Treating the sewage from 8.6 million New Yorkers is no small task. 

The city has a vast sewer system with more than a dozen wastewater treatment plants to treat the sewage before it ends up back in local waterways. 

What type of sewer system does the city have?

In about 60 percent of the city, there is a combined sewer system, which means wastewater from toilets and drains, storm water and street runoff, such as water from washing a sidewalk, go into the same sewers that flow into treatment plants.  

The rest of the city uses a separate sewer system, with pipes carrying wastewater into treatment plants and separate pipes carrying storm water and street runoff directly into local waterways. 

There are challenges with both types of systems as combined sewer systems can overflow when there is heavy rain or snow, and separate sewer systems don’t treat storm water that could have picked up pollutants from the street, according to the Department of Environmental Protection.