In a race to meet EPA guidelines, U.S. cities have installed dozens of these devices over the past decade.

Every so often, walking by the water in an American city, you see the sign: Wet weather discharge point. No swimming.

That's a nice way of saying: This is where your toilet flushes when it rains. Our sewer systems, poorly prepared to deal with the effects of even moderate rainstorms, dump billions of gallons of raw sewage into lakes, rivers and oceans each year. Superstorm Sandy alone caused more than 10.9 billion gallons of sewage to flow untreated into the East Coast ecosystem.

At the heart of this problem is a piece of antiquated infrastructure called the combined sewer, which funnels both waste-water and storm-water towards treatment plants in the same stream. During heavy rainfall, storm drains and sewage treatment plants can't handle the increased (if diluted) volume of sewage, and so into the river it goes. It's called "combined sewer overflow" (CSO):

Courtesy US EPA, via Wikipedia.

The EPA has required a number of cities -- there are 772 U.S. cities with combined sewers, in total -- to regulate CSO, at a cost of billions. This policy is likely responsible for your rising utility bill. Cash-strapped mayors have struggled to meet federally imposed goals.

Solutions are expensive. The back-end fix is an expansion of waste-water treatment plants, though this additional capacity lies unused during all but the most intense storms. Alternately, and at even higher cost, a city can take preventative measures by segregating disposal systems for sewage and storm-water. Environmentalists insist that sooner or later, cities will have to reduce run-off by turning impermeable surfaces from roofs to parking lots into green infrastructure, stopping the problem before it starts rather than making endless revisions to existing infrastructure.

Most money in most cities, though, goes to expanding storage capacity. Chicago has already spent $3 billion on the "Deep Tunnel," which remains decades from completion. D.C. is building a series of Metro-sized tunnels, which will cost hundreds of millions. Austin, Boston, Cleveland, Houston, Detroit, Los Angeles, New York, Minneapolis, and San Francisco have all undertaken similar projects.

But cities are also learning to make the most of what they have. It no longer suffices for sewers to be mere conduits. They must be storage tanks themselves.

Enter the inflatable dam. When activated, these giant sewer balloons turn roiling trunk lines into holding pens, rivers into lakes. They save raw sewage for a sunny day.

U.S. cities have installed dozens of these devices over the past decade. Made of industrial grade rubber and anchored to a cement base, the dam inflates on command to restrict sewer flow, like a plug in a giant drain. When the rain has passed, it deflates and allows CSO to flow slowly to the treatment plant.

The concept of using sewers for storage is nothing new. Many are equipped with hefty metal sluice gates that can perform the same function, and smaller tunnels can use pinch valves to adjust the flow.

But with federal grant money running low and EPA restrictions as strict as ever, inflatable dams are becoming more popular, especially in the biggest tunnels. Between 2002 and 2005, Detroit installed over a dozen of them. In Philadelphia's Main Relief sewer, a circular tunnel whose diameter is 13.5 feet, an inflatable dam completed in 2006 can hold back 6.2 million gallons of CSO during a storm, reducing overflow volume by 70 percent [PDF]. Two inflatable dams built in the Brooklyn sewers last year hold back 4 million gallons, and will save an estimated 100 million gallons of sewage per year from entering the New York Harbor. (See this diagram for details.)

If the prospect of sewage bubbling back up under your house alarms you, you're not alone. After a series of summer thunderstorms flooded sections of Washington, D.C.'s Bloomingdale neighborhood last year, there were concerns that D.C.'s inflatable dam was causing the sewer to back up through manholes and drains.

It was not the case: Water authorities in D.C. actually adjust the dam's capacity based on the slope of the sewer, so that even the lowest lying drains see no overflow. With sophisticated networks of sensors, managers at treatment plants perform a delicate balancing act: save as much sewage from the river as possible while ensuring that it never appears on the street.

The technology itself is not so new. Inflatable dams have been around for more than half a century, initially developed to stall flash floods in Southern California and allow rainwater to seep into aquifers. But aside from a few landmark, outdoor projects, like the Adam T. Bower Memorial Dam, which spans the Susquehanna River in Pennsylvania, or the Balgstuw bij Ramspol in the Netherlands, their use has remained largely restricted to Japan.

That is changing as U.S. cities begin to deal with more frequent flooding from climate change, and search for handy technology to divert water. The next frontier will be rail tunnels: New York's Metropolitan Transit Authority sustained nearly $5 billion in damage due to flooding from Superstorm Sandy. Parts of New York City were isolated for weeks.

On Thursday, the MTA inflated a plug in a tunnel at the South Ferry station. Fourteen feet in diameter and 30 feet long, this giant contraption and others like it may be expected to protect the network's longest tunnels from water damage. Plugs, like inflatable dams, would use a simple air barrier to hold back millions of gallons of fluid. Reckoned to cost $400,000 a piece, they are something of a bargain.

Top image courtesy of Dyrhoff.

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