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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES Alternative 1 - Modify Existing Secondary Treatment Facilities to a Step-Feed Plug-Flow System with Anoxic Selectors This alternative focuses on modifications to the existh'~g aeration bashes to convert them to a step-feed plug-flow with anoxic selector process. The anoxic, step-feed, plug-flow air- activated sludge process is an improvement on the complete-mix or plug-flow activated sludge process used historically at the WPCF. It combines the well-known step-feed, plug- flow activated sludge process with the anoxic selector process. In the anoxic, step-feed, plug-flow air-activated sludge process, effluent from the primary clarifier is fed to various points along the aeration tank, which is compartmentalized into anoxic and aerobic zones. Figure 6.2.1-1 is a basic process flow diagram for the step-feed process. Selective organism growth in the anoxic selectors allows for sludge settleability control. Nitrification and denitrificafion occur in the aerobic and anoxic environment, allowing for removal of nitrogen forms from the wastewater effluent. In addition to these benefits, the anoxic, step-feed process allows the use of smaller aeration basins and secondary clarifiers, eliminates the need for mixed liquor recirculation pumph'tg, and reduces process energy and alkalinity requirements. Thus it offers economic as well as process advantages. The basis for sizing the anoxic, step-feed, plug-flow air-activated sludge process is well-established and this process has been successfully used to meet similar criteria at a number of other treatment facilities. FIGURE 6.2.1-1 WPCF Anoxic, Step-Feed Plug-Flow Process MWMC Facilities Plan, Eugene-Springfield Cell 1 Cell 2 Cell 3 Cell 4 Influent ':, Aerobic ,~ Aerobic [ PE= PE= PE= PE= Pdma~ 25%/0% E~uent 0% 33% 33% 33% Dff Wea~r Wet Wea~h~ ~S Because the primary effluent is introduced in portions to each zone, the total dilution effect is delayed such that the mixed liquor concentrations in the early zones are higher than in subsequent zones. This is key to the step-feed process as the mixed liquor from the first, to the second, to the third, and to the fourth zones decreases. This configuration increases the average mixed liquor concentration in the basins, while reducing the solids loading rate on the secondary clarifiers. For a given mixed liquor concentration to the final clarifiers, the step-feed anoxic selector process can support a higher sludge inventory and solids retention time (SRT) than conventional desig-ns, increasing the treatment capacity of a given tank volume. Pumped recirculation of nitrified mixed liquor is not required for the step-feed m~oxic selector process. Nitrified mixed liquor flows out of each aerated cell directly into the MWMC 6.0 REV11.DOC 6-7