CEDAR RAPIDS — One bonus of the wastewater treatment plant at the city’s extreme southeastern tip is that most of the 170,000 metro area residents who depend on it don’t experience its ability at times to stink up the area around it.

Motorists driving by on Highway 13 some days confront the hydrogen sulfide smell coming off the plant, as do residents who live downwind if the weather and winds are right — or wrong.

This week, the City Council approved a contract for $590,465 with CH2MHILL Inc., of Mendota Heights, Minn., to engineer and manage a two-year, $4 million project to renovate and expand odor control facilities at the plant, called the Water Pollution Control facility.

The plant, which opened in 1980 at Bertram Road SE and Highway 13, treats more organic waste than any plant in the state, thanks to the city’s agricultural processing industries. It handles the waste equivalent of a city with 1.5 million people.

Utilities Director Steve Hershner said Wednesday the odor control project will renovate two bioscrubbers and add two new biotowers to fight the hydrogen sulfide that is released in the treatment process.

He said the new equipment will increase the plant’s ability to scrub 90,000 cubic feet of air per minute by 50,000 more cubic feet per minute and provide capacity to clean odor when a unit is down for maintenance.

The city has been studying the odor reduction needs for a few years, Hershner said, but the work also is coming as Linn County Public Health in the last eight months has established a monitoring station a mile from the city’s plant.

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“Our technology has advanced beyond our old capabilities,” Shane Dodge, Linn County’s air quality supervisor, said this week. “This is a very sensitive hydrogen sulfide monitor.”

Dodge said the county, like the state, doesn’t have the ability to enforce a standard. Nonetheless, he said his department has identified a “health-based threshold” of 30 parts per billion as being “important.” It is an odor standard used in California and one, Dodge said, that Iowa looked at a decade ago as it considered odor standards around animal confinement operations.

He said an odor producer should consider a remedy when the odor reaches the 30-parts-per-billion level at any time on eight days in a year.

In the eight months the monitor has been in place, that 30-parts level has been reached during seven hours on six days at the monitor, he said.

Hershner said it is helpful to have the data because it gives the city the science-based values that citizens’ subjective complaints have not.

“But let’s be perfectly clear, there is no rule,” he said. “There is no number that relates to compliance value.”

The city’s upcoming odor-control work follows a recommendation made by CH2MHILL as part of a $78,000 study completed last fall.

CH2MHILL concluded that none of the alternatives it considered would “completely eliminate off-site odors.” To do so would require additional odor polishing units, which would be costly.

Hershner has worked for the city since 1990, and was at the plant in the 1990s when the city brought in experts to come up with the best method to limit the odor.

The plant selected an approach pioneered in Los Angeles County that uses lava rocks. Bacteria that grows on the rocks eats the odor.

In 1999, the city brought in dozens of semi-trailer truck loads of lava rocks from Arizona, dumped them into two bioscrubber structures, and started a new frontal assault on odor.

In 2007, the city renovated the bioscrubbers with the lava rock medium, and now, in part, the new investment will do that work again.

Hershner said the special porous feature of the lava rocks, which allows the odor-eating bacteria to grow on an assortment of surfaces, is smoothed away over time, necessitating replacement.

Linn County’s Dodge said residents over the years have not been “too shy” about expressing feelings about odor at the Cedar Rapids plant, though his department doesn’t get as many complaints as it once did.

Even so, he said, “We know it smells out there occasionally. We drive through there, too.”