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    About CSO

    What is Combined Sewer Overflow (CSO)?

    Older parts of the City’s sewer system are known as a combined sewer, which must handle the combination of wastewater and stormwater in one single pipe. Combined Sewer Overflow occurs during heavy rainfall, when so much stormwater enters the sewer that it overloads the system ... causing it to back up and overflow at dozens of places around town.
    State and federal environmental mandates require us to eliminate sewer overflows, and to do this, the City’s CSO solution will be the largest capital improvement project the City has ever undertaken. The work, which affects more than 170 miles of underground pipe, will take years to complete and cost millions of dollars. Much of the funding will come from sewer usage fees, with the rest drawn from a variety of state and federal grants and loans.

    Fixing the Problem

    After years of study, the City devised a three-part plan to fix the problem of combined sewer overflow.
    1.  SEPARATION of the combined sewer into a two-pipe system, with one pipe for stormwater and another pipe for wastewater.
    2.  REPLACEMENT of the large “interceptor” pipes, which are the main arteries of the sewer system. This network of pipes carries sewage down to the regional wastewater treatment plant in the lower basin.
    3.  DISCONNECTION of gutter downspouts and other “rainleaders” from the sewer system, which keeps millions of gallons of stormwater from entering the sewers.
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