A remotely controlled robot handles an inert simulated chemical munition during training at the Pueblo Chemical Depot in southern Colorado, Jan. 29, 2015.

Brennan Linsley/AP

A Colorado plant destroying obsolete U.S. Army chemical weapons has taken a step toward resuming full operations.

Officials said Monday the Pueblo Chemical Depot restarted one of the final steps in the process, using bacteria to treat liquid hazardous waste.

The plant is destroying 780,000 shells containing 2,500 U.S. tons of mustard agent under an international treaty.

The biotreatment part of the plant has been out of operation for modifications but restarted on Oct. 5. Officials say biotreatment is running at 25 percent capacity and will be gradually increased.

While the biotreatment was out of operation, wastewater has been shipped to an incinerator in Port Arthur, Texas. Trucks have hauled 193,000 gallons so far and are expected to send up to 57,000 gallons more.