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MWMC FACILITIES PLAN 5.1.2 Regulatory Basis of Planning To plan for future regional wastewater facilities for the Eugene-Springfield metropolitan area, the following assumptions were made regarding the future regulatory requirements for Eugene-Springfield with respect to the current NPDES permit: · Dry season concentration limits will be set to the current Willamette River basin standards of 10 mg/L for carbonaceous biochemical oxygen demand (CBOD) and TSS on a monthly average basis. · Dry season mass limits for CBOD and TSS will remain the same as in the existing discharge permit and will be based on the dry season flow. · Wet season concentration limits will remain the same as in the existing discharge permit · Wet season mass limits for CBOD and TSS will remain the same as in the existing discharge permit and will be based on wet season flow. · Dry and wet season monthly average percent removal for CBOD and TSS will remain at 85 percent, the same as the existing discharge permit. · Wet season maximum day mass limits will be suspended when the plant flow is equal to or greater than twice the dry season design rating of the plant, the same as the existing discharge permit. · The dry season ammonia concentration limits will remain the same as in the existing discharge permit. · The excess thermal load limit in the dry season will remain the same as in the existing discharge permit. · The current limitation for effluent disinfection is based on E. Coli. It is assumed that the E. CoIi limit will remain the same as in the existing discharge permit. · The effluent pH limit will remain the same as in the existing discharge permit. 5.1.3 Effluent Quality CBOD and TSS Dry season mass limitations for both CBOD and TSS as outlined in the NPDES permit are based on the current average dry season flow of 49 mgd. The mass limit requirements must also be met for the highest 30-day flow period in the dry season (maximum month basis). Even if the constant concentration limits for CBOD and TSSare met, the mass limits imply a lower concentration requirement if the wastewater flows exceed the current dry weather design capacity or if the future dry weather design capacity of the facility is increased. Concentration limits as well as percent removal requirements are also specified in the NPDES permit. Figure 5.1.3-1 illustrates the anticipated maximum month CBOD and TSS concentrations that result from projected flows and unchanged mass limitations as specified in the existing NPDES permit. These are shown relative to the anticipated concentration limits. Percent removal limits apply; however, they are not a factor during the dry season because the mass and concentration limits are significantly more stringent. 5-4 MWMC_5.0_REV8_VKS.DOC