Charlie Lake Low-Pressure Sewer System Model

Charlie Lake is in Northeastern British Columbia, located within the Peace River Regional District. Charlie Lake has approximately 573 existing lots, and approximately 434 of those lots are currently connected to the sewer collection system. The lots have septic tanks and effluent pumps prior to flows being discharged into the low-pressure force mains (a STEP system).

Previously, system capacity was evaluated based on full flow of pipes and calculated in an excel spreadsheet. This method was overly conservative as pressurized lines were not considered in the calculations. The PCSWMM model considers capacity in terms of level of service and system pressures. If the individual effluent pump is not rated for the pressures in the force main, then the pump will struggle to introduce flows into the force main and may cause septic tanks to overflow. The PRRD didn’t have details regarding the individual effluent pumps, so a local contractor who has worked in the community for years was consulted to gain an overall idea of what the pumps in the system typically are. We looked at modeling the system using the probability method and the rational method. As we didn’t have good data on the effluent pumps, but we did have flow data from the lift station, we decided to use the rational method. The higher HGLs pressurized the pipes, and the resulting pressures were compared to the typical pump capacities.

This model allowed us to:

• identify areas where homeowners may need to upsize their pumps,

• identify which collection lines are not being well utilized,

• look at opportunities for redirecting flows, and

• determine if the system had capacity for new development.

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