Drippings of milk, whey, water and foam from the chlorinated alkaline-based cleaning solution were spilt in a dairy processing plant. At around 11:00 a.m., while conducting his regular rounds, the on-call electro-mechanical technician noticed that the groundwater sump from the production site was overflowing and that the lift pumps had stopped. Upon seeing this, he closed the containment pond’s shut-off valve and primed the two pumps again.

The overflow was caused by the pumps shutting down at around 7 a.m. The wastewater reached the waterproof surface of the cooling towers and the containment pond by gravity. The cooling tower zone was equipped with a small pump that discharged the water directly into the aeration tank. However, the water collected in the containment pond passed through the separator-sludge trap before reaching the treated effluent metering station. This wastewater was considered as treated and was released directly into the MAYENNE River. At around 2 p.m., a resident informed the duty manager that foam was present in the river, as 125 t of the product had overflowed from the sump. Of the volume released, the operator estimated that 20 t had made its way to the river.

After analysis, the operator identified several causes:

• the valves located behind the lift pump were no longer airtight. Each time the pump stopped, the water column emptied, causing significant vibrations in the piping system downstream from the pumps. The vibrations caused the flowmeter to fail, triggering a safety shutdown of the lift pumps;
• the shutdown of the pumps triggered a fault on the monitoring system, which relayed an SMS to the on-call electro-mechanical technician. However, an intervention was performed 3 days earlier on the server managing these alarms, but the version loaded did not correspond to the correct update. The SMS was not sent;
• no overflow system had been included in the equipment’s design.

On the same day, the operator:

• modified the PLC program so that a flowmeter failure would no longer shut down the lifting pumps;
• entered the correct program into the server managing the SMS alerts;
• reinforced the security rounds.

The operator also plans to undertake the following actions:

• replacement of the valves to avoid vibrations ;
• replacement of the flow meter;
• weekly verification of SMS alerts;
• study the possibility of overflow management on the lifting station in the event of stoppage on the two pumps.