Systems of pipes and pumps that transport wastewater to wastewater treatment plants are called sanitary sewers. Water used in homes or industry is flushed through their pipes until it reaches local sewer mains owned and operated by municipal or regional sewer departments.

Sewer mains flow into progressively larger pipes until they reach the wastewater treatment plant. In the ideal case, a sewer system is completely gravity-powered. In situations where gravity cannot do all the work, the sewer system includes grinder-pumps or lift stations to move the wastewater to the treatment plant. Manholes allow access to the sewers by means of vertical openings and covers.

MassDEP has responsibility for ensuring that sanitary sewer systems are in compliance with the requirements of the Massachusetts Clean Waters Act and the regulations adopted under 314 CMR 1.00 through 9.00.

Proper operation and maintenance of sanitary sewers is critical to public and environmental health. This website has guidance for owners, managers and operators seeking to optimize operation of their systems as well as information about notifying MassDEP about emergency overflows, bypasses, and sewage backups.

Infiltration and inflow (I/I) is groundwater, rainwater and snow melt that enter sewer systems through defects in sewers or illegal connections. I/I reduces the capacity of sewer systems and treatment facilities to transport and treat wastewater. During periods of high groundwater and large or sudden storm events, I/I entering sanitary sewers may cause surcharging, wastewater backups into homes and businesses, and inadequate treatment. The policy available on this website is intended to help municipalities manage infiltration and inflow. In rare instances, property owners may experience flooding and sewage backups. 

Newer sewer systems were built with separate systems for sanitary and stormwater flows. However, older cities across the state may still have combined systems designed to carry both sanitary sewage and stormwater in the same pipes. Combined sewer overflows, or CSOs, were built as part of sewer collection systems that were designed to carry both sewage and stormwater in the same pipe. Combined sewer systems have regulator structures that allow overloaded systems to discharge into rivers, lakes and coastal areas subjecting them to higher pollutant loads. When there is not a lot of stormwater, this mix is transported to a wastewater treatment plant where it is processed. However, after heavy rainfall or snowmelt, stormwater and sewage overload the system. This can compromise a water body's uses and lead to water quality violations in the receiving waters. Without CSOs, this mix would back up into homes, businesses, and public streets.  CSO discharges are regulated by MassDEP and US EPA in accordance with state and federal CSO policies and the State Water Quality Standards. Massachusetts has 24 CSO permittees that have National Pollutant Discharge Elimination System (NPDES) permits issued by EPA Region 1 and MassDEP's Surface Water Discharge Permitting Program.