Civil Engineering November 2022 | Vol 30 No 10

T he City of Cape Town’s Water and Sanitation department identified an urgent need for an upgrade to the old Retreat Low Lift Sewer Pump Station. The old pump station had exceeded its design life and was reported to no longer be maintainable and unable to cope with the current peak flow conditions. High levels of sedimentation, rugs and foreign materials in inflow caused frequent clog- ging and blockages of pumps. BVi Consulting Engineers was appointed by the City of Cape Town to undertake the planning, design, tender documentation, project management and site supervision for the construction of the new Retreat Low Lift Sewer Pump Station, which forms part of the Cape Flats Sewage Reticulation System. The proposed upgrades to the sewage infrastructure comprised the following components: Q Q Constructing the new Retreat Low Lift Sewer Pump Station with increased capacity of 750 ℓ/s peak flows Q Q Constructing a grit chamber and installing automated screening equipment and an odour control system Q Q Cured-in-place-pipe refurbishment and rerouting of an ex- isting 1 050 mm diameter outfall gravity sewer Q Q Demolishing the old Retreat Low Lift Sewer Pump Station. UNIQUE DESIGN ASPECTS The new Retreat Low Lift Sewer Pump Station was designed for energy efficiency, ease of operation and maintenance, high secu- rity, and visual aesthetics. The incoming raw sewage has a high influx of rags and inorganic solids. Two automated mechanical raking screens (one duty, one standby) were installed to remove rags and reduce clogging and thus prolong the lifespan of new pumping equipment and downstream infrastructure. Three immersible sewage pumps (two duty, one standby) were installed with variable speed drives in a dry well configuration. Telemetry and Scada systems were installed for early detection and relaying of signals and faults to enable quick response as well as offsite monitoring of the pump station. Profuse vandalism and theft in the area highlighted the need for heightened security. To this end, all external walls and slabs of the pump station were constructed of reinforced concrete, and all access doors and frames are steel. No windows were provided to the concrete structure, with wall openings and sizes kept to an absolute minimum. Electrical junction manholes external to the pump station are fitted with lockable covers to secure and protect cables. Ventilation fans and air conditioning units were provided to ensure adequate fresh air supply and circulation inside the reinforced concrete structure. The pump station is situated in a residential area, and a bio-trickling filter odour control system was therefore installed to remove high levels of hydrogen sulphide gas from the raw sewage and release polished air into the environment. A standby generator has been installed for backup supply during power disruptions. A 1 050 mm diameter emergency bypass line was also installed, capable of handling the full design flow in the event of a total shutdown of the pump station. The old Retreat Low Lift Pump Station has been demolished and the precinct repurposed into a tennis and netball court which will be utilised by the local school and community.  SANRAL SA I CE 2022 Nat i onal Awards Category: Techn i cal Excellence FINALIST Technical Excellence category (Sponsored by SKI Civil and Structural Engineers) KEY PLAYERS Client: City of Cape Town: Water and Sanitation Department Consultant: BVi Consulting Engineers Western Cape Contractor: CSV Construction Retreat Low Lift Sewer Pump Station and Refurbishment of Existing Bulk Outfall Gravity Sewer 24 November 2022 Civil Engineering The new Retreat Low Lift Pump Station The new pump station has an increased capacity of 750 ℓ/s