In the fermentation unit of a food-processing plant, which manufactures baker’s yeast, 25% sodium hydroxide spilt from the sodium hydroxide transfer network that leads to a distribution buffer tank. The spill occurred in an area located outside the retention pit near the plant’s sodium hydroxide storage tanks. An alert that the pH of discharges to the canal was high was sent to the operating system, but the situation returned to normal in less than two hours. At 5:00 a.m. the next day, the pH began progressively rising, reaching a value of 9 at 6:00 p.m. On the morning of 15 June, the high pH was confirmed by samples taken by the on-duty technician at the inlet to the lifting pumps and at the plant’s outlet. The production staff found that a main control valve used for the plant’s retention was opened near the unloading area. The valve was closed on 15 June at 1:30 p.m. The pH progressively dropped back down in the late afternoon and overnight.
On the morning of 17 June, employees discovered that a drain valve on the transfer network had remained open. This valve is what caused the spill. It was opened by either the movement of a nozzle or vibrations. The leak initially filled the pit containing the main control valve leading to the canal and which was open.
An estimated 15 m³ of sodium hydroxide was discharged to the canal. The canal’s pH rose to between 8 (at 12:00 p.m. on 15 June at the WWTP’s outlet 2 km downstream) and 9.10 (behind the plant at 10:00 a.m. on 14 June and at 8:00 p.m. on 15 June). The network’s two drain valves were replaced and locked closed. The operator added a systematic closure timer to the valve.