Some older cities have a combined sewer system that treats rainwater with their sewage. The problem with sending rainwater to a treatment plant is that that there is just so much of it. For example, if one inch of rain falls on Durham that is almost 2 billion gallons of water. Durham has 2 wastewater treatment plants that are each permitted to treat 20 million gallons of wastewater per day. At that capacity the city would need 90 treatment plants to handle 1 inch of rain. Cities with combined sewer systems often have sewer system overflows with heavy rains. This releases bacteria, pathogens, toxic chemicals, and debris into the environment.
Currently the city uses other methods to treat stormwater that include practices such as stormwater wetlands, bioretention areas, and vegetated buffers. The cost of adding such items to a site after it is developed is about $50,000/acre of impervious area. These high costs to remove pollution from our stormwater are why it is so important to prevent the pollution.