This article arises from Future Tense, a partnership of Slate, the New America Foundation, and Arizona State University. On the evening of Wednesday, Oct. 24, Future Tense and Scientific American will be hosting an event in New York City on building resilient cities. To learn more and to RSVP, visit the New America Foundation website.

If you've successfully flushed a toilet recently, then you appreciate (at least subconsciously) the workings of a good sewer system. Waste disappears, no matter what time of day or night, or what the weather's like.

But weather is actually a challenge for a sewer system, especially precipitation. That New York City’s sewers can handle the waste of more than 8 million residents on a daily basis—plus the occasional downpour or major storm like Hurricane Irene—is a testament to its resilience.

How does New York do it? Simple: combined sewer outflows. When there's simply too much water in the sewers for the city's wastewater treatment plants to cope, the proverbial flood gates are opened and rainfall mixed with sewage flows into area waterways, such as Newtown Creek or the Gowanus Canal.

That's great for keeping sewage from finding other places to go—like back up into your toilet. But it's less good for places that consistently see such overflows, like the Gowanus Canal. The canal, affectionately called "Lavender Lake" by locals for the multi-hued sheen of its near constant water pollution, carries a raging case of gonorrhea thanks to all that outflow as well as some of the most toxic sludge in the country. In fact, an industrial legacy paired with these "combined sewer outflows" has been enough to turn the canal into a Superfund site—or one of the nation's most polluted localities.

That isn't exactly sustainable. Especially once you consider that downpours and the like are predicted to be on the increase as a result of climate change, as are little threats like sea level rise that could turn outflows into inflows.

In fact, though resilience and sustainability—two of the hottest buzzwords in urban planning—are practically used interchangeably, they are in fact in some tension with each other. A resilient system bounces back from challenges, unharmed, and a big part of building in resilience includes building in ways to fail safely, such as the combined sewer outflows. So, for example, the blackout of 2003 showed how the U.S. power grid remains less than resilient to challenges like untrimmed trees and power lines sagging in the heat. An example of a more resilient technological system is the Internet here in America, where if one route for data fails, another is found.

Sustainability, on the other hand, means efficiency, at least in part, as designers strive to strike a balance between human needs and environmental impacts. This century, the world's megacities will swell to become gigalopolises—vast tracts of urbanized land, like the metropolitan corridor between Boston and Washington, D.C., or the predicted one between Hangzhou and Shenyang. Efficiency in the construction of infrastructure will be vital as the world attempts to build in the next few decades the same amount of urban infrastructure we've cobbled together over the last several millennia. Does that mean foregoing a built-in margin of safety? The Internet may be resilient in the United States, but a reliance on single lines of connection to the rest of the world has disconnected countries across Africa, from Egypt to Uganda. Imagine the same thing happening to an “efficient” sewer system.

Some of the most obvious ways to become more resilient are not sustainable. For example, if you are concerned about reliable electricity, you can increase the resilience of your local grid by buying a diesel generator, or two, or more. In effect, that's what the Googles, Facebooks, and Twitters of the world do. But extra diesel generators are certainly not an efficient, or particularly sustainable, way to create electricity. It's not ideal for the environment to be burning all that extra diesel, with attendant air pollution and the like.

Fortunately, there are ways to think more creatively about how to achieve the same resilience. So, instead of buying a couple big extra generators, a thousand small generators on rooftops might be better, as in the case of solar power on homes. Or programs that reduce the overall demand for power—reducing the stress on the existing electrical grid—can help avoid issues like the 2003 blackout.

Some 5 billion people are projected to live in urban areas by 2030. These cities of the future—most of them cities of today, like New York—will have to cope with climate change, sea level rise, increasing demand for electricity and the logistics of 5 billion peoples' sewage, among other things. New York, at least, has been working on a plan to cope with climate change since 2007, thanks to subtle reminders like an August thunderstorm that year, which shut down the city's subway system during morning rush hour. Climate change is predicted to increase the severity of such downpours. That's just as true for heartland cities like Louisville, which has seen four weather-related disasters in as many years, spurring the development of a climate change adaptation plan, and even up-and-coming cities around the world, such as Surat in India, which is using money from the Rockefeller Foundation to better plan for a history and future of flooding.

As for those combined sewer outflows, well, a little greenery goes a long way. Roofs planted with vegetation, streets that end in rain gardens, even permeable pavement can all go a long way towards making the New York City sewer system both more resilient and more sustainable. In the end, any enduring city must be resilient to a whole host of challenges—from floods and fire to economic collapse—in order to last for the long-term. And what could be more sustainable than that?

This article also appeared on the website of Slate on October 22, 2012.

Image: Gowanus Canal water's sheen via Flickr / Listen Missy!