Recently you may have heard conversations about I&I and wondered what it’s all about.
Inflow and Infiltration (I&I) is when clear water (rain, storm, and ground water) is allowed to enter the sewer system. When this occurs, the system becomes inundated with water volumes it was not designed to handle.
We saw first hand the immediate impact and cost of I&I during the summer of 2014 when many of our community homes experienced backups. These spikes in sewer flow also resulted in close to $1 million of surcharge fees to the City of Mound.
A second effect of I&I is that water otherwise not requiring treatment is mixed with sewage… and now needs to be treated. This adds cost to the treatment process and the fees that we pay to the Metropolitan Council each year. The Met Council uses a city’s average annual flow compared to the total seven-county flow to determine its “fair share” of the total annual system cost for staffing, energy, plant operation, sewer line repairs, sewer line replacement, capital improvement, and debt.
Our normal share of the Met Council annual cost is about .3 percent – remarkably similar to our .3 percent of total metro population. During the wet 2014, Mound had an increase in flow of 30 percent while the seven-county flow increased just 7 percent indicating the severity of the problem here in Mound compared to other communities. Further this three-times normal increase in flow forced an adjustment through the Met Council cost allocation formula that drove our share of total system cost for 2015 to .4 percent… and a direct increase in service fee of $100,000. But; close to $200,000 over the rate had we held to our normal flows.
Normalized system flow in 2015 returned us to our typical .3 percent share of total system cost for 2016; but because of I&I encountered in 2014, we paid more in 2015 – between $100,000 and $200,000 more depending on your point of view.
If we had no I&I at all… probably not completely achievable; but… total sewer flow would be less than total supplied water. Think of the city the same as your sink… the only water than goes down the drain comes from the tap unless you pour it in from somewhere else. So our total flow would be less than the 200 million gallons of metered water sales.
Indirectly, I&I represents about one third of our annual sewer fee, or about $200,000 of the current billing.
In total: surcharge, direct fee variability, and indirect costs; I&I will represent almost $2 million in cost to the City of Mound from 2015 through 2018.
In response to this, city staff has resumed discussions with the Met Council, surrounding communities, our engineers, and our city council to determine methods to best attack and solve the problem. I&I can occur in both city sewer lines, and the private service lines to each home and business in the community.
All of these sources of I&I risk, and possible solutions and the programs to implement them will be discussed in these open forum sessions. After initiating this conversation in December 2014, we resumed the dialog at the Nov. 11 council meeting. A workshop was held on Dec. 15, and additional follow up is planned for the Feb. 23 council meeting.
You can find packets and minutes from the previous sessions and look for agenda items for future meetings on the City of Mound website. The staff and council welcome any and all community participation in these sessions.
Eric Hoversten is the city manager and public works director for the City of Mound.