Oscar Perry Abello wrote:

The acronym “CSO” stands for “combined sewer overflow,” and CSO points like these are a feature of combined sewer systems designed and built mostly more than a century ago. In combined sewer systems, storm drains capture rainwater runoff — and any pollutants it picks up along the way — and sends it into the same sewers that collect wastewater from homes and businesses. On most rainy days, it all flows to wastewater treatment facilities that are supposed to remove toxins and other pollutants before releasing it back into natural bodies of water.

During heavier rainfall, however, combined sewer systems can’t handle the volume, sending the mixture directly out from overflow points like CSO-049, one of around 50 such overflow points in the District. And today, because of climate change, heavy storms are occurring more frequently, causing more of these overflow incidents in cities that have combined sewer systems.


Most cities facing this challenge have responded using what’s known as “gray infrastructure” — digging massive new underground tunnels or enlarging existing ones to capture and store stormwater until it can flow to wastewater treatment plants. An alternative, which a growing but still small number of cities are starting to embrace, is “green infrastructure” — rain gardens, bioswales, tree trenches, permeable pavement, green roofs and other forms that combine old and new technology to absorb more rainwater where it falls.