How to Clean Sewage with Gravity

The science of sedimentation and the first step of wastewater treatment.

We often use chemicals, filters, and even gigantic colonies of bacteria to clean sewage on such a massive scale, but the first line of defense in the fight against dirty water is usually just gravity.

Errata: (1) At 6:16, the header should say "ALL PARTICLES SETTLE WHEN tD IS LESS THAN tL."

Practical Engineering is a YouTube channel about infrastructure and the human-made world around us. It is hosted, written, and produced by Grady Hillhouse. We have new videos posted regularly, so please subscribe for updates. If you enjoyed the video, hit that ‘like’ button, give us a comment, or watch another of our videos!

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This is not engineering advice. Everything here is for informational and entertainment purposes only. Contact an engineer licensed to practice in your area if you need professional advice or services. All non-licensed clips used for fair use commentary, criticism, and educational purposes.

Practical Engineering receives cash compensation from Wealthfront Advisers LLC (“Wealthfront Advisers”) for sponsored advertising materials. Practical Engineering is not a client and this is a paid endorsement. Practical Engineering and Wealthfront Advisers are not associated with one another and have no formal relationship outside of this arrangement. Nothing in this communication should be construed as a solicitation, offer, or recommendation, to buy or sell any security. Any links provided by Practical Engineering are not intended to imply that Wealthfront Advisers or its affiliates endorses, sponsors, promotes and/or is affiliated with the owners of or participants in those sites, or endorses any information contained on those sites, unless expressly stated otherwise. Investment management and advisory services are provided by Wealthfront, an SEC registered investment adviser. All investing involves risk, including the possible loss of money you invest, and past performance does not guarantee future performance.

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This video is sponsored by Wealthfront.
Stock video and imagery provided by Getty Images, Shutterstock, Pond5, and Videoblocks.
Tonic and Energy by Elexive is licensed under a Creative Commons Attribution License
Producer/Writer/Host: Grady Hillhouse
Editor/Production Assistant: Wesley Crump
Script Editor: Ralph Crewe

[crabmansteve]

The small scale demonstrations are what set this channel apart from the rest in the beginning. I hope they never go away. I really appreciate the work.

[billiondollardan]

When I was a teenager I seriously considered going into wastewater management. I visited to treatment plants and I found the process pretty darn interesting. In the end I picked a job that would pay better, but I sure love clean water

[Gitau_Kenn]

As an Environmental Engineer, it's amazing that wastewater treatment is getting the exposure that it deserves. This can go a long way into shaping people's opinion in processes that are crucial in our lives but are oblivious to.

[a.i.]

My father in law was a pipefitter in Chicago so we got to go to the grand opening of Stickney Treatment plant. They showed and explained the different steps and processes used to separate things then put us on a sludge train. At the end of the line they has a HUGE field/ train yard that the sludge was dump into, then turned several times like compose. They would sell this as fertilizer.

[ThePercent]

Keep these public works videos coming! I manage a small municipal electric, water, and sewer utility and these videos help me better understand some of the fundamentals that our crews work with on a daily basis.

[calholli]

"A moment of tranquility" Being used to describe sewage settling to the bottom is just not the words I would have used.

[RainyDayBricks]

My three little boys ( ages 7, 4, and 1) always come running for these waste water videos :) They are fascinated by our small town's sewage treatment plant and are so very disappointed that they don't give tours. These videos have been the next best thing. Thank you for another excellent video.

[TheWicho]

I'm a California Grade IV wastewater treatment plant operator and I take pride in what I do. Job security for sure. Excellent presentation you gave. Thank you

[dsnodgrass]

In my specific field we have a wastewater pretreatment process called "metals precipitation" which uses hydrated lime and flocculant to "settle" and separate the metal contamination out so we can release clean water. It also uses a clarifier and at least one "polishing tank" to allow the sludge to descend, and a clear water layer to form and flow out of the plant into the sewer system.

[hassleoffa]

As a student engineer in Toronto, I did a work term co-op at the Queen Street sewage treatment plant. 32 years later I can still smell it when I think about those days. It was a huge factor in choosing to focus on Structural Engineering.

[wolfielee]

As a non engineer working in the civil engineering world, these videos have been incredibly helpful in understanding not just the process flow but the math behind the processes I see in my day to day.

[ROMAQHICKS]

I work for a municipality in the US and it is amazing how fast water treatment plants can clean water. One of the plants we have can take sewage and turn it into water clean enough to discharge in only about 24 hours; it really surprised me when I was discussing it with our Public Works department. An treatment operator is a good job, I consistently see openings for these positions. Urban areas and water treatment needs are only going to grow.

[basantprasadsgarden]

Seeing the water treatment plant in my city, I use to wonder, what those circular pools are
Thanks to Practical Engineering for ending my doubt.

[rob]

Don't forget belt presses Grady! Thanks for doing this series! My dad was a wastewater plant operator when I was growing up, it was a lot more interesting than people would think, especially when the state would audit them because their lab results showed the effluent exceeded state drinking water standards. . . And then the audit would reveal they WEREN'T falsifying their results! 🤣

[noahlarson]

I learned engines by working with my dad at a waste water treatment plant. They had 4 big natural gas Cat engines (that we maintained) that turned the blowers to aerated the sewage as it entered the plant. I learned a lot, not only from dad, but from the engineers, operators and chemists at the plant. Really, really cool!

[erat]

Taking extremely complex systems and simplifying them is something that you do best with these videos. You did a dang good job explaining the basics of WWT in a 11 minute video. This video could be 30 hours long with how complicated and elaborate water treatment can be.

[TeddyGun]

Dear Grady, great to hear such a complicated topic presented so easily.
I am working in the wwtp field with newest technologies such as electro dialysis, ultrafiltration or reverse osmosis.
In case you need contacts to a full scale plant almost everywhere in the world, to shoot videos or do interviews, don't hesitate to drop a comment and I can forward you to some colleagues.
Cheers

[tokencivilian]

GF works at one of the Seattle area sewage plants, on the solids side of things. "Polymer" is a big part of getting the solids to separate from the liquids. Once settled out, the solids go to enclosed digesters where the "material" is more or less composted in an anaerobic process. The resulting methane is used to help provide process heat for elsewhere in the plant. Once sufficiently digested, the solids are dewatered (the watery "mud" first goes on a separating belt to let liquid drip out, then into centrifuges to spin out more water) to the consistency of wet soil and shipped to eastern Washington for disposal, er, I mean application as fertilizer on agricultural land. Seattle area folks - if you've ever seen double dump trucks on I-90 with "Loop - turn your dirt around" you now know that is what's left of Seattle's poo.

[bloqk]

I love the concept when someone presents a lesson where they explain to have gravity work in your favor.
An example of having gravity working in your favor is with removing a heavy object from a box. Instead of straining to lift the item out of the box, a person should tip the box on its end so that the lightweight box is lifted-up while the heavy object remains on the floor. That way you are having gravity working in your favor to remove the box from the heavy object.

[karlangerer]

Awesome job! This episode hit home. I was born and raised in Chicago and now live near the Northside water reclamation plant in Skokie IL.
The plant is connected to Stikney by 16 miles of pipe. The Skokie plant sends the primary and secondary settling tank sludge to Stikney to be digested and dewatered. The sludge is composted and used as fertilizer.
My wife has covered our front yard with the fertilized and is planting native plants in place of our lawn.
I guess you could say that my yard is the final step in the settling tank process. :)