One Saturday at around 1:00 p.m., two air sensors detected carbon tetrachloride (tetrachloromethane, CCl4, toxic and CMR) near a settling tank at an organic chemical manufacturing plant. Employees looked for the leak’s source over the weekend.

A pump was found to have malfunctioned. It was subsequently replaced. On Sunday, samples were collected from several points at the site’s wastewater treatment plant (WWTP’s). The results, which were not known until Monday, showed that CCl4 was in the effluent. The effluent was immediately diverted to the emergency pond.

The pump’s malfunction, due to the fouling of one of its valves, caused the CCl4 level to rise to the intake leading to the stripping column, which became full. The effluent, still laden with CCl4, was drawn into the chemical gutter network and carried to the wastewater treatment plant, where it risked destroying the WWTP’s trickling filter.

The sensors used to continuously measure the chemical oxygen demand (COD) and total organic carbon (TOC) did not detect the WWTP’s contamination as they were not able to detect CCl4.

An estimated 34 kg of CCl4 was sent to the WWTP.

The operator took the following actions:

• the sensor’s alarm threshold has been lowered;
• monitoring of the pump has been increased;
• the CCl4 concentration in effluent is measured daily at weekends;
• it is looking for sensors capable of detecting CCl4 as replacements for or supplements to the WWTP’s COD and TOC sensors;
• it is considering setting up a CCl4 detection system;
• it installed systems to reduce pump fouling.