Zinc-laden water from a metal manufacturing plant was released into a canal during restart operations following periodic maintenance of the lixiviation and electrolysis shops. The establishment is equipped with a polluted rainwater network, connected to a sump enabling it to be transferred to a 5,500 m³ storage tank and a neutralisation-settling tank that had been commissioned the previous year. The sump’s older pumps that discharge directly (without treatment) into the canal were kept in place to be used in exceptional situations (precipitation in excess of the decade rainfall level or a long-term malfunction of the neutralisation station) and provided that the quality is satisfactory for the release. On the day of the accident, leaks on the lixiviation exchangers began flowing into the rainwater network then, due to a pumping error, were released without treatment into the canal for 3 days; 700 kg of zinc were thus released into the natural environment.

An inquiry revealed that the operating error was possible owing to the fact that the older pumps were kept locked. The Inspectorate also noted a malfunction of the leak detection system and the process control transmission chain to the central computer. This accidental release appears not to have had a notable impact on the environment. The operator undertook the following measures: replacement of the exchangers, displacement of the conductivity measurement and recycling of the condensates from the evaporators, physical locking or electrical lock-out of the older pumps and implementation of a procedure concerning their use. A prefectoral order relative to additional requirements, notably requiring the pumps to be locked, was also proposed to the prefect.