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
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No mention of lift stations? Hopefully that's a future video. u/gradyh
Speaking of lift stations, the subject of this video - Les Swanson - had an amusing perspective ... https://youtu.be/sTk8GsPbd4w?t=27
"Honey wagon" :O