Sanitary Sewer Overflows (SSOs) occur when raw or diluted sewage is discharged from a sewer collection system before it reaches the sewage treatment plant.  SSOs threaten public health because they can expose people to disease-causing organisms called pathogens.  In the photo above, untreated sewage overflows from a sewer manhole onto a City street.  This SSO was caused by extraneous stormwater that entered the sewer system.    [Close window]