Human noses critical for odor control at wastewater facilities

The “nasal rangers” at the Metropolitan Council’s Odor Lab wish they were smelling flowers, but that’s not what they were hired to do.

Instead, these trained contractors sniff air samples taken from the odor control units at wastewater pumping stations, metering stations and treatment plants operated by the Council’s Environmental Services division (MCES). The goal is to determine when it’s time to take corrective action, such as replacing carbon media in a unit before it starts emitting easily detectable odors.

“We do odor testing to be a good neighbor,” explained Lisa Wolfert, senior environmental scientist for MCES. “No longer are there any state or federal regulations about what odors the facilities can emit, though some communities have a local ordinance.” 

The Odor Lab is in a nondescript but carefully designed room at Council offices on Saint Paul’s East Side. Panelists can’t eat, smoke, or drink anything other than water an hour before their testing shift. They need to wash their clothes in scent-free detergent, and avoid filling up their gas tank on the way to the lab.  

“Fumes can hang on people’s clothing and skew a person’s ability to detect and identify the odor,” Wolfert said.

Gathering samples to bring back to the lab

Wolfert and her colleagues regularly visit about 40 odor control units – a mix of chemical scrubbers, carbon units, and biofilters – and collect bags of air samples from the units. They bring the samples back to the lab where Wolfert and the odor panelists do their work using a machine called an “olfactometer.”

The panelists sniff a series of decreasingly diluted samples from each location and identify when they first detect and then recognize an odor. (This mimics the experience of being downwind of a potential odor source and walking towards it.) After they leave the room, they jot down on a form what the sample smelled like. The sooner they can detect an odor, the more likely it is the odor control unit from which the sample was taken needs some attention.

Each sample series is tested by five panelists, two times each, to ensure reliability of the results. While one panelist is in the room with the olfactometer, the others are in a waiting room, “giving our noses time to recover,” said veteran sniffer Lori Wachter, of Shoreview.

Wachter was hired and trained by MCES in 1985 as a odor panelist. Since 1988, MCES has contracted with St. Croix Sensory, in Stillwater, which recruits, trains and hires odor panelists for a variety of firms.

“We wish it was flowers,” she quipped. “It’s hard to explain to anyone what we do, that’s why we don’t talk about it much.”

Most people recognize that rotten-egg smell

The compound that creates the most odor problems in the wastewater system is hydrogen sulfide, or H2S. Most people recognize it by its foul, rotten-egg smell. Wolfert said that H2S has a very low odor threshold, which means there doesn’t have to be a great concentration of it to be a problem. But it isn’t the only smelly component of wastewater.

“We use humans, and conduct the odor testing using their noses, because they can determine odor concentration for us and tell us if it’s objectionable or if it’s acceptable,” she explained. The sensory method is also less expensive than instrumental analytical methods.

Odors are a cyclical issue in the wastewater system

Odors are more of a problem at certain times of year and even times of day. During the warm-weather months, biological and chemical reactions will increase potential odor problems, Wolfert said. Even though July and August tend to be the warmest months in the metro area, odor problems tend to bloom in September and October. That’s because the wastewater collection system is underground, and it takes time for the pipes to warm up, which drives odor production.

The Council has made a significant investment to purchase, install the and run the odor control equipment. Regular odor control testing ensures a good return on investment and that the communities we serve are protected.

You do WHAT for a living?

“When people find out what I do for a living, some are blown away, others, their eyes just glaze over,” Wolfert laughed. “You don’t even get to the ‘why.’ Others are completely fascinated and want to know more about it.”
Did she ever one day envision she would be running an odor lab?

“No, no. I didn’t even know it existed. When I started working for the Council, it was ‘you do what?’”

“I am also a ratepayer as well as an employee,” Wolfert said. “I look at the work that we do and I am glad that the Council is investing in this service. I would not want to live with a stinky facility next door to me. People expect their government to be doing a good job, to be treating all aspects of wastewater, which includes odorous air.”