Item A-MWMC Facilities Plan - Laserfiche WebLink

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MWMC FACILITIES PLAN Pretreatment Programs require monitoring of historical biochemical oxygen demand (BOD), total suspended solids (TSS), and ammonia contributions from the industries. Industrial flows are not anticipated to increase significantly over the next 20 years. 1.6 Project Needs 1.6.1 Existing Condition Design of the original WPCF was based on demographic and population data established in the mid-1970s. The facility was designed to provide adequate sewerage capacity through the year 2005 for a projected population of 277,100. This projection was made for the sewer service area that existed in the 1970s. However, the growth rate during the 1980s was significantly less than projected. This trend in the growth rate was common throughout much of Oregon because of depressed economic conditions during the mid- 1980s. The existing average dry weather design flow for the WPCF, as stated in the current NPDES permit, is 49 mgd. This is defined as the average day flow calculated from May 1 through October 31. Although stated as an average dry weather capacity, the facility must meet the effluent requirements on a 30-day average flow (monthly) basis. Because any 30-day period, including the maximum 30-day flow period (or maximum month flow) during the dry season, must meet the NPDES effluent flow and load requirements stipulated for the average dry season flow, it is prudent to compare the actual dry season maximum month flow (DSMM) to the average dry weather design flow in order to assess treatment capacity. This method was reviewed with DEQ staff and verified as the appropriate method. There is available treatment capacity during certain periods of the year, very little during others, and in some cases there is a capacity deficit. This is because, in part, permit limits change on discrete calendar dates, whereas the changes in influent wastewater flows and characteristics do not necessarily coincide with these permit dates. For example, dry season (May 1 through October 31) permit requirements are more stringent than wet season (November 1 through April 30) requirements, but wastewater temperature and flow in the month of May, and sometimes into June, do not increase and decrease, respectively, to allow the WPCF to easily meet the more stringent dry season requirements. In addition to flow, temperature is important because the warmer the wastewater, the easier it is to treat. Measured DSMM flows from 1992 through 2003 range from 52 percent to 100 percent of the design capacity. It is anticipated that sufficient dry weather treatment capacity exists to meet short-term growth through 2005 for BOD and TSS, but modifications to the WPCF are needed to address ammonia; however, peak wet weather flows, not influent wastewater characteristics, currently constrain the life span of the plant's design capacity. This capacity constraint exists during winter wet season months as well as during wet periods of the regulatory "dry" season. The plant has a wet weather peak design capacity of 175 mgd. High levels of wet weather flows are generated by I/I of stormwater into the sanitary sewer system. Infiltration is a process by which groundwater enters the system through cracks and joints in sewer pipes. Inflow is the process by which stormwater enters the system through improper connections of roof drains and other storm drainage facilities to the sanitary sewers, and by surface runoff entering through manholes. The amount of I/I entering the system varies in different areas depending somewhat on the type, age, system 1-6 MWMC_l.0_REVI3.DOC