New Scientist 2018 sep

34 | NewScientist | 8 September 2018

conditions. It found that many marine species

would probably be fine 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 an extensive review

of the evidence, 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.

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, below).

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 watt 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,

“ The ideal desalination

process will reap valuable

minerals, not just water”

Dilution solution

Desalination produces concentrated brine. Pumping this back out to sea is a potential problem for
the environment, but diluting it wastes water that could be otherwise recycled. Extracting energy
from the dilution could make it worthwhile

Salt water from the sea

REVERSE

OSMOSIS

PRESSURE RETARDED

FORWARD OSMOSIS

MEMBRANE

FRESH WATER

WASTE

WATER

Water treatment plant

Concentrated brine

returned to the sea or...

...mixed with treated

water before being

returned to the sea

PRESSURE

PRESSURE

Pressure used to

drive a turbine

producing electricity