Could Wide-Scale Rainwater Harvesting in New York City Actually Work? This Study Says Yes
The water towers on the rooftops in midtown New York are as iconic as the skyscrapers that cast shadows on them. A recent study published in Water Research suggests that New York’s rooftops could become iconic in another way as well: helping the city save water, not just consume it. The authors propose repurposing the city’s nearly 40,000 acres of rooftop space to catch and reuse rainwater.
“The idea of the study is definitely in line with what we’re doing,” said John Brock, a policy analyst for the city’s Department of Environmental Protection’s Integrated Water Management & Resiliency Group. “We’re trying to build a case for water reuse.”
All water supplied by the city to buildings is potable, meaning even the water used for laundry is drinkable. If rainwater harvesting systems were installed on every building in the city, rainwater could substitute for about one-third—about 300 million gallons—of the city’s non-potable water needs, the study suggests. This could lower building owner’s water costs and reduce rooftop runoff, which contributes to sewage overflow and flooding.
Combined sewage overflow merges water from homes and sewers then deposits them in nearby water bodies. NYC Department of Environmental Protection
During short periods of heavy rainfall, rainwater turns to stormwater when it reaches the ground and mixes with sediment.
The stormwater makes it to the city’s sewer system, where it combines with water from buildings. If pipes can’t drain all this water fast enough, the sewage is dumped into water bodies like the Hudson River.
The stormwater makes it to the city’s sewer system, where it combines with water from buildings. If pipes can’t drain all this water fast enough, the sewage is dumped into water bodies like the Hudson River.
That process—called combined sewage overflow—can cause flooding and introduce contaminants, like human waste, to rivers and coastal waters. Rainwater harvesting systems can prevent rainwater from turning into stormwater by collecting it and sending it down drains to a water tank until it needs to be used.
New York City was an ideal candidate for the study because of its abundance of rooftop space, wealth of water-use and rainfall data, and willingness to implement environmental measures. “New York City, among all the major cities in the U.S., is leading in green infrastructures and sustainable development,” said Dr. Ranran Wang, a co-author of the study from Leiden University in the Netherlands.
A rainwater harvesting tank at William A. Harris Garden in Washington Heights. Brandon Alexander
In the 2010 NYC Green Infrastructure Plan, the city committed billions of dollars to limit water pollution and help buildings integrate green design. Currently, Brock says, only about 50 buildings in the city have robust water reuse systems, and GrowNYC says over 140 public parks have rainwater harvesting systems.
The researchers categorized the city’s over one million buildings into 12 groups based on their primary function and non-drinkable water needs. For example, an office building uses non-drinkable water primarily for toilet flushing, whereas an apartment building uses it for toilet flushing and washing machines. To determine the size of the water collection tank, the researchers further classified buildings by square footage.
According to the study, washing units could run entirely on harvested rainwater. Brandon Alexander
New York City has an estimated 40,000 acres of rooftop space available to facilitate
wide-scale rainwater harvesting. Brandon Alexander
wide-scale rainwater harvesting. Brandon Alexander
Using rainfall data from the Northeast Regional Climate Center and daily water use patterns for each building type, the researchers estimated the effects of replacing non-potable water with stormwater over the next two decades.
The analysis found that New York could replace up to 94 million of the estimated one billion gallons of water consumed in the city daily.
The analysis found that New York could replace up to 94 million of the estimated one billion gallons of water consumed in the city daily.
Of all building types, low-rise houses benefited most from this rainwater collection system. Their rooftop surface area is high relative to their water demand, so they can meet their water needs more efficiently. This building type is common in New York’s lower-income neighborhoods, and the researchers were surprised to see an overlap between those areas and the reduction in rooftop runoff—nearly 90 percent in some areas.
The study suggests that these areas could use rainwater harvesting to combat climate change effects as these neighborhoods are typically at risk of flooding because of rooftop runoff.
“If we think about climate change and risk adaptation, it’s not the rich, who will be concerned about it, but the poor,” Wang said.
A rainwater collection system of this scale could also lower costs for the city because less water needs to be processed at treatment plants. By decentralizing some of the water supply, pressure is relieved from cities grappling with increasing demand and aging infrastructures.
“People have started looking at decentralized water solutions and how water infrastructure in the future looks,” said Paulina Concha Larrauri, a researcher at the Columbia Water Center at Columbia University and co-author of a 2020 study about establishing rainwater harvesting systems in multiple buildings throughout Mexico City.
Concha Larrauri’s research focused on areas with limited drinking water access, but, she said, studies like the new one are critical for finding global water conservation solutions. “In New York, the reservoirs are pristine. There is an over amount of water,” she said. “The solutions are tied to different places.”
Sewage and waste enter water bodies via outlets like this one at Sherman Creek Park in Inwood, Manhattan, potentially harming wildlife and the environment. Brandon Alexander
Despite the perceived benefits, it’s unclear if New Yorkers can expect rainwater collection systems on their buildings anytime soon: there are currently only around 50 buildings with robust water reuse systems. The city is taking steps to encourage water conservation. The DEP offers a Water Conservation & Reuse Grant that reimburses commercial and large apartment building owners up to 20 percent of equipment costs for large-scale water reuse projects.
Installing rainwater collection systems varies in cost. In commercial buildings, price ranges from $40,000 up to a couple hundred thousand dollars, depending on the building’s size and needs. Retrofitting older buildings is costlier because of the work it takes to re-run pipes, said Stuart Bailin, the co-founder and Director of Engineering at Wahaso, a company that specializes in rain collection system installation.
These costs are why the study authors recommend that private-public partnerships fund the installations. The city could get value from the reduced costs in water treatment and supply, and a more manageable stormwater system.
“It’s not about just getting more water; it’s about protecting,” said Amanda Lounsbury, a co-author of the study from the Department of Public Policy at Tel Aviv University. “It’s the idea that rainwater harvesting offers a multi-faceted approach to dealing with water problems.”