Excessive stormwater in wastewater system causes overflows and backups

Date: Thursday, June 19, 2014

The Metropolitan Council reports excessive amounts of rainwater flowing into the local and regional wastewater collection systems is causing overflows and sewer system backups.

Late this afternoon, officials at Metropolitan Council Environmental Services (MCES) reported to the Minnesota State Duty Officer and the Minnesota Pollution Control Agency that sewage spilled into Maxwell and Carmen Bays in Lake Minnetonka, as well as into the Mississippi River, originating at Wabasha Street and Humboldt Avenue in St. Paul. MCES also reported spills into Medicine Lake and Bassett Creek in Plymouth. 
“We’re experiencing a rain event similar to June 1, but more widespread,” said Bryce Pickart, acting general manager of MCES. “In parts of the region, we are running wastewater pumping stations and regional sewers beyond their designed maximum operating capacities in an effort to keep up with the rainwater that is entering the sanitary sewer systems.” The Metropolitan Council owns and operates the regional wastewater collection and treatment system, which conveys wastewater from 107 municipalities in the metro area to seven area treatment plants.

“Wastewater systems are designed and intended to accommodate wastewater flow, not rainwater. When rainwater is directed to or flows into the regional system from local wastewater systems, homes and businesses, the result is what many municipalities experienced June 1 and what we’re experiencing in those municipalities and more today.”

Metropolitan Council staff continue to monitor flows throughout the region. Communities with especially high levels of flow are those all around Lake Minnetonka, including Orono, Mound, Excelsior, Wayzata, Minnetrista, Victoria, Chanhassen, and Maple Plain.  In addition, Chaska, Savage, Burnsville, South St. Paul, Forest Lake, St. Bonifacius, Hopkins, Plymouth, New Hope and North St. Paul are also experiencing high flows.

Communities with high flows are at risk of sewage spills and sewer back ups into homes and businesses. Residents are advised to monitor basements for signs of water and to move valuable items upstairs if possible. Because water can take time to move through sewer systems, residents should monitor basements for up to 24 hours after the conclusion of the rain event.

“This is a clear cut example of inflow and infiltration of stormwater into local and regional sewer systems, an adversary of the wastewater system” said Pickart. “Stormwater that flows or is directed to the wastewater system can press wastewater capacity to the limit.   As a region, we need to continue to address this problem.”

What is inflow and infiltration?

Inflow and infiltration is clear water that enters the sanitary sewer system from a variety of sources. Infiltration occurs when groundwater seeps into sewer pipes through cracks, leaky pipe joints and/or deteriorated manholes.

Inflow, the much bigger problem, occurs in direct proportion to the amount of rainfall. Inflow is storm water that enters the wastewater system through rain leaders, basement sump pumps or foundation drains illegally connected directly to a sanitary sewer pipe.

What happens when inflow and infiltration exceeds capacities?

Excessive inflow and infiltration of clear water that mixes with the regular amounts of wastewater can elevate flows above six times the average wastewater flows. This can overload city-owned and Council-owned pipes and pumping stations, cause wastewater to back up, and place private property at risk for backups or lead to bypasses of wastewater to the ground or to surface waters.

What does the Council do to plan for major rain events?

The Metropolitan Council recognizes that reasonable amounts of extraneous clean water will enter sanitary sewers, and it offers cities sewer capacities of about two to three times their average daily wastewater flow so that sewers can handle non-excessive amounts of inflow and infiltration of clean water into the sewers. The Council’s regional sewers are built with capacity beyond that level to accommodate increased wastewater flows that will come with future growth. 

More information

See more information on inflow and infiltration.


Posted In: Wastewater & Water

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