New York City has decided to put chlorine into raw sewage to mitigate health and environmental impacts of sewage discharge into local waterways during rainfall events. Combined sewer overflows (CSOs) occur when sewage flow must be released into waterways during large rainstorm events to prevent urban flooding. City officials believe the addition of the right concentration of chlorine will decrease the negative affects sewage has on the environment. However, many environmental advocates believe the plan will only increase risks to local bodies of water. Both sides agree that mitigation techniques must be increased to adequately remediate the risks that sewage has on water quality and recreation. 20 billion gallons of sewage is released into three bodies of water in the Bronx annually to prevent sewage overflow and consequent flooding in the city. New York City has a combined sewage collection system designed to collect sewage and stormwater. Sewage from commercial, domestic and industrial sources flow through the same piping that collects storm water. Public water treatment works (POTWs) adequately disinfect flows during dry periods and small rain events, but CSOs occur when storm water events overcome sewage treatment plant capacity. Combined sewage collection systems are no longer installed because of the pollution that they produce in local bodies of water. Completely removing and replacing New York’s combined sewage collection system with infrastructure that separates sewage and storm runoff would be very costly for the city’s residents. Instead, the city’s Department of Environmental Protection has undergone many projects to adapt its current system. The city has spent over $10 billion to improve water quality and upgrade water treatment plants over the last decade. $1.5 billion more is expected to be spent on sustainable infrastructure by 2030. Chlorinating the sewage will come at a cost to residents; however city officials believe it is a cost effective measure to minimize impacts from sewage discharge. Dumping sewage water directly into waterways has ecological, health and economic impacts. Water will typically turn green and increase the risk of viruses in the water. Recreational use decreases due to health hazards and unaesthetic appeal, causing economic loss. New York City plans to mitigate these impacts by chlorinating its sewage water. “They’re using the most worrisome and unproven technique that we have in our toolbox,” said Sean Dixon, a staff lawyer at Riverkeeper, a local group that aims to protect the Hudson River and its tributaries. Chlorination is normally done in a more controlled setting, instead of directly discharging it into waterways. Also, it is challenging to add the correct concentration of chlorine because it is difficult to predict how much storm water will be flowing through the system. Chlorine causes low levels of environmental harm, although some aquatic animals could experience chlorine poisoning. City officials believe the benefits chlorination has of killing bacteria and other microbes in the sewage water outweigh its cost and minimal health and environmental impacts.