COMBINING EXISTING TRICKLING FILTER TREATMENT WITH NITROGEN REMOVAL TECHNOLOGY: OPTIMIZING A MODIFIED SINGLE —SLUDGE NITRIFICATION-DENITRIFICATION SYSTEM

The Pennsylvania State University Wastewater Treatment Plant (PSU WWTP) treats about 3 MGD of wastewater from the University and part of the Borough of State College. All treated effluent is discharged to land application. Increasing nitrate levels in groundwater under the land application fields caused concern because groundwater is the major drinking water supply in the area, and there is a maximum contaminant level for nitrate of 10 mg/L as N in drinking water. This led to the plant being upgraded (retrofitted) to include nitrogen removal from the wastewater. As part of the retrofit, new single-sludge nitrification-denitrification (SSND) activated sludge treatment (anoxic and aerobic suspended growth reactors) was combined with existing stone-media trickling filters resulting in a unique treatment process flow scheme. This paper presents the design and operational criteria for this modified single-sludge nitrification-denitrification process, and reports the results of an extensive optimization study performed at the plant.In the treatment process, primary effluent flow is split between trickling filters and the anoxic reactor (the primary effluent flow going to the anoxic tank is called the trickling filter bypass, for the purposes of this paper). Flows exiting the trickling filters and the anoxic tank are then recombined prior to flowing to aerobic reactors and final clarification. An internal mixed liquor recycle (IMLR) recirculates mixed liquor and nitrate from the aerobic reactors to the anoxic reactor. By utilizing the two existing trickling filters to remove BOD and nitrify part of the flow, it was possible to use existing tanks for aeration, even though the hydraulic detention time in the tanks is only 4 hours, rather than the 6 to 8 hours of detention time normally required for nitrification of municipal wastewater. Plus, it reduces the oxygen (and air) requirements by about 50%, saving on operating costs.Following start-up of the new process, the effluent total nitrogen often exceeded the target of 10 mg/L, and an optimization study was performed to maximize nitrogen removal. The two variables studied for optimization were the trickling filter bypass flow and the IMLR flow rate. Bypass flow rates of 10%, 30%, and 50%, and IMLR flow rates of 1Q, 2Q, and 3Q, were investigated. Maximum nitrogen removal was achieved at a trickling filter bypass flow of 50% of the influent flow, and an IMLR flow rate of 2Q (two times the influent flow rate). This mode of operation reduced the total nitrogen by 75% to approximately 8.5 mg/L (with a relatively low standard deviation of 1.8).