There is a mounting challenge throughout North America on how to manage stormwater in urban areas as land becomes more developed, resulting in more impervious surfaces and less space in which to install stormwater treatment devices.
Case in point: until recently, the metal recycling efforts of Sims Metal Management Municipal Recycling (SMR) in the Bronx had been a double-edged sword. On one hand, the facility (formerly known as Hugo Neu until a 2005 buyout) has successfully recycled hundreds of tons of metals and plastics daily for the industrial and commercial sectors. But there was a price to pay for that efficiency. The facility would often flood during rain events, with thousands of gallons of stormwater runoff heavily laden with metals, hydrocarbons, suspended solids, and other pollutants heading into the adjacent Bronx River.
Now, stormwater that once carried highly polluted runoff travels through a system of native plant meadow swales, an infiltration conveyance system, an evaporative wall, a created wetland, and a doubled-stacked underground detention/infiltration system consisting of 240 StormChambers and 15 SedimenTraps that collectively collect, clean, and safely infiltrate the stormwater in a marriage of proprietary and non-proprietary stormwater treatment methods.
This project was the first-ever zero-discharge industrial retrofit of a material handling facility in the city or state of New York. No project before the Sims installation had attempted to incorporate all runoff from a 10-year or larger storm into such an integrated system of stormwater techniques.
Third-party verification by Professor Joshua Cheng of the Environmental Sciences Analytical Center at Brooklyn College of the City University of New York has concluded that the system is indeed effectively filtering the pollutants before their entry into the Bronx River.
The construction of the treatment train after years of design and permit approvals stemmed from a discussion between Hugo Neu owners John and Wendy Neu and Dr. Paul Mankiewicz, who suggested they establish a system for effectively capturing the stormwater. Mankiewicz is a noted hydrologist, a consultant to New York City on stormwater and related issues, and executive director of The Gaia Institute in the Bronx. The not-for-profit institute couples ecological engineering and restoration with the integration of human communities in natural systems.
Mankiewicz believes human communities and natural systems can coexist to mutual benefit. The so-called waste materials generated by human activity, he maintains, can be cleaned and used to create habitat.
The SMR project exemplifies the need to find solutions for urban stormwater management, especially in areas such as New York City that still have combined sewer
“In New York City, even one inch of rain can create a huge problem, because all of the stormwater runoff goes into the sewer system and the sewer system eventually ends up in the wastewater treatment plants,” says Cheng. “If the wastewater treatment can’t handle it, it gets released into the water bodies around the city, and that includes all of the animal and human waste–whatever it is that is in there–that is going to the CSO.
“Now, one-inch rains are already creating huge problems for New York City. We don’t have the capacity to handle so much water. The idea here is to try to relieve the amount that goes into the sewer system and divert the water into green infrastructure.”
For the Hugo Neu project, Mankiewicz wanted an underground stormwater infiltration system that would be able to capture large amounts of stormwater and help remove pollutants while being strong enough to sustain the weight of 18-wheelers loaded with steel. The solution also had to be cost effective within a limited budget and receive approval from regulatory agencies.
Because the property holds a state pollutant discharge elimination system permit, additions or modifications required involvement from the New York Department of Environmental Conservation. However, this request was somewhat unusual because the runoff would not ultimately discharge to the combined sewer system. Treatment was confined to the site.
Because all modifications of existing drainage are required to connect overflow discharge to the combined sewer, the New York City Department of Environmental Protection had to approve.
The principal permit required was from the New York City Department of Buildings, which had to review structures used for catchment, as well as actual construction plans. Because of the high level of treatment that would occur and the reduced loading to the combined sewer system, the proposal was approved.
For his design requirements and from among his options, Mankiewicz chose StormChambers.
Owned and operated by the Maestro family, StormChamber is a product of the HydroLogic Solutions company based in Occoquan, VA. StormChambers are open-bottomed, arch-shaped plastic chambers made of 50% recycled HDPE. Each unit has 115 cubic feet of storage (with 6 inches of stone above and below), and they exceed the AASHTO H-20 Wheel Load Rating requirement by over four times (over 32,000 pounds per square foot, with 18 inches of compacted soil). The StormChambers also provide an additional level of stormwater quality enhancement. A biomat forms on the underlying stone and soil, which breaks down pollutants, including hydrocarbons and nutrients, to nontoxic byproducts, similar to the function of a septic drain field. Most states actually allow chambered drain fields to occupy up to 50% less square footage than traditional septic drain fields because they are so much more effective with pollutant removal.
Mankiewicz’ choice of StormChambers met the design requirements for strength in such a way that the chambers could be stacked three deep with 30 feet of cover, if needed.
The chambers were large enough to accommodate the volume of stormwater that would pass through the system and, according to Mankiewicz, also were 30% more cost-effective than other options. Additionally, the plan met with approval from the principal permitting agencies.
The primary aim was to achieve a “zero-discharge” facility for pollutants as well as stormwater volume. Pollutant removal was achieved primarily through constructing a large enough humic and rhizosphere filter to remove and capture all metals and hydrocarbons in the runoff from the metal, plastic, and glass material handling zone. The StormChambers also functioned to further remove any residual pollutants and provided for zero discharge of stormwater through infiltration.
“We aimed for, and I believe have achieved, something like the capacity to capture a 10-year storm–about a half million gallons of runoff,” says Mankiewicz. “Because much silt and clay fill was removed and replaced with a sandier structural material, the actual capacity may be beyond the original aim, perhaps as much as a million gallons of additional runoff.”
The double-stacked array of 240 StormChambers is a key factor to the large volume storage, Mankiewicz says. “They were chosen for both their vault shape and the very high load-bearing capacity,” he points out. StormChambers have been independently tested to exceed the AASHTO H-20 Wheel Load Rating by more than four times. The strength of the chambers was critical to the project. Tens of tons of metal move over the site on almost a daily basis.
The StormChamber SedimenTraps, incorporated in the conveyance system, and the double-stacked chamber system also contribute significantly to the capture and removal of the relatively high concentration of suspended solids. The SedimenTraps are significantly less expensive than pretreatment devices and have a longer effective life. They are also easy and highly cost effective to maintain. A vacuum truck is used to remove the sediment through a riser pipe positioned directly above the SedimenTraps.
The system intercepts 6.4 million gallons of runoff containing metals, hydrocarbons, and suspended solids each year from the 6.4-acre site.
Nutrients such as nitrogen, phosphorus, and potassium are intercepted, utilized, and recycled by growing plants and the biomat of the StormChamber system.
As for the treatment train, “The concept was essentially to use large-scale soil column filtration so that we could get all of the metals and hydrocarbons out of the water, because it sits adjacent to the Bronx River. The simple way to do that is to tilt the concrete work surface a couple of percents away from the Bronx River, so instead of water flowing east, it flows west back toward the street side,” points out Mankiewicz.
There, the runoff enters a long, heavily mulched infiltration meadow of native plants, which functions to capture some of the sediment and remove some of the heavy metals and nutrients. The stormwater is then conveyed via a single row of StormChambers to a point at which it enters a manmade aquifer consisting of the stacked StormChamber system and gravel sitting above the groundwater.
Mankiewicz choose to use the StormChambers instead of traditionally utilized reinforced concrete pipe (RCP) for conveyance for several reasons. The StormChambers were significantly less expensive and have a significantly longer life than RCP, he says. More importantly, the stone base and 14-inch weir wall on the downstream ends provides peak flow attenuation and increases the amount of stormwater that is infiltrated. The biomat that develops on the underlying stone and soil additionally assists with water-quality enhancement.
“The primary purpose of the StormChambers is to increase the volume so we’d have enough belowgrade capacity to hold water off of this site,” says Mankiewicz. “The water goes down to the soil column and is stored in the StormChambers. Four of eight solar pumps move water into the head of a wetland system adjacent to the scale of the recycling facility. It circles around a railroad spur. From there it runs into a wetland along the property line, and when that fills up, it drops back down into the groundwater and it’s filtered another time before slowly being released to the Bronx River.”
The other four solar pumps also withdraw some of the stormwater from the StormChamber system, but they discharge it over a 3,000-square-foot evaporative green wall, which helps reduce the amount of runoff through evapotranspiration. This helps to reduce the amount of water stored in the StormChamber system to maximize available storage capacity when storms occur in rapid succession. It also creates an aesthetically pleasing effect. The wall is covered with ferns, mosses, and other moisture-loving plant species. The sheet flow over the wall adds to the aesthetic presence.
Frogs have been living in the wetland pond, and ducks use it on a regular basis. Migrating birds use the habitat as they’re coming through on a regular basis in spring and fall.
Changes in the water chemistry in the StormChambers as well as the groundwater following a storm event also is being monitored, with an eye to long-term trends in water-and soil-quality changes. Current findings are promising, Mankiewicz says.
“Nothing is going into the groundwater and nothing goes into the river,” he says. “Everything is captured by the soil column, which is what it is designed to do.”
In a publication outlining that research titled “Biogeochemistry Within a Stormwater Capture System–the Sims Metal Recycling Site in Bronx, New York,” Cheng points out that the study conducted by the Environmental Sciences Analytical Center at Brooklyn College of the City University of New York provides the first such data about the structure and development of stormwater management systems that are more commonly being constructed in urban areas.
To date, soil sample results have shown that in the area that is not planted but where mulch was added on top of the manufactured sandy loam soil, lead content decreased significantly from near the paved area to the green wall, agreeing with the runoff overflow direction that carries metal-containing particles and dissolved metals. Cheng notes that organic material strongly binds dissolved lead.
In the planted area where sands cover the gravels, lead content has been found to be generally low. Cheng notes the possibility that the runoff hasn’t regularly flooded
In general, soil metal contents decrease quickly with depth, suggesting the topsoil horizon has effectively retained metal contaminants, Cheng writes.
With respect to water samples, the studies to date have shown that the pH of the samples is neutral or slightly basic, not favoring metal leaching. Well water has a strong smell of hydrogen sulfide and high turbidity, but relatively low total dissolved solids. Water samples from the wells and the manmade surface water features do not contain higher metal concentrations than the levels found in the Bronx River, suggesting that groundwater is not a source for metal transport to the Bronx River.
“It seems to date, the high concentration of metals is really retained at the top of the surface of the soil, and very little metals are being released into the groundwater, into the StormChambers water. The amount and concentration in the StormChamber water is sometimes even lower than what’s in the Bronx River,” says Cheng.
Understanding of the biogeochemical systems within the soil horizon may help to identify parameters to maximize water infiltration, contaminant capture, and nutrient utilization, Cheng adds. “These will lead to better design of engineered soil horizons for stormwater capture systems.”
One of the project’s biggest challenges was space, says Mankiewicz.
“It’s a relatively small area we’re dropping water into, and the trick there is to maintain enough velocity, so that water coming off the six-acre site can be dropped into the ground rapidly enough so it doesn’t flood or otherwise slow down work or make it more difficult for the material handling service,” he says.
The difficult part was to get the system sufficiently belowgrade and ensure the pores were well-enough developed to allow water to sink into the ground without puddling onsite for a long period of time after a storm.
“You can create a porous soil by having built it well to start with and having it composted, having good root growth and plant development,” says Mankiewicz. “Perennial plants get large quantities of root structure.”
The material handling site is often filled with track vehicles moving tens of tons of metal at a time over a concrete surface, “so you have a very fine kind of gnarly clay that comes off of the concrete surface every time it rains,” says Mankiewicz. “During big storms, maintaining the downward movement of water in that context is a pretty tall order of business.”
To create an effective system, one must not only have plants with enough root growth, but plenty of mulch, Mankiewicz says. Wood chips are brought to the site on a regular basis for that purpose; burrowing worms add more porosity, allowing the water to continue to permeate.
“Hydrocarbons are very prevalent in high organic compost-derived soil,” says Mankiewicz. “We”˜ve caught them in huge numbers over time. We’ll document that as well.”
The StormChambers are also highly effective in removal of hydrocarbons by the microorganisms in the biomat and in the soil column below the chamber systems. This process was demonstrated in an award-winning bioremediation project by the Navy at its Marine Corps Air Station in Beaufort, SC. The groundwater in one portion of the base was highly contaminated with hydrocarbons. Observation wells were installed to monitor the extent of the pollution plume. Before the excavation, treatment, and replacement of the contaminated soil (the traditional approach at that time), the plume was observed to be contracting.
It was discovered that its contraction was due to naturally occurring microorganisms in the soil metabolizing the hydrocarbons into byproducts of carbon dioxide and water. As a result, the Navy left it up to the microorganisms to finish the job, saving millions of dollars and preserving the natural habitat on the site. Additional detail on this and other examples is available in a 1996 report titled “Intrinsic Bioremediation of Petroleum Hydrocarbons,” (Technical Memorandum TM-2185-ENV-Battelle, Columbus, OH).
A 2001 EPA publication, “A Citizen’s Guide to Monitored Natural Attenuation,” (EPA 542-F-01-004) also provides a background and explanation on bioremediation.
The project is a first, notes Mankiewicz.
“It works exceedingly well,” he says. “It can get very complicated with the solar, but it basically does what it’s supposed to do. I hope people build a thousand of these projects. The first one is always the hardest.”“Climate change and global warming patterns creating more frequent and intense weather events will necessitate more attention to stormwater management,” adds Cheng.