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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES Current WWPH flows into the plant exceed 200 mgd. The current secondary treatment capacity is limited to 103 mgd and any primary effluent (PE) flows over 103 mgd must be diverted and blended with SE. These blended flows must meet the current NPDES permit for secondary treatment. Future PWWFs are anticipated to increase and approach 277 mgd. In peak flow scenarios it is crucial that the diverted PE be of sufficient quality to ensure that the blended effluent can meet the current NPDES permit for both carbonaceous biological oxygen demand (CBOD) and TSS. Increasing both the capacity and the treatment performance of the existing primary clarifiers would ensure that the WPCF will continue to meet current NPDES permit requirements. As part of the evaluation, an initial capacity assessment was completed using historical performance data from the WPCF. The assessment resulted in a capacity rating for the existing primary clarifiers of 86 mgd, removing 50 percent of the TSS. This capacity rating was then used as a baseline for evaluating three alternatives to enhance the existing primary treatment capacity and improve performance: Alternative 1 - No Improvements This alternative would include no modifications or improvements to the existing primary clarifiers. Alternative 2 - Add Baffling to Primary Clarifiers This alternative would add EDIs, a new adequately sized feed well, and density current baffles at the perimeter of each primary clarifier. Alternative 3 - Add Baffling to Primary Clarifiers and Operate Without a Sludge Blanket This alternative would add EDIs, a new adequately sized feed well, and density current baffles as described in Alternative 2. This altemative would also include operating the primary clarifiers without a sludge blanket by pumping a thin primary sludge at a high flow rate from the primary clarifiers to an external thickener. This alternative would require replacing the existing primary sludge pumps and is based on adding two new 50-foot- diameter gravity thickeners and a new thickened primary sludge pump station. Alternative sludge thickening approaches outside the primary clarifier are available, but for purposes of cost estimating and alternative comparison, gravity thickeners were evaluated. Results The capital cost comparison and non-monetary comparison are summarized in Table 6.2.3-1. TABLE 6.2.3-1 Summary of Primary Clarification Enhancement Alternatives Comparison MWMC Facilities Plan, Eugene-Springfield Capital Cost Alternative (millions of dollars) Non-Monetary Ratinga 1 - No Improvements $0 13 2 - Add Baffling to Primary Clarifiers $1.16 22 3 - Add Baffling to Primary Clarifiers and Operate $4.79 24 without a Sludge Blanket Notes: a Non-monetary score is out of a possible maximum score of 30 points. MWMC_6.0_REV11.DOC 6-13