DENVER — The Army plans to start operating a $4.5 billion plant next week that will destroy the nation’s largest remaining stockpile of mustard agent, complying with an international treaty that bans chemical weapons, officials said Wednesday.

The largely automated plant at the military’s Pueblo Chemical Depot in southern Colorado will begin destroying about 780,000 chemical-filled artillery shells soon after this weekend, said Greg Mohrman, site manager for the plant. He declined to be specific, citing security concerns and possible delays.

Robots will dismantle the shells, and the plant will use water and bacteria to neutralize the mustard agent, which can maim or kill by damaging skin, the eyes and airways. At full capacity, the facility can destroy an average of 500 shells a day operating around the clock. It’s expected to finish in mid-2020.

The plant will start slowly at first and likely won’t reach full capacity until early next year, said Rick Holmes, project manager for the Bechtel Corp.-led team that designed and built it.

Construction began in 2004, but until now, the Army has been vague about the start date, citing the complexities of building and testing the facility and training the workforce.

The depot has already destroyed 560 shells and bottles of mustard agent that were leaking or had other problems.

Those containers were placed in a sealed chamber, torn open with explosive charges and neutralized with chemicals. That system can only destroy four to six shells a day.

Irene Kornelly, chairwoman of a citizens advisory commission that Congress established, said her group had no remaining safety concerns.

The shells stored at the Pueblo depot contain a combined 2,600 tons of the chemical.

The Army stores another 523 tons of mustard and deadly nerve agents at Blue Grass Army Depot in Kentucky, which is expected to start destroying its weapons next year, finishing in 2023.

Mustard agent is a thick liquid, not a gas as commonly believed. It has no color and almost no odor, but it got its name because impurities made early versions smell like mustard.