At around 8:00 p.m., after viewing a video posted on social media showing a release of blue effluent into the LYS, an off-duty technician at a pigment manufacturing plant (producing notably ultramarine blue) alerted the supervisor on site. He stopped the treatment plant, cleaned several pieces of equipment and increased the plant’s treatment parameters. At around 11 p.m., effluent treatment resumed after the compliance of an instantaneous sample was checked against the authorised emission values. The technician had conducted his round at 6:30 p.m., at which time he had noted that the releases were light in colour. The operator estimated that 62.7 kg of pigments (polysulphide sodium aluminosilicate) had been released.

The overflow was attributed to saturation of the clarifier due to a significant generation of sludge following the leakage of blue pigment from the filter press pump.

The operator’s analysis also showed the following organisational causes:

• the technician, who had been on the job for three months, had not performed any preventive emptying of the clarifier. This can also be explained by the absence of tools, criteria or procedures that would allow a decision to be made;
• the automatic emptying system was not correctly adapted: a lack of relevant instrumentation, parameter settings, criteria;
• the clarifier had experienced clogging by solid deposits on the walls, the lamellas (some of which had been broken by the excess weight of the deposits), making the measurements of the sludge level unreliable;
• the ferric chloride injection rate was considered unreliable due to its design.

The operator undertook the following actions over one year to prevent the event from reoccurring:

• raising of staff awareness;
• improvement of operating procedures and site organisation;
• research of instrumented solutions to improve the control and monitoring of the station’s parameters;
• replacement of the clarifier’s lamellas;
• a workload study of the treatment plant’s technician and the supervisors.