How Sewers Work (feat. Fake Poop)
Summary
TLDRThis video from Practical Engineering explores the crucial yet underappreciated role of sewer systems in maintaining public health. It delves into the complexities of designing sewage lines, considering factors like population growth, precipitation impact, and the need for self-cleaning velocities to prevent blockages. The video also addresses challenges like 'fatbergs' caused by non-flushable items and the engineering solutions like inverted siphons. It highlights the intricate balance between utility and infrastructure in urban planning.
Takeaways
- đœ Sewage collection systems are critical for public health in urban areas, ensuring the safe removal of human waste to prevent diseases.
- đ§ Designing sewer systems is complex and involves technical challenges, including the need for long-term functionality and adaptability to urban growth.
- đĄïž Sewers must be designed to handle not just average conditions but also peak flows, accounting for inflow and infiltration during precipitation events.
- đ Joseph Bazalgette's foresight in doubling the size of London's sewer tunnels highlights the importance of planning for future needs in sewer design.
- đ« Sewers carry harmful substances and must be leak-proof to prevent contamination of the environment and drinking water supplies.
- đ Understanding expected wastewater flows is crucial, using population and land use data to estimate average and peak flows.
- đ§ïž Inflow and infiltration (I&I) can overwhelm sewer systems, causing overflows and environmental issues, necessitating careful system maintenance and design.
- đ§ Sewage systems rely on gravity for waste transportation, but engineers must balance flow rates and pipe slopes to prevent damage and blockages.
- đïž The slope of sewer pipes must consider both the necessary flow velocities and the topography of the land to avoid construction and maintenance issues.
- đ Inverted siphons are used to cross rivers or creeks in sewer systems, but they require careful design to prevent solids from settling due to reduced flow velocity.
- đŻ 'Flushable' wipes and grease can cause significant problems in sewer systems, forming 'fatbergs' that disrupt the flow and require special maintenance.
Q & A
What is the primary purpose of a sewage collection system?
-The primary purpose of a sewage collection system is to safely carry away human waste and prevent threats to public health such as plague and pestilence in urban areas.
Why is the design of sanitary sewage lines more complex than it seems?
-The design of sanitary sewage lines is complex due to the need to consider factors like the disruption and cost of installation, the intertwining with other underground utilities, and the requirement for the lines to function effectively for many decades.
Why is it important for sewers to be designed for future city growth?
-Sewers must be designed for future city growth to accommodate the increase in wastewater production as the population grows, ensuring the system does not become overwhelmed and continues to function effectively.
What is the significance of Joseph Bazalgette's decision to double the size of London's sewer tunnels?
-Joseph Bazalgette's decision to double the size of London's sewer tunnels was significant because it demonstrated foresight in anticipating future needs, allowing the system to handle increased waste flow without immediate need for further expansion.
What are the challenges associated with inflow and infiltration (I&I) in sewer systems?
-Inflow and infiltration (I&I) are challenges because precipitation can enter the sewer system through various means, potentially overwhelming its capacity during storms, leading to overflows, exposure to raw sewage, and environmental problems.
How do engineers ensure that sewage collection systems rely effectively on gravity?
-Engineers ensure effective reliance on gravity by controlling the size of the sewer pipe and its slope, which influences the flow velocity and helps maintain a balance that prevents damage to the pipe and keeps solids in suspension.
What is the concept of self-cleaning velocity in sewer pipes?
-The self-cleaning velocity is the minimum speed required to keep the lines clear by preventing solids from settling out of suspension. It is typically required to be at least three feet or one meter per second in most cities.
Why do sewer engineers face challenges when designing sewers to cross natural channels like rivers?
-Sewer engineers face challenges when designing sewers to cross natural channels because it is often not cost-effective to lower the entire sewer line or increase its slope to stay below the natural channel. Instead, they may use structures like inverted siphons to address this issue.
What is an inverted siphon and how does it help in sewer design across rivers or creeks?
-An inverted siphon is a structure that allows a sewer line to dip below a natural channel like a river or creek and then rise back up on the other side. It helps maintain the flow of sewage while navigating topographic features and avoiding the need for excessive lowering of the sewer line.
How do sewer systems deal with the issue of 'fatbergs' caused by non-flushable items?
-Sewer systems deal with 'fatbergs' by promoting proper disposal of non-flushable items, using maintenance to remove blockages, and designing sewers to minimize the accumulation of such substances. Public awareness campaigns also play a role in educating people about what should not be flushed down the toilet.
What is the role of Nebula in supporting independent creators and how does it benefit viewers?
-Nebula is a streaming service built by and for independent creators, offering a platform for them to try new ideas without the constraints of advertiser-supported platforms. It benefits viewers by providing an ad-free environment with exclusive content from a variety of creators, including those featured on Practical Engineering.
Outlines
đœ The Importance of Sewage Systems
This paragraph introduces the critical role sewage systems play in public health, emphasizing their importance in urban areas. It acknowledges the constant production of waste by humans and the need for safe disposal to prevent diseases. The narrator, Grady, celebrates the engineering behind these systems, which are often overlooked despite their complexity and the technical challenges involved in their design and maintenance. The video, sponsored by Curiosity Stream and Nebula, will delve into the intricacies of sewer design, including the challenges of installation, long-term functionality, and the need to accommodate urban growth and changes.
đ Designing Sewage Collection Systems
The paragraph discusses the complexities of designing sewage collection systems, starting with the estimation of waste flow based on population and land use. It highlights the issue of inflow and infiltration (I&I), where precipitation can overwhelm the system's capacity, leading to overflows and environmental problems. The importance of gravity in moving waste through the system is underscored, along with the need to balance flow velocity to prevent damage to pipes or the settling of solids. The narrator demonstrates this with an experiment using a pipe and a slurry, showing how slope affects the flow and the need to maintain a self-cleaning velocity to prevent blockages.
đ Challenges in Sewer Pipe Design and Construction
This section delves into the specific challenges of sewer pipe design and construction, including the need to consider the slope of the ground above and the self-cleaning velocity required to keep pipes clear. It explains how the slope of a sewer pipe must be balanced to avoid being too close to the surface or too deep, which can lead to construction difficulties and costs. The paragraph also addresses the issue of crossing natural channels like rivers, where inverted siphons can be used to maintain flow below the surface. The narrator discusses the problems caused by non-traditional waste materials, such as 'flushable' wipes and grease, which can create 'fatbergs' and disrupt the sewer system's functionality.
đ Supporting Independent Creators and Educational Content
The final paragraph shifts focus to the support of independent creators and the educational content they produce. The narrator introduces Nebula, a streaming service free of ads and built for creators, allowing them to explore new ideas. Nebula is partnered with CuriosityStream, offering a bundle deal that includes access to both platforms at a discounted rate. The narrator encourages viewers to support the channel and other educational creators by taking advantage of this offer, highlighting the value of the content available on both platforms.
Mindmap
Keywords
đĄSewage Collection System
đĄPublic Health
đĄWastewater Treatment Plant
đĄInflow and Infiltration (I&I)
đĄSelf-Cleaning Velocity
đĄSlope
đĄInverted Siphon
đĄFatbergs
đĄFlushable Wipes
đĄHydraulic Grade Line
đĄCuriosityStream and Nebula
Highlights
A sewage collection system is a critical pillar of public health in urban areas, essential for safely carrying away human waste.
Sewers are designed to convert the figurative stream of waste into a literal one, flowing away from public spaces.
Designing sanitary sewage lines involves complex engineering challenges, including installation and integration with existing infrastructure.
Sewers must be designed for long-term functionality and to accommodate future urban growth and changes.
Joseph Bazalgette's foresight in doubling the size of London's sewer tunnels demonstrates the importance of planning for the future.
Sewers carry waste without leaking into the ground or contaminating nearby drinking water supplies, highlighting the high stakes of sewer design.
Estimating average and peak wastewater flows is crucial for designing sewage systems, taking into account population and land use.
Inflow and infiltration (I&I) from precipitation can overwhelm sewer systems, leading to overflows and environmental issues.
Gravity is the primary force for transporting waste in sewer systems, but it requires careful design to avoid issues with flow velocity.
The self-cleaning velocity of a sewer pipe is essential to prevent solids from settling and requires maintaining a minimum flow speed.
Sewer pipe slope must balance the necessary flow velocities with the slope of the ground above to avoid construction and maintenance challenges.
Inverted siphons are used to cross natural channels like rivers, managing the hydraulic grade line to prevent solids from settling.
Sewers must contend with non-design substances like grease and 'flushable' wipes, which can create blockages known as 'fatbergs'.
The video demonstrates the breakdown of toilet paper versus 'flushable' wipes, illustrating the challenges they pose to sewer systems.
Sewage collection systems are not magical but carefully planned and tested to manage waste effectively.
The video promotes Nebula and CuriosityStream as platforms for independent creators and educational content.
Transcripts
A sewage collection system is not only a modern convenience but one also of the Â
most critical pillars of public health in an urban area. Humans are kind of gross. Â
We collectively create a constant stream of waste that threatens city-dwellers with plague Â
and pestilence unless it is safely carried away. Sewers convert that figurative stream Â
into a literal one that flows below ground away from public view (and hopefully public smell). Â
There are a lot of technical challenges with getting so much poop from point A to point B, Â
and the fact that we do it mostly out-of-mind, I think, is cause for celebration. So, this Â
video is an ode to the grossest and probably most underappreciated pieces of public infrastructure. Â
Iâm Grady, and this is Practical Engineering. In todayâs episode, weâre talking about sewers. Â
This video is sponsored by Curiosity Stream and Nebula. More on that later.
As easy as it sounds to slap a pipe in the ground and point it toward Â
the nearest wastewater treatment plant, designing sanitary sewage lines - like Â
a lot of things in engineering - is a more complex task than you would think. Â
It is a disruptive and expensive ordeal to install subsurface pipes, especially because they are so Â
intertwined with roadways and other underground utilities. If weâre going to go to the trouble and Â
cost to install or replace them, we need to be sure that these lines will be there to stay, Â
functioning effectively for many decades. And speaking of decades, sewers need to be designed Â
not just for the present conditions, but also for the growth and changes to the city over time. Â
More people usually means more wastewater, and sewers must be sized accordingly. Joseph Â
Bazalgette, who designed Londonâs original sewer system, famously doubled the proposed sizes of Â
the tunnels, saying, âWeâre only going to do this once.â Although wantonly oversizing infrastructure Â
isnât usually the right economic decision, in that case, the upsizing was prescient. Finally, these Â
lines carry some awful stuff that we do not want leaking into the ground or, heaven forbid, into Â
the drinking water supply whose lines are almost always nearby. This all to say that the stakes Â
are pretty high for the engineers, planners, and contractors who make our sewers work.
One of the first steps of designing a sewage collection system is understanding how much Â
to expect. There are lots of published studies and guidelines for estimating average and peak Â
wastewater flows based on population and land use. But, just counting the number of Â
flushes doesnât tell the whole story. Most sanitary systems are separated from storm Â
drains which carry away rainfall and snowmelt. That doesnât mean precipitation canât make its Â
way into the sewage system, though. Inflow and infiltration (referred to in the business as I&I) Â
are the enemies of utility providers for one simple reason. Precipitation finding its way Â
into sewers through loose manholes, cracks in pipes, and other means can overwhelm Â
the capacity of the system during storms. The volume of the fabled âsuper flushâ during the Â
halftime of the Superbowl is usually a drop in the bucket compared to a big rainstorm. Â
I&I can lead to overflows which create exposure to raw sewage and environmental Â
problems. So utilities try to limit this I&I to the extent possible through system maintenance, Â
and engineers designing sewers try to take it into account when choosing the system capacity.
Once you know how much sewage to expect, then you have to design pipes to handle it. Â
Itâs often said that a civil engineerâs only concerns are gravity and friction. Iâll let Â
you take a guess at which one of those makes poop flow downhill. Itâs true that almost all sewage Â
collection systems rely mostly on gravity to do the work of collecting and transporting waste. Â
This is convenient because we donât have to pay a gravity bill - it comes entirely free. But, Â
like most free things, it comes with an asterisk, mainly that gravity only works in one direction: Â
down. This fact constrains the design and construction of modern sewer systems Â
more than any other factor, and Iâve built some demonstrations in the garage to show Â
you how. Iâm pumping a slurry of sand and water through this clear pipe which represents a sewer, Â
and weâll take a look at the factors engineers consider in designing these systems.
We need some control over the flow in a sewer pipe. It shouldnât be too fast so as to damage Â
the joints or walls of the pipe. But it canât flow too slow, or you risk solids settling out Â
of suspension and building up over time. We canât adjust gravity up or down to reach this balance, Â
and we also donât have much control over the flow of wastewater. People flush when they Â
flush. The only things engineers can control are the size of the sewer pipe and its slope. Â
Take a look at what happens when the slope is too low. The water moves too slowly and allows Â
solids to settle on the bottom. Over time, these solids build up and reduce the capacity Â
of the pipe. They can even completely clog. Pipes without enough slope require frequent Â
and costly maintenance from work crews to keep the lines clear. If I adjust the slope of the Â
line without changing the flow rate, watch what happens. The velocity of the water increases. Â
This not only allows solids to stay in suspension, but it also allows the water to scour away the Â
solids that have already settled out. The minimum speed to make sure lines stay clear Â
is known as the self-cleaning velocity, and you can see why in the demo. It can vary, Â
but most cities require that flow in a sewer pipe be at least three feet or one meter per second.
So far Iâve been using sand to simulate the typical âsolidsâ that could be found in a Â
wastewater stream. But, you might be interested to know that weâre, thankfully and by design, Â
only scratching the surface of synthetic human waste. Laboratories doing research on urban Â
sanitation, wastewater treatment, and even life support systems in space often need a safe and Â
realistic stand-in for excrement, of which there are many interesting recipes published in the Â
academic literature. Miso (or soybean) paste is one of the more popular constituents. This Â
polymer slime toy is as realistic as I want to be while keeping this video family-friendly, Â
but feel free to take your own journey down the rabbit hole of simulated sewage after this. Â
I mean that figuratively, of course.
The slope of a sewer pipe is not only constrained by the necessary range of flow velocities. Â
It also needs to consider the slope of the ground above. If the slope is too shallow compared to Â
the ground, the sewer can get too close to the surface, losing the protection of the overlying Â
soil. If the slope is too steep compared to the ground, the sewer can eventually become Â
too deep below the surface. Digging deep holes to install sewer pipes isnât impossible or anything, Â
but it is expensive. Above a certain depth, you need to lay back the slopes of the trench Â
to avoid having it collapse. In urban areas where thatâs not possible, you instead have to Â
install temporary shoring to hold the walls open during construction. You can also use trenchless Â
excavation like tunneling, but thatâs a topic for another video. This all to say that choosing a Â
slope for a sewer is a balance. Too shallow or too steep, and youâre creating extra problems. Â
Another topographic challenge faced by sewer engineers is getting across a creek or river.
It is usually not cost-effective to lower an entire sewer line or increase its slope Â
to stay below a natural channel. In these cases, we can install a structure called Â
an inverted siphon. This allows for a portion of a line to dip below a depressed topographic Â
feature like a river or creek and come back up on the other side. The hydraulic grade line, Â
which is the imaginary line representing the surface of the fluid, Â
comes up above the surface of the ground. But, the pipe contains the flow below the surface. Â
The problem with inverted siphons is that, because they flow full, Â
the velocity of the flow goes down. That means solids are more likely to settle out, something Â
that is especially challenging on a structure with limited access for maintenance. This is similar Â
to the p- or u-trap below your sink, that spot where everything seems to get stuck. Notice how, Â
even though the pipe is the same size along the full length, settling is only happening within Â
the siphon. To combat this issue, inverted siphons often split the flow into multiple smaller pipes. Â
This helps to keep the velocity up above the self-cleaning limit. A smaller pipe obviously Â
means a lower capacity, which is partly why siphons often include two or three. You can see Â
that, even though thereâs some settling happening, itâs not increasing over time. The velocity of the Â
flow in the smaller siphons is high enough to keep most of the solids in suspension.
The volume and hydraulics of wastewater flow arenât the only challenges engineers face. Â
Sewers are lawless places, by nature. There are no wastewater police monitoring what you flush Â
down the toilet, thank goodness. However, that means sewers often end up conveying (or at least Â
trying to convey) substances and objects for which they were not designed. Â
For a long time, grease and oil were the most egregious of these interlopers Â
since they congeal at room temperatures. However, the rising popularity of quote-unquote Â
âflushableâ wipes has only made things worse. Grease and fat combine with wet wipes in sewers Â
to create unsettling but aptly named, âfatbergs,â disgusting conglomerates that, among other things, Â
are not easily conveyed through sanitary sewer lines. Just to illustrate the issue, this is how Â
quickly toilet paper breaks down when agitated in a mixer. And this is a wet wipe labeled flushable. Â
You can imagine the problems this would cause. Conveniently, Â
most places in the world have services available to carry away your solid Â
wastes so you donât have to flush them. But they usually do it in trucks - not pipes.
Obviously, this issue is more complicated than my little experiment. The labeling of wipes has Â
turned into a controversy that is too complex to get into here. My point though, and indeed Â
the point of this whole video, is that your friendly neighborhood sewage collection system Â
is not a magical place where gross stuff goes to disappear. It is a carefully-planned, Â
thoroughly tested system designed to keep the stuff we donât want to see - unseen. What happens Â
to your flush once it reaches a wastewater treatment plant is a topic for another video, Â
but I think the real treasure is the friends - sewers - it meets along the way.
If youâre here at the end of the video, Iâm guessing that means youâre pretty thoughtful about Â
the kind of videos you spend your time enjoying. In other words, you probably prefer learning new Â
things about the world more than run-of-the-mill television programming. You probably also donât Â
enjoy watching ads like this one, which is great, because Nebula doesnât have any. Nebula is a Â
streaming service built by and for independent creators like MinutePhysics, Real Engineering, Â
Wendover Productions, and a bunch of others (including me). Itâs a way for us to try new Â
ideas that might not work on advertiser-supported platforms like YouTube. My videos go live there Â
the day before they publish here, with no ads or sponsorships. And, weâre super excited to Â
be partnered with CuriosityStream, a service with thousands of documentaries and non-fiction titles Â
on pretty much every subject you can imagine. CuriosityStream loves independent creators and Â
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