Introduction for CSO Public Notification

The Commission is conducting a program to notify the public when overflows from combined sewers occur. This web page is part of this pilot program which the Commission will use to assess the feasibility of providing the public with information about overflows and to determine how long it will take to post this information. Overflows; for a number of reasons which will be discussed later, can't be measured and posted onto the Commission's web page right after they occur.

Combined sewers convey wastewater as well as stormwater. In the areas with combined sewers in Boston, this type of system provides the most practical solution for collecting water and conveying to a treatment facility. Combined flows as well as separate wastewater flows are collected by the Massachusetts Water Resources Authority's (MWRA) which treats these flows at its treatment plant on Deer Island.

During very large storms, some of the combined flows can overflow into Boston Harbor and consequently are not treated at the Deer Island Treatment Plant. About 50 storms with rainfall from 0.25" to over 4" occur on average each year. Only the largest of these storms can generate enough additional flow to cause overflows.

Whenever it rains, the water levels in the combined sewers rise to accommodate the additional stormwater. The combined sewers which are typically larger than the separate wastewater sewers, are designed to carry this additional flow. The combined sewer system contains provisions when it is necessary, to allow the flows to escape into Boston Harbor. Overflows into Boston Harbor are necessary to avoid flooding in the public streets. An explanation of how flows are diverted to an overflow pipe is provided later in this discussion.