processes. That Sydney bulletin also
explained that the cost of desalination
would necessitate a 6% increase in water
usage charges, backdated to October. Shortly
afterwards, storms refilled most of Sydney’s
dams. But a refund is unlikely.
Another problem with desalination is the
waste product of brine – salt water with a
higher concentration of salt than ordinary
ocean water. Brine is typically released back
into the ocean, but the high salt concentra-
tion can harm animals and plants.
However, scientists are exploring new
opportunies to extract metals and salts such
as magnesium, calcium, lithium, and sodium
chloride, for use in different industrial
processes and in agriculture. Scientists from
the Massachusetts Institute of Technology in
Boston have developed a method to extract
sodium hydroxide – also known as caustic
soda – from ocean water. Lots of caustic soda
is now purchased by desalination plants to
pre-process ocean water so that the
membranes that desalinate the water don’t
get congested. So now, part of the waste from
desalination could be reused.Reverse osmosis drives out salt
Desalination methods can be divided into
two general categories: thermal and
membrane-based. Up until 2000, thermal
desalination was the most common, in a
method known as multi-stage flash distilla-tion (MSF). Ocean water flows through
several chambers with different pressures,
temperatures and heat exchangers, causing
evaporation and condensation of the water
without its salt content, leaving brine as a
waste product.
This process requires a lot of energy, and
so another membrane-based technology
known as reverse osmosis has been able to
outcompete thermal desalination in recent
years. The process is the opposite of osmosis- the natural phenomenon that occurs when
fresh water is separated from salt water, for
example at a cell membrane that allows only
water molecules to pass. In an effort to even
out the difference in salt concentration on
both sides of the membrane, water molecules
flow into the brine.
In reverse osmosis, salt water under high
pressure is forced through a membrane such
as cellulose acetate, moving to the opposite
side where there is a normal atmospheric
pressure. The membrane allows the water
molecules to pass, but blocks salt molecules
3 methods
to desalinate
ocean water
A collective of buoys that put water
under pressure, and ultrathin graphene
and wood discs – new environmentally
friendly methods are to make reverse
osmosis more climate-friendly, and
salty ocean water drinkable.1.1
billion litres of ocean water
will be desalinated per day
in Dubai as from 2030.THEUNI.^ OF^ MANCHESTERUNIVERSITY^ OF^ MARYLANDATMOCEAN.COM46 | SCIENCE ILLUSTRATED
TECHNOLOGY WATER