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
- 🚽 Sewage collection systems are critical for public health in urban areas, ensuring the safe removal of human waste to prevent diseases.
- 🔧 Designing sewer systems is complex and involves technical challenges, including the need for long-term functionality and adaptability to urban growth.
- 🌡️ Sewers must be designed to handle not just average conditions but also peak flows, accounting for inflow and infiltration during precipitation events.
- 📏 Joseph Bazalgette's foresight in doubling the size of London's sewer tunnels highlights the importance of planning for future needs in sewer design.
- 🚫 Sewers carry harmful substances and must be leak-proof to prevent contamination of the environment and drinking water supplies.
- 📊 Understanding expected wastewater flows is crucial, using population and land use data to estimate average and peak flows.
- 🌧️ Inflow and infiltration (I&I) can overwhelm sewer systems, causing overflows and environmental issues, necessitating careful system maintenance and design.
- 💧 Sewage systems rely on gravity for waste transportation, but engineers must balance flow rates and pipe slopes to prevent damage and blockages.
- 🏗️ The slope of sewer pipes must consider both the necessary flow velocities and the topography of the land to avoid construction and maintenance issues.
- 🌊 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.
- 🚯 '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
🚽 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.
🔍 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.
🛠 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.
🌐 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
💡Sewage Collection System
💡Public Health
💡Wastewater Treatment Plant
💡Inflow and Infiltration (I&I)
💡Self-Cleaning Velocity
💡Slope
💡Inverted Siphon
💡Fatbergs
💡Flushable Wipes
💡Hydraulic Grade Line
💡CuriosityStream 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
most critical pillars of public health in an urban area. Humans are kind of gross.
We collectively create a constant stream of waste that threatens city-dwellers with plague
and pestilence unless it is safely carried away. Sewers convert that figurative stream
into a literal one that flows below ground away from public view (and hopefully public smell).
There are a lot of technical challenges with getting so much poop from point A to point B,
and the fact that we do it mostly out-of-mind, I think, is cause for celebration. So, this
video is an ode to the grossest and probably most underappreciated pieces of public infrastructure.
I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking about sewers.
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
the nearest wastewater treatment plant, designing sanitary sewage lines - like
a lot of things in engineering - is a more complex task than you would think.
It is a disruptive and expensive ordeal to install subsurface pipes, especially because they are so
intertwined with roadways and other underground utilities. If we’re going to go to the trouble and
cost to install or replace them, we need to be sure that these lines will be there to stay,
functioning effectively for many decades. And speaking of decades, sewers need to be designed
not just for the present conditions, but also for the growth and changes to the city over time.
More people usually means more wastewater, and sewers must be sized accordingly. Joseph
Bazalgette, who designed London’s original sewer system, famously doubled the proposed sizes of
the tunnels, saying, “We’re only going to do this once.” Although wantonly oversizing infrastructure
isn’t usually the right economic decision, in that case, the upsizing was prescient. Finally, these
lines carry some awful stuff that we do not want leaking into the ground or, heaven forbid, into
the drinking water supply whose lines are almost always nearby. This all to say that the stakes
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
to expect. There are lots of published studies and guidelines for estimating average and peak
wastewater flows based on population and land use. But, just counting the number of
flushes doesn’t tell the whole story. Most sanitary systems are separated from storm
drains which carry away rainfall and snowmelt. That doesn’t mean precipitation can’t make its
way into the sewage system, though. Inflow and infiltration (referred to in the business as I&I)
are the enemies of utility providers for one simple reason. Precipitation finding its way
into sewers through loose manholes, cracks in pipes, and other means can overwhelm
the capacity of the system during storms. The volume of the fabled “super flush” during the
halftime of the Superbowl is usually a drop in the bucket compared to a big rainstorm.
I&I can lead to overflows which create exposure to raw sewage and environmental
problems. So utilities try to limit this I&I to the extent possible through system maintenance,
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.
It’s often said that a civil engineer’s only concerns are gravity and friction. I’ll let
you take a guess at which one of those makes poop flow downhill. It’s true that almost all sewage
collection systems rely mostly on gravity to do the work of collecting and transporting waste.
This is convenient because we don’t have to pay a gravity bill - it comes entirely free. But,
like most free things, it comes with an asterisk, mainly that gravity only works in one direction:
down. This fact constrains the design and construction of modern sewer systems
more than any other factor, and I’ve built some demonstrations in the garage to show
you how. I’m pumping a slurry of sand and water through this clear pipe which represents a sewer,
and we’ll take a look at the factors engineers consider in designing these systems.
We need some control over the flow in a sewer pipe. It shouldn’t be too fast so as to damage
the joints or walls of the pipe. But it can’t flow too slow, or you risk solids settling out
of suspension and building up over time. We can’t adjust gravity up or down to reach this balance,
and we also don’t have much control over the flow of wastewater. People flush when they
flush. The only things engineers can control are the size of the sewer pipe and its slope.
Take a look at what happens when the slope is too low. The water moves too slowly and allows
solids to settle on the bottom. Over time, these solids build up and reduce the capacity
of the pipe. They can even completely clog. Pipes without enough slope require frequent
and costly maintenance from work crews to keep the lines clear. If I adjust the slope of the
line without changing the flow rate, watch what happens. The velocity of the water increases.
This not only allows solids to stay in suspension, but it also allows the water to scour away the
solids that have already settled out. The minimum speed to make sure lines stay clear
is known as the self-cleaning velocity, and you can see why in the demo. It can vary,
but most cities require that flow in a sewer pipe be at least three feet or one meter per second.
So far I’ve been using sand to simulate the typical “solids” that could be found in a
wastewater stream. But, you might be interested to know that we’re, thankfully and by design,
only scratching the surface of synthetic human waste. Laboratories doing research on urban
sanitation, wastewater treatment, and even life support systems in space often need a safe and
realistic stand-in for excrement, of which there are many interesting recipes published in the
academic literature. Miso (or soybean) paste is one of the more popular constituents. This
polymer slime toy is as realistic as I want to be while keeping this video family-friendly,
but feel free to take your own journey down the rabbit hole of simulated sewage after this.
I mean that figuratively, of course.
The slope of a sewer pipe is not only constrained by the necessary range of flow velocities.
It also needs to consider the slope of the ground above. If the slope is too shallow compared to
the ground, the sewer can get too close to the surface, losing the protection of the overlying
soil. If the slope is too steep compared to the ground, the sewer can eventually become
too deep below the surface. Digging deep holes to install sewer pipes isn’t impossible or anything,
but it is expensive. Above a certain depth, you need to lay back the slopes of the trench
to avoid having it collapse. In urban areas where that’s not possible, you instead have to
install temporary shoring to hold the walls open during construction. You can also use trenchless
excavation like tunneling, but that’s a topic for another video. This all to say that choosing a
slope for a sewer is a balance. Too shallow or too steep, and you’re creating extra problems.
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
to stay below a natural channel. In these cases, we can install a structure called
an inverted siphon. This allows for a portion of a line to dip below a depressed topographic
feature like a river or creek and come back up on the other side. The hydraulic grade line,
which is the imaginary line representing the surface of the fluid,
comes up above the surface of the ground. But, the pipe contains the flow below the surface.
The problem with inverted siphons is that, because they flow full,
the velocity of the flow goes down. That means solids are more likely to settle out, something
that is especially challenging on a structure with limited access for maintenance. This is similar
to the p- or u-trap below your sink, that spot where everything seems to get stuck. Notice how,
even though the pipe is the same size along the full length, settling is only happening within
the siphon. To combat this issue, inverted siphons often split the flow into multiple smaller pipes.
This helps to keep the velocity up above the self-cleaning limit. A smaller pipe obviously
means a lower capacity, which is partly why siphons often include two or three. You can see
that, even though there’s some settling happening, it’s not increasing over time. The velocity of the
flow in the smaller siphons is high enough to keep most of the solids in suspension.
The volume and hydraulics of wastewater flow aren’t the only challenges engineers face.
Sewers are lawless places, by nature. There are no wastewater police monitoring what you flush
down the toilet, thank goodness. However, that means sewers often end up conveying (or at least
trying to convey) substances and objects for which they were not designed.
For a long time, grease and oil were the most egregious of these interlopers
since they congeal at room temperatures. However, the rising popularity of quote-unquote
“flushable” wipes has only made things worse. Grease and fat combine with wet wipes in sewers
to create unsettling but aptly named, “fatbergs,” disgusting conglomerates that, among other things,
are not easily conveyed through sanitary sewer lines. Just to illustrate the issue, this is how
quickly toilet paper breaks down when agitated in a mixer. And this is a wet wipe labeled flushable.
You can imagine the problems this would cause. Conveniently,
most places in the world have services available to carry away your solid
wastes so you don’t 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
turned into a controversy that is too complex to get into here. My point though, and indeed
the point of this whole video, is that your friendly neighborhood sewage collection system
is not a magical place where gross stuff goes to disappear. It is a carefully-planned,
thoroughly tested system designed to keep the stuff we don’t want to see - unseen. What happens
to your flush once it reaches a wastewater treatment plant is a topic for another video,
but I think the real treasure is the friends - sewers - it meets along the way.
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