how is sewer water treated? what’s the process it goes through?

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is it done mechanically or by people?

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It goes through a series of filters. Larges ones then incrementally smaller ones to catch various size particles. From there chemicals are added. Some are added to break the surface tension and any suspended material settles to the bottom. Then stuff like chlorine to kill any bacteria. I’m sure there are some detailed videos on YT

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.

Wastewater is treated by physical, chemical, and biological means.

A standard wastewater treatment plant begins with screening. This removes large items – everything from diapers to wet wipes to milk cartons – from the water.

Grit removal (settling out though gravity) removes dense things that don’t belong in further treatment, like gravel and rocks.

Historically, wastewater treatment plants have done primary sedimentation – settling out of the more solid parts of human waste. A coagulant, or chemical that causes larger particles to clump together, is usually added before this step. Nowadays this step is followed by biological treatment and secondary sedimentation (having microbes eat as much waste as possible, then settling them out of the water). This typically involved aeration, or adding small bubbles of air or even pure O2 to the water to keep aerobic bacteria growing.

This is followed by filtration, generally, to remove smaller particles.

Finally, a chemical disinfectant like chlorine is added to the water – and then removed, because disinfectants can harm wildlife. Other disenfection methods include UV light and ozone – since these only affect water when in contact with it, it’s better than having to add and then remove disinfectant. Just know that disenfection is a combination of method or chemical strength and contact time needed to kill or prevent cells from reproducing.

There are a few other processes wastewater can go through – for example, some plants have a third main step (“tertiary treatment”) designed to add more air to the water.

What’s allowed in the water when it leaves the wastewater treatment plant is based on local and federal regulations, ensuring that the plant effluent is clean enough to go into the environment.

I have a sewage treatment plant for my house that can process 500 gallons per day. It has three main chambers and a fourth, separate, collection chamber.

The first chamber is anaerobic – the really nasty, harmful to humans, smelly, awful bacteria and fungi are kings in this chamber. These basically eat/digest everything. This is one of the two Chambers that needs to be pumped for maintenance every decade or so, because everything that can’t be digested easily sinks to the bottom and just stays there. For instance, plastics, should we accidentally flush some.

The second chamber has an air pump, and aerobic bacteria will outcompete the anaerobic in an oxygen dense environment. This section is basically making alcohol and vinegar with everything that is left in the sewage at this point. This part is usually not pumped out, as the particulates are light enough that they fall into the third chamber.

The third chamber is the clarification chamber. The water here is quite still and the particulates left in the effluent fall out of solution and collect on the bottom. This chamber also needs a once a decade pump out. From this chamber, near the top, the effluent spills into a pipe and goes to a large (100 gallon?) Tank with a pump in it. When that tank is full, the pump moves all of the effluent out. On my property it used to spray over an area the size of a suburban back yard, but I’ve changed to a french drain, downhill, since I ran over the pipe with my tractor. Whoops.

From my wife, a chemist at a waste water treatment plant that took ELI5 way too seriously:
First thing that happens the waste water goes through screens which takes out material people shouldn’t flush and sends the water into tanks. There are bugs that grow in tanks of wastewater that eat the yucky poopy pollution. After bugs are full the water goes in different tanks and the bugs are separated from the water, some bugs go back into first tank and rest are thrown away. Then just enough strong bleach is added to kill extra germs before the water is put back into environment.

We first try to separate the raw waste into liquids and solids. Incoming wastewater is typicaly 99+% water. We slowly settle out the ~1% solids using high-tech gravity in large tanks called clarifiers. From there, the solids, which are still 90+% water, go on to even larger tanks where bacteria are bred to break down much of the material into gasses. Mainly methane, which we burn for process heat and electricity. After a few weeks, the remaining sludge is sent off to a mechanism that squeezes even more of the water out. The “cake” that produces is sold to farmers to fertilize non-food crops. The squeezed water is returned to the beginning of the process.

The liquid stream previously separated in the clarifiers goes on to reaaaaly big tanks called aeration basins. Here, air is injected to encourage specific bugs that break your pee and harmful pathogens down into less harmful things like nitrogen gas. These bugs are the true workforce of most modern plants- mine has literal tons of them for every human worker.

From the basins, the liquid stream is clarified again. Solids(mostly bugs) go back to the basins to work some more. Liquids go on to filters. At this point, you can expect the liquid to look almost like tap water. It’s not done, though. We still have…

Disinfection. We inject the water with chlorine gas and push it through concrete mazes. Chemistry happens. 20 minutes later the water is tested for residual chlorine. If the result is high enough, we can be confident most of the nasty bits have been destroyed. Then we inject sulfur gas into the water. More chemistry happens. The sulfur reacts with the remaining chlorine, and the water is clean and safe for the fishes in the river we release it into.