Whilst making his rounds at a hydrogen peroxide concentration workshop at around 1:00 a.m., a technician at a chemical plant noticed that a 70% hydrogen peroxide tank was overflowing into its secondary containment area. The tank is part of a hydrogen peroxide concentration cycle, which was on hold when the overflow occurred. The operator had to restart the sequence to empty the tank. He diluted the overflow and then recycled it into the workshop’s effluent network. He called in the plant’s firefighters to clean up the area at around 1:30 a.m. The incident was declared over at 2:00 a.m. The operator tested the effluent at a wastewater drainage pipe outlet into the natural environment.

He estimated that 13 m³ of hydrogen peroxide had overflowed from the tank into the secondary containment area. Most of the product was sent to the treatment plant after analysis. Based on the measurements taken at the drainage pipe outlet, 1 m³ of spilled hydrogen peroxide had flowed into the sewer network via a non-watertight manhole in the secondary containment area. The operator monitors this discharge point daily. An estimated 360 kg of hydrogen peroxide was discharged via this point, exceeding the regulatory concentration limit for discharges for a two-day period. The aqueous discharge then flowed through a 550 m factory channel before entering the natural environment. Due to the length of this channel and the decomposition of the hydrogen peroxide, the operator considered that there had been no impact on the aquatic environment (the concentration of hydrogen peroxide in the environment remained below the acute toxicity threshold for fish). For reference, hydrogen peroxide breaks down into oxygen and water.

The tank had overflowed because the control room operatives did not respond to two high-level alarms (one on the tank at 8:50 p.m. and another on a sump in the secondary containment area at 10:50 p.m.). According to the operator, the manhole for collecting effluent from the demineralised water banks was not watertight because of work carried out the previous week. 

The operator has implemented the following actions:

• provided feedback to the teams and conducted interviews with the relevant technicians;
• modified the alarms and mimic panels to ensure that information is displayed more clearly;
• improved the seals on the effluent collection manhole;
• checked the plant’s other secondary containment areas;
• analysed the high-level control system of the tank when the concentration process is shut down.