Combined Sewer Overflow
In some of the older sections of Corvallis, sanitary wastewater and storm water are collected in the same pipe. During wet weather, these combined flows can exceed pipeline and wastewater treatment plant capacity. This results in the overflow of a mixture of untreated wastewater and storm water (called "combined sewer overflows" or "CSOs") into the Willamette River. The City undertook a program to change this practice in order to respond to water quality issues and federal/state requirements that Corvallis remediate CSOs by December 31, 2001.
Construction on the CSO Project facilities began in June, 1998, and was substantially completed in February, 2001.
The City has successfully captured all overflow events (that have occurred so far this 2001-2002 winter storm season) in the new Combined Sewer Overflow (CSO) treatment facilities. This beneficial environmental impact is occurring one year ahead of the original schedule, due to the hard work of staff and Wildish Building Co., who will continue to perform start-up activities until the system is completely on line.
Approximately 45 inches of precipitation falls on Corvallis each year, the majority falling from mid-October to late April. Historically, about 80 to 120 combined sewer overflows (CSOs) have occurred annually, discharging an estimated average of 1.4 billion gallons of combined sewage into the Willamette River.
In 1992, the Oregon Department of Environmental Quality (DEQ) and the City signed a Stipulation and Final Order (SFO) setting an unprecedented level of CSO control and an ambitious 9-year compliance schedule. The SFO set a control limit of 1 overflow in 5 years during the winter and 1 overflow in 10 years during the summer–an overflow reduction of more than 99 percent! Because they take great pride in our community’s quality of life, the citizens and City leaders of Corvallis accepted the challenge posed by this unprecedented high level of overflow control.
A series of public workshops were conducted to evaluate CSO remediation alternatives. The First Street Relief Interceptor alternative was selected as being the most reliable and cost effective. This plan would capture CSOs in a new pipeline located on 1st Street and route them to a wet weather treatment facility to be located near the existing Wastewater Reclamation Plant, rather than allowing them to flow directly into the Willamette River. A storage lagoon near the facility will store peak CSO events for later processing.
During the CSO project design, City staff worked closely with businesses and downtown residents to lessen potential disruptions caused by construction of a large diameter pipeline through the downtown area. With strong business and community support, a CSO Construction Mitigation Plan was adopted encompassing innovative aspects such as hiring a construction liaison, installing a CSO construction information hot line, providing construction allowances for additional mitigation activities such as extra street sweeping and a contractor financial incentive award for meeting construction mitigation performance criteria established by the downtown business community.
Recognizing that conventional (and lowest cost) cut-and-cover construction of the 24-foot deep pipeline would have significant construction impacts, the City implemented an innovative tunneling alternative to substantially reduce construction impacts.
The project will be completed under budget, with current expenditure projections of just over $29.5 million, or about 8 percent under the project’s original budget of $32 million. Financing for the project was secured with a $21.2 million, 3.69 percent interest, DEQ loan and a $8.1 million revenue bond. The total financing costs for borrowing $29.3 million will be around 4.6 percent, due to the low interest loans, saving about $2.9 million over the life of the financing. This savings translates to wastewater rates that are approximately 3 percentage points lower than they would have been if the City had sold revenue bonds for the full project.
The CSO remediation program resulted in these indicators of success:
- A dramatic reduction in CSO events
- Improved water quality with significant reductions in biochemical oxygen demand, suspended solids, and fecal loads
- Greater citizen confidence in the City’s ability to maintain water quality in the river
- Contribution to the City’s response to the Endangered Species Act salmon listing for the Willamette River
Special recognition goes to the Operations and Technical Services staff at the Corvallis Wastewater Reclamation Plant for providing support for the CSO construction activities and continuing the operation and maintenance of the existing treatment facilities. Plant staff have been instrumental in the day-to-day construction success.
For more information about the successful CSO project, contact Public Works at 541-766-6916.
