The City of Eden Prairie is about to embark on a public education campaign to help our residents understand the City’s concern with Inflow & Infiltration into the City’s sanitary sewer collection system. It’s a complicated subject to explain, but I’ll give it a try.
The City’s sanitary sewer collection system collects sewage from Eden Prairie’s homes, businesses and public buildings. After you flush your toilet, the water and whatever goes into the sanitary sewer collection system is transported to the wastewater treatment plant. The wastewater treatment plant cleans up the sewage by taking the pollution and solids out of the water; cleaning up the water; and then putting the water back into a nearby river or stream.
The sewer collection pipes (see the illustration) that make up the sanitary sewer collection system have been designed by engineers to have a capacity that will accommodate the sanitary sewage collection needs of the various homes, businesses and other buildings that are connected to the system. These pipes are designed to carry the sewage to the wastewater treatment plant, which also has been designed to accommodate a certain population size’s sanitary sewage treatment needs. If rain water, river water or other non-sewage sources of water get into the sanitary sewage collection system it can consume the carrying capacity of the pipe which can force raw sewage into your basement or into our rivers and streams. If 50,000,000 gallons of sewage + rain water, storm water run-off or river water flows make it to wastewater treatment plant that is designed to treat a maximum of 30,000,000 gallons of sewage per day, the excess 20,000,000 gallons of sewage diluted storm water may have to be bypassed around the plant, which means it may be directed right into the nearby rivers and streams without being cleaned or treated. I’ve seen this happen, and it is disgusting.
In theory, the sanitary sewer collection system is supposed to be a “closed system.” The only water that is supposed to get into the sanitary sewer collection system should be from your tubs, showers, sinks, toilets, washing machines, dishwashers, etc. But sometimes the sewer pipes can be cracked, or a manhole lid has too many holes in it or a storm water intake gets accidentally connected to a sanitary sewer line. These are examples of Inflow and Infiltration (I&I). We need to find them and correct them.
Other sources of I&I aren’t so easy to find and aren’t so easily corrected. These sources are typically in private homes and businesses. Probably the most common residential I&I source is the sump pump that is connected directly to a home’s sanitary sewer plumbing or directed via hose into a laundry tub or floor drain. The water being pumped by the sump pump is rain water. Putting that rain water into the sanitary sewer system is not permitted, but it’s likely there’s some of that going on in Eden Prairie. We need to find it and help people correct it. It may not seem like the sump pump water from a single home is a big deal, but if you start multiplying that number by 1,000s of other homes that are doing the same thing, this source of I&I can get to be a big deal really quick.
Why? Why should you care about this? Two reasons. First, it’s costing you extra money. The wastewater treatment plant that treats our sewage is owned and operated by the Metropolitan Council’s Environmental Services division (MCES). MCES charges the City a monthly charge for treating the sewage that comes from Eden Prairie. Our charge is based on the amount of sewage that is delivered to the wastewater treatment plant from our City. The higher the volume, the higher the charge. If we can get I&I out of our collection system, we can reduce our monthly treatment charge from MCES.
MCES also keeps track of what our peak flows are and how they change when we get rain. MCES fines the City on an annual basis for the amount of I&I that we deliver to their wastewater treatment plant. Our projected 2008 I&I surcharge is going to be $500,000. In Bloomington, it’s going to be $50,000. That’s a big difference, which of course is merely passed on to the City’s utility customers in the form of higher utility fees.
The second reason is environmental. The invention of a reliable sanitary sewer system connected to a reliable wastewater treatment plant has probably done more to improve our natural environment than anything in human history. Just think about what our oceans, lakes, rivers and streams would be like without them. It’s important to make sure these systems work according to their designs. Allowing I&I into our sewage collection and treatment system compromises those systems and puts our environment at risk.
We’re going to start talking a lot more about I&I in Eden Prairie in 2009. I&I problems are something that we can, and should, do something about.
