I’d like to know more about on-site wastewater treatment and green roofs in NYC. Can you help?

Asked by Cesar Yoc, New York, NY

What do you know about on-site wastewater treatment? I heard that the Solaire apartment building in Battery Park has a "membrane bioreactor" that treats wastewater so it can be reused. Is that type of thing cost effective? Also, what is the average cost of green-roof installation in New York? I would like to know more about green roofs and what incentives are available to reduce the installation costs.


Paul Gleicher

Answered by Paul Gleicher

New York, NY

Gleicher Design Group

June 12, 2009

First, let's clarify the term wastewater. There are two categories of wastewater: blackwater and graywater.

  • Blackwater contains human and food wastes, and is not easily reused. Sources of blackwater include toilets, urinals, kitchen sinks, and dishwashers.
  • Graywater is wastewater that has been used in clothes washers, showers, bathtubs, and bathroom sinks. Because it does not contain human and food wastes, it can be more easily stored and reused for outdoor irrigation.

Typically a separate drain-piping system is created for collecting the graywater. The graywater system's piping must be clearly marked so that it is distinguished from the potable water system, to prevent any chance of contamination through mixing of the two systems. The graywater is filtered and then temporarily stored. Later it can be used to flush toilets, or it can be distributed in subsurface outdoor irrigation. Not only does this divert water that would normally be sent to the sewage treatment plant, it also reduces the expense of your irrigation system by reusing water you've already paid for.

Graywater systems for an entire building are worth considering when there are outside landscape areas where it is feasible to use subsurface irrigation. But graywater systems can be rather expensive, and they are not simple to put in place. As water shortages or drought conditions become more prevalent, these relatively complex and expensive systems will become more feasible, and we will be motivated to install them based on relieving these conditions rather than just for financial returns.

Green roofs

You also asked about green roofs.  Green roofs help counteract the Urban Heat Island Effect, a phenomenon in which urban centers become significantly warmer than surrounding rural areas as a result of the covering of natural land masses with concrete or asphalt. By covering asphalt roofs with a blanket of live vegetation, a more natural balance is attained, helping to reduce the average temperature in the city.

Green roofs can also absorb up to 80 percent of a heavy rainfall. This is significant because our sewer systems are combined with our stormwater systems, so when there is a heavy rain, all the rainwater overtaxes the sewage treatment plant, resulting in raw sewage being dumped into waterways.

Green roofs help reduce the cooling load placed on a building by reducing the amount of solar gain on the building during the summertime. Another benefit is that they filter pollutants and carbon dioxide out of the air (not to mention that they look great).

Green roofs come in two varieties: intensive and extensive.

  • An intensive green roof requires much more soil depth than an extensive green roof and, as the name implies, requires more intensive efforts to maintain, including feeding and irrigation. Deeper soil enables the growth of large plants or conventional lawns, even small shrubs or trees. An intensive green roof weighs appreciably more than an extensive green roof and may require significant upgrading of the underlying structural support of the building.
  • On the other hand, extensive green roofs are only about three inches thick, thus much lighter (about 15 pounds per square foot when fully saturated). They perform much better in drought conditions than in wet conditions. After the initial installation has stabilized, which usually takes about a month, watering should really be kept to a minimum. Extensive green roofs are virtually self-sustaining, relying on natural rainfall to fit the bill.

Extensive green roofs are comprised of several layers on top of a waterproof roof. (I particularly like the system from XeroFlor.) You start with a root barrier installed directly over the waterproofing membrane. This barrier prevents plant roots from compromising the building components. Above this barrier, a drainage mat of non-woven nylon filaments fused to a geotextile particle barrier allows surface flow beneath the vegetated layers. Then a water-retention fleece is applied, followed by a growing medium of lightweight aggregate. Finally, a pre-planted mat containing sedum and moss vegetation completes the installation.

A green roof system costs, on average, about $15 to $25 per square foot in New York City.

  • I recommend most homeowners stick with the extensive systems. They are much simpler to install and maintain, and they weigh much less than an intensive green roof, so you do not necessarily need to add expensive structural supports.
  • Last August, Governor David Paterson signed into law a bill (A.11226) that provides a tax abatement for construction of green roofs in New York City. Under this law, a New York City building owner can receive a property tax credit of $4.50 per square foot of green roof, or about one quarter of the roof's cost. In order to obtain the abatement, construction of any green roof must have begun on or after August 5, 2008.

The application for the abatement must be signed and sealed by a licensed architect or engineer, and must include signed and sealed drawings and a three-year maintenance plan.


For more information:

More details about green roofs in New York City is available on the websites of the NYC Department of Buildings and the S.W.I.M. (Storm Water Infrastructure Matters) Coalition.

Tagged In: green roof, greywater

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