Something is rotten along the banks of the Mississippi River in Minneapolis — or at least it smells that way.

Responding to a wave of complaints about a "decaying animal" stench, the Metropolitan Council is spending up to $200,000 under an emergency authorization to upgrade an odor control facility for the region's sewage.

The nauseating scent is emanating from a key point in the Twin Cities wastewater system, just north of Lake Street, where all of south Minneapolis' waste meets before crossing the river to be treated in St. Paul.

The council says it is also investigating similar complaints elsewhere in the system. The council oversees 600 miles of large pipes and eight treatment plants.

The stench chased nearby resident Garry Peterson off his porch this summer. Peterson, the retired Hennepin County Medical Examiner who oversaw autopsies for two decades, said the aroma was a familiar one.

"I said, 'That smells like a dead mammal. It's putrefying flesh,' " Peterson said. "So I just assumed that some animal had died down on the riverbank."

The Met Council has an extensive system for eliminating bad smells that relies on dozens of odor control facilities. They relieve pressure and filter air that's released from the sewers.

For quality control, workers collect samples of that air and take it to a laboratory in St. Paul, where trained "odor panelists" sniff it.

Incidents like this one are very rare; the council says it generally receives about 30 complaints a year about odors for a system that serves more than 100 cities.

It's not the first time the Minneapolis facility has had troubles, though in the past they were related to more typical sewer smells. To fix them, the council recently spent $3.8 million installing carbon filters. That has removed the rotten-egg smell caused by hydrogen sulfide, but not the dead meat aroma.

"It smells like rotting garbage," said Ariana Lindquist, while walking her dog Duke one recent morning beside the facility. Lindquist, a photojournalist, said it reminded her of what she encountered covering the 2008 Szechuan earthquake in China, which killed thousands of people.

"This is like a very minor version of the same smell," Lindquist said.

The council's interceptor engineering manager, Scott Dentz, speculated at a committee hearing Tuesday that the hydrogen sulfide smell was previously masking this more unpleasant odor.

"Once we remove [the hydrogen sulfide], these others come to the surface," Dentz said.

Peterson isn't so sure. He said there have been typical sewer smells since he moved into his home in the 1980s, and he thinks he would have detected this odor. He added that council staff have always been very responsive to his concerns.

He said the new smell has been "literally nauseating" this year.

"If we got a wind from the east, if the wind isn't blowing, we basically are prisoners in the house," Peterson said.

The emergency declaration, which will be ratified by the full council, authorizes staff to spend up to $200,000 on new filtration equipment to handle the new smell. The process will take about two weeks.

Eric Roper • 612-673-1732 Twitter: @StribRoper