TRAVERSE CITY — Steve Graziano of Jacobs Engineering outlined a proposal Monday for a comprehensive odor control study at the city’s wastewater treatment plant on Hannah Avenue, telling commissioners during a study session that the project would identify what’s causing foul smells, where they’re coming from and what can be done to stop them.
Graziano, who works alongside plant operations staff, walked commissioners through the science of wastewater odors, the tools used to measure them and a six-task study framework designed to evaluate offsite impacts and recommend improvements.
Most wastewater odors are tied to sulfides, Graziano explained, which begin as sulfate entering the sewer system from drinking water, groundwater infiltration and industrial or domestic sources. In the oxygen-deprived conditions inside sewer pipes, that sulfate converts to sulfide and then to hydrogen sulfide gas.
“In the collection system, it’s a nuisance because the collection system extends out to where people live, work and play,” Graziano said. “So if there’s any venting in the sewer, those odors can escape.”
While hydrogen sulfide gets the most attention, Graziano noted that a treatment plant produces a range of odorous compounds across various processes, each with a different character, persistency and intensity.
“At low concentrations, it may not be as significant of an impact from a health perspective, but at higher concentrations, it can lead to some serious injuries and even death,” Graziano said. “And there have been reported cases of that, for example, when an operator enters a tank that is high in H2S.”
Corrosion is another costly concern. Graziano explained that hydrogen sulfide gas converts to sulfuric acid, which attacks the cement in concrete pipes, exposes rebar and tarnishes copper piping. The plant spends significant money each year addressing the problem.
The proposed study would use two main approaches to measure odors. Hydrogen sulfide monitors — devices called OdaLogs — would be installed for real-time readings at specific locations. Grab samples of ambient air would also be collected using vacuum pumps and bags, then shipped to a lab for analysis.
At the lab, samples can be run through a gas chromatograph to identify individual compounds, or tested for total odor impact using trained human panels.
“If it’s somebody who’s got a really sensitive nose, they excuse them from the panel. And if it’s somebody who can’t smell anything, which I think we all know people who are on both sides of that spectrum, they also don’t want them on that panel,” Graziano said. “So they get a good average nose that sits on the panel.”
Those panelists are exposed to increasing concentrations of the gas sample until they reach a detection threshold — the point at which they first smell something — and then a recognition threshold, when they can identify what the odor is.
The results allow engineers to calculate emission rates and prioritize sources. Graziano showed an example from another project where final clarifiers posed little concern, while headworks processes like grit chambers, primary clarifier weirs and screening channels were the main culprits.
From there, the study would use air dispersion modeling — specifically, an EPA program called AERMOD and a companion tool called CalPuff — to simulate how odors travel offsite. CalPuff was included because of the Hannah Avenue plant’s proximity to the Great Lakes.
“The reason we would use both of them is because AERMOD is really good at capturing this information on a regional basis, but CalPuff has this added feature that allows you to better capture meteorological effects by being close to a large body of water,” Graziano said.
The modeling allows engineers to run what-if scenarios. Graziano showed graphics comparing current conditions with no odor control, a process change like replacing media in a treatment system and a more substantial upgrade such as a biological filter with carbon polishing.
“Ideally we can get this where the risk doesn’t extend beyond the plant fence line,” Graziano said. “Or if it does, that it happens at such a reduced frequency that you effectively don’t have any real impact to neighbors.”
The proposal also includes a community engagement framework. Graziano described an “odor wheel” system in which residents who call in complaints describe the smell — rancid, putrid or other characteristics — allowing operators to trace the odor to a specific part of the process.
“You don’t know if there’s an odor issue unless people tell you about it,” Graziano said. “It shows your residents, your ratepayers, that you care and that you are competent in following up and seeing these things through. And that leads to a lot of trust and transparency.”
Graziano noted that the Hannah Avenue plant’s location close to the beach and downtown makes the study particularly important, and that the graphical outputs from the modeling could be useful in conveying results to residents.
The study would also address odorous air currently being fed to a high-speed turbo blower at the plant. While the blower can treat odors, the hydrogen sulfide in that air is corroding the equipment. Graziano recommended disconnecting it to extend the blower’s service life.
A commissioner asked whether the plant currently monitors hydrogen sulfide levels. Staff said some buildings, particularly the headworks, have gas monitors, and operators carry personal detection units, but there is no comprehensive monitoring across all plant processes.
The study would conclude with a cost-benefit analysis of potential improvements and a framework for future community engagement. No formal action was taken during the study session.