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Deciphering Desalination

As conventional water resources are strained to meet the growing national demand desalination is increasingly being explored as an alternative. Severe drought in the Southern Cape between 2009 and 2011 fast-tracked the successful development of small municipal desalination plants to remove salt from seawater along the Garden Route. But to date, no large-scale desalination plants have been developed, largely due to high capital and operating costs. The evolution of technology and cost improvements are, however, positioning desalination as a strong future contender for South African coastal cities.

Desalinated seawater currently costs between R14 and R15 per kilolitre, with energy and treatment costs as well as infrastructure capital being key contributing factors.

“Historically, South Africans have been paying far below the real cost of water,” says Louise-Mari van Zyl, director of Cape Environmental Assessment Practitioners (Cape EAPrac), which facilitated the environmental impact assessment process for South Africa’s first desalination plant in Sedgefield. Low water costs extend beyond under-valuing of natural resources. “Many of South Africa’s large dams, water infrastructure and treatment plants have been

built and paid off a long time ago. This drastically reduced the costs of conventional water,” says Kevin Meier, Manager of Umgeni Water’s Planning Services Department. The cost of water from a new conventional water treatment works varies depending on location as it is very site specific, but generally could range from R8/kl to R12/kl. By comparison, any alternative will be more costly, even more so for advanced technology.

Technology and Advances

All South Africa’s municipal and industrial saltwater desalination plants use reverse osmosis (RO) membrane technology. Intake water is forced through membranes at high pressure, allowing purified water through and retaining salt as brine. From the intake water, 45% becomes potable, while the remaining 55% of concentrated saltwater is discharged into the ocean. These processes and supplying of water require an operational energy consumption average of 4kWh per kilolitre.

However, this is not a limiting factor in the South African context. “There is a major international drive for lowering energy requirements and some new technologies, such as Desalitech and niche applications, can be expected for the future,” says Abrie Wessels, Veolia Water Technologies South Africa’s Cape regional general manager.

Design for Demand

Operational costs are currently preventing local Southern Cape municipalities from operating their desalination plants. The possibility to mothball the seawater plants when not required was a key design planning criteria. “Desalination remains an emergency option for when all other resources have been entirely depleted or are no longer cost-effective,” says Van Zyl. Likewise for the Mossel Bay plant.

Large-scale desalination plant planning for Durban and Cape Town is ongoing.

Maintenance and National Funding

“The maintenance cost of repeated annual repair and protection against corrosion is seriously underestimated for the marine environment,” says Wessels. Using higher cost stainless steel offers some mitigation but siting again comes into play: “Because of the high cost of the large seawater delivery infrastructure, inlet and outlet pipeline distances have to be kept to the minimum,” says Meier. RO membranes need to be replaced every five to seven years.

Maintained in zero mode, full operation of the Southern Cape plants is attainable within a short re-instatement period of between one and two months. Whether securing water for significant future town or industrial expansion or drought, “the Mossel Bay Municipality is now positioned to generate water, which was previously not possible. The current challenge is that a zero mode phase can extend over a long period, but constant maintenance of plant components is still needed. Under a specialist service provider contract, the cost for annual zero mode maintenance was R4.5 million. To manage this, the Mossel Bay Municipality established an internal municipal management team, which has drastically reduced zero mode maintenance costs,” explains Amanda Murray, manager of the Mossel Bay Municipality’s technical services department.

The Southern Cape desalination projects were largely possible because of National Treasury drought relief funding after the region was declared a disaster area. In general, “the cost of desalination will come down significantly if given State grant development funding, making it more viable,” says Meier.

Assess for Environmental Risks

Environmental factors are critical. “If the marine environment is not carefully assessed and chosen, desalination could become extremely problematic,” says Melissa Mackay, environmental assessment practitioner of Cape EAPrac. Brine discharge is one environmental risk. “A high energy coastline is necessary, with sufficient water movement to disperse the brine, otherwise brine accumulation could potentially suffocate marine life.

It means that desalination in its current form is not viable for all coastal locations,” says Mackay. Yet there are indications the development of desalination plants weigh less heavily on the environment than conventional water supply alternatives such as off-stream dams and in-stream weirs.

Environmental Resilience

Besides human and industrial use, water reserves need to be designated for ecological functioning as well.

“This is justification for municipalities to motivate that exhausting all natural resources before looking into alternatives such as desalination is not possible,” says Mackay. Climate change also necessitates alternative solutions. “The Southern Cape hardly ever relied on water storage because of constant rainfall across seasons. But suddenly, there are consecutive months with no rain,” says Mackay.

Future Planning

Previous droughts may have focused attention on desalination, but “do not wait for drought. The current desalination plants provide specific South African references and experience on planning, operation and maintenance costs,” says Wessels.

“Determining the viability for a city is complex and a site specific calculation. But for coastal cities, it is definitely a resource option that should be considered,” concludes Wessels.

By Francini van Staden

See earthworks issue 31 Apr-May 2016 for the full feature.

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