At around 6 a.m., a leak occurred on a digestate tank in a wastewater collection and treatment firm. 850m³ of residue of organic matter resulting from the anaerobic digestion process, rich in nitrogen and phosphorus, were discharged into the Fontaine Despagnet, including 200m³ in Lourden Lake. The creation of a dyke around the opening, then stopping up the opening 1 hour later, stopped the flow. The digestates retained and those remaining in the digester were recovered with the firm’s pumping machinery. They were stored in a silo on the site. The digestate storage tank was flushed and cleaned. The ditch coming from the firm was cleaned. The contaminated waters were recovered over a period of 4 days. No health issues were identified, because there was no potable water supply intake downstream of the discharge point, and no bathing area. The two samples taken, with fast result analyses, showed weak dilution of the digestates in the stream, but strong dilution as soon as the lake was entered. The results of 4 other samples taken the next day showed that the impact of the event was limited to the confluence area.

On the morning of the event, the production manager acknowledged an “agitator failure” alarm which had been triggered, because this alarm was recurrent (problem of an agitator electrical fault, without consequences for the production process). 45 min. later a technician noted that the post-digester was leaking through the opening of one of the 2 agitators whose motor had moved from its position. Moreover, work in progress had required opening the retention basin under the wall, and the basin no longer performed its function. No compensatory measures had been implemented by the operator. The cause of the incident that had resulted in the motor moving from its position had not been determined. The bolts attaching the motor’s mounting to the post-digester structure had been severed, but the technicians stated that the bolts underwent a verification every week and that no defect had been detected during the most recent verification.

Following the event, the operator:

• requested an expert appraisal of the agitator;
• installed vibration sensors on the facility’s 2 lateral agitators and considered installing a mounting for the electric motor;
• considered distinguishing between its alarms;
• changed its work-related procedure.