Combined Sewer Overflows (CSOs)

How to use the Combined Sewer Overflow (CSO) locations map:
  • Select one of the highlighted municipalities to display a pop-up window. 
  • Selecting "zoom to" will center the map on that municipality and display the location of the individual CSO locations. 
  • For additional information about a specific location, select the location to display a pop-up window.  (Additional information is available in the fact sheets.)
  • The map may also be zoomed in by selecting the + button or zoomed out by selecting the - button. 
  • To pan the map hold in the left mouse button and move the cursor. 
  • Selecting the legend button will display the map legend.
Note: Map may be slow to load

Sanitary sewers are pipes which carry water from homes and businesses (i.e. wastewater from toilets, laundry, bathing, dishwashing). 

Storm sewers are pipes that carry rainwater from storms. 

A combined sewer is a pipe which is designed to carry both types of flow.

In Connecticut, like in many other states, there are combined sewers that transport both wastewater (water and sewage that goes down the drain in homes and businesses) and stormwater (rain or snow that washes off streets and parking lots) in one pipe to a wastewater treatment plant.
When combined sewer systems were designed over 100 years ago, the combined sewer system was considered to be an economical way of managing both wastewater and stormwater together in the same pipe.  At the time, wastewater treatment plants did not exist, and all wastewater and stormwater was dumped into the nearest water body without any type of treatment.  It was less expensive to build a combined sewer system than to build and maintain two entirely separate systems of pipes and tunnels to transport wastewater and stormwater.

Combined sewer overflows (CSOs) were purposely designed in combined sewer systems to prevent back-ups of untreated wastewater into homes and businesses, flooding in city streets, or bursting underground pipes during heavy rains.  As sewer systems were updated, the CSO locations were left in place to act as safety valves when the pipes get too full to handle the high volume of water during heavy rains.  Overflows were considered to be a necessary way to manage excess water. 

The advantage of a combined sewer system is that, most of the time, both stormwater and wastewater are treated to meet water quality standards prior to discharge into a water body. 

The disadvantage of a combined sewer system is that during heavy rains, untreated stormwater and wastewater may be discharged at CSO locations.  Untreated wastewater can contain pathogens, excess nutrients, and chemicals, which is why CSOs represent a public health and environmental concern.

There are far fewer CSOs remaining in Connecticut today, as a result of construction projects that have been performed to control these overflows in most parts of the State.  Unfortunately, the mitigation of the remaining CSOs is not a quick or easy process.  Sewer separation projects are expensive and are complex to coordinate, especially in more densely populated urban areas.  Municipalities in Connecticut that still contain CSOs throughout their collection systems are actively working to reduce and eliminate sewer overflows.

Bypass / CSO Live Map:  View "real-time" sanitary sewer and combined sewer bypass information. The map will have a query function with the ability to export information.

Frequently asked Questions about Combined Sewer Overflows - General Information about CSOs

Fact Sheets describing the status and progress toward mitigating remaining CSOs:


Content last updated: March 4, 2020