The event occurred in an atmospheric pressure tank storing effluents. The plant was in the production phase. The head of the overfill prevention valve located at the top of the tank was thrown a distance of 77 m. The tanks shell was deformed (curved downward) but not leaking. Employees noted a noise similar to the opening of a valve, and a low-pressure alarm in the tank appeared in the control room, despite the fact that the level and temperature were normal. The tank receive is filled with i) organic compounds normally producing hydrogen, and ii) formic acid containing a minor faction of hydrogen peroxide. Because of the presence of hydrogen the tank is maintained under positive nitrogen pressure to inert its vapour space. Two days before the incident, the second effluent was slightly contaminated with suspended solid particles of a catalyst used in the production unit feeding the storage tank. The catalyst consist of palladium on carbon in very fine size se particles ( 10 ppm). The presence of these particles is abnormal and results from a process malfunction. The palladium catalysed the decomposition of hydrogen peroxide into water and gaseous oxygen. The hydrogen and oxygen mixture in the vapour space then exploded in contact with an undefined heat source, causing an overpressure in the tank resulting in its deformation and the ejection of the liquid seal cap. According to the ARIA report, a post-accident study showed that, even in the absence of palladium catalysis, oxygen production in the tank through decomposition of hydrogen peroxide was higher than initially estimated.