RETROFITTING WALLINGFORD'S RBC'S FOR NITROGEN REMOVAL

Faced with the prospect of having to purchase nitrogen credits under Connecticut's new General Permit and nitrogen credit trading program, the Town of Wallingford opted to study ways in which a denitrification process could be added to their existing 30,000 M3 /Day (8.0 MGD )wastewater treatment plant. The Wallingford plant uses 56 Rotating Biological Contactors (RBC's ) for BOD removal and nitrification.Wallingford currently discharges an average of 295 kg/d (650 lb/d ) of nitrogen to the Quinnipiac River. Effluent total nitrogen limits established in the General Permit for Wallingford vary from 244 kg/d (537 lb/d ) in 2002 to 122 kg/d (269 lb/d ) in 2014. Nitrogen credits will be bought and sold by the Connecticut DEP, penalizing those municipalities not meeting their permit limits and rewarding those municipalities removing nitrogen in excess of their permit requirements.Three alternatives were studied in detail:Fluidized Bed Denitrification ReactorsSubmerged RBC'sSuspended growth pre-denitrification supplemented by attached growth post-denitrificationThe selected alternative utilizes an anoxic basin upstream from the RBC's which is fed by Primary effluent and recirculated RBC effluent. Underflow from the secondary clarifiers is pumped to the anoxic basin to develop mixed liquor. This process was piloted to verify its feasibility and to develop design criteria. A secondary anoxic basin, using submerged RBC's to provide a media for growth of denitrifying microorganisms, will be included in the project to trim remaining nitrates.