About Combined Sewer Overflows and Sanitary Sewer Overflows

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 occurs in response to rainfall and/or snowmelt because the carrying capacity of the 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). Both the combined sewer overflow structure and the discharge from the structure are referred to as "CSOs".

Find out who to contact for more information on CSOs.

Sanitary Sewer Overflows (SSO)

Sanitary sewer overflows (SSOs) differ from combined sewer overflows (CSOs).  CSOs are overflows from older sewer systems designed to carry both domestic and storm water loads.  SSOs are discharges of raw or inadequately treated sewage from municipal seperate 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 20 years, the MDEQ and its predecessor agency 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 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 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; and power failures.

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.   Recently, the MDEQ entered a settlement agreement with Clinton Township that required a multi-million dollar corrective program and the payment of $250,000 in penalties for past illegal discharges.  More recently, the MDEQ has become aware of additional SSO discharges.

Oftentimes, 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 expand efforts to identify and address these currently unknown SSOs.

The entire SSO strategy is available in .pdf format below.  This file can be viewed and printed on any computer with Adobe Acrobat Reader, available free of charge from http://www.adobe.com/.

Strategy for the Regulatory Control and Correction of Illegal Overflows from Separate Sewer Systems in Michigan.(18K - pdf format)