In a metal processing plant, an explosion occurred while removing sludge from an alkaline degreasing bath. As an exceptional measure, the plant operator commissioned a subcontractor for this biennial operation that did not entail any specific procedure. The “clear part” of the (50-m³) degreasing bath was racked so that the sludge stored at the bottom could be pumped; this sludge was then temporarily stored in 2 tanker lorries before transfer into 2 tanks located in an “acid room” (that also contained a 15-m³ HCl tank) dedicated to used acids. One of these tanks had been poorly drained by the operator; while tank no. 1 was being filled, a subcontracted employee informed the operator during the second transfer of the inconsistency between the quantity of alkaline product poured (17 m³) and the tank’s display (23 m³), in addition to the presence of yellowish vapours at the vent outlet. The transfer was halted during the lunch break. 90 min later, product incompatibility (acid/base) triggered a violent explosion that blasted tank 2 through the building roof all the way into the plant courtyard. Notified by the operator, fire-fighters and a chemical response squad undertook site security measures: ventilation of the acid room by removing siding and pumping liquid substances (degreasing agent and acid), the majority of which had been contained in the retention basin shared by all 3 tanks. Spattering was trapped by the rainwater recovery system (pipe, scrubber, shutoff valve). All liquid discharges were channelled to a certified waste treatment centre. The quantity of product lost in the accident was estimated at between 17 and 23 m³. The building (roof and siding), separation walls, adjacent machinery and electrical installations (oil separator, lighting, pump controls), and the acid room (tanks, pipes, retention basin) were all destroyed. Inspection Authorities recorded these findings and proposed that the Prefect issue an injunction intended to ensure compliance of Prefectural order prescriptions (specific procedures to adopt during use of installations capable of generating hazards; retention basins and pipelines unsuitable for incompatible product mixes), along with a draft supplemental decree (semi-annual analysis of on-site groundwater quality).