Mary Fralish, a Mankato public works administrator, compares a bottle of store-bought water to a bottle of water treated at the wastewater plant. Its water has long been well treated, but it was the solid stuff — called “biosolids” or “sludge” — that was recognized at a Monday event.

John Cross
The Free Press

Flush your toilet, and its contents could end up on Daryl Guentzel’s fields.

“We prefer it,” the Eagle Lake farmer said. “We wish we could go all-natural.”

Mankato has been giving its treated sewage to area farmers as fertilizer since the 1950s. But the steady improvement in the city’s wastewater plant — which also serves North Mankato, Eagle Lake, South Bend Township and Skyline — has left it among the cleanest in the state.

Those improvements were recognized Monday at the treatment plant as the city received a certification from the National Biosolids Partnership. Mankato is the first city in Minnesota and the 28th nationwide to earn that distinction.

Biosolids, traditionally called “sludge,” are basically processed waste. The plant makes about 1,000 dry tons of sludge annually.

The certification isn’t the result of a single technology or accomplishment but rather an indication that Mankato’s wastewater system produces some quality sludge. A third-party auditor verified in January that Mankato’s plant is exceeding regulatory standards and deserves to be certified.

The work starts before the waste even reaches the Pine Street plant.

The city works with industry to prevent metals and other contaminants from entering the plant. The auditors specifically noted the success of the city’s work with dental offices to prevent mercury teeth fillings from entering the water supply.

The facility itself is worth visiting. Swallows circle the open-air pits hunting for the mosquitoes that spawn in standing water. Nearby, bubbles shoot up from brown liquid amid a smell that is, while not exactly pleasant, certainly not overpowering.

The plant’s job is to convert wastewater — anything that goes down area sinks and toilets — into water, biosolids and methane. Ninety-five percent of the methane is used to help power the facility.

The water it creates is so clean that it can be used to irrigate the nearby Riverfront Park and cool the Calpine power plant.

“Who knows, in the future it might be a closed system” where wastewater could be cleaned and reused as drinking water, City Manager Pat Hentges said.

Mayor John Brady praised the plant and its works as “critical ... but underappreciated by the average citizen.”

Most of the 11 workers at the plant got some rare appreciation during Monday’s event as the blue-shirted workers shuffled to a corner for photos and a round of applause.

“They’re kind of the unsung heroes,” Hentges said.

The biosolids themselves are digested by bacteria very similar to those in a person’s stomach. They both prefer a temperature of about 98.6 degrees. The biosolids are dried in the facility until they reach the consistency of mud, or about 23 percent solid, before city workers drive loads to farmers’ fields. There’s a waiting list to receive the biosolids, which are free of charge.

The Guentzels have been using Mankato sludge for more than 20 years.

Daryl said it’s superior to chemical fertilizer partly because it helps the soil maintain organic matter, which acts as a sponge for air and water. Biosolids provide nitrogen and phosphorus aplenty, but lack potassium, which is supplied by potash.

The city is researching technology upgrades to improve its biosolids into a Class A program. Other goals are to eliminate contractor spreading of biosolids and get 5 percent more Mankatoans knowledgeable of the wastewater system.


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