A closer look at Toronto's water quality control design criteria for bioretention cells

Bioretention cells in the Toronto region are usually sized to accommodate runoff from the 90th percentile storm, which has a depth of about 25 mm. This research examines the water quality control performance and cost of bioretention cells sized to satisfy alternative design criteria ranging from 5 to 50 mm. The long-term average runoff-capture efficiencies provided by representative bioretention cells are determined, and their capital as well as operation and maintenance costs are estimated. Results indicate that the current design criterion of 25 mm is probably too high and not cost-efficient. In fact, above some threshold levels, little improvement in runoff-capture and pollutant removal performances may be achieved if the design criterion is increased further, but cost would still linearly increase. Presented here is a methodology that can be used to properly consider both the performance and cost of bioretention cells for establishing a more cost-efficient design criterion. It is shown that a more cost-efficient design criterion for Toronto could be lower than the current one, and significant cost savings can be realized if a lower design criterion is implemented. Some inappropriate ways of quantifying the effective volume of storage provided by bioretention cells are also identified in this paper.