Innovation showcased on new Retreat pump station project

The site of the old Retreat Low Lift Sewer Pump Station in Cape Town is now home to tennis and netball courts, while adjacent to it is a new state-of-the art facility that went into service in mid-2021.

The new Retreat Low Lift Sewer Pump Station in the background. In the foreground are the tennis and netball facilities built on top of the demolished pump station

Forming part of the Cape Flats Sewage Reticulation System serving the Cape Flats Wastewater Treatment Works (WWTW), the old pump station had exceeded its design life and was unable to cope with peak flow conditions. This was exacerbated by high levels of sedimentation, rags and foreign materials in the inflow, which caused frequent clogging and pump blockages.

A further complication was identified when a condition assessment of the 1 050 mm diameter outfall gravity sewer downstream of the station feeding to the Retreat Main Pump Station revealed that an urgent intervention was required. The assessment, carried out by the City of Cape Town’s Water and Sanitation Department and BVi Consulting Engineers in 2018, indicated excessive corrosion up to 96 mm deep in the concrete pipe wall, with steel reinforcement in several sections exposed due to corrosive chemicals.

Additionally, the outfall line had silted up above 50%, significantly reducing the flow capacity. This, along with regular local pipe collapses on the outfall gravity sewer, resulted in continuous surcharging of manholes and regular incidences of sewage flooding the site.

Another key concern was the fact that a portion of the deteriorated outfall gravity sewer crosses underneath a densely populated informal housing settlement. Ongoing pipe collapses therefore posed a high safety and health risk to residents.

Value engineering

BVi was subsequently 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 is surrounded by a primary school, community hall, residential houses and flats.

The key components of the infrastructure upgraded entailed: • construction of a pump station with a capacity of 750 ℓ/s peak flow • construction of a grit chamber, plus the installation of automated screening equipment and an odour control system • cured-in-place-pipe (CIPP) refurbishment and rerouting of the existing 1 050 mm diameter outfall gravity sewer • demolition of the old Retreat Low Lift Sewer

Pump Station. The new Retreat Low Lift Sewer Pump Station was designed for energy efficiency, ease of operation and maintenance, high security, and aesthetic appeal. From an operational perspective, the facility features a conventional dry well (dry and wet well) assembly running three immersible endsuction pumps (two on duty, one on standby) with variable-speed drives.

Telemetry and Scada systems were also installed for early detection and the relaying of signals and faults for off-site monitoring and rapid service response.

Overpumping and CIPP

At the onset, a precisely executed overpumping scheme was implemented to bypass the problematic outfall gravity sewer main between Retreat Low Lift Pump Station and Retreat Main Pump Station.

This intervention was designed to mitigate the ongoing risk of further pipe collapses and surcharging, which could have led to sewage flooding the new pump station construction site. The overpumping methodology established further ensured ready access for the rerouting and refurbishment of the outfall gravity sewer. This solution also negated the need for additional local overpumping during the CIPP stage of the project.

Forming part of the trenchless works, CIPP technology allowed for a 490 m section of the cleaned 1 050 mm diameter pipeline to be refurbished with resin-impregnated liner. In turn, the rerouted section was constructed using new

reinforced HDPE-lined concrete pipe. The old outfall gravity sewer underneath the informal settlement was abandoned.

Mitigating sewer spillage

A key challenge was experienced during the installation of a new flow meter to the existing 800 mm diameter rising main from Retreat Main Pump Station up to the Cape Flats WWTW. This required perfect coordination between the Sand River Pump Station, which discharges into the Retreat Low Lift Pump Station, and the Retreat Low Lift Pump Station, which discharges into Retreat Main Pump Station. A top priority was to ensure no sewage overflow and mitigate the environmental risk of sewage spillage into stormwater canals and vleis.

The three pump stations had to be pumped down to increase buffer and storage capacity in the upstream network to allow the team to complete the overpumping diversion and install the flow meter during off-peak, low-flow conditions within a limited time frame.

Deep excavation and dewatering

The underground section of the new concrete structure reaches depths of up to 9 m. During construction, this posed a major challenge because the high water table in the area sits at around 1 m below natural ground level. To counter this, a cofferdam with wellpoint dewatering system was constructed from sheetpiles up to 12 m deep. The excavation process for the rerouting of the bulk gravity sewer also entailed shoring and dewatering of the trench excavation at depths of up to 5 m below the natural ground level. The dewatering phase for the pump station was finally decommissioned after the below-ground reinforced concrete base, walls and ground level slab had been constructed and backfilled. This was a critical point, as the concrete structure needed to have sufficient weight before the water table could be allowed to rise without resulting in the structure floating due to buoyancy effect.

Corrosion protection

As is standard for pump station designs, the grit chamber and inlet works will be permanently exposed to corrosive gases from the raw sewer. Protection of the internal walls and roof surfaces is therefore crucial for the durability and longterm maintenance of the overall concrete structure. Proactive design elements included the installation of anchor-knob HDPE sheets with excellent chemical resistance, which were cast into the concrete. The walls were finished with epoxy-modified cement protective coating.

Aesthetics with added security

Above ground, the pump station’s reinforced concrete structure was designed for maximum visual appeal, while factoring in the need for added security given the high rate of crime and vandalism in the area. External walls were constructed with colour-coded concrete recesses and fitted with concrete recessed lights to match the colours of the City of Cape Town’s logo.

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. To ensure adequate fresh air supply and circulation, ventilation fans and air conditioning units were installed inside the reinforced concrete structure. Electrical junction manholes external to the pump station were also fitted with lockable covers to secure and protect cables.

PROJECT TEAM

Client: City of Cape Town Engineer: BVi Consulting Environmental: Silito Environmental OH&S: Safe Working Practice Civil contractor: CSV / Mazarin JV M&E contractor: Tricom Africa

Built for the community

First and foremost, though, the construction of the new Retreat Low Lift Sewer Pump Station was designed and delivered for the community. It is highly functional and cost-effective to run, with no smell, which is especially important when you live next door.

The facility’s new bio-trickling filter odour control system constantly processes the hydrogen sulfide gas build-ups that form in sewer systems and release it as polished air into the environment.

Then, as a contingency provision, a standby generator was installed for backup supply during power disruptions. And in the worst-case scenario, a 1 050 mm diameter emergency by-pass line was installed, which is capable of handling the full design flow in the event of total shutdown of the pump station.

In the meantime, the old Retreat Low Lift Sewer Pump Station has a new lease on life as a vibrant sports and recreational facility.

Odour control room

North-east view of the new Retreat Low Lift Sewer Pump Station

Dry well pumps, non-return valves and isolating knife gate valves Two automated mechanical raking screens (one on duty, the other on standby) were installed to remove rags, reduce clogging and thus prolong the lifespan of the new pumping equipment and downstream infrastructure