At around 6:00 p.m., sludge from the wastewater treatment plant (WWTP) of a pigment manufacturing plant was discharged to the water supply networks of two buildings following a malfunction. A technician noticed the sludge when he opened a tap at around 7:30 p.m. The internal drinking water network of both buildings was immediately isolated. The employees in these buildings were told not to use the water. This network was disinfected three times (once with 2.5% hydrogen peroxide and twice with bleaching extract) and then flushed until all traces of chlorine disappeared. The water contaminated by the non-hazardous sludge and the water used for disinfection were treated on-site at the WWTP. Daily microbiological monitoring and pH checks were set up at the WWTP’s outlet.

Failure of the water distribution solenoid valve following a malfunction of the centrifuge is what caused the sludge to backflow from the WWTP to the water supply networks of both buildings. There were no backflow preventers between the water distribution circuits of these buildings and the WWTP’s water network. The sludge backflowed into the buildings’ water supply networks because the pressure in the water mains dropped when the malfunction occurred (P < 3.5 bar). The centrifuge failed while the plant was closed for Christmas weekend. The incident revealed that the night staff were insufficiently trained in managing the WWTP.

The operator implemented the following actions:

• WWTP training for the night shift;
• annual maintenance of the centrifuge;
• a backflow preventer and check valve on the centrifuge;
• creation of an internal water network map;
• identification and elimination of dead legs.