Ontario’s Centre Wellington Township has removed and replaced the 30-year-old roof of the Fergus Wastewater Treatment Plant’s secondary digester for biosolids, which had been leaking gas, officials said.
The $2-million project included the removal of the worn-out fibreglass roof and the installation of a new 15.3 m-diameter stainless steel floating gasholder roof that weighs nearly 50,000 lbs.
The replacement will minimize life cycle costs due to the long service life and minimal maintenance needed for stainless steel, the township said, when it approved the project in September 2020. Stainless steel is characterized by its high resistance to corrosion due to its minimum chromium content of 10.5%, as unlike iron, chromium is not susceptible to oxidation.
“Additionally, the floating roof has operational and technical advantages over a fixed roof for this application, including the ability to store biogas for use in the boiler heating system, the capability for sludge thickening to reduce off-site sludge disposal costs, no structural concerns given that there is no increase in the loads applied to the digester tank structure, and no Environmental Compliance Approval amendment would be needed for a floating roof,” a township council report states.
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The cost of a floating stainless steel roof was only $60,000 more than a fixed stainless steel roof.
Township staff had attempted to seal the leak in the roof in 2016 by applying a polyurethane sealant coating along the inside of the roof skirt up to 300 mm beyond the joint where the skirt meets the roof dome. In March 2017, officials concluded that the repair was unsuccessful and operations were adjusted to limit gas production and storage in the secondary digester.
The secondary digester is used to contain and treat wastewater sludge, or biosolids. The digestion of sludge creates methane gas as a byproduct that is captured and stored by the new digester roof and then used as a heating fuel for the wastewater treatment plant.
The work was performed by Jacobs, serving as the township’s engineering consultants, and Glover-Hill, the design builder who worked in conjunction with WesTech as the roof designer and manufacturer. CIMA+ acted as project engineers and Ayr Welding as the roof installation subcontractor.
In a statement, the township thanked its wastewater services operators, who successfully managed the treatment process to avoid any costly off-site waste disposal.
The Fergus wastewater treatment plant is an 8,000 m3 average per day, conventional activated sludge plant with primary clarifiers, aeration tanks, final clarifiers, effluent filtration and UV disinfection.
The roof project was funded through the township’s Wastewater Capital Reserve.