At a fertiliser plant, a dense and whitish cloud was released into the atmosphere during a 30-minute period subsequent to a pH disorder on the gas washing device set up in the granulation workshop. Technicians did not respond quickly enough to this operational drift given that the installation was running in manual mode since the regulation system had been disassembled the same day for maintenance. The site operator modified the alarm sound signal to make it perceptible beyond just the control room. The automatic regulation system for neutralising washing juices was improved.