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Cokato wastewater treatment pond overflows
June 6, 2011
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City council to discuss issue at next meet; possible fines from the MPCA

By Jennifer Kotila
Staff Writer

COKATO, MN – One of Cokato’s wastewater treatment ponds, north of Cokato on Wright County Road 4, began overflowing May 24, less than three weeks after Eric Lembke of Bonestroo, Cokato’s engineer, warned the Cokato City Council that this could happen.

The wastewater treatment ponds, which have a maximum discharge rate of 950,000 gallons per day, have been inundated with an influent (water flow into the facility) of over one million gallons per day throughout the month of May, according to Lembke.

With more than 3 inches of rain falling the weekend of May 22, and another 2 inches falling the weekend of May 29, the wastewater treatment ponds have not had an opportunity to discharge the increased influent and recede to comfortable levels.

“In order for the system to catch back up, we need a good three weeks to a month of no rain,” Lembke said.

In May, 36 million gallons of water flowed into the wastewater treatment ponds, and 29 million gallons was discharged, leaving a net gain of 7 million gallons of additional water in ponds already filled to capacity, Lembke said.

If there is a silver lining to the problem, it is that the pond which is overflowing is the “polishing pond,” said City Administrator Don Levens.

The polishing pond is the final treatment pond in a three-pond system. Wastewater is treated in the first two ponds before going into the polishing pond.

Once the water has been treated in the polishing pond, it is discharged directly into the Crow River.

The city has reported the event to the Minnesota Pollution Control Agency (MPCA), according to Levens.

The fact that one of the wastewater treatment ponds overflowed is not a violation in the sense that Cokato did something wrong, Levens said, and he does not anticipate the city receiving a fine from the MPCA.

He noted that many cities are having the same issues right now because of the unusually high amount of precipitation that began last fall and continued throughout the winter.

Aside from the wastewater treatment ponds filling to over capacity, the city of Cokato is meeting all other discharge requirements mandated by the MPCA, which is good, Lembke said.

Lembke anticipates about a 50 percent chance the city will be fined following this event, saying, “It is hard to know what the MPCA will do.”

Although other cities are experiencing unusually high flow into their wastewater treatment facilities, Cokato’s actually overflowed, Lembke said.

The MPCA may decide to be more lenient because of the high amount of precipitation this spring, as long as the city agrees to fix the problems causing the wastewater treatment facility to run over capacity.

“Generally, when there are flooding situations, as long as a facility is communicating with the agency (MPCA) about the situation, enforcement is not an immediate option,” said Stephen Mikkelson, MPCA information officer. “But it’s a case-by-case situation, too.”

At this time, the MPCA is just now finding out about the incident, and it will be under investigation, said Eric Pederson, the MPCA compliance staff person assigned to Cokato.

Solutions to the problem

At last month’s city council meeting, Lembke presented his findings regarding inflow and infiltration (I and I) into the sanitary sewer system.

Inflow is due to direct storm sewer connections such as downspouts, foundation drains, sump pumps, and holes in manhole covers.

Infiltration is water that leaks into the sanitary sewer system through leaks in the pipe underground.

Inflow from residents’ and businesses’ sump pumps, downspouts, and foundation drains accounts for 60 to 80 percent of the unnecessary influent into the treatment ponds, Lembke said.

Infiltration due to poor infrastructure in the city’s sanitary sewer system accounts for 20 to 40 percent of the unnecessary influent.

Lembke monitored the flow through the sanitary sewer system between March 30 and April 13 using flow meters placed in manhole locations throughout the city.

It was concluded that there is a significant problem citywide with direct connections of sump pumps, foundation drains, and downspouts to the sanitary sewer system.

This is evidenced by the fact that the peak flow rate for the residential areas monitored occurred during a rain event April 10.

If sump pumps, downspouts, and drains are properly discharged, there would not be a such a significant spike in the flow of the sanitary sewer system during rain events.

Therefore, Lembke recommended the city implement a citywide sump pump inspection program, and use a combination of smoke testing and televising to identify any direct storm sewer or foundation drain connections to the system.

It was also recommended that the city systematically inspect all manhole structures and related items, to identify and make repairs to those that need it.

“The city is taking the recommendations seriously. The staff will work with the council to get this taken care of as quickly as possible,” Levens said.

The city council will be discussing sump pump compliance at its next meeting Monday, June 13.

“We would rather not go onto people’s property to inspect the sump pumps,” said City Council Member Carl Harju. “We hope that the people will take care of the problems without the city having to do an inspection.”

The city is in the process of working with Lembke and the public works staff to inspect the city’s infrastructure to identify needed repairs in order to eliminate infiltration, Levens said.

Any residents who have questions regarding the proper connection of sump pumps, downspouts, or foundation drains are encouraged to contact the city at (320) 286-5505.

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