Pollution
Humain
Environnement
Economique

In an ammonium nitrate-based fertilizer plant, a technician noticed that water had risen above the central gutter of a 50,000-t horizontal silo storing solid urea. This gutter was intended to collect upwelling water next to the silo and convey it to a sump equipped with a lift pump. An inspection of the pump determined that it had been blocked by crystallised urea, and as then replaced the next day. The following day, analyses, conducted in the well receiving the water pumped from the sump before being discharged into the wastewater network, revealed abnormal nitrogen concentrations (18 g/l, or 14 times the authorized discharge limit).

The investigation showed that the failure of the lifting pump caused the groundwater, collected in the gutter, to overflow into the storage area. The urea had dissolved and the ureic-nitrogen-charged effluents, present in the drain and the sump, were pumped into the well.

Pumping operations into the well were stopped. The discharge from the silo drain’s lift pump was diverted to a storage tank that had been placed nearby. The concentration in the sump returned to normal after 48 hours. The contents of the well were transferred to a polishing basin. The effluent from the urea unit, whose quality had returned to normal, was then sent to another well.

A daily checking procedure was set up for the lift pump, and the monitoring frequency of discharge points was increased from once to twice a month. A level sensor with a visual alarm was installed in the gutter’s collection sump.