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These images show the difference between a combined sewer overflow system found in many older cities, and a sewer system where sanitary and stormwater are completely separated. During heavy rains combined sewer overflow systems mix raw sewage with rainwater runoff and discharge it directly into the nearest waterbody without treatment. Photo: Washington DC Water and Sewer Authority.

completely separate sanitary and stormwater sewer system

 

 

combined sewer systems

 

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