The city’s costliest building project has a fresh-air payoff.

If you take the Border-to-Border trail past Geddes Dam, or walk the grounds at St. Joseph Mercy Hospital, you might have noticed that the air is pleasanter lately. Credit the most expensive building project in Ann Arbor’s history: the $140 million reconstruction of the city’s wastewater treatment plant downriver from Gallup Park. Its $45 million first phase is just about done–on schedule and under budget.

“Wastewater” is what people in the business call sewage. And the foul smells that used to waft from the plant came mainly from the machinery that separated what are politely called “biosolids” from liquid waste.

When that equipment was last updated thirty years ago, “solids handling was a plate-and-press process,” explains senior engineer Mike Amicangelo. “It put the biosolids in a membrane and slowly squeezed the water out–and boy that stunk!”

By the 2000s, the machinery was also wearing out. So when the city set out to renovate its wastewater treatment plant, replacing it was a top priority.

“The project started in spring 2009,” says Amicangelo, when we meet in his temporary office in a pair of double-wide trailers. The major work was done by early summer 2012, on time and under budget.

“It cost $45,451,155.33,” Amicangelo tells me. “The original contingency, the part of the budget for add-ons and changes, was $4,244,000, and the actual contingency so far is $3,000,228.33. We replaced a steel water line with a stainless steel water line, and the fire marshal wants additional fire alarms and strobes in some locations, altogether about $130,000, and there’s not much more to do.”

A big man with an easy laugh, Amicangelo shows me around the plant. Replacing the solids processing equipment without halting service was the toughest part of the job, he says, because the plant handles an average of nineteen million gallons a day–and the flow never stops. As Amicangelo says, “It’s always coming down the pipe.”

And there was a deadline. During the warmer months, he explains, “we produce land application, a liquid farmers use as fertilizer. But when the ground freezes in December, we switch to making cake [solid biowaste mixed with lime] for disposal in a landfill. The new equipment had to be operational by December [2011], or what are we gonna do with the stuff?”

They did it by working inside the existing building. “We tore out the incinerator that took half the space–it’s not been in use since ’97–and installed new floors. Then we added new processing equipment there and put that online.” The plate presses were replaced by enclosed centrifuges that get more water out while containing spills and odors. They also added new boilers and electrical systems, plus new blowers that are 20 to 30 percent more efficient than the old ones. “They use 50 percent of our electrical power, so you can imagine the savings,” Amicangelo says. And they closed gaps that let pigeons into the �xADbuilding–followed, sometimes, by a Cooper’s hawk that killed and ate them on the spot.

Once the centrifuges were up and running, Amicangelo adds, they took out the plate presses and used the space to install “HVAC and odor-control equipment”–huge white tubes that snake out of the floor and vent through enormous carbon filters.

The foulest odors in the new building are next to the cake storage bunkers and in the truck bays. The latter smells powerfully of the ammonia produced by the lime mixed into the cake, while the former reeks of processed biosolids. But the plant no longer shares those smells as freely as it once did: “The only time the odor is really noticeable is when the cake is moved from the storage bunkers to trucks for removal,” he says, “and that’s done inside the building.”

Sure enough, when he takes me to an outside balcony, the air smells as fresh as a spring day.

“St. Joe’s used to complain about the smell,” says Amicangelo. “But now, not so much.”

They’re already at work on the project’s $95 million second phase. Amicangelo points to a gargantuan hole filled with earth-moving equipment.

“The new administration building will be away over by the river,” he says. “The old building was right in the middle of the plant. We’re replacing it with two new treatment trains [for liquid waste], plus a third if we need it. We’ve already got four in the other plant, and they will remain in service.” There’s nothing Amicangelo loves better than multiple backups–�xADbecause otherwise, a failure can lead to discharging raw sewage into the Huron River.

In fact, the plant did discharge 10,000 gallons of wastewater into the river during a massive rain on June 27. “The amount of storm water that came into the plant with the sanitary sewer flow tripled in half an hour,” plant administrator Earl Kenzie explains in a later phone call. “I’ve never seen it increase that quickly in the twenty-five years I’ve been here.

“Staff got it under control in ten minutes, which minimized the spill,” Kenzie says. “If we’d had the West Plant up and running, we could’ve handled it more quickly–maybe.” But the West Plant–originally built by the New Deal’s WPA in 1936–was shut down in 2006. It’s now being demolished, and its replacement should be online in about two years. Kenzie says the additional capacity–and, he hopes, the resumption of the city’s footing drain disconnection program–should go a long way towards minimizing the risk of future spills.

The final part of the project is renovating the East Plant, constructed in 1977 to handle the sanitary sewers’ liquid waste. “We had to do solids first, because it was failing,” says Amicangelo. “Plus we didn’t want to do both projects at the same time and put the expenses on the customer at once. We spread it out over time with 4 percent [sewage fee increases] per year” from 2005 to 2014.

The entire construction cycle is scheduled to take five years, but “we got a good contractor, and he anticipates finishing in four to four-and-a-half years,” says Amicangelo. Adding contingencies and engineering fees, “the total project will cost $114 million.”

When the work is done, Amicangelo figures the new buildings and equipment will be good for twenty to fifty years, depending on how much sewage flows their way. The plant currently serves the city and parts of the townships of Ann Arbor, Pittsfield, and Scio. While the city is already close to its maximum population, he reckons the townships may grow: “You can never tell what political deals will be made that’ll increase the flow.”