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MWMC FACILITIES PLAN TABLE 3,2.2-2 WPCF Primary Clarification Unit Processes and Equipment MWMC Facilities Plan, Eugene-Springfield Capacity, (each/total Equipment Type Quantity firma/totalb) Primary Clarifier 135-foot-diameter, 12-foot side water depth, 4 21/64/86 with standard scraper mechanism, 44-inch influent column Primary Sludge Pumps 4-inch air-operated diaphragm pumps 4 110/330/440 gpm Primary Scum Pumps 4-inch air-operated diaphragm pumps 2 110/110/220 gpm Notes: a Total firm capacity is with largest unit out --of service. b Total capacity is with all units in service. Secondary Treatment The MWMC WPCF currently has a total of eight activated sludge basins to provide biological secondary treatment. The original design was intended to operate in complete mix, plug flow, or contact stabilization mode. The original aeration system consisted of six centrifugal blowers and coarse bubble aeration. In 1996 three of the eight aeration basins were modified and the coarse bubble aeration systems were replaced with fine bubble membrane diffusers. The secondary treatment average dry weather capacity was estimated at 49 mgd prior to the issuance of the new 2002 NPDES permit. This capacity was based on an adequate solids retention time (SRT) to remove 5-day carbonaceous biological oxygen demand (CBODs) only and does not account for any nitrification. At the same time, the peak hour wet weather treatment capacity was estimated at 103 mgd, limited by secondary clarification capacity. The introduction of nitrification in the 2002 NPDES permit stresses the facility's operation during dry weather as a result of the high solids concentrations maintained in the aeration basins to provide the SRT required for nitrification and the secondary clarifier's inability to handle those additional solids. Historically for wet weather peak flows, primary effluents in excess of 103 mgd are diverted around secondary treatment and blended with secondary effluent and disinfected prior to disposal. Secondary clarification takes place using the original eight secondary clarifiers constructed in 1984. Mixed liquor is split to each secondary clarifier using sluice gates. Each secondary clarifier is 130 feet in diameter, uses inboard launders, and contains a rapid sludge removal (RSR) mechanism that draws return sludge from the clarifier floor with the use of PVC suction tubes. Secondary scum is removed from the surface of the clarifier and flows by gravity to the secondary scum pump station located in the secondary control complex. Recent data indicate the wet weather peak hour capacity of the secondary clarifers to be 111 mgd, somewhat more than the 103 mgd estimated originally. Operations staff have been able to pass these higher peak flows; however, flow management techniques are required for the facility to stay within permit limits. Return sludge is removed from each clarifier through the RSR mechanism. The return sludge flow can be adjusted from each clarifier using electrically operated and controlled 3-10 MWMC_3.0_REV1 I.DOC