It doesn’t look like much, but this overflow tank at 60th Ave E and London Rd. in Duluth keeps 300,000 gallons of diluted sewage out of Lake Superior.
"What’s the difference between a sanitary sewer and a storm sewer?"
Between the sanitary and stormwater sewer systems, there is a lot happening below the streets to protect streams, lakes, and wetlands. The Sanitary Sewer System collects the dirty water that goes down our drains and toilets (i.e. sewage). In our area, this wastewater travels to the Western Lake Superior Sanitary District or to the City of Superior’s Waste Water Treatment Plant. It is discharged only after meeting strict regulatory standards for removal of pollutants.
The word "sanitary" is confusing because it refers to the "sanitation system," not the water itself, which is definitely not sanitary! The entire drainage network of pipes must be well sealed to keep raw sewage from leaking out before the water gets treated. We also don’t want clean groundwater to enter the system because then we’re treating clean water. Too much water can also overflow the pipes and pumping stations before it gets to the treatment plant. This is the "Inflow & Infiltration" (I & I) problem that has caused Duluth to construct large tanks to temporarily store this water and then gradually bleed it back to the plant for treatment.
The Stormwater Sewer System collects rainwater and snowmelt runoff from streets, roads, driveways, sidewalks, and parking lots to prevent flooding. Most of the open grates you see on the streets lead to the stormwater sewer. In Duluth, stormwater pipes drain directly into one of the city’s 43 named streams, all of which drain into Lake Superior. Many are sensitive trout streams. This water does not get treated, so all pollutants in the water are carried directly to streams and lakes. In fact the surge of water after a big storm, even if clean, can cause stream banks to erode and generate excess muddiness that degrades both stream habitat and the nearshore zone of Lake Superior and the St. Louis River Estuary. Superior, Wisc., has similar issues, and also has to deal with a combined system in certain areas where a single set of pipes collects sewage and stormwater.
The City of Duluth maintains more than 431 miles of underground storm sewer lines, 100 miles of roadway ditches and culverts, two lift stations, 13 sediment boxes, and 5,600 manholes. WLSSD has nearly 75 miles of sanitary sewer pipes, 16 pump stations and receives an average of 40 million gallons of wastewater per day. You can find out more about these systems at www.lakesuperiorstreams.org