At 8:15 a.m. in a wastewater treatment plant, due to a connection error during a delivery operation, 100 l of aluminium chloride (AlCl3) was transferred to a tank containing 11,000 l of sodium hypochlorite (NaClO). The reaction between these two products resulted in the release of chlorine gas. Two technicians were overcome by the gas and transferred to the hospital for evaluation. The gaseous release was able to be confined within an 800 m³ building. The reaction continued until around 1:40 p.m. The amount of chlorine formed was estimated at between 1 and 11 kg, corresponding to a concentration of 500 to 5000 ppm in the building. The firefighter’s atmospheric measuring devices were saturated. In the evening, the fire brigade’s mobile chemical response unit installed ventilation to reduce the chlorine concentration in the building. An extractor, located at the entrance, directed a flow of chlorinated air through a water curtain which allowed its concentration to be lowered before release into the atmosphere. The chlorine concentration of the outflow varied between 0 and 10 ppm. The presence of chlorine was not detected at the site’s property limit. Ventilation was continued with the injection of fresh air to render the process more efficient. Residual puddles contained in the containment areas were diluted with water.

The operator monitored the site over the weekend. On 20/01/14, the residual chlorine concentration in the building did not exceed a few ppm. The operator proposed a protocol for removing the sodium hypochlorite, the water from containment areas and the salt crystals that had formed. Once the protocol had been validated by the inspection authorities for Classified Facilities, the waste was removed to a specialised company for treatment the next day. On 22/04/14, the inspection authorities for Classified Facilities checked the results of the cleaning operation and the building ventilation was halted.