The Program Executive Office, Assembled Chemical Weapons Alternatives (PEO ACWA), the U.S. Army Pueblo Chemical Depot and the surrounding communities worked together to select neutralization followed by biotreatment to destroy the chemical weapons formerly stored at the depot. PEO ACWA was responsible for completing stockpile destruction operations by the Chemical Weapons Convention treaty commitment of Sept. 30, 2023. The last munition in the chemical weapons stockpile in Colorado was destroyed June 22, 2023, and PEO ACWA is now focused on the safe and environmentally protective closure of the plant.

Prior to destruction operations, the stockpile comprised 2,613 U.S. tons of weaponized mustard agent in three types of munitions: 105mm and 155mm projectiles and 4.2-inch mortar rounds.

The process of neutralization followed by biotreatment at the Pueblo Chemical Agent-Destruction Pilot Plant (PCAPP) used hot water to neutralize the chemical agent, effectively destroying the mustard agent molecules. The resulting hydrolysate was mostly water and thiodiglycol, a common industrial chemical that was readily biodegradable. Ordinary sewage treatment bacteria, or microbes, consumed the organics in the hydrolysate.

Step One: Removing the Energetics

Robotic equipment removed the munitions’ energetic components, including the fuze and the burster. Removing these parts first made the remaining processes safer. The non-contaminated energetics were shipped to and destroyed in the Static Detonation Chamber (SDC) unit in Anniston, Alabama.

Step Two: Removing the Mustard Agent

Once the energetic components were removed, the munition body containing chemical agent was processed. To remove the agent, the body was robotically accessed and the agent was washed out with pressurized water and collected for destruction.

Step Three: Neutralization of Mustard Agent

The mustard agent was mixed with hot water and a caustic solution, producing hydrolysate. It had a high pH, so acid was added to reduce the pH to neutral, making it suitable for digestion by the microbes used in biotreatment, the next step.

Step Four: Biotreatment

The hydrolysate generated in step three then went through a biotreatment process, in which microbes in large tanks digested and further broke down the solution.

Water released from the process was recycled, leaving various salts and biosludge. Biosludge, which was made up of microbe waste products and other bacterial matter, was filtered to remove water and shipped to a permitted Treatment, Storage and Disposal Facility (TSDF) for disposal.

Step Five: Disposing of Metal Parts

The final step was treating the munitions’ metal parts. Although the metal parts were cleansed of energetics and agent in step one and step two, they still might have contained traces of energetics and agent, and needed to be decontaminated to a higher level. To accomplish this, the metal parts were heated to 1,000 degrees Fahrenheit for 15 minutes. The metal was then safely recycled.

Secondary Waste

Secondary wastes, which included both solid and liquid products from the PCAPP SDC units, were stored in either a less-than-90-day hazardous waste accumulation area or a permitted storage area, pending shipment to a TSDF for further treatment and/or ultimate disposal.