The region of Halton in Ontario, immediately west of Toronto, recently expanded its facility for storing treated biosolids during winter months and the spring rainy season. As part of the project, it constructed two new storage tanks and installed fixed aluminum dome covers on the tanks.
Halton has a program of spreading biosolids on agricultural land as part of its sludge utilization program. The regional utility uses a two-stage anaerobic digestion process to treat biosolids from the town of Acton, the city of Burlington, the town of Georgetown, the town of Oakville, and the town of Milton.
Since it is not possible to spread biosolids during winter or excessively wet periods, the region stores its biosolids until the weather improves. In the mid-1980s, it constructed an eight-tank storage facility to hold approximately 65,500 m3 (14.5 million gallons) of treated biosolids.
Each tank is 140 feet in diameter, 21 feet deep, with a capacity of approximately 1.8 million gallons. To minimize costs, floating cover systems were installed on the tanks. The covers are made of closed cell insulation sealed between 80 mil high-density polyethylene sheets. They are attached to the reinforced concrete walls using a drape fabric system that allows the covers to float up and down with the sludge level.
Two additional tanks were completed in December 1996. The new storage tanks are similar in design to the existing tanks: in-ground, circular concrete tanks with flexible membrane floors and a capacity of 8,200 m3 each.
Simcoe Engineering Group Limited was involved in both the original project and the expansion. The tanks were designed by Simcoe and constructed by Woodbow Limited.
The new tanks have been provided with Temcor fixed aluminum dome covers, and the region has begun replacing the floating membrane covers on the other tanks with similar domes. A dome was installed on tank eight in 1998. Two more replacements are planned for 1999/2000.
The floating covers require continuous maintenance to remove precipitation from the top of the membrane. When the tanks are empty, the covers sit on the tank floor, making access for cleaning difficult. The fixed domes shed precipitation and allow access to the tank floor. They also provide the required odor control and prevent precipitation from entering the storage tanks.
Similar to the existing eight storage tanks, each new tank has been provided with mixer platforms and mixer support posts to allow portable Flygt mixers to be installed. The mixers are used to mix the tank contents before loading tanker trucks that transport the biosolids to the agricultural areas for application.
As part of the expansion project, a new tanker truck unloading/loading station was constructed. It is similar to the system serving the other tanks. The tanker trucks unload into the tanks by gravity feed. When it is time to apply the biosolids, the trucks are loaded using the same station with a submersible pump. The loading pumps were installed in pump chambers that share a common wall with the storage tanks.
The tanks also feature individually valved, supernate piping located at selected levels. The piping design allows the biosolids loading pumps to also be used to remove supernate from the tanks.
