CSOs 101: What is a “CSO”?

 CSO Diagram Source: Jersey Waterworks

Combined Sewer Outfalls (CSO) are where sewage overflows occur when the combined sewer system is overwhelmed by stormwater during rainy weather and sewage mixes with stormwater and overflows or discharges into a waterbody like the Alewife Brook. CSOs are a relic of outdated Combined Sewer Systems (also known as “one pipe systems”) from the 1800s, when flush toilets were an exciting technology that was being quickly adopted. Many older systems such as in Cambridge and Somerville were built this way.  Arlington’s was built later and has no combined sewers and no CSOs.

During dry weather, the sanitary sewage, which is what you flush, goes to the wastewater treatment plant (“POTW” or publicly operated treatment works). During wet weather, sanitary sewage mixes with stormwater in the same pipe and flows over a weir, or dam, and out into the receiving waterbody.

How should the sewers work?

Stormwater runoff, which is rainwater from the street, should be filtered through rain gardens or wetlands, where the water is naturally filtered and cleaned, and then it should flow through storm drains into our rivers and eventually out to sea. Sanitary sewage should all go to the waste water treatment plant.

How many CSOs are there in the Alewife Brook?

Twenty years ago, there were twelve CSOs in the Alewife Brook. As part of the landmark Boston Harbor Cleanup court case, Cambridge and Somerville closed six of the CSOs over the period of a decade or so. They didn’t close them all because they couldn’t afford to do them all at once. So they closed half of the CSOs and we now have six CSOs that discharge untreated sewage water into the Alewife Brook.

Alewife CSO Map from the MWRA website

Don’t we have to have CSOs if we don’t want sewage backing up into people’s homes and businesses and roads?

We do not have to have CSOs. They are a relic of very old and aging sewer system which is begging to be upgraded. If the sewer system infrastructure is updated by being separated and possibly by adding capacity, then the sanitary sewage can be pumped out to the wastewater treatment plant and it will not end up into homes, roads, or businesses.

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