Pre-Anoxic – The pre-anoxic process is designed for denitrification. Without oxygen, the biomass oxidizes organics using oxygen molecules from nitrate. The reaction reduces nitrate to nitrogen gas, which is released to atmosphere.
Aerobic - After denitrification, wastewater is pumped to the aerobic zone, where air is introduced using medium bubble diffusers. Here, residual organic materials (BOD) and ammonia are biologically oxidized to carbon dioxide, nitrate and water. Caustic soda is added automatically to control pH.
During the process, nitrified biomass in the aerobic tank is re-circulated to the pre-anoxic tank. The recirculation rate can be adjusted based on ammonia and TKN concentrations.
Post-Anoxic – Residual nitrate in the wastewater is converted to nitrogen gas and carbon dioxide gas. The biologically treated wastewater enters the membrane filtration portion of the process.
Membrane - The wastewater is filtered through the submerged membranes. Modules consist of polymeric flat sheet membranes for removal of micron size particles. Permeate pumps pull clear liquid (permeate) through the membranes under low trans-membrane pressure, leaving concentrated wastewater behind. An air diffuser assembly located at the base of the membrane unit generates an upward cross-flow of air over the membranes for cleaning.
The membrane reactor operates with solids levels ranging from 8,000–12,000 mg/l. Excess biomass is wasted to sludge holding tanks and hauled off-site for proper disposal. The permeate stream from each membrane module is pumped to Ozone and Ultraviolet units.
Ozone is injected into the wastewater stream to eliminate residual color and UV is used to disinfect the wastewater. The water is then directed to the final effluent tank. This fully treated water is either pumped to a remote storage tank for reuse or to the designated leach field for disposal. Approximately 80 percent of treated water is pumped back to the facilities and 20 percent goes to subsurface disposal.
Conclusion
This wastewater recycling system treats an average of 90,000 gallons per day and saves an estimated 12 million gallons of water per year, easing the strain on the town's potable water supplies and infrastructure. The system consistently meets stringent limits for water use, and the system is designed to produce a lower volume of sludge than traditional wastewater treatment plants. All told, Gillette Stadium serves as a model for "green" building in the 21st century. WW
About the Author:
Tim Davies is President of Applied Water Management, a group within American Water. The group offers customized drinking water and wastewater solutions to real estate developers, industrial clients, and new and expanding communities. Services include designing, constructing and operating decentralized water and wastewater facilities.
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