Municipal Wastewater

Most communities operate a municipal wastewater treatment plant to clean wastewater created by homes and businesses. Wastewater moves through the sanitary sewer pipes by gravity or with the use of pumping stations. Wastewater flowing to a treatment plant will typically go through four phases of treatment, described below:

Primary Treatment: As wastewater arrives at the treatment plant, it passes through a series of grinders and screens to remove large debris, like sticks and rags. When the wastewater flow is slowed down, coarse sediment (sand and gravel) settles out and grease is skimmed off the top. Debris removed from wastewater is disposed of in a landfill.

Secondary Treatment: Microbes break down organics and pollutants in the wastewater as they feed on the nutrients. The wastewater is aerated to allow microbes to thrive in the wastewater environment. Solids, or “sludge”, settle to the bottom and are removed. This process mimics what happens naturally in rivers and streams.

Tertiary Treatment: Water passes through a filter to remove fine particles and it may also be disinfected. Before the water is released to a river or lake, it is must first meet strict regulatory standards for pollutants.

Solids Management: Solids that are removed during the treatment process may either be disposed of in a landfill or processed into biosolids (what we used to call “sludge”). Biosolids are treated further to remove disease causing microorganisms (pathogens) and can then be safely used as a fertilizer and soil amendment.

Here are some more specific details on the City of Duluth sanitary sewer collection system.

Superior, WI

The City of Superior is the largest contributor of wastewater to Lake Superior in the state of Wisconsin. Wastewater throughout the City is conveyed through approximately 80 miles of sanitary sewers and 80 miles of combined sewers to the treatment facility at the foot of E Avenue on Superior Bay. The Operations Section of the Wastewater Treatment Division of Superior's Public Works Department is responsible for operating and maintaining collection and treatment equipment that handles 3 to 5 million gallons of sewage entering the system each day. The main structures include 16 lift stations, one advanced secondary treatment plant, and 3 combined sewage overflow facilities. The Collection System Section is responsible for maintaining the City's 10,000 manholes and catch basins and over 160 miles of sewer pipelines.

The main treatment facility site contains of two independent treatment plants, the advanced secondary main facility and combined sewerage overflow plant #2 (CSO #2; see below). Plant processes include screening, grit removal, primary treatment, secondary treatment (activated sludge), anaerobic digestion of solids, phosphorus removal, disinfection and dechlorination. The main plant handles all sanitary dry weather flows up to 5 million gallons per day (MGD). This facility can also treat up to 12 MGD for short periods of time during wet weather events.

Like its neighbor Duluth, large increases in water enter Superior's wastewater system when it rains. Rainfall can enter sanitary sewer systems as inflow from yard, footing and roof drains and as infiltration from leaking sewer pipes, joints and manholes. Inflow and infiltration (I&I) of relatively clean water into the sewer pipes has caused sanitary sewer overflows (SSOs). Excess flow of water into wastewater plants can reduce effectiveness of the treatment process and exceed facility capabilities.

The City of Superior's flat topography combined with predominately clay soil, cause the sewer system to act as a trench drain at times. The City began a sewer separation project in 1975, which was intended to eliminate combined sewer overflows by installing stormwater relief lines. Approximately 50% of the combined sewers were technically infeasible or too expensive or to separate. The City also constructed retention basins adjacent the lift stations along the East 2nd street interceptor to prevent SSOs. (See the EPA website for more information.)

The City of Superior was one of the first municipalities in the nation to construct and operate combined sewer overflow (CSO) facilities. Three CSO facilities store and treat combined wastewater from sewer districts #2, #5, and #6. CSO2 serves the downtown area of Superior. CSO5 is located in South Superior. Billings Park is served by CSO6. Whenever significant rainfall occurs and the combined sewers fill to capacity, dilute wastewater in the combined districts flow to these CSO facilities to be stored until the waste stream can be pumped back to the main treatment plant. If needed, stored wastewater can be treated at each CSO facility prior to release to the environment.

Critical elements of Superior's pollution prevention program include a comprehensive pretreatment program, its membership in the Regional Stormwater Protection Team and other projects to educate and involve the public.

Visit the websites for the Western Lake Superior Sanitary District (servicing Duluth and surrounding communities) and the City of Superior for a more detailed description of their wastewater treatment processes. For more information about wastewater treatment in other communities, contact local officials.