Yet another view of the mouth of the Hudson, just north of where it joins New York Harbor. The Statue of Liberty is at the far left and downtown Jersey City is at the far right. I took the picture to showcase the ship in the center, which deserves both a blog post and a close-up.
If you look closely at the bow, you’ll see “NYC” in the chunky lettering the city government adopted some years back as part of its visual branding. The ship is one of three belonging to the city’s Department of Environmental Protection, and its existence tells a good story about the engineering of large systems.
New York produces, all joking aside, a lot of sewage. Eight and a half millions residents and another couple of million commuters every day, and all of them use sinks and toilets. Because the street piping was largely laid out before split waste-water/rain-water systems were the standard, we have combined sewers, so all of the waste is mixed in with a lot of rainwater. (We get some 50 inches of precipitation per year, which is more than twice than London’s 23 inches and a lot more than Seattle’s 38 inches. (Note that a good chunk of New York’s total is snow, unlike London and Seattle.) There’s a lesson here about stereotyping: London and Seattle beat us for overcast days, but get their rain (on average) as steady drizzle. New York has a lot of sunny days and a lot of heavy downpours.
The sewage used to dump into the rivers and harbor, just like it did everywhere else. Now it goes to 14 treatment plants scattered around the city, to make it something less than toxic. Some of the solid material that is left after treatment is disposed of, some is turned into fertilizer.
What does this have to do with the three ships? The 14 plants don’t all have equal capabilities in treating sewage. Specifically, some of them can’t perform the final stages of treatment of the material known, really, as “sludge,” which is simply partially-treated sewage. So the sludge is loaded onto the ships, transported to the plants that can treat the sludge, and then treated there. The ships run back and forth around the city, day after day, distributing sludge. It is poetic, in a way.
In an ideal world, all of the plants could handle whatever comes their way, and the ships would not be necessary. To get to that point will require expansion of some of the plants, which is a slow and expensive process. To give an example at the other end of the water-supply system, the Third Water Tunnel, which helps deliver water from the upstate reservoirs, was planned in the 1950s, mostly constructed in the 1970s, 80s, and 90s, and is still not quite finished. Someday the ships will be unneeded, but that is not today.
So does this all make sense? Engineering design usually has to deal with contingent circumstances. In this case, we have old combined sewers, changes to land-use patterns that have changed where sewage is created, and increases in environmental awareness that were eventually codified in law. If we had infinite money and the construction could be done without annoying literally every single person in the city, it would be a great idea to replace the entire system with separate storm and sanitary sewers leading to all state-of-the-art treatment plants. Since we don’t and it can’t, we have to slowly adapt the system inherited from the past into a more modern form.
The system is slowly being iterated through design in its built, physical form. And the ships are a stop-gap until the desired form is reached. And that’s good engineering practice.
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