New York City's sewer system is over a century old. In much of the city, stormwater and sewage still flow through the same pipes. When it rains, even just a 10th of an inch in an hour, the system overflows. These combined sewer overflows (CSOs) happen around 90 to 100 days a year, releasing an estimated 27 billion gallons of untreated waste directly into local waterways. That's the water oysters are expected to filter.