Near where the morning shadows of the World Trade Center used to fall is the 27-story Solaire, America's first “green” high-rise residential building. This 293-unit rental building in New York City has received a LEED Gold Certification from the U.S. Green Building Council.

A significant green component in the Solaire is a membrane bioreactor-based wastewater treatment and recycling system installed in the basement, which provides reclaimed water for use as fixture flushwater and cooling tower makeup. This is the first urban, residential treated wastewater reuse application permitted in the United States. The wastewater system, with a capacity of 25,000 gallons per day, also supplies reclaimed water to an adjacent apartment building currently under construction. With further treatment, the water is used for subsurface irrigation in the adjacent Teardrop Park. The wastewater system and its associated equipment was designed and furnished by Hillsborough, N.J.-based Applied Water Management, and began operation in January 2004.

The use of multiple features enabled the Solaire to capture all of the LEED Water Efficiency Credits and to qualify for the financial incentives offered under the New York City Department of Environmental Protection Comprehensive Water Reuse Program, which offers a 25% reduction in water and sewer charges for buildings with reuse systems that reduce potable water consumption by at least 25%. Also, implementation of the reuse system and other water conservation measures, along with compliance with other parts of Battery Park City Authority's Green Guidelines, enabled the Solaire to qualify for New York State Green Building tax credits.

The designers set up the wastewater treatment system to provide flushwater for all the toilets in the Solaire and the adjacent building at site 18B, to provide a portion of the cooling tower makeup for both buildings, and to provide water for subsurface irrigation in nearby Teardrop Park. The wastewater and recycling system at the Solaire was designed with a nominal capacity of 25,000 gallons per day with an influent waste stream of typical domestic wastewater: biochemical oxygen demand (BOD) concentrations in the range of 250 to 280 mg/L, total suspended solids (TSS) of 220 to 250 mg/L, and total nitrogen of 40 to 50 mg/L.

The building's operating engineers established a testing protocol that included initial operation of the system for two weeks with effluent going to drain. During this two-week demonstration period, the effluent was sampled three times per week for the several parameters and, after demonstrating successful compliance, was introduced into the treated effluent storage tanks. This began the initial operation period with a three-month duration during which the effluent was sampled weekly. The system was granted final approval by the Health Department when it met quality standards of less than 10 mg/L BOD and TSS, fecal coliform less than 100 colonies/100 mL, and turbidity less than 0.5 nephelometric turbidity units.

Treatment tanks consist of a series of common wall, cast-in-place concrete tanks partially recessed below the basement floor level to provide sufficient headroom over the tops of the tanks. Treated effluent is stored in two fiberglass tanks.

Tanks and Pumps

The building sewer is routed to a 6000-gallon-capacity interceptor or feed tank, which is located adjacent to the trash trap tank but at greater height above the basement floor. This tank is aerated using coarse-bubble diffusers to keep the tank mixed and to avoid septic conditions. Duplex solids-handling centrifugal pumps are mounted on top of the adjacent trash trap, with suction lines passing through the wall of the feed tank. The discharge line from these pumps rises to an elevation above the invert of the overflow from this tank, which connects with the sewer mains in the street.

An air/vacuum relief valve is installed at the high point in the pump discharge line to prevent siphoning when the pumps are not operating. The pumps are controlled by levels in the water storage and treatment tanks to provide sufficient influent to the treatment system to meet the recycled water demand. The volume of the building's wastewater not needed to meet reuse water demands flows through this feed tank to the public sewers. Valves are installed on the building sewer lines so that the treatment system can be bypassed.

Wastewater from the feed tank is pumped into a 9890-gallon, cast-in-place concrete tank termed the “trash trap” for removal of non-biodegradable solids. Effluent from the trash trap flows by gravity into the subsequent treatment tanks. This tank is vented through the odor control system to the building roof.

Biological Treatment

The biological system combines both aerobic and anoxic biological processes. Wastewater from the trash trap tank is transferred directly into the anoxic reactor section of the treatment tank. The incoming wastewater provides a carbon source for denitrifying bacteria that reduce nitrate and nitrite in nitrified mixed liquor. The anoxic reactor is mixed via periodic injections of air. A coarse-bubble diffuser system is provided to distribute air within the anoxic tank. Even though there is no nitrate limit for this system, an anoxic zone was incorporated in the process in order to recover some of the alkalinity consumed by nitrification.

Wastewater flows by gravity from the anoxic reactor into the aerobic treatment tank. This tank provides aerobic biologic treatment for carbonaceous oxidation and nitrification, and is aerated and mixed using fine-bubble diffusers. Positive displacement blowers supply air to the feed, anoxic, aerobic, and membrane chambers.

Automatic pH control is provided in the aerobic chamber through the automatic injection of sodium hydroxide. The aerobic system requires minimal alkalinity adjustment due to the alkalinity produced by denitrification in the anoxic reactor.

Membrane Filtration

Liquid/solid separation is accomplished via membrane ultrafiltration technology. Hollow-fiber filter membrane modules are submerged in the filter tank chamber of the final aerobic chamber and the mixed liquor is separated by the membranes—with a pore size of less than 0.4 microns—reliably producing permeate free of suspended solids. The small pore size excludes many pathogenic organisms, thus providing the first barrier to carryover of pathogens. Sufficient filter modules are provided with a design permeation rate such that the entire daily design flow can be filtered in 14 to 16 hours. Duplex membrane pumps apply a small vacuum to extract clarified water (permeate) from the membranes and convey it to the ozone system.

To prevent excessive solids buildup in the membrane tank and to maximize nitrogen removal, an internal recirculation loop is provided between the membrane chamber and the anoxic reactor. A submersible pump in the membrane tank pumps mixed liquor rich in nitrates into the anoxic reactor for denitrification. The flow in the internal loop is controlled using a percentage timer.

The biological process is designed to accumulate solids until a mixed liquor suspended solids (MLSS) level of approximately 12,000 mg/L is reached. Solids are wasted periodically from the system into the gravity public sewer system to maintain the MLSS level between 8000 and 12,000 mg/L.

Additionally, a backwash system automatically reverses the flow through the membrane racks and forces clean water out through the membranes. Backwash water will be clear ultrafiltration effluent to ensure that the membrane tubes remain clean and unobstructed. A hoist system enables removal of the membrane cassettes from the treatment tank and placement into the adjacent “dip tank” for a periodic thorough clean and soak procedure in order to maintain long-term membrane permeation rates. A programmable process controller controls the treatment system.

Disinfection

Treated, filtered effluent first passes through an ozone generator and contacting system to remove the traces of color that typically are present in the permeate, and to provide the second barrier to the transmission of pathogenic organisms. This system includes an ambient air ozone monitor, which will shut down the ozone system and trigger an alarm in the event any ozone is detected.

Disinfected effluent from the ozone system flows to an ultraviolet (UV) light disinfection system using germicidal lamps. A manually cleaned, enclosed unit is used. The lamps operate fully submerged in the effluent flow and are specifically designed to kill pathogens that may remain in the treated wastewater, thus providing the third barrier to the transmission of pathogens. A UV intensity sensor is installed with an alarm to alert operators when the UV intensity falls below acceptable levels.

An online turbidimeter is installed to indicate the turbidity of the final, disinfected effluent flowing to the water storage tanks.

Future projects of a similar nature need to provide means to discharge cooling tower blow-down downstream of any wastewater treatment and recycling system. Due to the wastewater flowing into the feed tank being very fresh, with little time available for paper products such as paper towels to disintegrate, problems were experienced with clogging of the feed pumps, even though the pumps selected were heavy-duty solids-handling pumps that are routinely used for pumping raw sewage. This problem was solved at the Solaire by the installation of a “horizontal” trash basket in the feed tank, which is regularly cleaned by the operator with removed material being disposed of as normal solid waste. Future systems will incorporate either a similar device, provide for the use of submersible grinder pumps, or install in-line grinders on feed pump suction lines.

Zavoda is a senior project engineer with American Water, Applied Water Management Group, in Hillsborough, N.J.

The pieces of the Solaire's puzzle

The unit processes for the Solaire's treatment and recycling system are:

  • Three-stage membrane bioreactor consisting of:
  • Anoxic mix tank
  • Aerobic digestion tank
  • Filter tank containing ultrafiltration membrane assemblies to remove suspended solids, with recirculation back to the anoxic tank
  • Ozone oxidation for color removal
  • Ultraviolet light disinfection
  • Finished water storage tanks
  • Booster pumping system and recycled water distribution piping
  • Aerated influent feed tank
  • Trash trap to intercept non-biodegradable solids.
  • Associated equipment includes:

  • pH adjustment to enhance the nitrification process, using sodium hydroxide
  • Liquid sodium hypochlorite feed system for membrane cleaning
  • Automatic filter backwash and cleaning system
  • System controller and automatic telephone dialer/alarm system.
  • Green roof produces its own energy

    When designing the Solaire, authorities from Battery Park City hired a team of experts to develop what has since become a definitive model for all green, high-rise residential buildings—the Battery Park City Authority Residential Environmental Guidelines. One of these guidelines focuses on energy savings and, in the design of the Solaire, took root in the development of “green roofs.” In addition to saving energy, the green roofs provide Solaire residents a number of benefits conducive to sustainable living.

    One of the building specifications required the integration of photovoltaic (PV) panels to generate solar energy equal to 5% of the building's base electrical load. PV panels are essentially laminated glass with silicon cells inside, wired to create energy. The two primary benefits of PV are energy independence and environmental compatibility—essential components to sustainable design. For this system, silicon wafers recycled from old computer parts were wedged between two pieces of glass; when sunlight hits the silicon surface, free electrons are produced, yielding energy without noise or moving parts.

    When building the Solaire, the design team faced an enormous challenge deciding in which areas to place PV panels. Ultimately, the team decided to incorporate half of the panels on the front of the building, and the other half on the roof. The PV panels were installed on the walls of the “bulkhead,” which screens the cooling tower and house tank on the roof of the building. The panels became not only decorative add-ons, but also an integral part of building design, acting as both an energy producer and an enclosure. The incorporation of PV panels into the Solaire has become the most prominent design signature of the building.

    The Solaire's green roofs are located on the flat roofs of the 19th and 28th floors. In addition to saving solar energy, the roofs have many other beneficial uses. For example, they serve as a receptacle for collecting rainwater, which is then used for irrigation. The green roofs also help keep the average apartment temperature down by approximately 10° F daily during the summer.