Every day, wastewater goes down toilets and drains in homes, schools, businesses and factories and then flows into sewer system. Runoff from rain, street and sidewalk washing, and other outdoor activities flows into catch basins in the streets and from there into the sewers. In some neighborhoods, runoff from the streets is carried by separate storm sewers directly to local streams, rivers and bays. In most areas of the City, sanitary and industrial wastewater, rainwater and street runoff are collected in the same sewers and then conveyed together to the City\'s treatment plants. This is known as a combined sewer system. Sometimes, during heavy rains or snow, combined sewers fill to capacity and are unable to carry the combined sanitary and storm sewage to the plants.When this occurs the mix of excess storm water and untreated sewage flows directly into the City\'s waterways. This is called combined sewer overflow. Approximately 70 percent of the City sewers are combined.
Wastewater treatment plant, also called sewage treatment plants or water pollution control plants, remove most pollutants from wastewater before it is released to local waterways. At the plants, physical and biological processes closely duplicate how wetlands, rivers, streams and lakes naturally purify water. Treatment at these plants is quick, taking only about seven hours to remove most of the pollutants from the wastewater. In the natural environment this process could take many weeks and nature alone cannot handle the volume of wastewater that produces.
At the City\'s wastewater treatment plants, wastewater undergoes five major processes: preliminary treatment, primary treatment, secondary treatment, disinfection and finally, sludge treatment. Primary and secondary treatments remove about 85% to 95% of pollutants from the wastewater before the treated wastewater is disinfected and discharged into local waterways. Sludge, the byproduct of the treatment process, is digested for stabilization and is then dewatered for easier handling. The resulting material, known as biosolids, is then applied to land to improve vegetation or processed further as compost or fertilizer,