Like many older American cities, much of New York's sewer system has one set of pipes for both stormwater and wastewater. The major issue with combined sewer systems is that during heavy rain (or even light rain), water treatment plants are overwhelmed, and excess water, a mix of raw sewage and runoff, must be shunted directly into waterways. Every year, the overflows total over 17 billion gallons of polluted water in the city's waterways.
Combined sewer overflows are dangerous to both the environment and public health. Raw sewage carries disease-causing pathogens. Industrial run-off and water flushed from streets is contaminated with solvents, heavy metals, pesticides and debris. Overflows are the biggest source of pollution in the region's water, and are a special problem in New York's two Superfund waterways, the Gowanus Canal and Newtown Creek.
The sewer system is under the charge of Department of Environmental Protection. DEP has modelled what rain storms lead to overflows, and helpfully publishes alerts when rainfall indicates an upcoming overflow. @combinedsewer rephrases those alerts, inserting news about often-ignored waterways into Twitter's real time stream of news and talk.