Mitigation of combined sewer overflows include sewer separation, CSO storage, expanding sewage treatment capacity, retention basins, screening and disinfection facilities, reducing stormwater flows, green infrastructure and real-time decision support systems. For example, cities with combined sewer overflows employ one or more engineering approaches to reduce discharges of untreated sewage, including:
* utilizing a green infrastructure approach to improve storm water management capacity throughout the system, and reduce the hydraulic overloading of the treatment plant
* repair and replacement of leaking and malfunctioning equipment
* increasing overall hydraulic capacity of the sewage collection system (often a very expensive option).
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. In 2009, 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 the frequency of combined sewer overflows.
Rehabilitation of combined sewer systems to mitigate CSOs require extensive monitoring networks which are becoming more prevalent with decreasing sensor and communication costs. These monitoring networks can identify bottlenecks causing the main CSO problem, or aid in the calibration of hydrodynamic or hydrological models to enable cost effective CSO mitigation.
Municipalities in the US 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  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  per year. This investment that has yielded an 85 percent 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 US are undertaking similar projects (see, for example, in the Puget Sound of Washington). Cities like Pittsburgh, Seattle, Philadelphia, and New York are focusing on these projects partly because they are under federal consent decrees to solve their CSO issues. Both up-front penalties and stipulated penalties are utilized by EPA and state agencies to enforce CSO-mitigating initiatives and the efficiency of their schedules. Municipalities' sewage departments, engineering and design firms, and environmental organizations offer different approaches to potential solutions.