Systems Planning

Sanitary Sewer Overflows

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​​The City of Ann Arbor sanitary sewer system collects and transports residential and non-residential wastewater and very limited amounts of stormwater and infiltrated ground water through over 500 miles of pipes to the waste water treatment plant for treatment. Sanitary sewers are different than combined sewers, which are designed to collect large volumes of stormwater in addition to wastewater.

Occasionally, sanitary sewers will release raw (or untreated) sewage. These types of releases are called sanitary sewer overflows (SSOs). SSOs can contaminate our waters, causing serious water quality problems, and back-up into homes, causing property damage and potentially threatening public health.3Ps-graphic.png

   Possible causes of SSOs include:

  • blockages,

  • pipe breaks,

  • sewer cracks that allow stormwater and groundwater to overload the system,

  • power failures, and

  • poor sewer design


The City of Ann Arbor is proactive in the maintenance of its system to prevent backups that are costly risks to public health and our water quality.  The most effective way you can prevent backups by flushing only the 3 p's (pee, poop and toilet paper) and to dispose of fats, oils and grease in the trash bin. SSOs that reach waters of the U.S. are point source discharges. Like other point source discharges from municipal sanitary sewer systems, SSOs are prohibited unless authorized by a NPDES permit. 

All SSOs are reported to the State of Michigan and are available for search here:

​Date(s)​Volume Discharged​Location​Reason for Discharge
​3/14/2020​~450 gallons​University of Michigan Arboretum​Debris (large piece of concrete brick) in sanitary pipe
​1/11/2020​~300 gallons​Wooded area in Mallet's Creek​Sewer discharge due to high flow during a wet weather event
​1/11/2020​~4,500 gallons​Swift Run Creek vacant land area, scrub/shrub area​Sewer discharge due to sewer capacity issue during a wet weather event
​8/26/2019​~2,000 gallons​Road surface in Allen Creek​Concrete debris blocking flow in sanitary pipe
​7/5/2019​~3,000 gallons​220 block of W. Liberty​Sewer main break adjacent to a 24" storm pipe
​6/23/2019​~500 gallons​Maple Village storm water catch basin​Grease clog in sewer pipe
​5/8/2019​~1,800 gallonsField near University of Michigan Arboretum​Mower knocked lid of manhole structure where gravel accumulated and blocked sewer pipe
​5/1/2019​~75,000 gallons​Wastewater Treatment East Plant Stormwater outfall​Plant effluent water from tertiary backwash wet well did not receive ultra-violet treatment
​4/23/2019​~10 gallons​Private residence​Non-standard sewer connection from a private property owner
​1/5/2019​~675 gallons​Outfall of enclosed system into Mallets Creek, north of Packard Rd.​Tree roots bl​ocked sanitary line causing a surcharge from sanitary manhole
​10/26/2018​~10,500 gallons​Wooded area on South East side of  Concordia University ​Flushable wipes clogged sewer line
​6/10/2018​~30,000 gallons​University of Michigan Nichols Arboretum​Sewer main blocked by 3,000 ft of rope
​4/3/2018​~110 gallons​Intersection of Warwick Rd and Glenwood RdTree roots in the sewer line
​2/21/2018​~600 gallonsWooded area rear yard in Millers Creek​Tree roots in sewer line
​2/20/2018​~25 gallons​Waste Water Treatment Plan Outfall 1​Snow and rain melt caused flow to go over weir spraying a light mist over the outfall to the river
​8/24/2017​~10 gallonsDexter Rd, east of Maple Rd​Grease blockage in a sewer line
​6/22/2017​~1,350 gallons​S State and Hidden Valley Club Drive​Line failure caused sewage backup
​5/7/2017​~10 gallons​Pine Tree Dr and Miller Rd​Tree roots blocking sewer line
​9/26/2016​~20 gallons​Geddes Ave, North of Devonshire Park​Broken bypass vale caused release to ground
​9/23/2016​~20 gallonsGeddes Ave, North of Devonshire Park​Broken hose bypass pump during sewer rehabilitation construction
​9/17/2016​~600,000 gallons​Geddes Ave North of Devonshire Park​Sewer main broken during construction activities
​5/13/2016​~3,600 gallons​Wooded area and vernal pond near private residence​Tree roots in sanitary pipe
​3/23/2016​~400,000 gallons​Huron Parkway in woodlot area​Tree root clog causing manhole overflow
​3/17/2016​~50 gallonsNear 325 Washtenaw Ave​Broken sewer pipe


​~30 gallons​300 S. Maple Road​Grease in a private lead plugged sewer line