At 5:00 a.m., the pH monitor in the neutralisation tank of a textile finishing plant failed, indicating a pH of 10. Normally, the pneumatic sulphuric acid injection valve had been programmed to inject acid into the neutralisation tank starting at a pH of 9.8. The valve injected 4,300 l until 10:00 a.m., after which the WWTP controller stopped injecting acid. The amount of acid discharged was retained in the buffer tank, upstream of the aeration tank, thus preventing acid from being discharged into the biological tank. The pH of the tank was measured at 8 (normal value), thereby preserving its bacterial activity. The operator neutralized the buffer tank.

The pH sensor had failed due to an electrical cable failure. The obsolescence of the measuring equipment and the harsh weather conditions were the cause of the event.

In addition to the existing audible and visual alarms (level control in the neutralisation basin to prevent overflowing, operation of the floating aerator in the buffer tank, operation of the neutralisation tank’s lift pumps to the buffer tank, and operation of the traveling crane’s clarifier), the operator installed a remote monitoring system which is triggered when the pH sensor in the neutralisation tank is faulty.

Although the equipment is already mounted in hermetic enclosures and designed to withstand inclement weather conditions, additional protective enclosures were installed.

The pH sensor in the buffer tank, which is connected to the valve that disconnects the communication link between the buffer tank and the biological tank in the event of a pH drift in the buffer tank, was replaced.