How Sewers Work (feat. Fake Poop)

Summary

This 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

00:00

🚽 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.

05:01

🔍 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.

10:05

🛠 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.

Keywords

💡Sewage Collection System

A sewage collection system is a network of pipes designed to transport wastewater and sewage away from urban areas to treatment facilities. It is integral to public health, preventing the spread of diseases by safely carrying away human waste. In the video, the sewage collection system is described as one of the most critical yet underappreciated pieces of public infrastructure, highlighting its importance in modern society.

💡Public Health

Public health refers to the organized efforts of society to protect and improve the health of its citizens through the prevention of disease, promotion of health, and prolonging life. The video emphasizes the role of sewage systems in maintaining public health by safely disposing of waste and preventing the spread of plague and pestilence in urban environments.

💡Wastewater Treatment Plant

A wastewater treatment plant is a facility designed to receive, process, and ultimately purify wastewater from residential and commercial sources before it is released back into the environment. The script mentions these plants as the destination point for the sewage carried by the sewer pipes, indicating the broader process of waste management beyond just collection.

💡Inflow and Infiltration (I&I)

Inflow and infiltration refer to the unwanted entry of water into the sewer system through cracks, loose manholes, or other openings. The video describes I&I as a significant issue for sewer systems because it can overwhelm their capacity during heavy rainfall, leading to overflows and environmental problems. This concept is crucial for understanding the challenges in maintaining sewer system integrity.

💡Self-Cleaning Velocity

Self-cleaning velocity is the minimum flow speed required in a sewer pipe to prevent the accumulation of solids and maintain the pipe's capacity. The video demonstrates this concept by showing how an increase in the slope of a pipe can achieve this velocity, preventing solids from settling and keeping the sewer lines clear.

💡Slope

In the context of sewer design, slope refers to the incline of the pipe, which affects the flow rate of the wastewater. The script explains that the slope must be balanced to ensure effective flow without causing the pipe to be too close to the surface or too deep underground, both of which can create construction and maintenance challenges.

💡Inverted Siphon

An inverted siphon is a structure used in sewer systems to allow the pipe to cross under a natural feature like a river or creek. The video describes how it works by dipping below the feature and then rising back up, with the hydraulic grade line above ground level but the flow contained within the pipe. This design is crucial for understanding how sewer systems navigate challenging topography.

💡Fatbergs

Fatbergs are large masses of congealed fat, oil, and wet wipes that can accumulate in sewers and cause blockages. The video uses the term to illustrate the challenges faced by sewer systems due to non-traditional waste materials that do not break down easily and can disrupt the flow of sewage.

💡Flushable Wipes

Flushable wipes are a type of personal hygiene product that is marketed as being safe to flush down toilets. However, the video points out that these wipes can contribute to the formation of fatbergs when they combine with fats and oils in sewers, highlighting the impact of consumer products on sewer system management.

💡Hydraulic Grade Line

The hydraulic grade line is an imaginary line that represents the surface of the fluid in a pipe. In the context of an inverted siphon, the video explains that the hydraulic grade line can rise above the ground surface, while the actual flow remains below, which is important for understanding how sewer pipes can cross under topographic features.

💡CuriosityStream and Nebula

CuriosityStream and Nebula are mentioned in the video as sponsors. CuriosityStream is a streaming service offering documentaries and non-fiction content, while Nebula is a platform built by independent creators, including the video's host. The video promotes a partnership between the two services, offering a subscription deal that supports independent creators and provides access to a wide range of educational content.

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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