In a refinery employing 2,700 employees, analyses of samples taken on 13/01 revealed Legionella concentrations of 4,800,000 CFU/l and 310,000 CFU/l respectively in two of the site’s 25 cooling towers, identified as DLL2 and CT17. After 20/01, the operator carried out a shock treatment (chlorine and biodispersant) in its installation and shut down DLL22 on 22/01. On 23/01/04, the local government authority issued an emergency measures order to requesting he shutdown, drainage, cleaning and disinfection of cooling tower DLL2 (attached to the lubricant logistics department), the continuous treatment of tower CT17 connected to the fluid catalytic cracker (FCC) alkylation units, and within eight days, recommendations for cleaning this tower. The checking of the site’s 23 towers and the towers of the neighbouring establishments was also requested. On 27/01, the operator sent a technical report to the environmental inspection authority describing the current and future schedule for maintaining a minimum free chlorine concentration of 0.3 ppm and the use of biodispersants. It therefore specified chemical treatment in the form of shock treatments twice a day for all of the site’s cooling circuits and chemical cleaning by the continuous injection of biodispersant for 4-6 weeks in order to eliminate the biofilm. According to the operator, shutting down the catalytic unit for over one month to clean the cooling circuit could result in potentially high-risk conditions (high t°C and P, atmospheric releases of SO2 and catalyst powder) with the implementation of non-routine, complex operating procedures for the staff. On 28/01, an emergency measures order requested the shutdown of cooling tower CT17 in order to empty, clean and disinfect it. The FCC installations were shut down on 02/02 and the CT 17 cells were shut down on 4/02. The same operations were scheduled for the alkylation unit but were staggered by three days, due to flow management and resource issues. The drainage of the circuits in question involved 4,500 m³ of water. CT 17 was restarted on 18/02, after a 16-day stoppage. The 26/02 and 11/03 analysis results gave Legionella concentrations of below 50 CFU/l. The cost of the cleaning and disinfection operations, excluding supervision costs, amounted to €1.1 million. The operating losses amounted to €7.9 million. The total cost of the shutdown was €9 million.