A combined sewer is a sewage ...

A combined sewer is a sewage collection system of pipes and tunnels designed to also collect surface runoff. Combined sewers can cause serious water pollution problems during combined sewer overflow (CSO) events when wet weather flows exceed the sewage treatment plant capacity. This type of sewer design is no longer used in building new communities ( current design separates sanitary sewers from runoff ), but many older cities continue to operate combined sewers. 

You can find the Combined Sewer Overflow Documents here :

• EPA's Combined Sewer Overflow (CSO) Control Policy
• Combined Sewer Overflows Guidance for Nine Minimum Control Measures  – Provides information on minimum technology-based controls for communities to use to address CSO problems.
• Combined Sewer Overflows Guidance For Long-Term Control Plan  – Describes how municipalities can develop comprehensive long-term control plans (LTCPs) that recognize the site-specific nature of CSOs and their impacts on receiving water bodies.
• Guidance: Coordinating Combined Sewer Overflow (CSO) Long-Term Planning with Water Quality Standards Reviews  – Addresses impediments to implementing the water quality-based provisions of the CSO control policy and actions states and CSO communities should take to overcome them.
• Combined Sewer Overflows Guidance for Monitoring and Modeling  – Provides guidelines for using monitoring and modelling development and implementation of a CSO control program.