PUEBLO, Colo. — The Army has begun destroying the nation’s largest remaining stockpile of chemical weapons in an automated plant in southern Colorado.

Officials at Pueblo Chemical Depot said they put the plant into operation on Wednesday.

The plant will destroy about 780,000 artillery shells filled with mustard agent under an international treaty. The Army said previously it planned to start this week.

Mustard agent can maim or kill by damaging skin, eyes and airways.

The plant uses water and bacteria to neutralize the mustard agent. Work is expected to finish in mid-2020.

The depot has already destroyed 560 shells and bottles of mustard agent that were leaking or had other problems that made them unsuitable for the plant.

They were placed in a sealed chamber, torn open with explosives and neutralized with chemicals.