City of Duluth Sanitary Sewer Collection System

The City of Duluth sanitary sewers have a total length of approximately 1,831,000 feet (347 miles). The sewer system is divided into 30 basins. The system gravity feeds to Western Lake Superior Sanitary District (WLSSD) interceptors. Forty-three Pump Stations assure the continual movement of wastewater through the system. The system services approximately 26,000 residential accounts, 1200 commercial accounts and 4 industrial accounts. Approximately 18 million gallons of waste water flow through the system daily from the City. Another 25 million gallons per day (mgd) meet it at WLSSD after arriving from Cloquet, Wrenshall, Esko, Scanlon, Carlton, the Sappi Paper Mill (formerly Potlatch Paper), Hermantown, Rice Lake township, the North Shore of Lake Superior and Proctor. The combined flow of a bout 43 mgd is still well below the plant's peak capacity of about 100-120 mgd.

Significant portions of Duluth's sewer system were constructed in the early 1900’s with some sewers dating back to the 1870s. New sewers have been added to the system up to the present time as additional residential, commercial and industrial development has occurred. Sewers are updated as needed and following review as part of Duluth's Street Improvement Program.

The sewer system consists of a variety of pipe construction materials, including vitrified clay pipe (VCP), concrete, and brick arch pipe in older portions of the system and polyvinyl chloride (PVC and acrylonitrile-butadiene-styrene (ABS) in the more recent construction. There are also cast iron and ductile pipes primarily as pump station forcemains.

The entire system is mapped and all manholes and pipelines are assigned identification numbers. Mapping is available as hard copy and as computer data in a Geographic Information System (GIS) format. Additional data on the system is available in the engineering vault at the City Engineering Department.

Going Undercover.The city of Duluth can investigate problems with the sanitary sewer using a special camera connected to a robot. These pictures show rocks in the sewer line (left) and a leaking pipe joint (right).

The City maintains all public piping. The City of Duluth has an active maintenance program that targets cleaning of all sanitary sewers in a five-year cycle. The City has the capacity to televise the inside of sewers and uses this information to target repairs of City lines.

Sewer laterals from private homes are privately owned and the responsibility of the owner of the property. These connect the main drain from the home to the public sewer lines deep under the street. Homeowners should clean and maintain these systems to prevent plumbing backups. If problems arise they should contact a local plumber or line cleaning service. Breaks or cracks in these laterals are difficult to detect but are thought to the City's stormwater and sewer problems because they allow relatively clean groundwater to enter the lines. Sewer line breaks can also lead to sewage either surfacing or entering the stormwater system into our streams. This is called exfiltration.

Prior to the 1970s private development was allowed to connect footing and roof drains to the system. These connections allowed storm water from rain or snow melt to enter the sanitary sewer system. This water should not be in the sanitary system because:

• The storm water does not require treatment by WLSSD. If the water gets into the sanitary sewer system, utility customers end up paying to clean water that is more similar to rainwater than sewage.
• During heavy rain and snow melt the storm water can overwhelm the sanitary system filling it to capacity so that manholes pop, lift stations by pass and basements back up resulting in environmental pollution and property damage. In a worst case scenario it can acutally impair wastewater treatment by excessive flushing and dilution in the advanced wastewater treatment process at WLSSD.
• When these bypasses and overflows occur, diluted sewage is discharged int area streams and Lake Superior without adequate treatment leading to increased ecological and health risks.

After 1970, footing drain and roof drain connections to the sanitary sewer were no longer legal. However a significant portion of developed property is older than 30 years. In the early 1990s a comprehensive program was launched to identify and disconnect roof drains from the system. To date the City has identified and disconnected almost all of the roof drains.

A significant number of footing drains remain connected to the system and a mandatory program is underway to disconnect these. To address disconnection of the footing drains the City has established the Inflow and Infiltration program. Inflow refers to direct pipe connections of clear water to the sanitary system influenced by rain or snow melt such as footing drains connected to the sanitary sewer and infiltration refers to clear water entering the sanitary system due to pipe cracks or leaks at joints. Cracked laterals (lines from homes ot the City line in the street) can allow significant infiltration. These are the homeowner's responsibility. The lines can can crack due to freeze and thaw or roots among other reasons. The City is constantly inspecting, cleaning, repairing and updating its lines to minimize infiltration in public lines.

The City of Duluth Inflow and Infiltration program addresses inflow from footing drains and is targeting specific basins for mandatory disconnection of footing drains. In the targeted basins, a grant program is available to assist building owners with mandatory disconnection of the footing drains and installation of sump pumps as appropriate. Ultimately the program will address all basins in the City, but is currently working in basins 1-5 (see map).