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Communities identify sources of clear water flow into sewers

Progress is made as date for surcharge on wastewater bills nears

Communities around the region are tackling the problem of clear water in the sanitary sewer system called infiltration and inflow (I/I) – in anticipation of a regional wastewater surcharge program that takes effect in January 2007.

In most communities the first step is identifying and quantifying the problem, explained Kyle Colvin, manager of the Metropolitan Council’s I/I surcharge program. For example, Lakeville installed nine meters to measure flow that might come from sump pumps or leaking manholes, primarily in the northern part of the city. The city also worked with the Metropolitan Council to identify portions of the regional distribution system that may be subject to infiltration during heavy rains or snow melt.

“We want to nip it [I & I] in the bud – find out where it is a problem and go fix it,” said Keith Nelson, Lakeville’s city engineer.

Eden Prairie’s effort to quantify I/I led it to install 40 manhole meters in areas of the city believed most subject to clear water flows. The city also arranged to have the Metropolitan Council install meters on its interceptors, in order to verify flows in the regional system. Eden Prairie plans to collect data through September, then prepare a response plan. The city will first address problem areas with public education. Ordinances and enforcement measures will follow as needed.

Clear water reduces capacity in the system

The Metropolitan Council adopted a program in December 2005 that will impose surcharges for clear water flows to the regional sanitary sewer system. Communities may avoid paying the surcharges, which are based on flows measured during rainfall events, by investing money to stop I/I at the source.

Clear water reduces capacity in the regional wastewater collection and treatment system and, in intense rainfall events, can cause sewer backups and overflows. In the future excessive I/I could, if not reduced, limit growth in the region. If successful, the Council’s program will help the region avoid these problems with local investments estimated at $150 million.

That figure compares with a minimum of $900 million the Council estimates that it would cost the region to build the infrastructure just to convey the excess flows to wastewater treatment plants. Expansion of the plants themselves would be very costly and, because of space constraints, not possible in many cases, explained Colvin.

The surcharge program ‘makes sense’

“The Metropolitan Council program makes sense,” said Ed Sorensen, special projects coordinator for Eden Prairie. “It is much better to stop inflows at the source rather than adding new facilities – after all, it is everybody’s money we’d be spending.” Sorensen believes most of his city’s clear water flows stem from sump pumps that illegally discharge to household drains. “We may have some manholes that need to be addressed,” he said, “but most of the pipes in our sanitary sewer system are less than 30 years old. I don’t expect they will require significant replacement dollars.”

Additional programs, new since the Council adopted its policy, include efforts by Medicine Lake to visually inspect its system and homeowner sump pumps. Golden Valley recently completed a program to identify its sources of I/I. Based on the data, the city plans to line portions of its sanitary sewer system and install drain tiles to direct drainage from residential sump pumps, and to continue its aggressive program of public education and outreach about the issue.

“We really see the program as a partnership between the city and the Met Council,” said Jeanine Clancy, Golden Valley’s public works director. “We’re concerned about the impacts surcharges could create, but we recognize the Council is working with us to improve its own system and help communities develop solutions.”

Colvin encouraged cities to contact him as they develop strategies to reduce excess clear water flows; he can be reached at 651-602-1151 or kyle.colvin@metc.state.mn.us.