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In 1992 the City of Bremerton, with a recently adopted Combined Sewer Overflow (CSO) Reduction Plan, embarked on a program to meet the State of Washington's compliance criteria of one untreated CSO event per year. The City's schedule for implementing sewer system improvements was accelerated by a Consent Decree resulting from the 1993 settlement of a citizen's group lawsuit. The City assumed that separating the combined sewer collection system would eliminate excess wet weather flows and achieve CSO reduction. In 1999, using new overflow data collected since 1994, the City recognized that its 1992 CSO Reduction Plan would not achieve the required level of reduction. As a result, the primary CSO strategy required immediate change to meet schedule requirements and avoid additional litigation. A revised CSO Reduction Plan was developed using a sewer system model that correlated measured rainfall and overflows. The City aggressively implemented the new approach, which included increased conveyance, diversion of flows to other basins, storage, CSO treatment, and separation of flows on public and private property. The hydraulic interdependency between basins also drove the schedule. By the end of 2004 the City successfully achieved reduction compliance at 10 of the 14 CSO sites and by the end of 2005 three more sites will be in compliance. The final site will be in compliance by 2007. The total estimated cost of the CSO program is 52 million.

In 2003, the Washington State Department of Health (DOH) allowed shellfish harvesting in an inlet located near the City, an indication that CSO reduction activities undertaken by the City of Bremerton were successful.

Developing and implementing a long-term CSO reduction plan requires accurate data. The City's 1992 CSO Reduction Plan used textbook assumptions about inflow and infiltration; when compared to actual data, these assumptions were found to not accurately represent the collection system's response to rainfall. A comprehensive and clear understanding of the entire system is also required to develop a successful plan with a schedule that can be sequentially implemented.
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Document Type: Research Article

Publication date: 01 January 2005

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