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 post is an ode to the grossest and probably most underappreciated pieces of public infrastructure. I’m Grady, and this is Practical Engineering. Today, we’re talking about sewers.

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.

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 you adjust the slope of the line without changing the flow rate, 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. 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 talking abou 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. 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 post. 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. Even though the pipe is the same size along the full length, settling only happens 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. Even though there’s some settling happens, 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. 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 post, 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 post, but I think the real treasure is the friends - sewers - it meets along the way.