At 2 am in a Seveso-rated site producing chlorine (Cl2) and chlorinated by-products, the base of a 150-m³ chlorinated brine tank containing mercury began leaking; a technician detected this leak an hour later.

For the most part, the brine spilled into the mercury effluent collection network, which was connected to the site’s treatment plant. Despite the unfavourable ground slope, some of the brine still reached the storm drain network 5 m from the tank where it had been diluted. The plant operator isolated this discharge point and directed storm drain flows towards a retention basin serving as the recipient of 3,200 m3 of polluted water. However, a breach in the knife gate valve seal, due to encumbrance by branches interrupting normal flow to the natural environment, allowed a portion of this discharge to reach the environment. The operator duly informed the local mayor, health authorities and the local government authority.

Most of the outflowing brine was collected by the network dedicated to treating water with mercury content. Due to this spill of some effluent into the storm drain, when the leak rate was highest, and a faulty seal on the storm drain bypass valve, the quantity of mercury discharged into the environment was evaluated at 2.54 kg, diluted into a non-process water outflow of 5,800 m³/hour.

The mercury electrolysis unit was being gradually brought back online after a 2-day shutdown for maintenance. The operator’s investigation revealed that this break was caused by a rise in tank pressure from brine reversing flow in a pipeline and reaching the 5th floor of the electrolysis room (by virtue of the water column effect). Under normal conditions, this additional hydraulic load would not have happened, given the presence of an overflow circuit. Yet during restart following maintenance works, the degassing and overflow circuit shut-off valves on one of the brine tanks had not been reopened.

The operator removed this valve from the overflow circuit. Also, a grating was installed on the storm drain discharge network upstream of the knife gate valve to prevent objects from interfering with valve closure.