While some communities and cities have separate systems to collect and divert stormwater and sewage, many older American cities were built on the model of a combined system, meaning that rainwater flows into the same pipes that carry waste from your home for treatment. Given that there is half as much pipe, CSOs are certainly easier and cheaper to install but their long term function brings an environmentally expensive drawback. When the rate of rainfall reaches a certain threshold (sometimes as low as 1/4″ per hour), the system of pipes becomes overwhelmed and treatment facilities can no longer handle the excess load. In these storm events, overflows are utilized that dump the combination of stormwater and untreated sewage directly into natural bodies of water. Pretty disgusting.
The grotesque nature of the practice could lead one to think this is a rare event that we are continually trying to fix as a society, but that would be wrong. According to the Environmental Protection Agency, there are 772 communities in the U.S. with CSO systems with a total of 9,348 outfalls into water bodies that affect 40 million Americans. These are not all small, rural hamlets either, but familiar destinations like Syracuse, Baltimore and New York City and many existing systems have components over 100 years old.
According to the National Swimming Pool Foundation, there are roughly 10 million swimming pools in the U.S. consisting of 6 million in ground pools and 4 million above ground pools (each holding an average of 20,000 gallons and 10,000 gallons respectively). This means that over 160 billion gallons of water is filled into residential pools every year. This stands as a pittance compared to estimate of national CSO outfall volume that the EPA reported to Congress in 2004: 850 billion gallons of untreated waste and storm water, or enough to fill all of our pools 5.3 times over.