CSO: Combined Sewer Outflow

During 1997, the Allegheny County Health Department issued health advisories on 45 days of the recreational boating season urging residents to avoid contact with river water. The need for these warnings traces to wet weather sewage overflows and sewage system bypassing occurring in municipal systems. Combined sewer overflows (CSOs) occur at structures designed to release mixed sewer and stormwater when flows in combined sewer lines exceed system capacity. They are legally permitable under the National Pollution Discharge Elimination System, but must be reduced. The regional sanitation provider, Allegheny County Sanitary Authority (ALCOSAN), is due to release its Long Term CSO Control Plan in late 1998 or early 1999.

Sanitary sewer overflows (SSOs) can result from illegal connections of roof and basement drains and infiltration into cracked or disjointed sanitary sewer lines. Excess flows leak out through cracks or by pushing off manhole covers as pressures mount during wet weather. In addition, when the ALCOSAN regional system was designed in the 1940s, dedicated overflow structures were built into sanitary as well as combined sewer lines, because older urban sanitary sewers were known and expected to include many roof and basement drain connections. Federal and state regulatory agencies now consider SSOs illegal. Responses to the SSO problem have been many. A number of communities have built or are considering water storage tanks and detention basins to hold excess wet weather flows. Projects to rehabilitate cracked sanitary sewer lines or replace those lines altogether are underway or in the works in several municipalities.

What are CSOs, and why are they important?

CSOs are remnants of the country's early infrastructure. In the past, communities built sewer systems to collect both storm water runoff and sanitary sewage in the same pipe. During dry weather, these "combined sewer systems" transport wastewater directly to the sewage treatment plant. In periods of rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, lakes, or estuaries.

Combined sewer overflows (CSOs) contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. This is a major water pollution concern for cities with combined sewer systems. CSOs are among the major sources responsible for beach closings, shellfishing restrictions, and other water body impairments.



Stormwater Management Charrette, NMRGP

EPA: Combined Sewer Outflows

Nine Mile Run Greenway Project
STUDIO for Creative Inquiry
Carnegie Mellon University