The U.S. Chemical Safety Board (CSB) will conduct a full investigation of the storage tank explosion that killed three workers and injured a fourth at the Packaging Corp. of America corrugated cardboard mill in Tomahawk, Wis., on July 29.

The accident occurred as workers were performing welding to repair a flange fitting on top of an 80' tall storage tank, which contained recycled water and paper fiber. The three workers were standing on a catwalk above the domed, cylindrical tank performing welding when an internal explosion ripped open the tank lid. All three workers died of traumatic injuries, including two who were found on the ground beneath the tank. A fourth, who had been observing the work from a further distance, survived with minor injuries.

CSB investigators were present at the mill in August, examining the accident scene, conducting interviews and gathering evidence. Among the issues the investigation will examine is whether anaerobic microbes, which grow in the absence of oxygen and feed on organic matter, produced flammable gas to fuel the explosion.

Large storage tanks at paper mills -- especially those that are stagnant, poorly mixed, or have long material retention times -- are at an elevated risk for the growth of anaerobic bacteria. Recycled process water, known in the industry as "white water," contains nutrients that promote growth of the organisms, producing hydrogen or other flammable gases that can be ignited by hot work or other sources of ignition.

The cause of the explosion remains to be determined. The CSB plans to conduct microbiological and chemical testing to understand what fueled the blast.

The investigation is expected to take approximately a year to complete, and will likely involve further work at the site.