What Would You Do with Eight Million Gallons of Water Every Day?

On a rainy mid-December weekday in New York City, underground pumps throughout the city were working hard to keep the subway dry, siphoning rainwater out of the tunnels and into the sewers. Up on the 40th floor of a building in lower Manhattan, a panel of experts gathered for a public brainstorm about how to put that water to good use.

The December 9 panel, hosted by the New York Academy of Sciences, was trying to make the Metropolitan Transit Authority more environmentally sustainable. And though the process is in its early stages, the MTA engineers and consultants assembled far above the rumbling trains seemed inclined to use the water for geothermal heating and cooling.

The MTA pumps eight million gallons of groundwater out of the subway system on a dry day. That’s enough to fill a dozen Olympic-sized swimming pools. When it rains, the agency pumps out 13 million gallons.

Nearly the same amount of potable water, seven billion gallons a day, is used by the MTA for everything from keeping employees hydrated to washing subway cars.

Cleaning all that wastewater and returning it to the drinking water supply would probably be prohibitively expensive, according to one panel member, Thomas Abdallah, the chief environmental engineer for New York City Transit. A centralized solution isn’t ideal, he added, because water is pumped out of subway tunnels all over the city.

Geothermal heating and cooling could be implemented at many locations, he said.

Geothermal systems work because a mere fifteen feet underground the temperature is about 55 degrees Fahrenheit year-round. That constant temperature can be used to make hot summer air cooler, and to make cold winter air warmer. “The earth serves as your boiler or your heating tower,” said John Rhyner, a geologist with P.W. Grosser consulting and a member of the panel.

In a pilot project begun in 2006, water pumped from the London Underground is used to cool Victoria Station, a major central London hub. Air is pushed across pipes containing 55-degree groundwater and into the station. The pilot is part of a $230 million effort to make London’s stations cooler in the summer.

In New York, stations are neither heated nor cooled, as is painfully obvious to anyone on the subway platform in August or January. A system like the London pilot could go a long way toward making the New York subway more comfortable. By keeping water out of the sewers, a geothermal system would also ease the river and bay pollution that occurs every time there’s a heavy rain and storm water overwhelms New York’s combined sewage and drainage system.

Beyond the geothermal proposal, Abdallah said the MTA was considering using the water for landscaping and — if it’s clean enough — washing subway cars and buses, a solution that would help reduce that daily demand for potable water.

The groundwater recycling program is clearly in its earliest stages, and the MTA is still considering all its options. It’s not clear what will come next — an efficient use of groundwater is a long-term goal for the organization.

Panel members were enthusiastic about the groundwater’s potential. One such consultant, Paul Mankiewicz, the director of the Gaia Institute in the Bronx, said “it’s wonderful to get in on the ground floor when something is being thrown away but everyone recognizes it has inherent value.”