On August 3, 1991, at about 12:20 pm, a deflagration occurred inside a wastewater treatment tank at the Allied-Signal Delaware Plant. The tank was being used to hold treated wastewater prior to discharge to the plant process sewer system. One minor injury occurred as a result of the deflagration. The tank sustained damage to its manway, conservation assembly, and inlet and discharge piping. The building in which the tank was located had to have relief panels and roofing replaced and required repairs to the walls where the roof girders separated from the masonry.
The exact cause of the deflagration is not known. Investigation of the incident indicated that the source of the deflagration was ignition of flammable vapors in the head space of the tank. The three most likely sources of ignition each involved the presence and use of hydrogen peroxide in the process.