Desalination: the Ideal Solution?

Due to the current water shortage, people are asking why st helena doesn't use desalination. But how feasible is desalination for st helena?

27 June 2019 By Andrew Turner & Emma Weaver

“Island surrounded by water, and still no water...”

“Why are they not converting sea water?”

“Build a desalination plant.”

News of an island-wide water shortage last week spurred people, like the Facebook commenters quoted above, to ask why St Helena doesn't use desalination.

More than 300 million people now get their water from more than 20,000 desalination facilities across the globe, according to the International Desalination Association. But St Helena, although surrounded by ocean, does not house one of those facilities.

If we could turn the ocean water into fresh water, then water shortages – like the current Stage 3 Drought and the similar droughts in 2013 and 2016 – might become a thing of the past.

But how feasible is desalination really, for St Helena?

 

What is desalination?

Desalination is the process of turning saltwater or seawater into drinking water by removing the salt.

Desalination is still a developing technology and many experts say that it is not ready to be used on a global scale. But that has not stopped countries such as China, India, South Africa and the United States investing in these plants. And according to the BBC, a number of small countries like the Maldives, Malta and the Bahamas meet all their water needs through desalination. Success stories like Israel where desalination saved the country from the worst drought the area had seen in 900 years.

There are two main kinds of desalination – Distillation and Reverse Osmosis.

Reverse Osmosis Distillation (ROD) is now seen as the more efficient method and has largely replaced Distillation.

In ROD, seawater is pumped to one side of a tank and pure water on the other side, separated by a semi-permeable membrane. Extreme pressure is applied to the seawater side of the tank to push the seawater through the filter. The remaining salt and chemicals are then disposed of, usually back into the ocean.

On St Helena, many believe desalination could be effective in preventing water shortages.

A stable supply of water is vital for St Helena currently. Most of the Government's Sustainable Economic Development Plan (SEDP) relies on a good supply of water – to facilitate increased tourism, development and investment; to increase exports (like coffee and honey, all which need water to be produced); to enhance the supply of local produce; and to substitute imports (like the £65,000 worth of mineral waters we imported in 2016/17).

But would desalination prevent drought-related water restrictions like we're currently experiencing?

Experts say desalination comes with its own issues, and may be less than the ideal solution.

 

It's expensive

For islanders already struggling with Connect's current water tariffs – desalination would be even more costly.

According to the BBC, "desalination may be getting cheaper but it is still prohibitively expensive for poorer countries, many of whom also suffer from water scarcity."

This is mainly due to the costs of the desalination process – and for places like St Helena where only a small population would split the cost of desalinated water, these costs are especially prohibitive.

Of course, though, ROD is used on neighbouring Ascension Island, which has an even smaller population. As rains are rare on the island, Ascension uses ROD as its main source of water. Ascension's system uses two ROD plants that fit into 20ft containers and are capable of producing 120 cubic metres of water per day.

For Ascension, desalination has proven very effective.

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.Ascension Island's desalination plant, photographed just before shipping to Ascension. Photo from SaltSeperationServices.

But importantly, Ascension is a working island and so residents benefit from a water allowance from their employers (and the US Air Force base has its own separate ROD facility funded for its employees).

Only when residents go over their allowances do they foot their own water bills.

"I was working for AIG back in 2015, and as a resident of Ascension your contract comes with some water allowance," said Emergency Planning & Sea Rescue Officer Simon Wade. "[But] after you use your allowance, you have to pay the true cost of water. That cost back in 2015 was £24 for 1,000 litres of water – and you quickly start realising that's unaffordable."

The cost on Ascension is now £29.30 per 1,000 litres.

The reason ROD is so costly, is that large amounts of electricity are needed to generate the high pressure that forces the water through the ROD filter. According to research from MIT, current methods require about 14 kilowatt-hours of energy to produce 1,000 gallons (3.8 cubic meters) of desalinated seawater.

Therefore it would currently cost approximately £91 to produce 1,000 gallons (3.8 cubic meters) of desalinated water on St Helena. (Although cheaper, renewable energy is being promised by SHG as part of Connect's deal with company PASH Global – the deal was publicised back in April 2018 but has yet to be finalised.)

But for now, desalination on St Helena would mean households with a quarterly water bill of around £40, would instead see a bill of around £900, according to the island's monopoly utilities provider Connect St Helena Ltd.

"The cost of desalination is prohibitively expensive at about £24 per cubic meter plus that water would need to be pumped up to Hutts Gate or Red Hill which would cost another £20, so say £44 per cubic meter," Connect CEO Barry Hubbard said. "We currently charge less than £2 per cubic meter so whilst [desalination] is technically feasible, people could not afford the increase. A household consuming 20 cubic meters of water per quarter would see their bill of approximately £40 increase to £900, which is unaffordable."

 

It would need environmental mitigation

St Helena uses diesel generators – and the amount of electricity needed to run an ROD plant would have clear associated environmental impact.

And diesel pollution wouldn't be the only worry. According to a report by Washington DC's Food & Water Watch, the by-products of desalination include salt, bisulphates and chlorines. This waste typically finds its way back to the ocean and the concentration of chemicals is harmful to marine life.

Some desalination plants try and reduce this affect by pumping the leftovers into areas with a heavy swell, so that the concentration mixes back in with the seawater quicker – but still, scientists are worried about the affects of the concentrated chemicals.

"High salinity and reduced dissolved oxygen levels can have profound impacts on benthic organisms, which can translate into ecological effects observable throughout the food chain," said Wageningen University's Edward Jones in a BBC article from January entitled "Concerns over increase in toxic brine from desalination plants."

Further environmental damage could arise because plankton, fish eggs and other organisms can be sucked up by the intake at the plant and killed during the purification process.

"As with most things, the better a system is designed from the beginning, the less impact there will be during operation," said Head of Marine Conservation at the National Trust Beth Taylor. "No system is perfect and mitigation is not cheap. Any desalination project would need an in-depth EIA – as with anything – but it could definitely work here – and its urgency could be supported by the current issues with water."

 

It's got associated health concerns

ROD would remove the salt from seawater – the fresh water would then be treated in the usual way.

However, seawater contains a greater amount of chemicals such as Boron that cannot be entirely removed by ROD or by the standard water treatment methods, according to the World Health Organisation (WHO).

A small amount of Boron is needed by the body as it helps the body produce oestrogen and assists in bone growth. Boron is found in very small doses in nuts and other plant foods, and some people take Boron supplements if they are Boron deficient.

However Boron can have negative effects on male fertility as well as being fatal in high enough doses. The WHO guideline value for Boron in drinking-water is 2.4 mg/l and many water companies have had to develop separate processes for removing Boron from ROD water.

And when salt is removed by ROD, some essential minerals like Calcium and Magnesium are also removed. According to a WHO study, this can negatively affect health because Calcium is a substantial component of bones and teeth and Magnesium plays an important role in digestion – and both minerals also help counteract the effects of metals (like Boron) in water.

 

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The Rupert's desalination plant in use in the 1900s. Photo from sainthelenaisland.info.

Wait... Didn't St Helena have a desalination plant?

Interestingly, St Helena used to have a desalination plant.

In 1901 St Helena was faced with housing thousands of Boer prisoners, but the island lacked enough fresh drinking water. A distillation desalination plant was built in Rupert's Bay. The plant was tested, though it was never really used (now only the large brick chimney remains).

 

Is there a solution?

Desalination technology is constantly advancing. For instance, American company Memsys in 2011 showcased an entirely solar-powered desalination plant that fit inside a shipping container.

The plant uses a process called vacuum multi-effect membrane distillation, which combines ROD with thermal desalination. Water is boiled in a vacuum, at low temperatures, and the steam is passed through several membranes at lower and lower temperatures and pressures.

The system recovers its own energy at each step – and that energy is then used to power the next step.

The shipping container unit can produce 265 gallons of fresh water daily. This system is cheap to run because of its energy efficiency and more environmentally friendly that standard ROD methods, as it runs on solar power.

Its health concerns are also significantly lessened, according to Memsys:

"This high quality of separation can be achieved for all mineral parts and organic elements including dangerous elements like Boron and Arsen which can pass mechanical filtration processes like reverse osmosis."

The Memsys system costs twice as much, though, to purchase as a standard ROD plant.