COMBINING EXISTING TRICKLING FILTER TREATMENT WITHNITROGEN REMOVAL TECHNOLOGY: OPTIMIZING A MODIFIEDSINGLE –SLUDGE NITRIFICATION-DENITRIFICATION SYSTEM

摘要

The Pennsylvania State University Wastewater Treatment Plant (PSU WWTP) treatsrnabout 3 MGD of wastewater from the University and part of the Borough of StaternCollege. All treated effluent is discharged to land application. Increasing nitrate levels inrngroundwater under the land application fields caused concern because groundwater is thernmajor drinking water supply in the area, and there is a maximum contaminant level forrnnitrate of 10 mg/L as N in drinking water. This led to the plant being upgradedrn(retrofitted) to include nitrogen removal from the wastewater. As part of the retrofit, newrnsingle-sludge nitrification-denitrification (SSND) activated sludge treatment (anoxic andrnaerobic suspended growth reactors) was combined with existing stone-media tricklingrnfilters resulting in a unique treatment process flow scheme. This paper presents therndesign and operational criteria for this modified single-sludge nitrification-denitrificationrnprocess, and reports the results of an extensive optimization study performed at the plant.rnIn the treatment process, primary effluent flow is split between trickling filters and thernanoxic reactor (the primary effluent flow going to the anoxic tank is called the tricklingrnfilter bypass, for the purposes of this paper). Flows exiting the trickling filters and thernanoxic tank are then recombined prior to flowing to aerobic reactors and finalrnclarification. An internal mixed liquor recycle (IMLR) recirculates mixed liquor andrnnitrate from the aerobic reactors to the anoxic reactor. By utilizing the two existingrntrickling filters to remove BOD and nitrify part of the flow, it was possible to use existingrntanks for aeration, even though the hydraulic detention time in the tanks is only 4 hours,rnrather than the 6 to 8 hours of detention time normally required for nitrification ofrnmunicipal wastewater. Plus, it reduces the oxygen (and air) requirements by about 50%,rnsaving on operating costs.rnFollowing start-up of the new process, the effluent total nitrogen often exceeded therntarget of 10 mg/L, and an optimization study was performed to maximize nitrogenrnremoval. The two variables studied for optimization were the trickling filter bypass flowrnand the IMLR flow rate. Bypass flow rates of 10%, 30%, and 50%, and IMLR flow ratesrnof 1Q, 2Q, and 3Q, were investigated. Maximum nitrogen removal was achieved at arntrickling filter bypass flow of 50% of the influent flow, and an IMLR flow rate of 2Qrn(two times the influent flow rate). This mode of operation reduced the total nitrogen byrn75% to approximately 8.5 mg/L (with a relatively low standard deviation of 1.8).