The City of Whitehorse in the Yukon Territory is the largest city in northern Canada, with approximately 77% of the territory’s population located in the Whitehorse area. In 2015, the City retained Associated to design a sludge management system for the lagoons at their Livingston Trail Environmental Control Facility, which treats wastewater from the community. Commissioned in 1996, the facility has two anaerobic lagoons, two facultative lagoons, one long-term storage lagoon, and a treated effluent discharge system that flows to the Yukon River. Each anaerobic lagoon holds a wet volume of 115,000 cubic metres. To manage sludge from the lagoons, the project team decided to build a sludge drying bed that could hold the sludge from one of the cells. Only one bed will be constructed, with provision for a second one in the future, if required.
Determining the depth of a sludge drying bed is based on the estimated volume of sludge and leachate drainage, and on the worst case scenario for rainfall and ice thickness to avoid the overflow of potential pathogens and contaminants from the bed into the environment. The depth of sludge must be determined so that frost can reach the bottom of the bed for efficient freeze-thaw cycles. The frost depth is commonly calculated using the normal freezing index. However, these values are based on observations made between 1931 to 1960, which are outdated due to climate change.
The project team recommended a safety factor of 60%, which was accepted by the City. The freeboard of the bed is based on a 15-minute rainfall event at the highest intensity recorded during a 100-year period of time. Rainfall Intensity-Duration-Frequency curves are commonly used to determine this value for a specific location. However, these curves cannot be fully relied upon as climate changes. The University of Waterloo has developed a new online tool called the Computerized Tool for the Development of Intensity-Duration-Frequency Curves under Climate Change, which the project team used to determine rainfall intensity.