Wastewater Treatment Process
The District's regional wastewater treatment plant at 27th Avenue West and Courtland Street in Duluth treats about 40 million gallons of wastewater from homes, business and industries every day. The treatment process utilizes beneficial bacteria similar to those found in area rivers. Clean water and biosolids are the end products of this process. Wastewater Treatment Fact Sheet (PDF).
Approximately half of the wastewater comes to WLSSD from industries. The remaining water comes from homes and businesses. Wastewater is transported to the regional wastewater treatment plant through a 75-mile network of interceptor sanitary sewers. Sixteen pumping stations are necessary to move wastewater from as far away as Wrenshall and Jay Cooke State Park.
Sanitary sewers carry dirty water from homes and businesses. They are completely separate from the region's storm sewer system, which drains into area lakes and rivers. The majority of sewer lines in the region are managed by individual municipalities, not the WLSSD.
Wastewater flows or is pumped from the eastern and western portions of the sanitary district and is lifted into the wastewater treatment plant by large screw pumps. From the highest point of the plant, water flows primarily via gravity through the rest of the facility. Operations mimic the actions of a natural river.
Bar screens and grit removal
After entering the plant, wastewater passes through large 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 settle out in the grit tanks as the wastewater is detained before entering the next stage of treatment.
Oxygen generation: The oxygen purification system removes impurities from air taken from the atmosphere. Two oxygen production plants produce sufficient high-purity oxygen to meet the biological needs of the living, biological treatment process, known as "activated sludge".
Activated sludge process
Step one - aeration: Wastewater is mixed with oxygen and older wastewater ("return activated sludge") in the aeration tanks. Each tank is a concrete box measuring 56 feet on all sides with a depth of 18 feet. Large mixers and the serpentine flow dissolve oxygen into the sewage and provide a good living environment for beneficial bacteria. These microorganisms feed on organic matter in the sewage. As they thrive and reproduce, pollutants are converted into more bacterial bodies. This process mimics what happens more slowly in natural waterways.
Activated sludge process
Step two - settling: After the active mixing and bacterial action, wastewater 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 stick together (flocculate) and sink to the bottom of the tanks. Most sludge is then returned to the aeration basins to be mixed with more wastewater and oxygen and to repeat the process. A small amount of solid "sludge" is pumped to the solids processing area.
Mixed media filters
After settling and clarification, 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 wash water is pumped to the head of the plant for treatment.
From the mixed media filters wastewater flows into disinfection contact tanks. There, a sodium hypochlorite solution is mixed with the water to kill any remaining bacteria. Uncombined chlorine from that process is then removed in the dechlorination tanks.
Flotation thickeners use dissolved air to thicken sludge for more efficient processing. Thickened sludge is pumped to one of four 1-million gallon tanks to be further processed to become 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 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° F for approximately 10 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 about 95° F for approximately 10 days.
When this process is finished, the remaining material is biosolids. To prepare the biosolids for land application 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 distributed through WLSSD's biosolids land application program.