New York’s earliest visitors found a vibrant natural ecosystem, with wooded hills surrounded by pristine waterways. That landscape began to change after colonisation, as the population of New York City grew rapidly over two centuries, from approximately 5,000 in 1700 to more than two million by 1900. As New York City grew into an international hub of commerce and manufacturing, its residents and industry discharged human waste, industrial pollutants and residential garbage into the waterways. By the late 1800s, more than 600 million gallons of raw sewage were directly discharged into the harbour every day from sewers that ended at the water’s edge.
Over the past 100 years, New York City undertook a significant campaign to capture sewage and rainwater by connecting those street end sewers with 146 miles of large interceptor sewers that run along (not into) the water, directing that flow from interceptors to one of 14 wastewater treatment plants, and by creating a network of 7,000 miles of underground sewers and 144,000 catch basins to accept wastewater and stormwater. But like most older urban centres in the northeastern and the midwestern United States that first addressed the public health threat of raw sewage discharges, New York City has a combined sewer system; in a majority of the city, the same pipes carry household and commercial wastewater (also known as dry weather flows) and stormwater runoff to treatment plants that handle 1.3 billion gallons of wastewater on an average dry day. In these areas, the capacity of our treatment plants is often exceeded when it rains, and to relieve pressure on the system during these high-flow periods, our interceptor sewers are equipped with regulators and overflow devices that divert stormwater and wastewater into the city’s surrounding waterways at 423 locations. This is known as a combined sewer overflow, or CSO.
With the construction of the last two of our 14 treatment plants in the mid-1980s, dry weather discharges of sewage into New York Harbour ended, and water quality has improved dramatically. But CSOs are our greatest remaining hurdle to even better water quality. The 423 CSO outfalls cannot simply be “plugged up,” because the captive water flow would simply wash out and disable our treatment plants, cause sewer backups, and dramatically degrade water quality. One solution is to build completely separate sanitary and stormwater sewer systems, but that would cost more than $60 billion in New York City, would take decades to complete, and would severely disrupt the quality of life in areas where the construction would take place.
Over the past 20 years the Department of Environmental Protection (DEP) has upgraded its plants and sewers to handle more wet weather flow and has built tanks to store combined flow that can be pumped to plants for treatment when the rain subsides. This programme has resulted in greater overall capture and treatment of total volumes of combined wastewater and stormwater flows, from approximately 30% of all flows in the city during the 1980s to more than 72% today.
But these traditional solutions have limitations. As the most densely developed city in the US, New York City generates a tremendous volume of stormwater runoff from rooftops, streets and other impervious surfaces. If runoff continues from these impermeable surfaces, the city will have to build additional tanks and tunnels–also known as “grey” infrastructure because of their reliance on steel and concrete-to manage stormwater flows. These facilities are difficult to site, are expensive to build and operate, and displace competing uses of scarce land. With greater amounts of precipitation and more intense storms expected due to climate change, we may well find that today’s solutions are obsolete in mere decades.
The need to rethink the traditional approach to managing stormwater led New York City’s mayor, Michael Bloomberg, to unveil the NYC Green Infrastructure Plan in September 2010. A bold and innovative vision to improve water quality, the plan proposed to invest $2.4 billion in “green infrastructure” over 20 years. These types of controls include green roofs, bioswales, tree pits; and other adaptive structural additions to public works that address the root cause of CSOs by absorbing and retaining stormwater before it can enter the sewer system and trigger a combined sewer overflow. For New York City, Mayor Bloomberg set an ambitious goal to capture the first inch of stormwater from 10% of impervious surfaces in combined sewer areas by 2030–which would eliminate 1.5 billion gallons of CSOs per year.
The plan targets the city’s two most prominent impervious surfaces: rooftops and roadways. Impervious streets and sidewalks make up more than 26% of the land in combined sewer areas. By developing standard designs for enhanced tree pits, streetside bioswales, and stormwater-capturing Greenstreets, we expect to achieve significant penetration for a comparatively small marginal cost above what the city would have had to spend on roads and public spaces anyway. Similarly, impervious rooftops cover more than 46% of the city. Specially planted “green roofs” can reduce energy costs and provide a beautiful, verdant ecosystem, and inexpensive “blue roofs” use simple mechanical devices to moderate peak flows by gradually releasing stormwater.
The good news is that the plan is gaining momentum. This month, thanks to the leadership of New York State Commissioner for Environmental Conservation Joe Martens and his team, New York City and State released a modified consent order for public comment that embodies the key milestones and initiatives in Mayor Bloomberg’s Green Infrastructure Plan–and puts the city on the path to spend $1.5 billion on green infrastructure over the next 20 years. In exchange, New York State has agreed to eliminate $1.4 billion and defer $2 billion that would have had to be spent on traditional grey infrastructure like storage tanks and tunnels.
The potential benefits of the plan are tremendous. Combined with existing investments in more traditional infrastructure, it will cut CSOs by more than 12 billion gallons annually by 2030–a 40% reduction-which is almost 2 billion gallons more per year than the previously required all-grey plan, and will be achieved for $2.4 billion less in public spending. This is smart government. And we estimate that green infrastructure will create up to $400 million a year in additional benefits from increased property values, increased shading and lower energy use. The plan will also green and beautify the city at a time when many cities are cutting capital investments, and public services are contracting as needs increase.
Together with the public and our partners in federal, state and local government, we are using all of the resources at our disposal to drive toward a greener, greater New York City. With green infrastructure, we are making innovative investments in a sustainable future–and making our waterways cleaner than they’ve been in over a century.
Global Forum on Environment: Making Water Reform Happen Paris, 25-26 October 2011
©OECD Observer No 286 Q3 2011