A surface treatment plant discharged metal hydroxide sludge to Les Hespérides, a stream. Sludge produced at the industrial wastewater treatment plant is stored in a recovery tank before being passed through a filter pressed and placed into flexible intermediate bulk containers (FIBCs). As no FIBCs were available, the sludge built up inside the recovery tank, which then overflowed. The overflow was discharged from the plant’s outlet and into the natural environment. Upon discovering this, the operator shut down the plant and stopped the discharge. Water quality analyses of the stream, performed the same day, revealed concentrations of 0.61 mg/l of nickel and 0.011 mg/l of chromium. No fish were reported to have died. New surface water and sediment samples were taken immediately downstream of the plant 12 days later, after the inspection authorities for classified facilities conducted their visit.

Multiple factors caused the discharge. Firstly, the plant’s design means that overflow from the final recovery tank is discharged at the plant’s outlet instead of being returned to the neutral recovery tank. Secondly, the settling tank contained an abnormally high amount of sludge because the FIBC supplier had stopped delivering them. There were not enough on hand at the plant and the operator failed to find an alternative solution. Thirdly, a technician had removed the float switch (used to regulate the water level) from the final recovery tank. Lastly, a lack of job training and skills in operating a treatment plant were identified.

The following preventive measures were implemented:

• overflow from the sludge recovery tank is no longer discharged into the natural environment;
• a contingency stock of FIBCs has been built up in the event of a possible supply disruption;
• line technicians now receive training in the plant’s operation;
• maintenance procedures are now documented.