Abstract: At approximately 9:30 a.m. on October 15, 2004, Employee #1 and Employee #2 were on top of hopper Number 2, one of four 26-ft-diameter, 18.5-ft high hoppers incorporated into building 860. They were replacing metal covers on the screw conveyor housings. The conveyors fed post-digester-centrifuged sewage cake into the hoppers. The conveyor housings were located on top of the hoppers and shared the same atmosphere with the hoppers. No obstruction existed at the 2- by-3-ft opening between the housing and hopper spaces. Employee #1 was cutting through one of the metal housing covers with a Dewalt heavy duty small angle grinder and a SAIT Type 27 grinding pad. The cut was being made above the opening to the hopper. The hopper and housing exploded, throwing pieces of the tank such as guardrails and a manhole cover. Employee #1 and Employee #2 were both thrown horizontally several feet but managed to avoid falling off the tank. The contractor's foreman and superintendent were on the ground below the tank and were also thrown several feet. Although heavy debris was thrown onto the ground, no one on the ground was hit. Employee #1 and Employee #2 both received bruises, contusions, and abrasions.