A Hybrid Suspended Growth/RBC Nitrogen Removal Process At Wallingford, Connecticut

The Wallingford Department of Public Utilities implemented a low cost, novel, hybrid nitrogen removal process at the Wallingford Wastewater Treatment Plant consisting of a suspended growth anoxic reactor coupled to their RBC secondary treatment system. The 8.0 MGD Wallingford plant is regulated by the “General permit for Nitrogen Discharges” issued by the State of Connecticut. The process implemented at Wallingford is designed to strike a balance between the cost of construction and operation of the system and the cost of purchasing nitrogen credits, and it was designed with provisions to add a second stage to achieve lower levels of effluent nitrogen.The Wallingford plant currently includes a nitrification/denitrification process consisting of two anoxic basins, fifty-six rotating biological contactors (RBC's) and four rectangular secondary settling tanks. An Intermediate Pump Station delivers primary effluent and recirculated RBC effluent to the Anoxic Basins. Underflow from the Secondary Settling Tanks is pumped to the Anoxic Basins to develop mixed liquor for the suspended growth anoxic denitrification process. Ammonia and organic nitrogen are converted to nitrate by nitrification in the latter stages of the RBC's. The nitrates are recirculated back to the Anoxic Basins where nitrates are reduced by the denitrification process.The process was started up in May 2005, and did not perform as well as anticipated based on the results of pilot testing. In collaboration with the design team from AECOM (Formerly Earth Tech), the operations staff at the Wallingford facility stepped up and did an excellent job of resolving the operational problems and developed an excellent understanding of the processes introduced at the plant. Over the next two years of operation the process was fine-tuned, providing optimum results.