During the review of sewersheds in the area and servicing of new development, a third pumping station (Reach Street Sewage Pumping Station) was proposed. The Reach Street Sewage Pumping Station would have an initial capacity of 30 L/s with an ultimate capacity of 120 L/s. In order to ensure proper flushing velocities were obtained within the forcemain without excessive headosses, two forcemains were proposed for the ultimate pumping station capacity. Initially one forcemain would be constructed and then as the station capacity was upgraded beyond 60 L/s a second forcemain would be constructed. This approach would have resulted in 4 separate forcemains.
In order to reduce costs of the Reach Street Sewage Pumping Station construction and to manage velocities within the forcemains, an analysis was undertaken to determine if the three pumping stations could be serviced using the existing forcemains during both normal operation (two forcemains in operation) and under emergencies (one forcemain in operation).
Forcemain And Sewage Pump Design
In Ontario good design practice is to select a forcemain diameter which will ensure that the sewage velocity within the forcemain Analysis is between 0.9 m/s (3 ft/s) and 2.6 m/s (8.5 ft/s). Maintaining minimum velocities ensures that solids in the raw sewage do not settle within the forcemain. Solids settling within a forcemain can be a source of odours and also reduce the forcemain capacity due to a reduction in the effective volume. Limiting the velocity to a maximum velocity ensures excessive headlosses are not developed within the forcemain reducing pumping costs. As well, the higher the velocity, the greater the potential that exists for forcemain transient pressures.
Sewage pumps are designed to pass solids within the sewage and in order to have this functionality require impellers with relatively large passages. These large open passages work well is pumping solids at high flows and low Totally Dynamic Head (TDH). However, they are relatively inefficient in pumping at higher head applications.