How do waste treatment plants filter and get rid of the sewage, where does it go? and do they return the filtered water back into the water supply?
In: Engineering
Depends on the plant.
When I was in high school in Utah, one class had us take a field trip to the wastewater treatment plant. That plant strained out all the solids, put some basic sanitizing chemicals into the water, and then let it flow out into natural water. They explicitly told us they don’t turn the water back into drinking water there, they just give it back to nature and a separate plant collects and treats drinking water.
However, there are sewage treatment plants that turn wastewater into drinking water using processes like reverse osmosis. These are more common in places with very dry climates like Australia.
Most sewage treatment plants use 2 stages to treat the sewage:
1. Primary treatment – getting rid of non-sewage components which can get mixed up with the sewage – grit and other heavy stuff for one, and floating stuff like sticks, plastic, fats/ grease for the other.
2. Secondary treatment – usually done in trickling filters or activated sludge bioreactors, followed by sedimentation tanks. The sewage is broken down by bacteria, which form a sludge, which gets removed in the sedimentation tank. (The sludge is also further treated in an anaerobic digester before being disposed).
The treated sewage is far less polluting than the raw sewage, and is usually released into the environment – into a river or lake.
Some plants go one or two steps further.
3. Tertiary treatment can take the form of lagoons, where the treated sewage meanders for a long time, effectively becoming clean water, before being released into the environment.
Alternatively, the treated sewage can be “polished” to become drinking water again, by aerating, followed by
a. Filtering and chlorinating or
b. Reverse osmosis.
First you filter out large solid objects and then the dirty water gets processed with various method to filter out the waste components of the water. Bascially if you let sit the water for a while, some of the waste will flow to the bottom, and the water abvoe gets cleaner. Then you can use microbes to convert the waste to some other form, which either flows away as gas or also goes to the ground or something. Then you might have some chemical treatment and in the end you have some more or less clean water, which you can give into a river or similar.
You also get mud, which contains basically the waste. This mud can be burned or converted to biogas, or used as fertilizer in farming.
Ooh I’ve had this job. This was for a chicken processing plant.
So, all the water that is used in the plant got sent over to my building through a huge zig zagging tube that fluctuates in different widths. As the water is zig zagging through, two different types of polymers(they just said positive or negative on the bags and would be mixed into the appropriate tank) are injected into the zigzagging tube and binds to the solids. The water then goes through a skimmer. Which is exactly as it sounds. It’s a giant conveyor that skims the binded polymers and solid off the top of the water. The skimmed off material is collected in a giant stack and then used for fertilizer in crop fields. Back with the water, the flow is then sent through many micron filters to collect any small pieces of bone or other gross chicken parts. After that, the water is transferred to our giant stacks(imagine a huge 1,000,000 gallon cup) where it is treated with bugs(bacteria) and heat. The bacteria eats away at the smaller bits of solids and the heat encourages growth of the bacteria(we did bug checks everyday and depending on how many were in a sample, we could add more bugs if needed.). Eventually the still dirty water is sent to the city’s water treatment plant. There are periodical checks throughout a shift to ensure that the polymers and bugs are doing their job. If our water comes out too dirty and is sent to the city’s treatment plant, we would get fined.
Been many, many years since that job but that’s what I remember.
Some water treatment plants allow for educational tours. If you’re really curious, it wouldn’t hurt to see if any by you do them.
For the second part of the question, most times, the treated water is put into a lake or river. The Plant converts sewage into river water. The water is tested to make sure it is of good enough quality to be put into the river.
Imagine a giant strainer and washing machine for dirty water. The treatment plant uses big screens to catch solid stuff, then tiny helpers (bacteria) eat the yucky bits. The water goes through more cleaning steps, like adding stuff to make dirt clump together and sink. Finally, it’s disinfected to kill germs. The clean water goes back to rivers or lakes, while the leftover gunk (sludge) is treated separately and often used as fertilizer or burned for energy. It’s like giving water a spa day before sending it back into nature
Practical Engineering has several series on modern infrastructure. here is his playlist on sewage specifically [https://www.youtube.com/playlist?list=PLTZM4MrZKfW8jvCLy9K0UzZb2C6yxU3p8](https://www.youtube.com/playlist?list=PLTZM4MrZKfW8jvCLy9K0UzZb2C6yxU3p8)
his videos on infrastructure that we use every day is very interesting and worth the watch to get a base level understanding of all the things that help make modern life possible
Generally:
The plant influent goes to giant settling tanks. The heavy, gross bits gravity out. The floaty bits get skimmed off the top. The not-water parts go to giant tanks where they are partially broken down into methane, CO2, and less dangerous chemicals over a few weeks. Those solids get cooked into relatively stable fertilizers, or landfilled.
The liquid stream goes into even bigger tanks, wherein specially adapted microorganisms break down target nutrients into, again, less dangerous chemicals. Ammonia breaks down mostly into nitrogen gas and bubbles back out into the air, for example. That liquid stream gets gravity settled again, the solids get recycled, and the liquids go on to be filtered and decontaminated. After the decontamination chemicals are canceled out, with more chemicals, the water gets released to either an existing body of water or further processed to make drinking water.
In general, non compostable trash (wipes, tampon applicarors, syringes, etc) gets screened out of the river then the stream is sent to settling tanks. Solids like suspended mud or sand sink to the bottom and is removed.
The other suspended sewage gets sent to a digester talk where microbes and bacteria eat the sewage. Chemicals are added so the bacteria and remains are settled out of the effluent.
The remaining water is treated with chlorine and other chemicals to make the water safe for the environment. The water is then either released into the environment (lake, stream or ocean) or sent to injection wells to eventually make it’s way to the water table and reuse.
Sometimes, the reclaimed (treated) water is used for agriculture or landscaping uses in nearby communities. That saves the expensive, treated drinking water for drinking.
Some plants are installing secondary processes to capture methane and other greenhouse gasses to generate electricity and reduce smells in surrounding communities.
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