The loud bang was heard mid-morning last Friday when a tank is believed to have ruptured, releasing chemical acid into a secondary containment facility.
No one was near the plant when the explosion occurred but about 100 men and women from building contractor Brookfield Multiplex were on the site, near Mundaring Weir, at the time of the incident.
The Helena Water Consortium, comprising ACCIONA Agua, United Utilities Australia, Brookfield Multiplex and Royal Bank of Scotland, were selected to design, build and operate the plant for 35 years in the first public-private partnership water project of its kind in WA.
Helena Water project director Patrick O’Leary said 27 people were treated at the plant site for possible exposure to chemical vapours and one man suffered a burn from the chemical after he was the first person on the scene and it made contact with his skin.
‘The 27 treated were all construction staff,’ Mr O’Leary said.
‘There was a leak of fluorosilicic acid which, when it leaks, it does so as a vapour.’
Mr O’Leary stressed that no one was in the building at the time of the incident.
He said with the chemical, known as hydrofluorosilicic acid or HFS, there were risks, but the design of the plant and the response procedures in place ensured those risks were minimised.
‘The chemical was being delivered into the storage tanks when a bang was heard from within the storage building,’ Mr O’Leary said.
The acid is used in water treatment plants to purify the water.
Mr O’Leary said there was no impact to or on the water supply.
‘Investigations are still taking place but we believe the primary containment tank or associated pipe work may have been damaged and the secondary containment contained the released material,’ Mr O’Leary said.
The company has alarms which were activated and staff were mustered to a central point and a roll call completed.
Mr O’Leary said the 27 men were all treated as a precautionary measure.
They also used the shower facilities provided by emergency services on site.
It is a requirement that anyone working on site and within areas containing hazardous materials be appropriately dressed for dealing with chemicals and Mr O’Leary said all staff had complied with the safety measure.
An DFES spokeswoman said units from Welshpool, Midland and Belmont attended the site and were there for seven hours after the initial call.
Volunteer Fire and Rescue units from Mundaring and Kalamunda also attended the scene.
‘Once they were happy the material had been contained they released the site back to the company,’ Mr O’Leary said.
‘A specialist team with the required safety clothing and equipment has now removed the material and cleaned the area.’
He said the site was secure and was not near a public area.
‘Workforce staff accessed all other parts of the plant on Monday and repairs to the damaged tank will be undertaken after the investigation into the explosion is completed,’ he said.