Tackling inflow/infiltration in sanitary sewers

Council, communities make good progress to solve vexing problem

What looked like a looming crisis in regional wastewater capacity is turning into a success story, thanks to the efforts of local communities in the seven-county region and the Metropolitan Council.

For the second year, the Council in 2008 will defer proposed surcharges on community wastewater bills because of progress made and additional commitments by communities to reduce excessive inflow and infiltration (I/I) of clear water and stormwater into sanitary sewers.

illustration of water inflow and infiltration near a home

Both private properties and public facilities contribute to inflow and infiltration, though about 80% of sources are private. See expanded version of diagram. (Graphic courtesy King County Department of Natural Resources and Parks, Wastewater Treatment Division, Regional Infiltration and Inflow Control Program.)

Two years ago the Council adopted a program to impose a surcharge on wastewater bills starting in January 2007 for communities with excessive clear water flows into the regional wastewater system. Communities could choose to undertake a program to reduce the I/I in lieu of the surcharge.

“Communities have stepped up to the plate to solve the problem,” said Bill Moore, general manager of the Council’s Environmental Services division (MCES). “A substantial amount of excessive I/I has been taken out of the system.”

Communities target I/I sources

Of the 46 communities identified as having excessive I/I in 2007, six have already spent the equivalent of their entire five-year surcharge to reduce the problem. All of the other communities have made progress and submitted work plans to continue their efforts in 2008.

One city discovered two large stormwater pipes that were connected to the sanitary sewer system. Another found a breach between adjacent stormwater and sanitary sewer pipes that allowed stormwater to enter the sanitary sewer. And many communities are attacking a major source of inflow – foundation drains at individual properties that are illegally connected to a sanitary sewer pipe.

Council offers disconnection grants

The Council initiated a grant program to provide matching funds, starting in January 2008, for local spending to disconnect foundation drains from the sanitary sewer system. Communities are eligible for grants up to half the cost of individual disconnections, with a cap of $1,000 for each disconnection. A total of $700,000 is available in 2008.

According to Kyle Colvin, assistant manager of Engineering Services for MCES, an estimated 80% of I/I originates from private property sources like illegal foundation drain connections.

Big savings for the region

Excess clear water robs the regional wastewater collection and treatment system of capacity needed for future growth. It also can cause sewer backups, resulting in threats to human health and the environment.

If successful, the Council’s program will help the region avoid these problems with local investments estimated at $150 million. Adding capacity at the regional level to collect and treat the excess clear water would cost more than $900 million and may not have even been feasible because of capacity constraints at the Metro Plant in St. Paul and other Council facilities.

“At this rate of progress, by 2012 we will have ensured adequate capacity in our wastewater collection and treatment system for decades to come,” Moore said.