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Saving rainwater on the rooftop. Scattering wastewater for plant irrigation. Careful conservation measures like these have been going on since the Stone Age, but designing them and other “green” features into a new mixed-use urban cityscape in Lower West Side Manhattan seems almost surreal. For one thing, New York City enjoys ample rainfall—44 inches in an average year—and the region isn’t exactly lacking in big-pipe infrastructure. But water is water, and 44 annual inches multiplied by the square footage of a big rooftop adds up to several hundred thousand gallons, according to plumbing designer Peter Costa.
Costa designed the rooftop collection systems that have been catching rainwater om high-rise apartment buildings in Battery Park City. A stormwater storage system keeps it for eventual use in irrigation of Battery Park’s lush, green, waterfront promenade.
Costa has a system going for a third structure, construction for which is now under way: Goldman Sachs’ 43-story world headquarters, which will use 100,000 gallons of retention water. Another of Costa’s forthcoming systems will also collect steam condensate and air-conditioning condensate. Together with more collected rainwater, the flow will supply a cooling tower, saving the city water department thousands of gallons. Besides these, three more rainwater collection systems are also “on the boards,” notes Costa, who works for a nearby engineering services firm, Cosentini Associates.
Rain collection is but one of several novel features being designed into what will eventually total 20 cutting-edge “green” buildings now under construction in Battery Park City. It’s a costly undertaking, and the developer (Albanese Development Corp., of Garden City, NY) and contractors are candid in conceding that this probably isn’t going to pay back, at least not monetarily. One benefit will be to win the US Green Building Council’s coveted LEEDS award; however, there’s more to doing this project than the prestige, as James Cavanaugh, president of the Battery Park City Authority (BPCA) explains. A key reason is simply the novelty of water reclamation in an urban setting—“Because it is unusual,” he says—and there’s an experimental element that’s tempting to explore. In addition, Cavanaugh notes that water reclamation fits in with Battery Park City’s mission, which is to offer a kind of pilot showcase of environmentally progressive, resource-conserving urban design. Elsewhere on the 92-acre redevelopment tract, which has dozens of buildings either done or forthcoming, microturbine power, solar panels, high-efficiency energy saving, and recycling are all being incorporated. “The idea is to build sustainable buildings,” Cavanaugh explains. Water conservation, he says, is a part of that—although perhaps not a critical issue quite yet. It may well be someday, though. “For that reason, Battery Park City wanted to be on the cutting edge in terms of exploring options,” he notes.
Black, Gray and Green
In practical terms, urban onsite sewage disposal and water reclamation present some strategic difficulties for plumbing design. Answering these up front is important, notes Cosentini project manager Marvin Lewin, because the resulting green design concepts are then incorporated into building systems and architecture. For example, regarding septic collection, there’s a distinction between “gray water” coming from bathroom sinks and showers, and higher-strength “black water,” which includes toilet flushes and greasy flow from kitchen sinks. Lewin explains that gray water is easier to treat because it is cleaner in the first place. When it enters a septic tank that lacks biological material, the treatment process must include additives like chemicals or sugar “to make the bacteria in the tank work, to start eating up everything that you don’t want recirculated.”
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| Rain collection is one of several novel features being designed into "green" buildings now under construction in Battery Park City. |
Kitchen grease and the water closet aren’t in the gray water mix; both are too hard to decompose, Lewin says, so the plumbing can be arranged for this waste to leave the building via the sewer.
The downside with a gray-water system, though, is that lots of water is lost to the sewer instead of being reclaimed. So at Battery Park’s pilot green project in the Solaire Building it was decided to plumb everything—including high-strength kitchen and bathroom waste—into a septic tank. Although this meant more challenging waste processing, the simpler plumbing made the up-front cost cheaper. “The whole building’s drainage system is connected and returned back to the system,” he says. “You’re also saving more water, because all that water is also being recycled. You have much more water that you’re utilizing and sending back.”
Onsite Septic—In a Parking Garage?
Thus, the first “greened” residential high-rise—in Battery Park or anywhere in the nation—is the 27-story Solaire, completed in 2003. Engineering consultant Ed Clerico, president of Alliance Environmental LLC (Hillsborough, NJ) selected and installed the Solaire’s somewhat exotic, 17,500-gallon, membrane bioreactor-equipped septic tank, which uses UV and ozone for purification. Tank design—using components that have been available for about 15 years—was the work of Applied Water Management and American Water, with which Clerico is affiliated. Sizing—the tank can process 25,000 gallons a day—was based on New York City data showing per-person water-usage characteristics; these would likely be reflected by the estimated 770 Solaire residents.
As for manufacture, Clerico notes that the massive concrete tank itself was poured as part of the foundation wall in the parking garage. At 1,500 square feet of floor space, it nevertheless took up surprisingly little of the least usable garage area—perhaps as few as 10 parking spots. Its height measures about 16 feet, but most of the tank is recessed below the floor; only 5 or 6 feet rise to eye level. Related equipment rests atop and alongside.
Clerico notes that he’s installed three dozen similar tanks over the years in a variety of other buildings in the Northeast, including schools, malls, and even a football stadium. Until the Battery Park project, however, all previous jobs were more or less rural in character, and the advanced tank design was chosen as an alternative in situations where adequate wastewater systems were not available. The difference between a big-city septic design and a similar tank installed in a rural setting isn’t all that radical, he says, and boils down to different emphasis. Out in the country, the issue is how to reduce the discharge of pollutants, perhaps with an eye on aquifers or surface water and nitrogen, phosphorous or other contaminant reduction. He explains that, in an urban locale, “You’re more interested in reducing the burden on a wastewater infrastructure,” the objective being to recycle or—to use his preferred term—reclaim the water. In a city there are different water-usage characteristics, and different reuse opportunities, so these were important factors in specifying the tank.
The Membrane Bioreactor Package
Ultrafilter membranes are combined in the tank with a biological treatment process, UV rays for disinfection, and ozone. Wastewater enters the tank and is pumped through the filters; air is also pumped in to build up the bacteria for digesting. “Ozone provides final oxidation and color removal,” Clerico explains. Solids empty into the sewer, and the treated water exits to a storage tank.
Flow-through control is largely automated too. Reclaimed water drawn from the storage reservoir is continually replaced with newly processed gray water—minimizing the need for makeup water from the city pipe.
“So it’s a neat little package,” he sums up. “A little sophisticated, but it’s also highly automated.” Despite the high volume and high performance, there’s no daily maintenance requirement. Further data on MB technology is available from the Water Environment Federation Web site (www.wef.org) and from American Water (www.amwater.com) and its applied water management group, he suggests.
Clerico adds that tank operation does consists of a weekly inspection check by the technician, to make “sure the biology is intact and performing well, and that the equipment is operating correctly.” There’s also annual maintenance.
Michael Grady of the Solaire’s maintenance staff does the routine oversight. Two sets of filter membranes are periodically removed for cleaning and soaking, and this is really the maintenance. “We also test the tank for ... things like phosphate, iron levels, pH,” he notes. The staff keeps extensive logs, which city inspectors look at every day during the initial period.
Recently, to decrease the salinity and improve the water’s usability, a phosphate-removal mechanism was upgraded.
On the back end, Cosentini’s Lewin notes that “there’s no smell to the water coming out. And I’ve tasted it. I’ve even drunk it.” Although it’s not tasty, owing to the lack of chlorine and minerals in city water, nor is it legally potable, it is tested every month “to such high standards that it actually can be tasted safely.”
The Solaire is, of course, dual-plumbed. Loops for both city water and non-potable reclaimed water are incorporated (the latter running to assorted reuse). Likewise with drainage. Conventional plumbing lines run to the sewer, but a primary line takes raw black water to the tank in the garage, Lewin says. It’s the best of both worlds, in a sense: Two redundant systems allow for environmentally conscious experimentation to see what works and is optimal—thereby providing invaluable field data for future designs—there’s still ever-present reliability from the conventional system.
Reuses
As it turns out, Solaire residents produce about 36,000 gallons per day (gpd) of wastewater, according to carefully logged figures that Clerico and others have studied. Of this, the garage tank handles and reclaims about 17,000 gpd, with the remaining 19,000 gpd going to the sewer.
What happens to the reclaimed and recycled 17,000 gpd? Currently, over half the amount is being cycled back to the loop for use in flushing toilets—this reclaimed water meeting the required standard as “crystal clear and odor free,” notes Clerico.
A second use began in May 2005: irrigation for nearby Teardrop Park. A subsurface drip rather than a spray system was specified, as the latter would have required more permits and inspections, raising the cost. Recycling for irrigation water is routine, of course; one notable design consideration with urban landscaping is the selection of appropriate vegetation. Gray water typically has higher salt content; hardy plants and shrubs are thus needed. Lawn grass, fortunately, is usually very tolerant. As an added precaution, reverse osmosis was added to remove any salts—“Just to be doubly sure the vegetation succeeds,” says Lewin. All in all, the irrigation has been problem-free although, due to adequate rainfall, very little used so far.
Outside the Solaire, a hose bib allows reclaimed water to be used for maintenance chores such as cleaning the sidewalk—but, for safety, its valve is under lock and key.
Cooling-Tower Innovation, Challenges
Far more experimental is a third reuse the Solaire is pioneering, which could someday have wide repercussions.
City buildings often sport big evaporative cooling towers, which on hot days consume thousands of gallons of water; they provide a prime opportunity for extensive wastewater reclamation and reuse. The Solaire tower holds 20,000 gallons and recycles often. “We had an opportunity here,” says Clerico, “really to reuse much more of the water, to fill and refill. This was a first.”
Once again, however, a prime technical issue to face was the gray-water salt content. Salts readily condense, encrusting and corroding the pipes. Even a cooling tower filled with city water often needs softeners for chemical balance. After several evaporative cycles, the cooling-tower loop must be evacuated—a process known as a blowdown—and refilled with fresh supply water.
Thus, a system of reclaimed gray or black water would naturally need more frequent blowdowns. It would also require closer monitoring and chemical fine-tuning. The question was how much more. “That’s kind of the key to the whole conversation,” Clerico says. “Understanding the exact behavior of your water use is important to getting the operation right.” Lastly, because of the salts, piping would have to empty the blowdown into the sewers rather than shunting it to the reclamation cycle.
As it’s turned out, blowdown with reclaimed water is needed twice as frequently. Instead of getting 10 complete cycles per city-water load, says Clerico, “you’re maybe getting five or six.” This relationship will vary, of course, depending on specific conditions. Water usage also fluctuates with the season: In wintertime, “You’re evaporating only a thousand gallons a day,” notes Clerico, “but up to 20,000 gallons or more a day [i.e., the cooling tower’s full capacity] in the summer.”
Blowdown or flushing takes place automatically, being triggered by an electro-conductivity gauge that responds to increased salinity. Depending on the season and total dissolved solids (TDS) buildup, blowdown usually occurs at least once a day.
During the second cooling season (2005), chemistry analysis showed that even after frequent blowdowns, the salts were potentially damaging to tower pipes; hence, more city water should be added. “We’re learning,” says Lewin, “that, basically, a cooling tower ... can’t take all-reclaimed water. There has to be a mixture of reclaimed and city water together for our systems.” Currently the mix is 50/50. During summer, cycles become more frequent, and thousands of gallons of makeup are needed.
Black Water, Brighter Wash?
Recycling for cooling is novelty enough; but coming soon to a new high-rise now under construction (Battery Park City 1617), the reclamation envelope will be pushed a little further. A boiler will be added, and the water piped into a common laundry room.
Each apartment has a small, washer-dryer unit (using potable water), but eventually, for doing large loads, residents will be able to avail themselves of several large-capacity washers located in the basement. These will soon be demonstrating yet another gray-water breakthrough.
A chief concern in attempting this first-ever effort is that, if the septic tank’s UV or ozone purification should fail, the water will discolor and be unusable. (Discoloration was not a big worry, of course, with irrigation or the cooling tower.) In any case, as a precaution against such a prospect, the septic tank is being equipped with additional sensors able to signal a shutdown of either. Also, notes Lewin, a duplex ozone system is being installed as backup should the primary fail.
As still another precaution, completely separate hot-water heaters on the city water will duplicate the black-water heater. Thus, when the system is taken offline for servicing—or if it should fail to provide completely clear and odor-free water—the laundry can be serviced by city water via heaters that have never contained black water. The reclaimed-water inlet “is not mixed in with any other system,” says Lewin.
Although the investment for all this is big, the resulting conservation will also be considerable. Comparable washing machine arrays use hundreds of thousands of gallons yearly. “You’re talking a lot of water,” says Lewin. “This will be a big savings.”
Lessons learned and technical innovations from this experiment will be incorporated into future designs. Moreover, just a bit down the road, after several years of success, the loop for reused water will be expanded to provide service to the individual apartment washers—and savings will grow dramatically. Tenants will first receive notification letters and will have their questions answered, Lewin notes; for that matter, there’s already a Web site where they can log in and see how much water is being conserved now. Lewin muses: “Whether this will be accepted by individual apartment dwellers ... is still questionable. But, again, it’s something that we have to learn as we go.” Worst case, if reuse stirs too much tenant resistance there’s already the capability to revert to city water as needed.
Saving Water, By The Numbers
How much water is the Solaire saving today—and how much more will it, down the road?
This historic first urbanized tank began producing reusable outflow early in 2004, at which time in-depth data gathering also started.
Eighteen months later, head to head against a hypothetical “control” or non-conserving design, the black-gray system has been reducing dependence on city water “by about 50%,” Clerico says. Better still, sewer discharge has come in even higher, at about 60%. “And what’s also interesting,” he adds, “is that water-reuse potential in large commercial buildings”—which is being pioneered only now—“is much more significant” than in a residential setting, due mostly to the cooling-tower opportunity.
Moreover, he believes, as the methodology steadily improves, reuse rates should climb to 75% or even 90% for gray and black water in nonresidential facilities. Purely residential usage will probably always be more restricted, however, and people will still require chlorinated tap.
Solaire inflow/outflow reclamation numbers thus come out as follows (all figures in gallons-per-day):
- Potable water used by residents: 26,956
- Additional for cooling-tower makeup: 1,894
- Wastewater produced: 36,392
- Discharged to NYC Sewers: 19,801
- Transferred to tank (i.e., reclaimed): 16,591
- Reused for cooling tower: 7,155
- Reused as flush water: 9,436
(Irrigation reuse negligible)
As for a payback curve, this really isn’t part of the story, because—as all participants agree—the Solaire was never meant to “beat the waterworks,” as it were, on billings. BPCA’s Cavanaugh points out, “The financial return for water reclamation is not comparable right now to the financial return for energy conservation.” Nevertheless, “We think this is quite likely to change,” he adds, “as water treatment becomes more expensive, and as water becomes scarcer.”
Clerico agrees that recouping the investment would, at this point, take decades. But he also foresees rising rates for water and sewer, and installation of a cost-justifiable system is easily imaginable. For now, he says, urban water reclamation probably represents “a good futuristic model ... where either the water or wastewater infrastructure is limited.”
Cavanaugh suggests, further, that the real value at the Solaire is its ground-breaking first effort. “Not only did the experiment work,” he adds, “but the city has embraced the idea” as a result, and has recently sweetened the investment by providing future adopters a 25% reduction on water usage and sewer fees.
In fact, he goes on, the original idea for onsite water conservation actually came from City Hall, which also granted the developer a first-ever exemption to city health codes in order to allow septic treatment and reuse there.
Cavanaugh sums up: “We’ve done exactly what we set out to do, which is show that a new technology can work and to make it ‘mainstream.’... It’s now part of New York City policy to encourage this.”
He adds: “And what we do here is not meant to be just about New York City. There are other parts ... of the nation where water is much more expensive and will become more so in the future. So what we’re trying to do here is provide a national model for resource conservation. That’s the thinking behind this, and it’s worked out very well.”
La Mesa, CA-based writer DAVID ENGLE specializes in construction-related topics.
OW - May/June 2006 |
