8 September 2018 | NewScientist | 35an 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 breathednew 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. ■Katherine Bourzac is a freelance science journalist
based in San FranciscoCalifornia is building 8 desalination plants. The
effect of waste salt on kelp forests is unknownRICHARD HERRMANN/FLPA
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