Your Sewer System Is Probably Not Full

I’ve never actually been in a room with an engineer who claimed their city couldn’t add more housing because the sewer system couldn’t handle it. I’ve had lots of people relay a similar discussion to me. I’ve had lots of people pass along the limitation, expressing it to me as if it were law, some code handed down from Mount Sinai that they were just passing on. But I’ve not heard an actual engineer make this claim.

Which makes me unsure if engineers really believe this or not. I doubt it, actually. If they do, it’s absurd, and today I’m going to explain why.

When I was a young engineer, one of the things I was tasked with doing was I&I studies. I&I stands for inflow and infiltration, which are two different ways that water gets into a sanitary sewer system even though it isn’t sewage.

Inflow tends to happen during a rainstorm. The storm sewer is connected somehow to the sanitary sewer, maybe through a drain or a downspout. We try really hard to get rid of those connections because, when it rains, they can suddenly put a lot of water into pipes that weren’t meant to carry it. At its most destructive, inflow can overwhelm a system, resulting in untreated (though diluted) sewage being discharged.

Infiltration is different. That is groundwater leaking through cracked pipes or joints that have separated. It is less dramatic than inflow. More of a chronic condition.

The cheap way to do an infiltration study was to go out in the middle of the night. People weren’t taking showers or doing much laundry, and relatively few toilets were being flushed after midnight. On a dry night, there shouldn’t be much water moving through the system. So I would walk around, pop manhole covers, look down into the pipes and see what was flowing.

This wasn’t a very sophisticated technique. If the entire system leaked a little everywhere, which many old systems did, it wasn’t especially helpful. But if one section of pipe had a serious infiltration problem, you could find it pretty quickly.

What made this crude method useful was the daily rhythm of sewer flow. The same pipe that carried very little water overnight might carry much more only a few hours later. That seems obvious, but it changes how we should understand the claim that a sewer pipe is “at capacity.”

Let’s examine how other infrastructure systems work. For years, I had an electric water heater on an off-peak program. The utility would heat the water at night, when demand on the electrical system was low, and then shut the heater off during the day. I never noticed because I still had hot water when I needed it. The only thing that changed was when the electricity was used.

Utilities do this because they understand that the problem they need to solve is not just how much electricity people use, but when they use it. If everyone demands the same amount of electricity at the same time, the system has to be sized for that peak. If some of that demand can be shifted to another time, the same infrastructure can serve more people. Batteries do the same thing more directly. Charge them when demand is low, draw them down when demand is high, and the battery smooths out when the demand hits the system.

Sewer systems are not that different. If the problem is that too much sewage reaches a pipe during the peak hour, we don’t necessarily need to make the pipe bigger. We can change when the sewage reaches the pipe.

The sewer equivalent of the battery is a holding tank. There is nothing exotic about this. Septic systems hold sewage in tanks. Lift stations hold sewage in wet wells. Treatment plants use storage and equalization to manage fluctuating flows. If a development is going to push a downstream sewer over its peak capacity, the sewage can be held temporarily and pumped into the system later, when flows are lower.

The tank can be sized to serve one property or a hundred. The pump can be controlled by time of day or by actual conditions in the system. I’m not suggesting that every project should do this, only that this is an ordinary engineering problem with ordinary engineering solutions. It’s all very simple.

Which is why I keep coming back to the assertion that we can’t add more housing because the sewer system can’t handle it. Maybe there are places where that is literally true. Maybe there is no treatment capacity, which is a situation that does exist, although I’ll note that it would affect new and old parts of the collection system equally. This is all very different, however, from saying a pipe reaches its peak flow for part of the day.

There are complications to all of this, of course. Pumps do fail, as do control systems. Tanks need to be maintained. Sewage can create odor problems. All of that is real, but none of it is unusual. We know how to handle it. If a pump failure would create a serious problem, we put an alarm on it. If redundancy is warranted, we add redundancy. If something needs to be maintained, we maintain it.

That’s not to suggest that the maintenance concern isn’t a real tradeoff. If we add tanks and pumps and controls throughout a city, then we have added more things that need attention. That makes the system somewhat more complicated. But cities become more complicated as they mature. Ongoing maintenance is not a defect in the system; it is part of having the system.

This is also where I think the argument begins to turn back on itself. A city with an aging sewer system needs money to maintain and eventually replace that system. One of the ways a city increases its ability to do that is by adding more productive development to places where the infrastructure already exists. More homes mean more utility customers, more taxable value and more private investment spread across essentially the same network of streets and pipes.

If we insist that the sewer system first be rebuilt to some ideal standard before we allow any additional development, we may be denying ourselves the very growth in financial capacity that would make rebuilding possible.

For generations, American cities have had access to enormous amounts of capital, from developers, from state and federal governments and through their own borrowing capacity. In that environment, there has not been much reason to become especially creative about working around a peak-flow constraint. We could always just make the pipe bigger.

I suspect that kind of affluence has narrowed the range of solutions we instinctively reach for. We have gotten used to designing the optimal system and, over time, started treating that as the only acceptable system. That works reasonably well when someone is willing to pay for the optimum. It works much less well in a city where the pipes are old, the maintenance backlog is large, and the money for wholesale replacement simply is not there.

Those are often the places where I hear about this sewer-capacity argument. Again, I am hearing it secondhand, so I want to be careful about putting words in anyone’s mouth. But the message relayed to me is often some version of this: the development can happen if the developer pays to upsize the sewer. If the developer will not do that, then there is no capacity.

I understand why that is attractive to a city. If you have a pipe that needs to be replaced anyway and a developer shows up with a project, there is a strong temptation to use that moment to get the larger improvement paid for. Cities have enormous maintenance backlogs, and opportunities to have someone else cover the cost do not come along every day. Why not have developers (and the buyers of the new homes) pay for that?

I think the sequencing is wrong in that logic. Forget housing for a moment and just think about the sewer system. The pipes are aging. They need to be replaced. How does a financially struggling city ever get into a position where it can afford to do that?

One option is to wait for the big project. Wait for a grant. Wait for a developer willing to pay for the improvement. Raise rates, raise taxes, borrow the money and do the entire thing at once. In the meantime — years and perhaps decades — restrict new development because the existing system is already at or near its peak.

The problem is that waiting can lock the city into exactly the condition it is trying to escape. The pipes continue to age. The tax base does not grow. The number of utility customers does not grow. Private investment goes somewhere else. The city remains financially weak while waiting for the large infusion of capital that will supposedly make everything work.

The obvious alternative is to make the system you have work. If peak flow is the immediate constraint, manage the peak. Add the holding tank. Add the pump. Let the next increment of housing happen. Allow your neighborhoods to mature. None of this eliminates the need to eventually replace the pipe, but it keeps the city moving while building more of the financial capacity needed to do that future replacement work.

That is an important distinction. I am not talking about extending miles of new sewer to serve development on the edge of town and pretending that any growth is good growth. I am talking about adding productive development to places where the basic network already exists and where the city is already responsible for maintaining it. If that existing network can support more private investment with relatively modest adaptation, that is a very different, and quite advantageous, financial proposition.

A city with old pipes, weak finances and a need for more investment cannot afford to sit still until someone arrives with enough money to fix everything at once. It has to find ways to keep moving with what it has. That may mean doing something less elegant. It may mean adding a tank and a pump. It may mean accepting another maintenance item. It may mean solving the immediate problem without pretending the larger one has gone away.

But that is still engineering. And, in a city that needs to mature, one that wants to become a Strong Town, it is the more responsible kind.