A boiler failure occurred in a wastewater treatment plant. While one of the three boilers of the sludge treatment facility was shut down for maintenance, the operator noticed a hot spot and deformation on piping in the other 2 boilers. These boilers were then also shut down for repair work. With all 3 boilers out of service, treatment of sludge from wastewater treatment operations was thus interrupted. As a substitute for heat treatment, the operator installed a chemical treatment facility to process the sludge. A mobile centrifuge was brought on site. The wastewater treatment plant’s operation was adapted to the degraded situation (modification of the process parameters on specific treatment units, reduction of the flow of wastewater treated by the plant, etc.). The partial bypass of specific treatment steps caused a decrease in the suppression of nitrogen and phosphorus compounds.

The manufacturer’s intervention on the 2 deteriorated boilers shows that the deformation of the fluidisation air pipes was caused by a reduction in the air flow rate, causing them to overheat. A deposit had reduced the flow cross-section of these pipes by half. This deposit, consisting of dust and metal particles, was related to the work being carried out on the boiler undergoing maintenance.

Following the accident, the operator and the manufacturer had modified the shape of the piping (90° elbows replaced by 45° elbows) and the position of the boilers’ air intake to make them less sensitive to deposits of dust generated by the work. The operator has set up maintenance procedures to isolate the section being worked on from the boilers in operation and to inspect the equipment after each intervention. The pipes are now periodically checked for obstructions.