Demolition at the City of Toronto’s Humber Wastewater Treatment Plant is a big job, never mind a battle with the “Green Monster” — a massive, multi-tentacled odour-control facility that’s stood guard at the plant for decades.
The demolition side of the project is being undertaken by Priestly Demolition Inc. as a subcontractor to Alberici Construction Limited, which is rebuilding the plant in phases. Demolition work includes removal of all concrete aeration tank covers and clarifiers, and demolition of a ferrous chloride facility, an activated sludge building and selective equipment demolition in various buildings and access tunnels.
Demolition work began in April 2017.
“This is an active wastewater treatment plant,” says David Ohashi, senior project manager with Priestly.
“All demolition work has to be coordinated with the contractor and the city. We had to schedule work around shut downs, work by five other contractors on site and plant activities such as chemical deliveries, equipment testing and other operational restrictions.”
Much of the work is selective demolition. Specific elements of various structures must be preserved so they can be incorporated into future construction. The contractor must also work around existing pipes, fibre optic lines, communication system components, high voltage electrical cables and cable trays.
As Alberici construction workers build new infrastructure, demolition workers will remove the old units. For example, following the commissioning of the new return activated sludge pumping station, demolition workers will remove the existing station.
“It requires incredible timing to phase the work just right,” says Ohashi.
No two areas slated for demolition are alike. For example, one tank was covered with soil.
“Neither the engineers nor the city could tell us what the allowable loading was for the top of the tank,” says Ohashi. “We had to assume the lightest equipment possible, so we carefully removed dirt that was the equivalent weight of the equipment we were using.”
And then there’s the Green Monster, a nickname bestowed on the odour control equipment by staff at the wastewater plant. About 35 feet tall, the steel-and-fibreglass structure sprawls across a footprint measuring about 30 by 200 feet.
“They collect the stinky air from the tanks and then blow it through air scrubbers in ducts made of steel and fibreglass,” says Ohashi. “I can only describe it as green and ugly. They sprayed green insulation all over it.”
The challenge?
“There were tunnels on the clarifier side so we had a weight restriction and on the other side we had aeration tanks covered with metal plates at ground level,” says Ohashi. “We couldn’t place demolition equipment on top of them.”
The demolition procedure was planned with drawings prepared by Priestly’s structural engineers to ensure safe disassembly.
“We had to use handwork and saw-cutting or smaller equipment, including hammer-equipped excavators,” Ohashi says.
“The closest we could position our 200-ton crane was 60 feet away from the plant, on the road. As we cut the Green Monster into chunks, we would move them close enough to the road for the crane to reach them. It took about five weeks to demolish it. I can’t say the wastewater plant staff was sorry to see it go.”
Equipment brought to the overall project to date includes scissor-lifts, excavators equipped with buckets and shears, and a fleet of bobcats. Priestly has fielded about a dozen workers at the peak of operations.
Structural steel and beams made of both steel and fibreglass were recycled. Concrete was delivered for use in the Toronto Region Conservation Authority’s (TRCA) land reclamation program. Even the entrails of the Green Monster — plastic media used to treat the odour — were placed on the TRCA’s Partners in Project Green material exchange website.
“Work at the plant is ongoing,” notes Ohashi.
“We may be returning to the site in phases for as many as three years.”
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