Skip Navigation
MI.gov
DEQ - Department of Environmental Quality | DEQ Department of Environmental Quality | DEQ
Department of Environmental Quality | DEQ
Email this Page
Share this Link on Facebook
Tweet this page on Twitter!

Retention Treatment Basin, Combined Sewer Overflow, and Sanitary Sewer Overflow Discharge Information

Sanitary sewer overflows (SSOs) differ from combined sewer overflows (CSOs).  CSOs are discharges from older sewer systems designed to carry both domestic sewage and storm water, collectively referred to as combined sewage.  CSO discharges can either occur from untreated outfalls or treated outfalls.  Treated CSO discharges pass through Retention Treatment Facilities (RTFs), which are designed to capture combined sewage long enough to provide adequate treatment and disinfection.  Many RTFs generally comprise of screening/skimming equipment, storage basin for settling, and disinfection equipment.  Such facilities are referred to as Retention Treatment Basins (RTBs) and are designed in accordance with approved plans and operate under National Pollutant Discharge Elimination System (NPDES) permits.  SSOs are discharges of raw or inadequately treated sewage from municipal separate sanitary sewer systems, which are designed to carry domestic sanitary sewage but not storm water. These overflows may also contain industrial wastewater that is present in the sewer system.
 
When an SSO occurs, raw sewage may be released into basements, city streets, properties, rivers, and streams.  SSOs are illegal and often constitute a serious environmental and public health threat.  The number of communities that have SSO problems is not known.  The frequency and duration of SSOs are often unknown.   For the past 30 years, the MDEQ and its predecessor agencies have been working with municipalities across the state to identify SSOs and correct SSO discharges.
 
Most SSOs are associated with wet weather conditions, when sanitary sewer systems receive storm water via inflow and infiltration.  The SSOs may occur during extreme hydrologic events in many separate sanitary sewer systems, even though systems are intended to collect and contain all the sewage that flows into them.  When SSOs happen frequently in any given system, then the system is not functioning properly, and chronic problems must be addressed to eliminate the SSOs.
 
Problems that may cause chronic SSOs include too much infiltration and inflow into the sanitary system from groundwater infiltrating through cracks; rain water or snow-melt flowing into the sanitary system through roof drains connected to sewers; groundwater from footing drains and house leads connected to the sanitary system; undersized sanitary systems with sewers and pumps that are too small to carry all the sewage; system failures due to tree roots growing into the sewer, sections of sewer pipe settling or shifting so that pipe joints no longer match, or sediment and other material building-up causing blockages; equipment and pump failures; power failures; or other system failures or defects.
 
The MDEQ has broad statutory and regulatory authority to deal with SSOs under Part 31, Water Resources Protection, and Part 41, Sewerage Systems, of the Natural Resources and Environmental Protection Act, 1994 PA 451, as amended.
 
Under its authority, where the MDEQ has become aware of chronic SSOs, the MDEQ has taken enforcement actions requiring corrective measures to address the SSOs and their causes.   For many of these cases, the MDEQ entered into a settlement agreement with system owners that requires corrective programs to meet State and Federal requirements and the payment of penalties for past illegal discharges.  In some cases, the MDEQ is not aware of the occurrence of SSOs, as they are illegal and often not reported to the MDEQ by the municipalities.  It is the intent of the MDEQ to continue to expand efforts to identify and address unknown SSOs.