On 8 June, a police officer alerted the operator of a PVC flooring plant that a ditch had been filled with a black liquid since 30 May. The valve used to discharge polluted water to the municipal network was closed. This water contained a mixture of ink residues and sodium carbonate solution.

The operator realised that the electrical cabinet powering the sump pumps used to feed the wastewater to the buffer tank had short-circuited at 11:18 p.m. on 29 May. When production resumed at around 7:00 a.m. the next day, the effluent had flowed into the municipal network. The network is above the bypass line used to divert stormwater into the wastewater network if a pollution event occurs. An overflow installed at an even higher level is used to manage this risk. The cabinet was switched back on at around 2:30 p.m. At 11:23 p.m. on 31 May, the cabinet again short-circuited; it was switched back on at 2:56 p.m. on 1st June. At the same time, a leak sprung on the bypass line’s valve (stormwater and wastewater networks). The wastewater flowed by gravity into the stormwater network and then into the ditch.

The bypass valve, which was old, was scheduled to be replaced in the following days. However, as no leak test plan was in place, the leak had not been detected.

To prevent such an accident recurring, the operator:

• replaced by the bypass valve and now annually tests all its valves for leaks;
• replaced the cabinet’s GFCI and installed a system that alerts in case the cabinet shuts off;
• changed the programming so that both wastewater sump pumps operate simultaneously to cover the flow rate.

On 11 October 2017, the inspection authorities for classified facilities visited the site and found two nonconformities:

• the plant’s electrical equipment was in poor condition;
• the map of the stormwater and wastewater networks and the equipment was out of date.