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Council plans I/I workshop for information sharing

A workshop for city officials to share ideas, challenges and success stories in reducing excessive inflow and infiltration (I/I) in their sanitary sewer systems will be held on Thursday, Feb. 16, 2012.

Metropolitan Council Environmental Services (MCES) will host the workshop, which will include a panel of representatives from four communities that had excessive I/I in the first phase of MCES’s I/I reduction program, which began in 2007. 

A new ongoing I/I mitigation program begins in January based on measured wastewater flows in the first six months of 2012. If excess I/I occurs during this period, surcharges will be billed in 2013. The new program was recommended in 2010 by the Council’s Demand Charge Task Force, which had broad community representation. See the task force's final report (pdf).

This month most of MCES’s municipal customers will receive a letter that shows the adjusted average and peak hourly flow thresholds for all metersheds in their community for the 2012 measurement period.

Efforts to reduce inflow appear to be working

Under the original program, municipal wastewater flows after peak rainfall events showed that 47 communities had an excessive amount of clear water entering the sanitary sewer system. Those communities were required to reduce the excess I/I or face a surcharge on their municipal wastewater bills.  All but four communities, which were granted an extension to do the work, will have completed it by the end of next year.

Measured flows in 2010 and 2011 showed some dampening of peak wastewater flow after storm events in those communities that took aggressive action in I/I mitigation efforts.

“We’re having some success but we need to keep up the good work to ensure that our wastewater collection and treatment system has the capacity that was built into it to handle the region’s anticipated population and household growth,” said MCES Finance Director Jason Willett.

Severe rain events or overall higher-than-average precipitation in the first six months of 2012 where I/I has not been well mitigated could lead to some communities again being required to spend money on mitigation or required to do so for the first time. 

Local fixes will save hundreds of millions of dollars

The cost to fix I/I at the local sources was originally estimated at about $150 million, compared with nearly one billion dollars that would be needed to add collection and treatment capacity to handle excessive I/I. Both cost estimates have likely gone up, according to Willett, but it’s still clearly better – both financially and environmentally – to eliminate I/I at the source.

Inflow is when clear water enters the wastewater system through rain leaders, sump pumps or foundation drains that are connected to the sewer lines (illegal in Minnesota since 1968), or directly through openings in the wastewater collection system such as open, broken or leaking manhole structures. Infiltration is when groundwater seeps into cracked or broken wastewater pipes.

Inflow is the biggest problem for MCES, because during major rain events it quickly consumes pipe capacity needed for future growth. And, in more extreme rain events, inflow can cause sewer backups into homes and businesses. Infiltration, while it takes up pipe capacity, is a less variable contributor to the problem.