When restarting the urea production unit of a fertiliser plant, the clearing by steam of a line clogged by urea and the seal leakage of 2 pumps caused losses of concentrated nitrogen solution in the sewers. These effluents were sent to a pit. In the event of an abnormally high nitrogen concentration, the effluents are automatically sent to the polishing pond. Since the continuous analyser’s filter was clogged, no measurement was able to be performed by the analyser and it returned a null value, preventing sending of the effluents to the polishing pond. Moreover, the pump for transfer of the effluents to the polishing pond and the associated piping were clogged, which caused overflowing of the pit. The level in the pit became very high, but since it produced no visual or audible alarm in the control room because it was considered unimportant, the technician was unable to react.

During these 3 events, the laboratory noted that the threshold of nitrogen discharge into the natural environment was exceeded. The urea concentration in the plant’s sewerage network discharging into the Le Havre Grand Canal was diluted by the cooling water.

These 3 events demonstrated the ineffectiveness of the existing barriers to prevent pollution of the natural environment:

• clogging of the pumps and piping from the pit to the contaminated water recovery pond;
• failures at the monitoring level, which impacted the response time resulting in the discharge.

The pit water lift pumps were cleaned, part of the piping was cleared and replaced, and the pit was cleaned. The systems were restored to operation during the day.

The operator took the following measures to:

• prevent discharge analysis faults: verification of the existence of a flow on the analyser and modification of monitoring, filter cleaning and maintenance of the analyser pump;
• prevent clogging of the pit lift pumps: addition of thorough purging during the technician’s round;
• enhance pit monitoring: establishment of an emergency response procedure associated with a very high level in the pit;
• check the entire circuit to be able to prevent discharge from the pit into the natural environment before start-up: updating of the start-up checklist.