but in my mind, wastewater treatment operators are microbe herders. Though I suppose thinking of them as bog technicians is also accurate.
This has been the short version of my “wastewater treatment is fricking awesome” rant. I generalized a lot but the gist is still true. You asked for something cool, behold: Wastewater treatment, first wonder of man’s interface with nature.
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