[Note that this article is a transcript of the video embedded above.]

For as straightforward as they are, there’s a lot of mystery to sewers. They’re mostly out of sight, out of mind, and ideally out of smell too. But there’s one familiar place you can get a hint of what’s happening below your feet, and that’s the manhole. Sanitary engineers know that there’s actually a lot of complexity in this humble hallmark of our least-glorified type of infrastructure. So, I set up a see through sewer system so you can see what’s happening inside. I’m Grady, and this is Practical Engineering.

There are a lot of kinds of manholes. If it’s a utility of any kind and you put it underground, there’s a good chance you’ll need some access to it at some point in time. But I figure if you picture a manhole in your head, it probably leads to a sewer system: either a sanitary sewer that connects to your drains and toilets, a storm sewer that connects to storm drains, or a combined system that carries it all. Unlike what you see in movies, most sewer systems aren’t huge tunnels full of totally tubular turtles and their giant rodent mentor. They’re mostly just simple pipes sized according to the amount of flow expected during peak periods.

Sewer networks have a tributary structure. Gravity carries waste along downward sloping pipes that converge and concentrate into larger and larger lines, like a big underground river system…but grosser. Terminology varies place to place, but in general, it goes like this. Pipes that service individual buildings are usually called laterals, and those servicing particular streets are branches. Larger pipes that collect wastewater from multiple branches are called mains or trunk sewers. And the most significant lines furthest downstream in the system are usually called interceptors. And connecting each one is a manhole.

This is my model sewer system. I’m just pumping water into an upper manhole and letting it flow through the system by gravity. I chose to do this with nice blue water for anyone watching while having lunch. In real life, the color in a sewer isn’t quite this nice. Unlike regular plumbing, where you use “fittings” to connect lengths of pipes together, sewers lines are connected with manholes. Any change in size, direction, alignment, or elevation is a place where debris can get caught or turbulence can affect the flow. So instead of elbows or tees in the pipe, we just put a manhole instead. In fact, unlike many underground utilities, you can usually trace the paths of a sewer network pretty easily, because it’s all straight lines between manholes. They provide a controlled environment where the flow can change direction, and more importantly, a place where technicians can get inside to inspect the lines, remove clogs, or perform maintenance (hence the name).

Unlike fresh water distribution systems that can usually go a long time without any intervention, sewers are a little… more hands-on (just make sure you wash your hands afterwards). There’s just no end to the type of things that can find their way into the pipes. Fibrous objects are particularly prone to causing clogs, which is why so many sewer utilities have campaigns encouraging people not to flush wipes, even if they say “flushable” on the package. Fats, oil, and grease (or FOG, in the industry) are also a major problem because they can congeal and harden into blockages sometimes not-so-lovingly known as “fatbergs”. Of course, a lot of people aren’t aware of what’s safe to flush or wash down the drain, and even for people who know, it’s easy to let something slide when it’s not your problem anymore. And in most cases, the rules aren’t very strictly enforced outside of large commercial and industrial users of the system. But if you use a sewage system, in a way, obstructions really ARE your problem because a portion of your taxes or fees that pay for the sewer system go toward sending people - not always men (despite the name) - into manholes to keep things flowing. And the more often things clog up, the higher the rates that everyone pays to cover the cost of maintenance.

There’s quite a lot of sophistication in keeping sewers in service these days. It’s not unusual for a city or sewer district to regularly send cameras through the lines for inspection. Technology has made it a lot easier to be proactive. In fact, there’s a whole field of engineering called infrastructure asset management that just focuses on keeping track of physical assets like sewer lines, monitoring their condition, and planning ahead for repairs, maintenance, and replacement over the long term. A lot of the unclogging and cleaning these days is done by hydro jetting: basically a pressure washer scaled up. Rotating nozzles blast away debris and propel the hose down the line. In fact, one of the benefits of manholes is that, if a sewer line does need maintenance, it can be easily taken out of service. You can just run a bypass pump from one manhole to another and keep the system running. But maintenance isn’t the only thing a manhole helps with.

You can see a few more things in this demo. For one, manholes provide ventilation. Along with the solids and liquids you expect, gases can end up flowing through sewer pipes too. You can see the bubbles moving through the system. Air bubbles can restrict the flow of fluid in a pipe, and air pressure can cause wacky problems like backflow. Along with regular air, toxic, corrosive, or even explosive gases can also build up in a sewer if there’s no source of fresh air, so ventilation from manholes is an important aspect of the system. Sometimes you’ll even notice condensed water vapor flowing up from a manhole cover. In a few cities, like New York, that might be related to an actual steam distribution system running underground. But it can also happen in sewers when the wastewater from sources like showers and dishwashers is warmer than the outside air, especially in the winter.

I added a third manhole to my model so you can see how a junction might look. It just provides a nice way to confluence two streams into one pipe, which is an important job in a sewer system, since a “sewershed” all has to flow to one place. The manhole acts kind of like a buffer, smoothing out flows through the system. At normal flows, that’s not a super important job. It’s basically just a connection between two pipes. But the peak flows for most sewers, even if they’re not storm sewers, happen during storms. Drains may be improperly connected to sanitary sewers, plus surface water often finds a way in through manhole covers and other means. In fact, a lot of places require sealed and bolted covers if the top of the manhole is below the floodplain. That’s why you sometimes see these air vents sticking up out of the ground. Many older cities use combined systems where stormwater runs in the same pipes. So rainwater in sewers can be a major challenge. And you can see when you get a big surge of water, the manhole can store some of it, smoothing out the flow downstream.

These storm flows are actually a pretty big problem of the constructed environment. You may have heard about the trouble with holding swimming events in the Seine River in Paris during the 2024 summer olympics. Same problem. Wastewater treatment plants can only handle so much flow, so many places have to divert wastewater during storms, often just discharging raw, if somewhat diluted, sewage directly into rivers or streams. In fact some of the most impressive feats of engineering in progress right now are ways to store excess wastewater during storms so it can be processed through a treatment plant at a more manageable rate. But overflows can also happen way upstream of a treatment plant if the pipes are too small. Sometimes that storage available through manholes isn’t enough. I can plug up the pipe in my demo to simulate this. If the sewer lines themselves can’t handle the flow, you can get wastewater flowing backwards in pipes, and if things get bad enough, you can get releases out of top of manholes. And of course, this doesn’t have to be the result of a storm. Even a blockage or clog in the line can cause wastewater to back up like this. Obviously, having raw sewage spilling to the surface is not optimal, and many cities in the US pay millions of dollars in fines and settlements to the EPA for the contamination caused by backups.

Another thing this model shows is that not all pipes have to come in at the bottom. They call this a drop manhole when one of the inlets is a lot higher than the outlet. The slope of a sewer line is pretty important. I’ve covered that topic in another video. There’s a minimum slope to get good flow, but you don’t want too much slope either. Wastewater often carries rocks and grit, so if it gets going too quickly, it can wear away or otherwise damage the pipes. So if you’re running a line along a steep slope, sometimes it’s a better design to let some of that fall happen in a manhole, rather than along the pipe. It’s not normally done this way where my pipe just juts in. You usually don’t want a lot of splashing and turbulence in a manhole, again to avoid damage, but also to avoid smells. So most drop manholes use pipes or other structures to gently transition inlet flow down to the bottom.

I hope it’s clear how useful manholes are by now. Doing it this way - by making the plumbing junctions into access points - just provides a lot of flexibility, while also kind of standardizing the system so anyone involved, whether its a contractor building one or a crew doing maintenance, kind of knows what to expect. In fact, if you live in a big city, there’s a good chance that the sewer authority has standardized drawings and details for manholes so they don’t have to be reinvented for each new project. In many cases, they’re just precast concrete cylinders placed into the bottom of an excavation. Those cylinders sit on temporary risers, and then concrete is used to place the bottom, often with rounded channels to smooth the transition into and out of the manhole. I did a video series on the construction of a sewage lift station and showed how a few of these are built if you want to check that out after this and learn more.

Constructing manholes reminds of that famous interview riddle about why manhole covers are round. There’s a lot of good answers: a round object can’t fall down into the hole, it can be replaced in any orientation, it’s easy to roll so workers don’t have to lift the entire weight to move it out of the way. A professor of mine had an answer that I don’t think I’ve heard before. Manhole covers are round because manholes are round. It’s almost like asking why pringles lids are round. And manholes are round for a lot of good reasons: it’s the best shape for resisting horizontal soil loads. It’s easier to manufacture a round shape than a rectangular one. For those reasons, manholes are usually made of pipes, and pipes are round because it’s the most efficient hydraulic section. It’s one of those questions, like the airplane on a treadmill, that can spawn unending online debate. But I like pipes, so that’s my favorite answer.