Desalination: how it will work
Desalination plans are in the pipeline to boost water supplies in drought-stricken parts of SA. How will it work?
‘We need to bring consumption down to 500 million litres a day to help us get through as much of the summer as possible’
FOR months people have been hoping for a miracle – for the skies to open up and break the crippling drought that’s gripped Cape Town. But as dams get emptier by the week city officials are short on answers and residents are either in total denial or flat-out panic as the metro inches ever closer to “Day Zero” when the taps will run dry.
With drinkable water supplies currently at around 27% due to below-average winter rainfall, that dreaded day isn’t too far off – according to estimates come early March the Mother City might be left high and dry.
Cape Town isn’t the only place that’s facing a crisis. It’s a similar scenario throughout the Western Cape and in parts of the Eastern Cape, especially Port Elizabeth.
But surely authorities aren’t just going to sit back and wait for Day Zero to arrive? There are all kinds of weird and wonderful plans being mooted but right now it seems it’s desalination – or the harvesting of seawater – that seems to offer the best prospect of resolving the crisis. But what exactly does this involve, and is it really the best way to avoid disaster?
HOW TO KEEP THE TAPS RUNNING
Cape Town’s total consumption at the end of October was about 585 million litres a day, compared to 1,1 billion litres before the crisis hit – so it seems the urgency of the situation has finally dawned on water guzzlers.
The next few months until May are going to be critical, says Craig Kesson, the City of Cape Town’s chief resilience officer. “We need to bring consumption down to 500 million litres a day to help us get through as much of the summer as possible, towards winter 2018.”
But it’s clear that with the current dam levels the city can’t only rely on reduced water usage to see it through.
Officials had several ambitious plans – from accessing groundwater through aquifers and boreholes to waste-water purification. But they quickly realised that desalination, although expensive, may hold the key to alleviating the city’s water woes.
A total of about R3,3 billion was initially allocated for these water augmentation plans but environmental impact studies and cost analysis forced the city back to the drawing board.
Late last month Cape Town mayor Patricia de Lille announced new desalination plans. Tenders are going out for mobile reverse-osmosis units – they’ll be small enough to fit into a shipping container and will be dotted along the coastline, supplying water incrementally from February and eventually hopefully about 100 million litres a day altogether.
In addition, from April dedicated desalination barges will be anchored in Cape Town harbour, producing up to a planned 50 million litres a day.
A permanent desalination plant should come online in May in Cape Town harbour contributing 50 million litres a day.
So that gives about 200 million litres. Nowhere near the 500 million litres a day (including water from groundwater and wastewater-recycling projects) envisaged in the metro’s original augmentation plans. But it’s a start. To build bigger desalination plants would take up to two years – and right now the city just can’t afford such a lengthy process.
Meanwhile over in the Eastern Cape the Nelson Mandela Bay metro (Port Elizabeth and surrounds) is in a similar boat, with dams only about 30% full, compared to 60% in other parts of the province, which is a mixed rainfall area.
With Day Zero fast looming, the metro is pinning its hopes on assistance from the National Disaster Fund to help it raise the R3,23 billion it needs for three desalination plants, which are part of an overall drought mitigation plan that will take four years to roll out.
THE INS AND OUTS OF DESALINATION
It’s essentially the removal of salt and other minerals from seawater to make it drinkable. The technology that dominates today – seawater reverse osmosis (SWRO) – has been around since 1960, explains Dawid Bosman, an engineer and independent strategic adviser in the water sector. He says desalination is an excellent alternative water source in coastal areas.
South Africa already has about a dozen smallish SWRO plants in various parts of the country, including Mossel Bay, Richards Bay, Knysna, Lambert’s Bay and Boknesstrand in the Eastern Cape.
“Nearly all these plants were built as an emergency response to a severe drought,” Bosman says. This resulted in several poor technical decisions being taken, he adds, pointing out that operations at most of the plants were suspended once the droughts were broken. He says officials tend to view desalination as a last-resort drought response measure but it actually has much larger potential.
A MAGIC BULLET FOR CAPE TOWN?
Bosman says desalination is now of critical importance for drought-ravaged Cape Town. But as officials are looking for a quick solution – small units that can be up and running within months – it’s going to be expensive, he warns.
But Bosman says once the short-term crisis has been dealt with, a long-term desalination strategy should definitely be explored as bigger plants will be a lot more cost-effective. If officials are willing to think long-term they could avoid a crisis like this ever happening again. Costs: The principle of economy of scale definitely applies to desalination, Bosman says. At plants producing between 150-200 million litres a day the capital cost (construction cost) works out to about R13,74-R19,24 a litre. But at a 5-10 million litre-producing plant the capital cost increases to a range of between R27,48-R41,22 a litre.
When it comes to the eventual cost for the consumer international consulting group Global Water Intelligence says the international benchmark for large-scale SWRO should be $0,60-$1,20 (about R8,40-R16,80) a kilolitre (1 000 litres). That way you’ll still be able to get water at a cost that’s in line with municipal water prices in rand terms – which are R8-R12 a kilolitre, Bosman says.
GrahamTek, an international company of SA origin that’s involved with largescale desalination plants in India and Saudi Arabia, has said in a proposal it could provide Cape Town with 450 million litres a day at an estimated R8,5 billion. The company’s estimate for a kilolitre of water is about R11,50.
But desalination plants can easily surpass cost estimates if the design and implementation are done badly, Bosman warns. Of the six large plants built in Australia between 2005 and 2012, only one (in Perth) is truly costeffective. Bosman says desalinated water is usually costlier than dam water, groundwater or recycled water, “but if the project is done right, not much more expensive”. Sustainability: Bosman reckons if population and economic growth patterns continue as they are, desalination will ultimately be the primary water source for a coastal metro such as Cape Town.
Perth in Western Australia is a comparable example. “Ten years ago they were still massively reliant on surface-water resources. Today, they have more than 400 million litres per day desalination capacity, supplying nearly 50% of their water needs.”
Water specialist Anthony Turton says Israel is a prime example of how successful desalination can be. “Twenty years ago it was one of the most water-scarce countries in the world. Today they’re no longer water-stressed because they have massive desalination projects.”
He also points out that if it hadn’t been for desalination, “about the entire Arab world wouldn’t exist today”.
He concedes cost is an issue, but asks, “How expensive is expensive water? If there’s no water for the economy, we can’t create jobs and people are rioting in the streets – that’s the cost of not having water . . . So the cost will ultimately be determined by the cost of no intervention.”