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MWUC FACILITIES PLAN 5.4 Design Capacity of Conveyance System and Wastewater Treatment Plant 5.4.1 Conveyance System Conveyance system alternatives were developed and evaluated in the WWFMP (CH2M HILL, 2000). The current facilities planning effort focused on the analysis of wastewater treatment alternatives regarding influent pumping, preliminary and primary treatment, secondary treatment, disinfection, and biosolids management. As part of the current facility planning effort, collection system modeling was performed by updating the MWMC MOUSE hydraulic model developed for the WWFMP to provide an estimate of WWPH flows to be used in the analysis of wastewater treatment alternatives and pumping capacity. A detailed report of the wastewater collection system hydraulic modeling analysis is given in the March 2004 "MWMC Wastewater Facility Plan--Wet Weather Peak Flow Analysis" technical memorandum (see appendix). The most significant tasks in the modeling update process were the following: 1. Conversion of the wet weather flow estimating method from a spreadsheet-based regression analysis to the new RDII modeling module that is a part of the MOUSE model 2. Calibration of the model based on more recent system flow monitoring data 3. Use of a new planning time frame (2025) not addressed in the WWFMP The WWFMP did not explicitly evaluate peak flows for 2025, although it did evaluate buildout flows. Current modeling efforts were also undertaken, using the limited additional flow monitoring data (from six permanent monitoring locations), to assess the effectiveness of ongoing RDII reduction efforts. In addition, the current modeling effort has incorporated system network configuration modifications, such as physical upgrades to pump stations and/or pipelines. In accordance with DEQ guidelines, the 10-year summer and the 5-year winter rainfall events were compared to determine which storm produced the defining (worst case) flow condition in the wastewater collection system. The 10-year, 24-hour storm event produced an unadjusted peak flow at the WPCF of 181 rngd for the 2025 condition, far less than the 277 mgd for the 5-year event. Therefore, the 5-year rainfall event was used as the design rainfall event. The 2003-04 modeling effort indicated that WWPH flows generated in the collection system were comparable to those projected in the WWFMP. The best current estimate of 2025 WWPH flow at the WPCF is 277 mgd. The buildout flow rate at the WPCF is 294 mgd. Based on these values and the 290 mgd peak buildout flow rate predicted in the WWFMP, a value of 300 mgd as the 2025 peak flow rate was used in evaluating peak flow management alternatives under the overall facility planning effort, to account for the uncertainty associated with projecting flows 20 years into the future. The peak flow in 2025 (based on the 5-year, 24-hour storm) that MWMC will control and manage for, however, is 277 mgd. 5-26 MWMC_5.0_REV8_¥ KS.DOC