On 11 May, a contractor fitted sodium hydroxide/sulphuric acid pipes and nozzles with an in-line mixer on the inlet to the aeration tank of a cheese factory’s wastewater treatment plant. Water was used to test the pipe for leaks, but no visible leaks were found. The pipe was commissioned with 96% sulphuric acid. A small leak was found on the pump’s suction connections. While the contractor was performing a new water test to check the seals and tightness of the connections, an exothermic reaction occurred, rupturing the suction pipe and heating the discharge pipe. The contractor replaced the suction pipe and recommissioned it with concentrated acid in the late afternoon. The system ran without any problems until two days later. At around 8:30 a.m. on 13 May, the operator noticed that the effluent in the pretreatment plant had been registering abnormal pH curves for several hours. The sulphuric acid injection pump was operating but with no visible impact on the pH. The injection point on the wastewater pipe was found to be leaking sulphuric acid, most of which had fallen into a trench. The system was shut down. The on-call manager and the firefighters were notified at 9:30 a.m. The emergency pollution response team sucked up 800 litres of acid and stored it in a container. Work on cleaning up this spill ended at around 12:30 p.m. That afternoon, all the valve’s nuts were retightened and the system was restarted.

The leak of sulphuric acid was caused by incorrect tightening of several couplings. On 15 May, the pipe was dismantled and it was found that the collar of the retaining nut between the injection valve and the last valve was deformed. On 16 May, the contractor noticed that the non-return valve on the pipe downstream of the acid injection system was defective (it lacked a spring). This is what caused wastewater to be drawn into the pipe. The exothermic reaction between the acid and the wastewater deformed the PVC pipe (which has a maximum upper temperature limit of 60 °C) and caused the leak.

The following actions were taken:

• the deformed collar was replaced on 15 May;
• a monitoring round was organised during the night of 15 May;
• on 16 May, the contractor temporarily removed the injection from the pipe and reinstalled direct injection in the tank;
• the polluted soil was excavated and the container of sulphuric acid was removed on 15 and 16 May;
• the sulphuric acid injection pipe made of PVC was scheduled to be replaced during the week of 5 June by one made of PVDF, which has a higher upper temperature limit;
• a retention system with level detection for protecting around the injection points is currently being looked into.