Combined Sewer Overflows (CSOs), Sanitary Sewer Overflows (SSOs), and Nonpoint Sources

Combined Sewer Overflows (CSOs), Sanitary Sewer Overflows (SSOs), and Nonpoint Sources.

Overflows from combined sewer and sanitary sewer collection systems have been recognized as difficult problems requiring solution, especially for many of the older cities in the United States. The problem has become more critical as greater development changes the amount and characteristics of storm water runoff and increases the channelization of runoff into storm, combined, and sanitary collection systems. Combined systems carry a mixture of wastewater and storm water runoff and, when the capacity of the interceptors is reached, overflows occur to the receiving waters. Large overflows can impact receiving water quality and can prevent attainment of mandated standards. Recreational beach closings and shell-fish bed closures have been attributed to CSOs (Lape and Dwyer, 1994). Federal regulations for CSOs are still under development and have not been issued at the time of writing this text (2001).

A combination of factors has resulted in the release of untreated wastewater from parts of sanitary collection systems. These releases are termed sanitary system over- flows (SSOs). The SSOs may be caused by (1) the entrance of excessive amounts of storm water, (2) blockages, or

(3) structural, mechanical, or electrical failures. Many overflows result from aging collection systems that have not received adequate upgrades, maintenance, and repair. The U.S. EPA has estimated that at least 40,000 overflows per year occur from sanitary collection systems. The untreated wastewater from these overflows represents threats to public health and the environment. The U.S. EPA is proposing to clarify and expand permit requirements for municipal sanitary collection systems under the Clean Water Act that will result in reducing the frequency and occurrence of SSOs (U.S. EPA 2001). At the time of writing this text (2001) the pro- posed regulations are under review. The U.S. EPA estimates that nearly $45 billion is required for constructing facilities for controlling CSOs and SSOs in the United States (U.S. EPA, 1997).

The effects of pollution from nonpoint sources are growing concerns as evidenced by the outbreak of gastrointestinal illness in Milwaukee traced to the oocysts of Cryp- tosporidium parvum, and the occurrence of Pfiesteria piscicida in the waters of Mary- land and North Carolina. Pfiesteria is a form of algae that is very toxic to fish life. Runoff from pastures and feedlots has been attributed as a potential factor that triggers the effects of these microorganisms.