New sewage treatment system could help clean Minnesota lakes

Ceramic filter
A vacuum pulls water though microscopic pores in ceramic plates. The filter is a barrier to dangerous bacteria like E. coli.
MPR photo/Dan Gunderson

Just south of Detroit Lakes on Lake Melissa, 36 homes along Ravenswood Beach share a common septic system. Sewage from each home is piped to a central septic tank.

Longtime resident Clayton Jenson says he started to notice problems with the septic system about four years ago.

"We noticed it was ponding. That water on top of the drain field indicates that the water is not going down and being saturated into the soil," he explained. "So we knew the drain field was going to be a problem for us."

Traditional septic systems separate liquid from solids when you flush the toilet or take a shower. Solids settle to the bottom of the tank where bacteria breaks them down. The liquid flows through pipes into a drain field where the waste is filtered by the soil.

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Over time the soil gets plugged with sewage sludge and the drain field no longer absorbs water.

Sludge tank
Sewage is mixed in this ten-foot-deep tank. Clean water is pulled through the ceramic filters stacked inside the tank.
MPR photo/Dan Gunderson

The usual solution would be to build a new drain field. But the homeowners here on Ravenswood Beach decided to try another option.

SJE Rhombus, a Detroit Lakes company that designs wastewater treatment control systems, created a sort of micro sewage treatment plant.

The system is designed to work best with a minimum of ten homes, and can handle up to one million gallons of waste per day.

The treatment filter at Ravenswood Beach is in a metal-covered building not much larger than a backyard storage shed.

"We're going to treat the wastewater before it gets to the soil so we're not going to rely on the soil to treat it," said SJE Rhombus Marketing Director Mike Metelak. "Therefore you don't need a big piece of land and good soils to do that final drain field dispersal and treatment."

The technology is called a membrane bio reactor. It's essentially a filter system.

All the waste collected from homes ends up in a 2,000 gallon tank about ten feet deep, where it's constantly churning and mixing.

In the middle of the tank, a column of thin ceramic plates are stacked about eight feet high. They're hollow in the middle and filled with holes; very, very tiny holes.

"The pore size of this ceramic membrane, the pore size is .20 micron. The E.coli bacteria are about 35 times larger than that pore size so physically E.coli can not pass through," explained Metelak. "The membrane is a physical barrier so it won't let anything pass through it."

Clean water
Water flowing out of the membrane bio reactor is clear and SJE Rhombus officials say it's clean enough to pump into a lake or wetland.
MPR photo/Dan Gunderson

A vacuum pump pulls water through those tiny holes and it comes out clear. That's something that impressed lakeshore resident Clayton Jenson.

"We saw water that looks like distilled water. So there wasn't any way we would not go ahead and put something like it in," said Jenson. "It's just the most sophisticated sewer system I think in Minnesota or perhaps the United States with this type technology."

The filter system is monitored and controlled remotely through an internet connection. About once a month SJE Rhombus staff take water samples and check the equipment.

Membrane bio reactor technology isn't new, but the ceramic filters make it unique. Other systems use thin plastic filters that are easily damaged and have a short life.

Water regularly flushes the filters that are also cleaned with bleach to keep them working efficiently. The company expects the ceramic filters to last 20 years.

The ceramic filters were developed in Germany. SJE Rhombus designed the controls to allow the system to automate the system.

The Ravenswood Beach test site is the only system operating in the United States. The company is also testing the system in Canada.

SJE Rhombus Marketing Director Mike Metelak said in addition to lakeshore homes, the company plans to market the sewage filter system to small towns and industrial processing plants.

Clayton Jenson
Ravenswood Beach resident Clayton Jenson says cost and environmental impact were key reasons local residents chose to replace their septic system with a membrane bio reactor.
MPR photo/Dan Gunderson

The Minnesota Pollution Control Agency is watching this experiment with interest.

"From what I see it's a very promising technology, they're getting very good numbers. And it's something we definitely want to look at further," said Wastewater Treatment Plant Permit Writer Denise Oakes.

Oakes, who is studying the data collected from the Ravenswood Beach site, says she has some questions about how well the membrane filter system will remove phosphorus and nitrogen.

Company officials said their Canadian test site lowered phosphorus levels to the point where the water is discharged directly into a stream.

Oakes hopes to see more data, but with thousands of Minnesota septic systems requiring replacement, she thinks this kind of sewage treatment technology could be a good option in many areas.

"We're going to have to move away from some individual septic systems, because the older lots that aren't so big, the lot sizes are too small for them to have an additional drain field area for a new drain field system especially with the regulations we have today as compared to maybe 20 years ago," explained Oakes.

Moving away from individual septic systems means homeowners will need to put in pipes to carry sewage from each home to a central treatment location.

SJE Rhombus says it costs each homeowner about $12,000 to $15,000 to install sewer lines and a filter treatment. A new individual septic system typically costs about $8,000 to $10,000.

Tougher pollution standards that require septic system improvements will force thousands of Minnesota homeowners to face an expensive choice.

Clayton Jenson is happy he and his neighbors on Ravenswood Beach will never again need to worry about their septic system failing.