Mitigation of Combined Sewer Overflows (CSOs)
The United Kingdom Environment Agency identified unsatisfactory intermittent discharges and issued an Urban Wastewater Treatment Directive requiring action to limit pollution from combined sewer overflows (CSOs). The Canadian Council of Ministers of the Environment adopted a Canada-wide Strategy for the Management of Municipal Wastewater Effluent including national standards to (1) remove floating material from combined sewer overflows, (2) prevent combined sewer overflows during dry weather, and (3) prevent development or redevelopment from increasing frequency of combined sewer overflows.
Municipalities in the United States have been undertaking projects to mitigate CSO since the 1990s. For example, prior to 1990, the quantity of untreated combined sewage discharged annually to lakes, rivers and streams in southeast Michigan was estimated at more than 30 billion US gallons (110,000,000 m3) per year. In 2005 with nearly $1 billion of a planned $2.4 billion CSO investment put into operation, untreated discharges have been reduced by more than 20 billion US gallons (76,000,000 m3) per year. This investment that has yielded a 67% reduction in CSO has included numerous sewer separation, CSO storage and treatment facilities and wastewater treatment plant improvements constructed by local and regional governments. Many other areas in the United States are undertaking similar projects.
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