Overflows
What is a Sanitary Sewer Overflow?
Average, daily flow of water through a community’s Sanitary
Sewer system can be very different than the flow seen during a
large rain storm or in Spring during high snowmelt runoff. This
increase in water does not mean that residents are suddenly creating
more wastewater (water that goes down drains and toilets). It
usually means that “extra” “clear” water
is getting into the sanitary sewer system through the process
of Inflow and Infiltration.
 Wastewater plants are sized to handle much more than “average” daily
flow. In Duluth, average daily flow to the Western Lake Superior
Sanitary District is about 48 million gallons per day. The treatment
facility can handle up to about 120 million gallons per day. On
rainy days, this flow may rise to well over 100 million gallons
per day, and in extreme situations, considerably more. See what
happened at WLSSD from a huge
storm on July 3, 2003. How can this happen?
When a lot of “extra” water that should be handled
by the storm sewer system, that drains water from parking lots
and roads, seeps into the sanitary sewer system, the capacity
of the pipes in the collection system can be overloaded. Water
cannot reach the treatment plant, as the pipes become pressurized
and the system overflows before the water is delivered to the
treatment plant. This is called a sanitary sewer overflow (SSO).
This problem exists in many cities around Lake Superior and throughout
the U.S.. This can also occur during rain events in cities that
have a combined sanitary sewer and storm sewer system. Combined
sewer overflows (CSOs) are perhaps a more difficult problem because
the systems were never separate to begin with.
These overflows mean that untreated or partially treated wastewater
and sewage can back up into basements, run down streets or directly
enter water bodies such as creeks and rivers or even Lake Superior.
Untreated wastewater contains pathogens that can make humans sick,
and contain high levels of nutrients, organic matter and solids
that can cause algae blooms and other eutrophication impacts
in the water bodies.
Do Overflows cause Beach Closures?
Sanitary Sewer Overflows are undesirable situations that all residents
and agencies want to see stopped. It is true that overflows contribute
many pollutants including disease causing microorganisms (pathogens)
to water bodies. However, it is not evident that the indicator
bacteria seen during beach monitoring water
testing always come from sanitary sewer overflows. In fact, many
if not most beach closures occur during times when there have
been no sanitary sewer overflows. It is believed that bacteria
seen during beach tests comes from a variety of sources, including
animal feces (gulls, geese, pets), improperly disposed of diapers,
children not properly cleaned after using the bathroom, a vomiting
or fecal accident in the water, swimmers with diarrhea, malfunction
in septic systems in the vicinity, nearby boat moorings or marinas
releasing sewage into the water, and stormwater runoff into streams
and from direct sheet runoff that potentially includes all of
these other sources, as well as sanitary sewer overflows or treatment
plant malfunctions. Find out more about pathogens here and
about how to minimize your risks at the MPCA
Beach Website.
What is being done about Overflows?
The Environmental Protection Agency and state environmental
agencies such as the Minnesota Pollution Control agency are working
with cities to address this serious issue. The City of Duluth
and the Western Lake Superior Sanitary District have developed
a Plan
of Action (2.5 MB pdf) to prevent overflows.
Find out more about the progress of the plan at I&I or WLSSD.
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