Combined Sewer Overflow Program
Massachusetts Water Resources Authority

What is a CSO?

Some older sewer systems, like Boston's, have "combined" sewers that carry both storm water and sewage flows together. During heavy rains, when flows sometimes double and even triple, these systems can become overloaded.

Combined Sewer Systems (CSOs) are designed to act as relief points. They discharge excess flows upstream from sewage treatment plants, into the nearest body of water, to prevent backups into homes and area streets.

Boston, Cambridge, Chelsea and Somerville have combined sewer systems that connect to MWRA's sewer system.

Boston Harbor and the Charles and Mystic Rivers are subject to overflows of combined storm water and sewage during heavy rains.

How CSOs work
Dry Weather During dry weather, CSOs do not discharge into bodies of water. Their flows are carried to treatment facilities.	Larger image
Heavy Rains In heavy rainstorms, CSOs act as relief points for the sewer system. They discharge into the nearest body of water in order to prevent sewage backups into homes and onto streets.	Larger image

December 15, 2004