Every year nearly 30 billion gallons of wastewater filled with untreated sewage and pollution overflows into New York City’s waterways. Last week New York Mayor Michael Bloomberg and the Department of Environmental Protection unveiled a proposal for a new “NYC Green Infrastructure Plan” to help reduce the city’s serious sewage overflow problem.

The proposal has two key components: widespread use of green infrastructure in public spaces — such as green roofs, parks, tree boxes, roadside plantings, porous pavement in parking lots, cisterns, and rain barrels — as well as the establishment of design requirements for developers to ensure that all private property projects manage a minimum volume of stormwater on site. Green infrastructure not only naturally absorbs excess stormwater and thereby reduces runoff and sewage overflow, but it also improves the city’s livability by adding more greenery, boosting local jobs, and leading to cooler overall temperatures. Bloomberg proposes these measures, alongside improving existing ‘gray infrastructure’ such as storage tanks and tunnels. It’s a hybrid strategy that the city believes is cheaper and more efficient than simply building more traditional infrastructure.

During heavy rains New York City’s combined sewer systems — which funnel wastewater full of raw sewage to the city’s treatment plants — become flooded by rainwater that is loaded with pollution and litter from city streets. When it rains just a tenth of an inch, the sewer systems can become overwhelmed, dumping sewage and garbage into the city’s waterways — from the Hudson River and New York Harbor to the Bronx River and even out to the beaches on Long Island (watch a video of a sewage overflow into Brooklyn’s Gowanus Canal caught on camera by a couple of bystanders).

This happens more than 50 times a year. “Not only does this muck contain disease-causing bacteria and toxic chemicals that can make us sick,” said Larry Levine, senior attorney for NRDC’s Water and Oceans Programs. “It also fouls our waters with excessive nutrients that breed algae blooms, essentially choking ecologically sensitive marine life.”