Following tank cleaning operations, a fertiliser plant exceeded the limit value for arsenic discharge into the natural environment. The laboratory raised the alarm the next day, following the daily analysis of discharge waters. At around 2 p.m., the operator took an isolated sample which showed a content of 0.03mg/l, indicating the end of peak discharge. 1.15kg of arsenic was discharged into Le Havre Grand Canal.

Two days before the event, the operator had a container of solid matter with a high arsenic content cleaned by a subcontractor. The technicians took into consideration the arsenic risk with a work permit and a risk analysis. The container was cleaned with water using a sewer cleaning truck. At the end of the day, the subcontractor cleaned its sewer cleaning truck. The next day, another emptying operation was performed with the sewer cleaning truck used the day before. The operator asked the subcontractor to transfer the water resulting from this cleaning to a pit, because the tank normally used was full. A nitrogen analysis was performed. The arsenic discharge was identified at the level of this pit, which was sent into Le Havre Grand Canal. The sewer cleaning truck’s hose had not been cleaned between the two operations and it still contained arsenic from the previous cleaning operation. The cleaning procedure was not formally recorded by the subcontractor.

Following the event, the operator decided on the following actions:

• every cleaning operation on the sewer cleaning truck must be thorough;
• rinsing water sampling and laboratory analyses must be performed after flushing and cleaning the sewer cleaning truck;
• no discharge should be performed at a point linked directly to Le Havre Canal without prior approval.