The biological plant for treating the industrial effluent of a major chemical site was struck by a phenomenon of sludge build-up subsequent to the development of filamentous bacteria (bulking). The sedimentation of organic sludge in the clarifiers had become impossible, causing sludge overflow towards the site’s final physicochemical treatment plant that had rapidly become saturated. Effluent at the plant outlet reached the natural environment (first the MERLE River then the ROSSELLE). The water turned brown and turbid. Significant noncompliance of discharge-related standards for the parameters of suspended matter, chemical oxygen demand and DBO5 (5-day biological oxygen demand) were recorded at the plant outfall.
The ROSSELLE River, which was potentially subject to pollution, emptied into the SARRE in Germany. German authorities were alerted by the Civil Protection Office in accordance with cross-border practices. The operator duly informed the classified facilities inspectorate and drafted a press release for communication on 4th August.
A set of measures were adopted once the anomaly had been discovered (lowering of the hydraulic load, alternative flocculant testing, chlorination followed by re-seeding of the plant). Before discharge into the natural environment, the effluent was treated by large mobile units (for filtration and coagulation / flocculation / sedimentation) as of the 5th day (cost of the operation: €500,000). The output of several site shops was slowed in waiting to restore performance of the treatment plants; one was totally shut down, while the activity of two others was scaled back 30% to 50%, resulting in significant operating losses. A portion of the incoming effluent to the biological plant was stored in order to limit the organic load arriving at the plant. Close monitoring of the natural environment over the medium term was instituted to assess the actual impact of this accident (ecotoxicological testing on daphnia, etc.). An unannounced inspection ordered by the classified facilities inspectorate 10 days after the accident revealed the compliance of effluent being discharged into the environment.
The operator considered several hypotheses in order to explain the appearance of filamentous bacteria: accidental destruction of biomass, lack of oxygenation, shortage of nutriments, sudden variation in load at the plant inlet, unfavourable climate conditions (high temperature).
In the long run, a study of the platform water treatment facilities was anticipated so as to justify their design (closing of several installations, leading to an overdesign).
