Category Archives: Engineering

A couple of weeks ago I took a day long class on urban stormwater design. It was a class designed for Professional Engineers and others who needed to earn some continuing education credit. It was taught by two Professional Engineers. Urban stormwater design is something I am in general familiar with but not anywhere close to an expert, so I thought this would be a worthwhile, intriguing class by which I could also earn needed continuing education credit.

The class subject was the important topic of urban stormwater, specifically in older cities like Boston, New York, Philadelphia, and Washington, where much of the city has combined sewer systems. Combined sewers are systems where both stormwater and sanitary wastewater (i.e. the discharge from household and business sinks, toilets, showers, etc.) go into the same set of pipes and eventually to a wastewater treatment plant. When it is not raining, this type of system works fairly well because it is mainly just the wastewater reaching the treatment plant. When there is precipitation, this type of system can cause problems because the amount of flow can overwhelm the treatment plant. If the plant is not big enough, or if there is not enough storage capacity to hold the wastewater until the treatment plant can treat it, then this results in raw, untreated wastewater (i.e. poop) being discharged to whatever body of water is downstream of the plant. The discharge of untreated wastewater is illegal under the Clean Water Act and is considered a permit violation under the National Pollutant Discharge Elimination System (NPDES) program.

The class covered ways that cities can try to prevent overwhelming the plant and thus permit violations. This can range from the more obvious, and often most expensive, ways of building more and bigger wastewater treatment plants and/or storage capacity to hold the wastewater until the plant can treat it, to the more innovative and sometimes lower cost of green infrastructure. Green roofs can absorb some of the stormwater, and thus reducing the amount of stormwater going into the sewers. Tiny little parks and green bump-outs into the street which trap perhaps an inch of rain can also help, especially when many of them are built around the city. In the area where I live, many redesigned streets have a green median which sinks below street level or have a depressed area with plants next to a wide sidewalk. The idea with these areas is to allow some of the stormwater to sink into the ground and also to simply hold some of the stormwater during the storm. This evens out the flow going into the sewers.

During this class, one of the instructors showed various models that he and his company had run for an older city to solve a problem where a particular neighborhood kept having the basements flood with sewage. The city had a combined sewer, and in that area, the pipes could not handle all the flow during the storm, and this would lead to sewage backing up into people’s basements. Quite obviously, the people living there did not like raw sewage flooding their basement every couple of years. Who would? So the instructor demonstrated how complex the issue was with maps that showed if you added extra capacity in this tiny area, it would solve the problem for this tiny part of the neighborhood. However, if you spent more money and built more capacity for a larger area, it would solve the issue for more of the neighborhood but ironically would make it worse for this other part of the neighborhood. His point was to show how the problem was complex, and it was not just a matter of adding more pipes.

However, in every single one of his models, the extra pipes to handle the extra flow had one or more outfalls to the river that flowed next to the neighborhood. The pipes were intended to connect to the combined sewer system, but to alleviate the extra flow that caused the flooding in people’s basements, they modeled adding extra outfalls where the raw sewage, instead of going into people’s basements, would instead go into the river next to the river. Now, I am sure most people would think, well, I would rather it go into the river than into my basement. However consider if you live in that area, you would still be dealing with raw sewage in the area, just maybe not in your basement. An improvement, but still a problem. The instructor said they were designing for a storm size that would happen about every five years. To be clear, dumping raw wastewater into a river ever five years is illegal. I was incredulous that they were actually designing the system to dump raw sewage into the river. I asked him why didn’t the design include a new interceptor (a giant pipe that generally conveys the flow for long distances to a plant), or some other large storage area until the system could handle the extra flow and convey it to the treatment plant. He cited cost. Cost is not a reason to violate the law. In fact cities that dump raw sewage can fined, so there’s a cost. [I don’t work in this area. I don’t know how often cities are fined for this, but I do know the idea is not to take money from them but to get them to spend the money so violations don’t occur. Use the money to build more wastewater treatment plant capacity, etc.]

I questioned the instructor about this. It is one thing to have an old system that dumped raw sewage into a river and then to keep building pipes and plants such that the city reduced or eliminated any discharge of raw sewage. It is an entirely other thing to actually design “improvements” to your system to do that. To be clear, sewage flooding into people’s basements is obviously a serious problem that the city needed to solve. But solving it to only create another problem is not a true solution. I kept asking him why didn’t the solution include a new interceptor or storage. A couple of the other people taking the class asked questions of a similar nature. Finally the instructor seemed to be tired of the questions and not being able to answer them to our satisfaction, and he said “I don’t know guys, I’m just the modeler.” He is a Professional Engineer. I accept that the reason he was teaching this part of the class is that he in an expert in modeling stormwater. I do not accept that because he was just doing the modeling, he had no responsibility for the solution that he was modeling. At some point, he should have asked what kind of solution was he modeling if by design it would lead to permit violations every five years or so. Part of the responsibility of being a Professional Engineer is knowing that is a not a solution. Part of being an engineer is planning for every contingency you possibly can. Part of being an engineer is knowing the law. That means you can’t shrink responsibility by simply saying that’s not my part. I just did this tiny area. Go talk to the other guy. Nope. You can’t do that. A Professional Engineer takes responsibility for the design. That’s your job. Find a solution that solves the problem you were asked to solve but does not create other problems.