Item A-MWMC Facilities Plan - Laserfiche WebLink

Home Browse Search

MWMC FACILITIES PLAN NPDES permit for wet weather. However, because peak wet weather flows are very dilute, it is the solids percentage removal requirement that limits effluent CBOD and TSS levels during critical wet weather flow periods and this is the most difficult to achieve. Peak wet weather flow, not wet weather influent wastewater characteristics, currently constrains the WPCF's design capacity. The plant has a wet weather peak design capacity of 175 mgd. Current peak wet weather flows to the facility exceed 200 mgd, and only 200 mgd can hydraulically be moved through the WPCF because of a redundancy that was required by DEQ beyond the rated wet weather capacity. SSO Limitations Collection system modeling efforts estimate the current peak wet weather flow in excess of 250 mgd, a situation that now results in SSOs. Current peak wet weather flows exceed 200 mgd, and are limited by both the collection system and plant capacity. An increase in both peak flow conveyance and treatment capacity is necessary to comply with the MWMC WWFMP objectives and policies to eliminate basement flooding and SSOs, as well as DEQ's requirement that the wet season flow associated with the 5-year, 24-hour rainfall event be accommodated by MWMC's facilities without resulting in SSOs. The MWMC WWFMP policies also direct that an increase in the level of full secondary treatment be provided through the expansion of the secondary treatment system. In addition, the treated peak flow must meet the secondary treatment standard. This federal regulatory requirement will take effect by January 2010 [OAR 340-41-0009 (6) and (7)]. Blending and WWFMP Secondary Treatment Policies Previous studies and collection system modeling efforts have concluded that peak wet weather flows resulting from the 5-year 24-hour storm could be conveyed by the collection system to the WPCF for treatment without SSOs. Under the 1997 Master Plan and 2001 WWFMP modeling, it was determined that increased raw sewage pumping capability (among other improvements) needed to be completed by 2007 to avoid non-permitted overflows, and to conform to the objectives and policies of the WWFMP. However, current experience and system modeling indicate that current peak flow estimate is in excess of 250 mgd. Recent modeling efforts and actual system performance indicate that plant expansions to avoid non-permitted overflows need to be constructed beginning in 2005. Current collection system modeling results estimate the 2025 projected peak hour flow to the WPCF to be 277 mgd. MWMC and the cities are continually collecting pre- and post- rehabilitation/construction collection system flow data to assess the effectiveness of their I/I reduction efforts. It is anticipated that the peak flow values will be periodically revised based on the more current data and collection model calibrations and output. The current data and information form the basis for increasing the peak flow capacity of the facility to comply with DEQ's requirement to treat the 5-year, 24-hour rainfall event without resulting in SSOs. Blending is not addressed in the current NPDES permit; however, blending of primary and secondary effluents is the current practice for treating the peak wet weather flows. Flows over 103 mgd (the secondary treatment capacity) receive primary treatment and are diverted around secondary treatment and blended with the secondary effluent. This blended effluent must meet current secondary treatment standards before it is discharged to the fiver. Increases in peak flows resulting from the elimination of SSOs will require that the 1-8 MWMC_1.0_REV13.DOC