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6 DEVELOPMENT AND EVALUATION OF ALTERNATIVES <br /> <br />Alternative 1 - Modify Existing Secondary Treatment Facilities to a Step-Feed Plug-Flow System <br />with Anoxic Selectors <br />This alternative focuses on modifications to the existiztg aeration bash'~s to convert them to a <br />step-feed plug-flow with anoxic selector process. The anoxic, stcp-fced, plug-flow air- <br />activated sludge process is an improvement on the complete-mix or plug-flow activated <br />sludge process used historically at the WPCF. It combines the well-known step-feed, plug- <br />flow activated sludge process with the anoxic selector process. <br /> <br />In the anoxic, step-feed, plug-flow air-activated sludge process, effluent from the primary <br />clarifier is fed to various points along the aeration tank, which is compartmentalized into <br />anoxic and aerobic zones. Figure 6.2.1-1 is a basic process flow diagram for the step-feed <br />process. Selective organism growth in the anoxic selectors allows for sludge settleability <br />control. Nitrification and denitrification occur in the aerobic and anoxic environment, <br />allowing for removal of nitrogen forms from the wastewater effluent. In addition to these <br />benefits, the anoxic, step-feed process allows the use of smaller aeration basins and <br />secondary clarifiers, eliminates the need for mixed liquor recirculation pumping, and <br />reduces process energy and alkalinity requirements. Thus it offers economic as well as <br />process advantages. The basis for sizing the anoxic, step-feed, plug-flow air-activated sludge <br />process is well-established and this process has been successfully used to meet similar <br />criteria at a number of other treatment facilities. <br /> <br />FIGURE 6,2.1-1 <br />WPCF Anoxic, Step-Feed Plug-Flow Process <br />MWMC Facil/ties Plan, Eugene-Springfield <br /> <br /> Cell 1 Cell 2 Cell 3 Cell 4 <br /> <br /> t <br /> DO Wea~r <br /> W~ Wea~ <br /> <br /> ~ WAS <br /> <br />Because the pfima~y effiuenl is introduced in po~Qons ~o each zone, the to~a] dilution effec~ <br />is delayed such that the mixed {iquo~ concenications in the eaHy zones a~e hi~he~ ~hau ~ <br />subsequent zones. ~is is key to the stepdeed p~ocess as the mixed liquo~ ~mm the fi~st, to <br />~e second, to the third, and to the ~ou~th zones dec~eases. ~s confi~u~aQon increases the <br />average mixed liquo~ concen[~a[io~ in the basins, while ~edudn~ ~he solids loadin~ ~ate o~ <br />the secondary clarifie~s. For a ~ive~ mixed llquo~ concent~atio~ to the Hnal cla~ifiers, the <br />s[epdeed anoxic se{ector p~ocess c~ support a hi~he~ sludge hwento~y a~d solids ~etentio~ <br />time (SRT) than conventional desi~, increasing the treatment capacity of a given tank <br />volume. <br /> <br />Pumped recirculation of nitrified mixed liquor is not required for the step-feed m~oxic <br />selector process. Nitrified mixed liquor flows out of each aerated cell directly into the <br /> <br /> MWMCJ O_REV11 DOC 6-7 <br /> <br /> <br />