City of Cape Town desalination plant

South Africa is classed as the 30th driest country in the world. As a semiarid country, it has a high within-season variability of rainfall and uneven distribution of rainfall (43% of the rain falls on 13% of the land). This means that even during a good year of average rainfall the country’s water resources are already stretched in some regions. 

The direness of the arid situation has certainly been exacerbated by the drought and extreme weather conditions in recent years. However, consideration must also be given to the adequacy of water supply infrastructure and management, and wasteful water usage.

Although not limited to, the impact of changes in weather patterns have certainly been harshly felt across the Cape-regions of South Africa. And, while the Western Cape has managed to fend of the threat of Day Zero indefinitely, this would not have been possible without serious interventions that were initiated by the City of Cape Town Municipality. Some interventions were aimed at addressing wasteful water usage and instilling a culture change towards using water much more sparingly and responsibly. Others focused on implementing a series of emergency water supply projects to supplement critically low water resource levels in the district.

One such project is the City’s desalination plant at the V&A Waterfront. 

Quality Filtration Systems (QFS), along with Osmoflo, were awarded the work as the main contractor – to build, own and operate the plant, as well as to decommission the plant at the end of the contract term. WSP tendered as sub-consultants to QFS.

Given the state of emergency of the water crisis in the region, this was a rapid execution, top priority project for all parties involved. To put this into context; the project was awarded to QFS and Osmoflo in January 2018, where WSP, as sub-consultants, immediately started working on the designs, drawings and specifications for the plant’s ancillary components. Practical completion of these components was reached within two months and within a total of three-months from the work being awarded the plant began overall commissioning.

For this project, WSP provided a number of key services, including:

• Concept to detailed designs,
• Construction drawings and specifications for the desalination plant’s ancillary works
• Project management services
• Site supervision for improved collaboration, quality assurance and quick decision-making on site during rapid delivery on the construction of the plant

“The three main components that make up the ancillary works include the marine intake pipeline and seawater pumping system, the brine discharge system and the injection system to convey the fresh water to the localised water infrastructure network. WSP provided the marine, civil and mechanical design of these components – where these components were also designed in such a way that the entire system can be upgraded for up to 50% more production, if required in future,” says Marthinus Retief, Principal Associate: Coastal, at WSP Maritime Africa. 

The desalination process itself is owned and operated by QFS, along with Osmoflo.

The seawater abstraction system is located close to the entrance to the V&A basins, but designed for optimised water quality and rapid construction. The desalination process separates the salts from the seawater and produces brine, which is then discharged back into the ocean – and the fresh water is fed into the City of Cape Town’s infrastructure networks. 

 Together with Southern Oceaneering, WSP designed a geotextile bag weighting system to provide stability for the marine intake pipeline. This enabled the contractor to more easily install the pipeline onto the existing revetment slope and down to the required depth on the seabed. The bags were concrete filled after installation to provide the required stability against wave action. Being an alternative solution to conventional pre-cast concrete weight collars, it saved time on construction, while also reducing risk during pipeline installation. WSP worked closely with the contractor to ensure that the solution was constructible and optimised.

“As this project was fast-tracked, significant focus was placed on where we could save time, but not compromise on quality of design or constructability,” add Retief.

Challenging the status quo

WSP designed the brine discharge system to operate under gravity, rather than a pumping system as was envisaged within the tender document. By changing the hydraulics WSP was able to optimise this process, which saved the client capital and operational costs on the project. Care was also taken to discharge the brine in an environmentally acceptable manner.

“The professional work and value added by WSP on the City of Cape Town V&A desalination plant played an integral role in timely and professional delivery of the project under extreme pressure. The engineering and project management skills showcased by WSP during the project planning and implementation phases were impressive and made the project for QFS so much easier to deliver according to specifications,” says Herman Smit, Managing Director, Quality Filtration Systems (QFS).

“Working on a high-profile project such as this and with a tight turnaround time truly shows the strength of your project team, and the importance of collaborating towards the same end goal. We were able to leverage on the world class expertise of our team of Coastal and Civil engineers to find solutions that were workable from a design perspective and quick to implement from a construction perspective. Thanks to the team - we are very proud of our contribution to this project,” concludes Retief.