The massive fires that torch most of the metro area's excrement are about to get a boost.

Partly to help keep up with the region's projected growth, the Metropolitan Council is installing a fourth incinerator at its massive Metropolitan wastewater treatment plant beside Pig's Eye Lake in St. Paul. The 1930s-era facility is the 10th largest of its kind in the country, by capacity, and purifies wastewater from about a third of the state's population.

The plant still has some spare room in its incinerators for extra material, said the council's plant engineering manager Rene Heflin, "but basically in 2020 we will be at capacity."

Treating wastewater begins by filtering out large objects, then using a gravity-based settling process to separate heavier "sludge" solids from the water. Incinerators reduce the sludge to ash at temperatures exceeding 1,300 degrees. The plant generates about 40 tons of ash a day, which goes to a landfill in Rosemount.

That process generates power in the summer and helps heat the plant in the winter. Microorganisms feast on the water to purify it further before it is expelled into the Mississippi River.

There are just over 200 sewage sludge incinerators in the country, according to the U.S. Environmental Protection Agency, including the three — and soon to be four — constantly churning away at the Metro plant. "The environment within the unit is very aggressive and harsh," Heflin said.

The council plans to spend $150 million for the new incinerator, covered by existing wastewater fees paid across the region. The agency's goal is to begin construction in 2021 and finish in 2024. That will allow the council to shut down other incinerators for refurbishment, at a cost of about $30 million.

Met Council staffers said they won't need similar capacity in the water-oriented portion of the plant, due to successful efforts to conserve water and reduce the amount of water that leaks into sewers.

The current incinerators have been used since 2005, when they replaced older incineration technology. The switch reduced annual odor complaints from 70 to zero in recent years, said Jeannine Clancy, assistant general manager in the council's environmental services division.

The council looked at other technologies before deciding to build a fourth incinerator. The top options included several that relied on anaerobic digestion — microorganisms breaking down material in an oxygen-free container — to reduce the volume of the sludge, which can then be incinerated or used as a fertilizer.

"The lowest-cost digestion alternative is twice the cost of adding a fourth incinerator," Heflin said.

Incinerators release air emissions, and Minnesota Pollution Control Agency data on permitted facilities show that the Metro plant is the sixth-largest emitter of air pollution in Ramsey County. But it ranks a distant sixth, with its emissions amounting to a fraction of the manufacturing and power plants that top the list.

The council says the incinerators have a great track record meeting EPA emissions standards, consistently producing less than half of what they are allowed. Heflin said their modeling shows a fourth incinerator will have an "undetectable" impact on air quality in the surrounding area.

Anne Jackson, principal engineer with the MPCA's air assessment section, said the plant's rank among county emitters is boosted largely by carbon dioxide rather than more immediately harmful pollution like fine particulates.

"The system they have there is state-of-the-art for sewage sludge. And it's performing very well," Jackson said. "They are well below allowed emissions."