Combined Sewer System
About half of Portland's neighborhoods, most of them built before the 1960s, are served by a combined sewer system.
Portland began a program to reduce combined sewer overflows (CSOs) in 1991 under an agreement with Oregon Department of Environmental Quality. The CSO program is scheduled for completion in 2011.
Combined sewer outfalls discharge combined sewer overflows, a mixture of stormwater and sewage. Stormwater outfalls discharge stormwater runoff from separated sewer areas.
In some areas stormwater drains into large perforated manholes formerly called sumps but now referred to as UICs. An underground injection control structure (UIC) is defined by DEQ as any system, structure, or activity that discharges fluid below the ground or subsurface. UICs are mainly used on Portland’s Eastside where porous soils provide adequate infiltration rates, and they've been used extensively to remove stormwater from the combined sewer system as part of the CSO Program.
Pump stations pressurize sewage flows for conveyance through a force main to a point were it can continue to flow by gravity.
Culverts are thin-walled pipes typically installed instead of a bridge to allow drainage to pass under a road. Culverts represent a barrier to fish passage, confine flows, and increase their velocity. Culverts can become maintenance problems if they are undersized and cannot handle the volume of stormwater flowing through them, or if they collapse or become blocked with debris.
Diversions & Interceptors
Until the early 1950s Portland's sewers discharged directly into the Willamette River or Columbia Slough. Diversion are small dams installed in the older sewers that divert flows into what are called interceptor sewers running parallel to the receiving stream. The interceptors, mostly built between the early 1950s and the 1960s, convey those flows to the wastewater treatment plant. The interceptors convey all of the dry weather sanitary flows but allow ,a href=http://www.portlandonline.com/bes/index.cfm?&a=47260&c=31030 to occur when system capacity is exceeded.
Separate Sanitary Sewers
Separated sewers serve areas of the city developed since the 1960s as well as some areas formerly served by combined sewers but now separated because of sewer capacity issues. Much of downtown Portland, for example, has been separated. Wastewater from these sewers is conveyed directly to either the Columbia Boulevard Waterwater Treatment Plant or the Tryon Creek Wastewater Treatment Plant, located in Lake Oswego but part of Portland's sewer system. On average, 95% of sanitary wastewater receives treatment, and the remaining 5% is discharged untreated through combined sewer overflows or sewer leakages (CSO Characterization Report, 1992)).
Stormwater flows through combined sewers, separate storm sewers, and open channels. A 25-year storm event is the design standard for capacity, and many older parts of the stormwater conveyance system are inadequate.
System deficiencies described in the characterization are primarily from BES's 1999 Public Facilities Plan. Other more recent issues have been documented by BES Maintenance staff and is included when available. BES is currently updating its Public Facilities Plan, which will provide an updated list of systems deficiencies for each subwatershed.