Launch Slideshow

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‘Green’ building gets the gold

‘Green’ building gets the gold

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    The Solaire has 293 rental units, ranging in size from studio to three-bedroom apartments. Photos: American Water

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    The membrane ultrafiltration pumps and related equipment are housed in the basement. Liquid/solid separation is accomplished using membrane ultrafiltration technology.

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    The green roof on the 19th floor of the Solaire is irrigated with captured stormwater.

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.