Inflow and infiltration a problem for Two Harbors
The city of Two Harbors held an informational meeting Monday about the problem of inflow and infiltration (I&I) in the city's sanitary sewer system.
I&I is clean water entering the sanitary collection system that is coming in the form of rain water, groundwater or snow melt. Brian Guldan, environmental project engineer at Bolton and Menk, said this is problematic because once the clean water mixes with the wastewater it all becomes wastewater and needs to be handled and treated as wastewater.
"If we can keep the clean water out of the wastewater, there is less volume of water to handle," Guldan said.
According to Luke Heikkila, Two Harbors water and wastewater plant superintendent, the treatment plant typically sees about 500 gallons per minute, but during a rain event or snow melt like the one the Two Harbors say in February, it can jump up to around 3,750 gallons per minute and peak out at about 7,500 gallons per minute.
"So what you are seeing here is additional pumping costs. We have lift stations around town that have to get this water to the wastewater treatment plant," Heikkila said. "We also have three separate lift stations in the plant itself. So every single gallon of water that comes into the treatment plant has to get pumped three times before it goes out."
Heikkila said they conservatively estimate that about 150 million gallons of water leaves the plant, is used by homeowners in Two Harbors and comes back into the treatment plant, but the Two Harbors wastewater treatment plant saw approximately 302 million gallons of wastewater last year.
"So if we are pumping 150 million out and treating 302 million, we are seeing double due to I&I. And that's a conservative number," he said. "Everything there is seeing twice the workload that it is designed for."
I&I can be introduced into the system in many different reasons such as direct connections from down drains or sump pumps or holes or cracks in manholes or pipes.
"You could also have a cracked or broken sewer lateral. Roots from trees break that lateral. Any opening in that lateral is going to let groundwater in," Guldan said. "There really is no way to meter that from the homeowner. There is no meter on that until its downstream and combined with our wastewater."
What the city is doing
Two Harbors put $127,000 in its Capital Improvement Plan for new manhole covers for the city. According to Heikkila, the problem is that in the past the casting for manhole covers were the same for the storm water sewer and the sanitary sewer systems, so there are a lot of holes in the covers where clean rain water can get into the sanitary sewer system.
"So if you get holes in low areas, it will just flood out and add a ton of inflow into the system," Heikkila said. "So we are identifying low line areas, areas that we need to address and are a cause for concern, and replacing approximately $25,000 to $27,000 in manhole covers a year."
The city is hoping to address all the manhole covers over the next five years and reduce the I&I into the sanitary sewer system by about 5 to 10 percent.
The city is also planning a smoke test of the sanitary sewer system July 19-21 to find cracks, holes or bad connections that would allow clean water into the sanitary sewer system. City crews will be looking for smoke coming out of the ground or entering homes. Once all of the problem areas are identified, the city will come up with a plan to address these issues.
What can residents do
The number one thing that Heikkila said residents can do is make sure their sump pumps are in compliance and working property.
"So sump pumps is a huge thing that homeowners can do to try and eliminate I&I. There are free sump pump inspections by city staff. So there are no ramifications to homeowners," Heikkila said. "We'll come in and tell you what you have to do, and a lot of time it's things you can do yourself and you don't have to have a plumber come in there."
Gas, water and sewer superintendent Paul Johnson said that in 2006 the city did a study on 10th Avenue and Ninth Avenue.
"In those comparative four blocks, we were able to prove that roughly 25 to 30 percent of a rain event was removed from sump pumps," he said. "That's a huge number."
Residents who wish to get their sump pumps inspected can contact the city at (218) 834-5631.