After heavy rains that fell during the night of Friday to Saturday, process water from a bottling plant was discharged into a pond after a lifting pump failed when its dry-run protection activated. The second pump took over, but its dry-run protection activated as well. This failure was acknowledged. Then, at 6:00 a.m., both pumps failed again.

In the morning, the wastewater treatment plant’s (WWTP) technician manually acknowledged the failures on both pumps. He checked their flow rates and the level of the upstream biological network. He saw that it was high, but not at the point of overflowing. The rain had filled the ponds to three-fourths their capacity. After starting the pump used to discharge the contents of the first pond to the River Loire and opening the connection between both ponds, he sampled the first pond’s contents. Seeing that they were out-of-limit, he immediately stopped the pump leading to the Loire and closed the connection between the ponds. He sampled the second pond; the results were also out-of-limit. He then pumped the contents of the first pond into the biological WWTP’s buffer pond (via the septic tank) and closed the valves used to bring in excess water pumped from boreholes to avoid feeding the ponds.

The next day, the first pond’s drain pump was found off and its strainer clogged with vegetation. As a result, the pond’s level did not go down. The pump’s intake was cleaned and its discharge to the buffer pond moved. On Monday, the pond’s surface water was within limit.

There were no environmental consequences other than a discharge that lasted just 5 minutes. The pollution was contained the ponds and treated by the WWTP.

The incident was caused by an issue with the flowmeter of the sump at the biological WWTP’s inlet. It sent a ‘zero level’ signal to the pumps, causing their dry-run protection to activate. The operator found that the first pond was being polluted by an unidentified connection between the building’s network and the pond.

A number of curative measures were implemented:

• The dry-run protection was placed on standby;
• The sump’s flowmeter was removed and checked;
• Efforts were made to locate the connection between the building’s network and the stormwater network;
• The seals on the isolation valves of the borehole sumps’ control valves were replaced;
• Preparations were made to drain the ponds as 12 mm of rain were forecast during the week.

Enhancing the safety and reliability of the sump if its dry-run protection activates is mentioned in the procedure for optimising the WWTP and improving its reliability. The relaying of a WWTP alarm to, say the guardhouse is also provided.