FOR the time being, Cape Town has dodged a bullet. After months of unrelenting drought, the recent winter rains have begun to refill its parched dams. That doesn’t mean things are easy. City residents are still limited to using 50 litres of water a day, scarcely enough to half-fill a bath. But at least so-called day zero, when the taps run dry and residents have to wait in line to collect survival rations of water, has been averted.
The South African city is an extreme example, but it is far from the only place facing a severe water shortage. To slake that thirst, many cities are turning to the ocean, a seemingly inexhaustible supply of water. They are doing this through desalination, a water purification technology that has been around for decades. Cape Town is bringing a couple of desalination plants online in a hurry and many others are being built elsewhere. As they spring up, however, attention is focusing on what they leave behind: concentrated brine, millions of litres of it a day.
Now scientists are sounding a note of caution about the impacts of dumping all that salt in the environment. “Increasing salinity is one of the most important environmental issues of the 21st century,” says engineer Amy Childress. But smarter methods of desalination are emerging and they have benefits far beyond providing clean water.
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Sao Paulo, Cairo, Beijing, Bangalore – the list of cities with water shortages runs long and touches every continent. Even London, often thought of as a wet city, only gets about 600 millimetres of rain a year and will probably have supply problems by 2025. As populations grow, things are set to worsen. In 2007, the UN found that about to supply them with drinking water. By 2025, the organisation expects 1.8 billion people, almost a quarter of the world’s population, to be living in areas where there is not enough water to sustain them.
Ideally, we would meet demand by tapping into stores of fresh water such as rain-filled reservoirs and rivers, or perhaps groundwater wells. But plenty of places don’t have sufficient, easily accessible sources of fresh water to support growing populations. One option is to recycle waste water on a mammoth scale. Another is to turn to salty water, like the ocean or brackish lagoons.
That is where desalination comes in, and there are plenty of ways to do it. For example, you can evaporate water from one spot and condense it in another, leaving salt and impurities behind. But the most energy-efficient method is reverse osmosis.
Imagine a strong salt solution and a weak one, separated by a membrane that allows only water through. In this situation, the water will flow naturally from the weak solution to the strong, evening out their concentrations in a process called osmosis. Do the reverse, forcing ocean water at high pressure through a salt-excluding membrane in the opposite direction, and you are well on the way to making drinking water.
This approach is becoming more common. In 2005, desalination produced about 40 billion litres of water daily, according to the International Desalination Association. By 2015 that had increased to 87 billion litres, produced by nearly 1900 plants around the world. The vast majority of those are in the dry countries around the Persian Gulf, but the technology is on the up elsewhere too. The Australian city of Adelaide gets roughly half of its water from a huge desalination plant. California already has a few plants and is spending more than $30 million on eight new ones.
Great news. Except what about all that brine left behind? It is about twice as salty as the starting water, depending on the desalination technique used, and most plants dispose of it by pumping it back out to sea. That is a source of concern for researchers, including , who is based at the University of Southern California.
The concentration of salts in the sea varies. On average it is 35 parts per thousand, meaning that for every 1000 grams of seawater, about 35 grams is salt. Once salt levels exceed what marine plants and animals are used to, there is a danger that their cells might cease to work properly.
The evidence so far for what happens to life around brine outflows from desalination plants is mixed. In 2012, a study for the California State Water Resources Control Board looked at how animals responded to increased salt concentrations in lab conditions. It found that in increased salinity. But some important ones, like giant kelp, would be at risk. Dense strands of these tall algae form underwater forests along the California coastline, and their canopies are home to a diverse range of species including sea otters and urchins.
Giant kelp reproduces most successfully at salinity levels between 25 and 35 parts per thousand, says Michael Foster at California’s Moss Landing Marine Laboratories. With the Pacific Ocean’s salinity already at 35 parts per thousand, any increase might be problematic for the kelp and the ecosystem it supports. The water control board study also found that red abalone, a prized edible mollusc, seems to be highly sensitive to salinity increases.
Heather Cooley, director of research at the Pacific Institute, a water think tank in California, is alert to the unintended consequences of brine disposal. “We don’t really know what the impacts will be on the marine environment,” she says.
Cooley has conducted , for example looking at levels of biodiversity and dissolved oxygen before and after the installation of brine outflows from desalination plants in Perth, Australia, and elsewhere. The results are not easy to interpret and do not necessary apply elsewhere. Wave patterns, for instance, significantly influence brine dispersion, she says.
Still, the potential risks mean that most desalination plants must already dilute their brine before discharging it into the ocean. That in itself is problematic. The stuff used for the dilution is often cleaned-up waste water or water used to cool industrial facilities. It is clean enough to dump in the ocean but not quite drinkable.
“The ideal desalination process will reap valuable minerals, not just water”
That whole procedure is drenched in irony. Desalination plants are needed only where there is a lack of fresh water, yet they are taking fresh water that could be easily cleaned to make it drinkable, and instead flushing it into the sea. “Why aren’t we reusing that water?” says Cooley. “You’re treating it, then dumping it back in with desalination brine. It just defies logic.”
Childress thinks innovative engineering could stop this. In order to desalinate water, you have to fight against its natural tendency to flow from areas of low to high salt concentration. But if you let nature take its course – allowing forward, not reverse, osmosis – it is possible to get more and more water to flow across a salt-excluding membrane into a container of brine, increasing the pressure. That pressurised water can be used to drive a turbine and generate electricity in a process called pressure-retarded forward osmosis (PRFO). Add this stage to a reverse-osmosis desalination plant and you not only dilute the waste brine, you also get power that can be fed back into the process or used however you like (see “diagram”).
Hybrid systems like this do not work perfectly yet. The first such facility, opened in 2009 and operated by Norwegian company Statkraft, closed after five years because it didn’t generate enough electricity to justify the building and operating costs. Childress is currently modelling similar systems in her lab to see if they can be made successful, though the details are under wraps.
There are hopeful signs elsewhere. Neal Tai-Shung Chung, a chemical engineer at the National University of Singapore, says his lab has developed the technology to a point where it makes economic sense. It comes not a moment too soon in his home country. “We don’t have energy and we don’t have water,” he says. Singapore buys a significant amount of water from neighbouring Malaysia, but the arrangement is set to expire in 35 years and has long been a political football.
Chung’s group set up a test system based on essentially the same idea as the Statkraft plant, but using the team’s own improved membranes. When the researchers ran the set-up for 500 hours, feeding it municipal waste water and seawater, its power consumption was just 1 kilowatt-hour per cubic metre of desalinated water made, a quarter of what is typically needed for reverse osmosis alone. A Singapore research incubator has taken up the designs and is planning a larger pilot plant.
Ultimate utopia
Some want to take desalination even further. Carry on removing water from brine, and you eventually get pure water and salt. Childress calls it the “ultimate utopian” desalination process. The technical name for it is zero liquid discharge (ZLD) desalination. Anyone who pulls off the feat would get three-fold rewards: zero brine, maximum fresh water and a haul of valuable compounds. In some cases that includes lithium salts, which would provide the crucial component of our best batteries.
“It’s amazing how much work has been done on this,” says Christopher Bellona, an environmental engineer at the Colorado School of Mines. Chemists have explored mining just about any mineral from brines: uranium, lithium, rubidium, plain old table salt. But turning briny trash into treasure has never quite hit the big time, principally because the various salts are present at low concentrations and in mixtures that are hard to separate. There are, however, a few places where the economics finally add up.
Most desalination happens at the shoreline, but the US is an exception. Much of the country has groundwater reserves that tend to be salty, which is why 95 per cent of its desalination plants are inland. With no ocean to discharge into, the waste brine is even more of a problem than usual. Most of the time, it gets dumped in rivers. Where land is cheap, it might be routed to evaporation ponds. In Texas and other places with favourable geology, it is injected into deep wells. But all these options have a limited capacity. That’s why Arizona is trying to get Mexican permission to build a brine-carrying canal to the sea, so far without success.
The largest inland desalination facility in the US is the Kay Bailey Hutchison plant in El Paso, Texas. It slakes the thirst of 2.7 million people and injects its brine into deep wells. That is expensive and the wells will be full before long, a combination that has breathed new life into the ZLD dream. The plant has teamed up with Enviro Water Minerals of El Paso and, in April, they finished building a brine-mining facility inspired by, and using some technology from, the petroleum industry. The firm’s CEO Hubble Hausman calls it a “water refinery”. Just as crude oil can be separated into many valuable products, the facility separates brine into about five different streams, eventually extracting nearly all the water and a handful of useful compounds. These include hydrochloric acid, sodium hydroxide, gypsum and magnesium hydroxide, all of which are either used in industry or in building materials.
The facility will soon be running at full scale, recovering an additional 7.5 million litres of drinking water a day from the plant. There is no new technology involved, just existing tech in a new combination. “It’s a brilliant idea,” says Michael Mickley, a hydrologist and consultant in Colorado. “Whether it makes sense economically is the question.” Only time will tell.
Oily treasure trove
Other engineers are looking to turn waste water into treasure under even more challenging conditions. Benny Freeman at the University of Texas at Austin has his eye on oil wells, which extract five times as much water as oil. “There’s been talk about using the water, but the least expensive thing to do is pump it back into the ground,” he says.
Yet oil well water in Texas contains 1000 parts per million of lithium. Freeman has collaborated with chemists at Monash University in Australia to develop membranes that can selectively separate the element from water. Even if mining the lithium in this way makes economic sense, Freeman is the first to admit that turning this system into a full-on desalination process would be a challenge, but he says it is an obvious thing to try next.
The ZLD dream won’t work everywhere. But desalination is a tool that city and state planners need to have ready, says Childress. She says the only way to solve the problems that come with it is to embrace a diverse set of technologies and pick the options that work locally. The reason many of these more cutting-edge ideas are not widely used is not that they don’t work. Rather, it is that “we aren’t desperate enough yet”. Perhaps the lesson from Cape Town is that we soon will be.
This article appeared in print under the headline “The briny deep”
Article amended on 28 September 2018
We corrected the power consumption of Neal Tai-Shung Chung’s desalination plant
