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About Combined Sewer Overflow (CSO), Retention Treatment Basin (RTB), Sanitary Sewer Overflow (SSO) and other discharges

Combined Sewer Overflows (CSO)

A combined sewer is a sewer that is designed to carry both sanitary sewage and storm water runoff. A discharge from a combined sewer system, referred to as a combined sewer overflow (CSO), occurs in response to rainfall and/or snowmelt because the carrying capacity of the combined sewer system is exceeded. These discharges do not receive all treatment that is available and utilized under ordinary dry weather conditions (normally during dry weather conditions the wastewater is transported to a wastewater treatment facility where it receives appropriate treatment prior to discharge). Untreated CSOs are required to be controlled by either elimination (via sewer separation projects) or adequate treatment (see RTB). Currently all remaining untreated CSOs in Michigan are under schedules to be controlled. These schedules are included in permits, orders, or other enforceable documents issued by the DEQ or by court action.

Find out who to contact for more information on CSOs.

Retention Treatment Basins (RTB)

RTB discharges are treated discharges from facilities installed to collect and adequately treat combined sewer system overflows. RTBs operate under National Pollutant Discharge Elimination System (NPDES) permits, to meet state and federal combined sewer overflow control requirements. RTBs are designed and constructed to be protective of water quality and public health.

Sanitary Sewer Overflows (SSO)

Sanitary sewer overflows (SSOs) are discharges of raw or inadequately treated sewage from municipal separate sanitary sewer systems, which are designed to carry domestic sanitary sewage only. 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.  In recent years, there has been much effort by municipalities across the state and the DEQ to identify SSOs and correct SSO discharges.

Most SSOs are associated with wet weather conditions, when sanitary sewer systems receive storm water in-flow or infiltrating groundwater.  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 occur frequently in a particular system, it is likely that the system is not functioning properly, and such chronic problems must be addressed to eliminate the SSOs.

Problems that may cause chronic SSOs include excessive 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; and power failures.

The DEQ 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. The DEQ has developed a SSO Policy and Clarification Statement to address SSOs. Many communities with known chronic SSOs are currently under corrective action programs in accordance with the SSO Policy and Clarification Statement, to meet state and federal SSO correction requirements. The DEQ continues to work with municipalities across the state to correct SSOs as they are identified.


As a result of wet weather events, some Wastewater Treatment Plants (WWTPs) experience higher influent flows due to significant levels of infiltration/inflow in the associated separate sanitary sewer system. In order to accommodate these higher influent flows in their wastewater treatment operations, some WWTPs have designed a “blending” option at their facilities (“blending” used in this discussion does not include combined sewer systems). Generally during blending, the wastewater receives primary treatment and disinfection but a portion of the wastewater will not receive secondary treatment. The term blending generally refers to the mixing of the wastewater that received full (i.e., secondary) treatment with a component of primary-treated wastewater that has been diverted around the secondary treatment process. This mixed wastewater is disinfected and then discharged from the facility.

The term blending is used specific to an anticipated wet weather treatment scenario approved by DEQ through issuance of a NPDES permit as an interim means to address higher influent flows as a result of wet weather conditions during storms at or below the DEQ remedial design standard (see DEQ SSO Policy & Clarification Statement). Currently there are very few approved blending scenarios in Michigan. It is important to note that the discharges associated with these approved blending scenarios are required by the NPDES permit to produce effluent quality that complies with the Michigan Water Quality Standards (WQS).


Bypassing is the diversion of waste streams from any portion of a treatment facility other than approved blending described above. Similar to blending, bypassing can be the result of elevated influent flows due to wet weather events. However, bypassing may occur in the absence of a wet weather event due to equipment failure or some other difficulties at the WWTP. In contrast to blending, bypassing is not a pre-approved treatment scenario and is not authorized in NPDES permits except to prevent severe property damage or loss of life. Further, unlike the approved blending scenarios, the discharges associated with bypassing events may or may not be compliant with the Michigan WQS. Instead, this wastewater is variable and often partially treated (usually through the treatment train up to the process unit that failed). For example, if a malfunction shut down the mechanical disinfection system, then the wastewater would have received secondary treatment but not disinfection, and this would be considered a bypass. In general, the DEQ requires that the factors contributing to the occurrence of the bypass event be corrected as soon and as aggressively as is feasible. Home   |  DEQ_Home  |   Online Services  |   Permits  |   Programs  |   Contact_DEQ
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