In a chemical plant, the daily effluent log from the treatment plant showed an accidental release, over last 48 hours, of heavy organic pollutants, equivalent to 337 kg of dinitrotoluene (DNT) and 170 kg of mononitrotoluene (MNT), into a river.

Two days earlier, the operator identified a source of organic pollution on a decanter consisting of slightly polluted effluents released by a manufacturing facility upstream: a leak on the bowl of the hydraulic seal of one of the workshop’s centrifuges. A damaged seal was replaced, but the leak continued. The bowl itself was replaced. At around 3:30 p.m. on the day before the discharge was detected, the content of the decanter, which was overloaded with organic pollutants, was sent by batch to the unit for treating effluents polluted by an oxidation process.

As the leak was chronic, the decanter had accumulated more than 400 kg of heavy organic pollutants (having a density of 1200 kg/m³). The treatment unit, which was undersized for such a concentration, was unable to process all the pollutants which had accumulated, batch after batch, in the buffer tank downstream.

The buffer tank’s TOC sensor detected abnormal concentration peaks each time a batch processed by the treatment unit was drained. The drainage operation temporarily put the organic pollutants in suspension, they then fell back down to the bottom. These peaks regularly exceeded the alarm threshold (250 ppm), which was programmed in the control room. Following a detection of concentration at 930 ppm, the shift supervisor of the crew on watch checked that the sensor was working correctly, at around 5:30 p.m., and performed analyses of a sample taken at the surface of the tank, which proved to be negative (TOC below 85 ppm). Thinking that the situation was normal, the control operators continued to convey the effluents from the buffer tank directly to the treatment plant.

For its part, the plant’s TOC sensor, as it was out of service, did not detect the effluent’s abnormal concentrations before it was released into the environment. Its sampler was saturated with suspended solids present in the effluents released just a few hours before by a neighbouring plant. The maintenance department had not informed the control room of the problem. Furthermore, there was no alarm on the sensor’s sampler to alert to the problem. The discharge of effluents, having a high concentration of heavy pollutants, continued until the next day, when a 24-hour average discharge report enabled the situation to be detected.

The operator had the faulty bowl evaluated and sent the effluents remaining in the buffer tank to be burned. The following corrective measures were implemented:

• Analysis of the concentrations of the hydraulic seals on the centrifuges each time the bowl is changed;
• In case of intervention on the centrifuges, the contents of the slightly-polluted effluent decanter are sent for burning, and not for treatment by oxidation;
• Installation of a TOC concentration alarm in the treatment plant with the instruction to check this concentration before a batch is emptied into the buffer tank;
• Reinforcement of the buffer tank’s TOC alarm in the control room (audible alarm and pop-up screen); in the event of an abnormal detection, instructions to divert effluents from the tank for burning;

Installation of a low flow rate alarm on the sampler of the treatment plant’s TOC sensor, and sample detection.