City holding tons of contaminated sludge
Published 12:00 am Friday, October 17, 2003
The city has a molybdenum problem. Its sewage sludge has an excess of the element.
The sludge is a product of the treatment of municipal wastewater. Different geographical locations have different types of sludge. According to Dr. Ron Reuter, who has a Ph.D. in soil science, Minneapolis has more industry, therefore its waste might contain a greater concentration of metal than Albert Lea.
But last year, Albert Lea’s industries dumped a high level of molybdenum into the wastewater system. Molybdenum (pronounced mah-lib-dih-num), is a metallic element used by industries to prevent corrosion and to strengthen other metals.
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Molybdenum is a naturally occurring element and is harmless at low concentrations. However, the Minnesota Pollution Control Agency (MPCA) has told the city that its level of molybdenum is too high, and the regular method of disposal is not an option.
According to City Engineer Dave Olson, the sludge is normally injected into farmlands, where it adds nutrients to the soil. According to the MPCA, many of the chemicals in the sludge are also found at harmless concentrations in the soil.
The MPCA said that sludge with concentrations of no more than 75 parts per million could be used on farmland. But Olson said that the level of molybdenum in last year’s sludge is as high as 107 parts per million.
&uot;That is high,&uot; said Carl Rosen, who has a Ph.D. in soil science. He said that dumping the sludge onto farmlands would not be dangerous for plants, even though it would accumulate in them. Instead, the animals that eat the plants could be harmed. The contaminated plants would injure the animals’ digestive systems.
He added that humans who ate an affected animal would not be harmed.
A letter was sent to local farmers, letting them know that the city would not be able to inject their farmland with sludge.
Four disposal options were considered, but the MPCA rejected three of them.
The first failed option was to mix the wet sludge with agricultural lime, thereby dispersing the concentration of molybdenum, and bury it. However, the sludge would have to be mixed and tested in batches, and the ground would freeze before all of the sludge could be buried.
Another method was to mix dry sludge with agricultural lime, but the MPCA determined that they would not adequately mix.
The third option was to use the sludge as a cover material at the city’s demolition landfill. However, through this method, rainwater could draw molybdenum into the soil.
City Manager Paul Sparks said that the method of disposal that the MPCA recommended to the city is to thicken the waste to form a solid and dispose of it in a lined landfill.
According to Gene Christenson, a chemical waste manager at the University of Minnesota, a lined landfill is different from a regular landfill in that it has a synthetic fiber and clay liners on the bottom. Water that is exposed to the waste can be captured and treated before it seeps into the soil.
Albert Lea has two full sludge-holding tanks, with capacities of 2 million gallons each. Once the sludge is turned into a solid, the city would be left with 4,000 tons of waste.
The city council has been considering offers from various landfills and, according to Olson, is currently considering two locations: in Steele County and Lake Mills, Iowa.
Scott Goldberg, director of Steele County Environmental Services, recently provided Albert Lea an offer through which Albert Lea would pay $20 per ton for disposal and $11.20 per ton for hauling, totaling $132,000. The council is waiting for an offer from Lake Mills.
Olson said the molybdenum problem has already been solved for next year.
Last July city environmental ngineer Steve Jahnke sent letters to Albert Lea industry heads, explaining the problem and asking them to cease the use of chemicals containing the element.
&uot;They responded immediately,&uot; Olson said. The new sludge has been tested and the molybdenum rate is back to normal.
(Contact Benjamin Dipman at firstname.lastname@example.org or 379-3439.)