A total of 400 m³ of sewage sludge having a COD of 11 g/l was discharged to the River Légère between 11:00 p.m. and 2:00 a.m. following maintenance on a pump at a chemical facility’s new wastewater treatment plant. The pump’s variable speed drive (VSD) failed, in turn causing its power supply transformer to fail and the filter system to completely stop. The operator measured the river’s COD at the WWTP’s outlet and upstream in order to determine the extent of the pollution. The inactivated biological sludge that was dumped into the river settled in its bed and then progressively became re-suspended and was naturally eliminated after a few days. No significant impact on wildlife was noted.

Amongst the causes identified, the operator noted:

• a high concentration of sludge in the aerobic digester, which tends to foul the filters;
• the division and responsibilities between the existing and new facilities needs to be clarified;
• management of the on-call crews: they were contacted but did not come in because they thought the maintenance crews could sufficiently tackle the problem;
• management spare parts, which could have enabled the VSD to be repaired quickly;
• the pond upstream of the filtration system reached its high level too quickly in relation to the time needed to handle the incident.

After the accident, the operator had the WWTP’s VSDs inspected by its supplier and all its electrical equipment inspected by a specialist company. The operator and the company that designed the WWTP set up a system to divert the flow of effluent to a 3600 m³ pond in the event of a problem. The inspection authorities for classified facilities asked the operator to be better prepared to respond to such accidents and to specify the measures taken to restore the river’s water quality. The operator updated its emergency procedure to include such accidents and set out the preventive measures decided on when analysing the causes.