To assess capacity constraints in the wastewater collection system under existing conditions, a dynamic hydraulic model was developed and calibrated. The model is fully dynamic and includes the City’s sanitary trunk sewer system as well as all the city’s pumping stations and the treatment plant.
System performance was assessed using the calibrated model during dry and wet weather conditions. From the model, it was determined that the wastewater collection system has adequate capacity to convey peak dry weather flows and no capacity concerns were identified.
To assess wet weather performance, several design storm events (2-year, 5-year, 10-year, and 25-year storm) were used as input to the model. The storm events were also adjusted to reflect the potential impacts of climate change. For all these storm events, surcharge conditions were predicted in some sections of the wastewater collection system. In some cases, surcharge conditions were excessive.
Model adjustments were made to include flows from future growth in the years 2021, 2036 and 2041. The model results were used to assess storage requirements for the WwTF when heavy rainfall events occur.
Modelling results for future peak wet weather flows were determined. Based on a peak capacity of the plant, it was clear that an equalization tank is required to manage peak flows regardless of the rainfall distribution.
The tank storage requirements were determined to be between 250 m3 and 12,000 m3, depending on the rainfall distribution utilized in the analysis. The City is incorporating this analysis along with other practical concerns such as land availability, geo-technical conditions and capital costs. Current capital plans for the city include a 12,000 m3 peak attenuation tank.