What's the Difference?  Storm/Sanitary & MS4/CSO

To combat this, modern storm sewers have moved away from the "one pipe system" of the past and separate wastewater from storm runoff.  Most of the storm sewers in the Greater Lansing area are what we call Municipal Separate Storm Sewer Systems (MS4s). Here, the sanitary sewer still directs sewage to the treatment facility, but stormwater is diverted directly to water bodies.

The MS4 system reduces the risk of large quantities of sewage entering the surface water, but since stormwater doesn't go to the treatment plant, it has it's own risks and must be "treated" in other ways.  As illustrated in the graphic to the right, MS4s rely on catch basins and built infrastructure to filter out large pieces of litter, sediment, and oils, but these systems only work if they are cleaned, maintained, and understood by the public.  If you see someone pouring anything down a storm drain, report it! Also report clogged or dirty catch basins to your municipality.  


By reducing runoff, picking up after your pet, and keeping illicit discharge out of the drains, you can keep it out of the watershed. 

It's easy to get confused.  

You wouldn't be alone in thinking that communities have one sewer system, but there are typically two: a storm and a sanitary sewer.

Storm sewers redirect runoff formed during rain or snow events into a system of catch basins and pipes.


Sanitary sewers collect the wastewater from toilets and taps.  This water is sent to a treatment plant to be cleaned.    

Here's where things get more tricky:  there are also two types of storm sewers.  Combined Sewer Overflows, or CSOs, are older systems but still remain in use in some areas.  This "one pipe system" combines both surface runoff (stormwater) with wastewater (sewage) and leads to a treatment plant for processing.  However, as the illustration shows, these systems were designed to overflow when a heavy rain or snow melt event backlogged the system.  When this occurs, the mixture of stormwater and raw sewage would flow directly into a river, stream or lake.  

Untreated sewage entering our surface water had obvious impacts to the water quality and ecosystems that depend on these habitats. 

(A typical catch basin)

(Combined Sewer Overflow "one pipe system")

(Diagram of an MS4)


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The GLRC is supported by the Tri-County Regional Planning Commission
3135 Pine Tree Rd. Suite 2C, Lansing, MI 48911