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Water and Wastewater Plant Directory
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Duluth, Minnesota, USA
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Western Lake Superior Sanitary District
2626 Courtland Street
Duluth, Minnesota, 55806
USA
Contact Information:
Contact:
Email:
Telephone: 218-722-3336
Fax: 218-727-7471
Plant Operation: Municipal
Processing: Wastewater
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Web site:
http://www.wlssd.duluth.mn.us/
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Plant/Process Description:
The District's regional treatment plant is
designed to treat an average daily flow of 43
million gallons of wastewater each day. Since
1985, the District has worked with local
businesses to monitor industrial discharges.
Major industrial contributors are regulated under
the Industrial Pretreatment Program through a
series of routine monitoring, self-monitoring and
reporting, and on-site inspections. The WLSSD
also operates a nationally recognized pollution
prevention program.
Pollution prevention is based on the simple but
powerful idea that it makes more sense to stop
producing waste, than to develop treatment and
disposal technologies to manage it. Through the
WLSSD's commitment to Zero Discharge and its
pollution prevention program, the District aims
to provide wastewater treatment and biosolids of
the highest quality.
Collection System
Wastewater from cities and industries throughout
the WLSSD is transported to the regional
treatment plant through a 50-mile network of
interceptor sewers. Sixteen pumping stations are
necessary to move wastewater from as far away as
Wrenshall and Jay Cooke State Park.
Preliminary Treatment
In-plant pumping: Wastewater enters the plant
from the east and west. Wastewater flows are
lifted into the plant by five 50 foot long screw
pumps. Each screw is 72 inches in diameter,
driven by a 75 horsepower motor, and pumps 8,333
gallons a minute.
Wastewater from the Knowlton Creek pump station,
including industrial flows from the mills in
Cloquet and Duluth, arrives via forcemain at the
total plant influent channel.
Bar screens and grit removal: After entering the
plant, wastewater passes through large
mechanically cleaned bar screens into grit tanks.
The bar screens remove sticks, rags and other
large debris which could damage process
equipment. Sand and gravel in the sewage flow
settle out in the grit tanks as the wastewater is
detained before entering the next stage of
treatment.
Secondary Treatment
Oxygen generation: The oxygen purification system
removes impurities from air taken from the
atmosphere. Two 40-ton per day oxygen production
plants produce sufficient high-purity oxygen to
meet the biological needs of the activated sludge
process. Two horizontal, liquid oxygen storage
tanks help met peak demand and provide a backup
source of liquid oxygen.
Activated sludge process - aeration
Wastewater, return activated sludge and pure
oxygen are mixed in the aeration tanks. Each tank
is actually a concrete box measuring 56 feet on
all sides with a depth of 18 feet. The plant has
four trains with 4 stages per train, a total of
16 tanks. Wastewater passes from the grit tanks
to the oxygenation tanks, where large mixers
dissolve oxygen into the sewage. The oxygen is
required to maintain a suitable living
environment for bacteria, which feed on organic
matter in the sewage. Natural processes convert
pollutants to microorganisms. The same process
occurs in a natural waterway, only at a slower
pace.
Activated sludge process - settling
Wastewater then flows into four large settling
tanks. Each tank is 160 feet in diameter. Here
wastewater is detained to permit solids suspended
in the water to flocculate and sink to the
bottom, or float to the top. Large sweeping arms
remove settled and floating materials from the
tanks. Most sludge is returned to the aeration
basins to be mixed with more wastewater and
oxygen and to repeat the process. A small amount
of sludge and floating materials are pumped to
the sludge processing area.
Tertiary Treatment
Mixed media filters: After flocculation,
wastewater flows through mixed media filters.
Each of the twelve filters is a layered bed
containing mixed media: (from top to bottom)
medium grain anthracite coal,silica sand, garnet,
fine gravel, and finally medium sized gravel.
Solid particles in the wastewater are trapped in
the media. The filters are automated to clean
themselves when dirty.
The washwater is pumped to the head of the plant
for treatment. These filters are similar to those
used at the Duluth Filtration Plant for our
drinking water.
Disinfection: From the mixed media filters
wastewater flows into chlorine contact tanks,
where chlorine is added to kill remaining
bacteria. Uncombined chlorine is then removed in
the dechlorination tanks.
Solids Management
Sludge processing dissolved air floatation sludge
thickeners: The floatation sludge thickeners draw
water out of sludge generated during the various
treatment processes. Floatation thickening
increases the solids content of the
sludge.
Biosolids production: Sludge from the thickeners
is pumped into one of four 1-million gallon tanks
where the sludge is further processed to produce
biosolids.
Biosolids are the nutrient-rich organic product
of wastewater treatment. Biosolids are a
valuable resource containing essential plant
nutrients and organic matter and can be recycled
as a fertilizer and soil amendment.
WLSSD's produces biosolids through a process
called Anaerobic digestion. Anaerobic digestion
is a two-stage process. In the first stage,
wastewater solids are heated to 131 degrees F for
5 days. The high temperatures kill harmful
bacteria and viruses. Heat-loving bacteria
reduce the solids and generate methane and carbon
dioxide in the process. In the second stage, the
material is further broken down at 95 degrees F
for another 15 days.
When this process is finished, the remaining
material is biosolids. To prepare the biosolids
for land application the WLSSD removes excess
water. The water is returned to the "head" of
the plant. The final biosolids product has an
ammonia-like odor and has the appearance of damp
soil. The biosolids are then managed through the
WLSSD's anaerobically digested biosolids land
application program.
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Owner:
Western Lake Superior Sanitary District
Web site:
http://www.wlssd.duluth.mn.us/
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Operating Company:
Western Lake Superior Sanitary District
Web site:
http://www.wlssd.duluth.mn.us/
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Directions:
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Disclaimer:
Water and Wastewater.com provides the information on
this page for reference purposes only. We do not certify
the accuracy of the information provided by third-party
individuals for this listing.
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