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.
If you’re here at the end of the video, I’m guessing that means you’re pretty thoughtful about
the kind of videos you spend your time enjoying. In other words, you probably prefer learning new
things about the world more than run-of-the-mill television programming. You probably also don’t
enjoy watching ads like this one, which is great, because Nebula doesn’t have any. Nebula is a
streaming service built by and for independent creators like MinutePhysics, Real Engineering,
Wendover Productions, and a bunch of others (including me). It’s a way for us to try new
ideas that might not work on advertiser-supported platforms like YouTube. My videos go live there
the day before they publish here, with no ads or sponsorships. And, we’re super excited to
be partnered with CuriosityStream, a service with thousands of documentaries and non-fiction titles
on pretty much every subject you can imagine. CuriosityStream loves independent creators and
wants to help us grow our platform, so they’re offering free access to Nebula when you sign up at
CuriosityStream.com/practicalengineering. I know there are a lot of streaming services right now,
and all those monthly subscriptions can be tough to keep track of. That’s why this bundle is such
an awesome deal. CuriosityStream is taking 26% off an annual plan - that makes it only
$15 for an entire year of access to thousands of awesome documentaries on CuriosityStream
AND everything on Nebula as well. You can watch awesome long-form content on CuriosityStream.
I loved this David Attenborough special called Light on Earth about all the different animals
that exhibit bioluminescence. Or, watch Tom Scott pit other YouTubers against each other in his
Nebula-exclusive gameshow, Money. It’s a great way to support my channel and a whole host of
your other favorite educational creators. Plus it’s just a great deal. Do us both a favor and
click that link in the description. Thank you for watching, and let me know what you think!