The high costs of desalination have led to sig-
nificant interest in less expensive methods. One
popular method is called reverse osmosis.
To understand how reverse osmosis works,
let’s first look at regular osmosis. When there
are two regions of a salt solution with different
concentrations, salts will move through the
water to reach equilibrium in a process called
If a semipermeable membrane is placed
between the two regions, the salts would be
unable to diffuse to reach an equilibrium concentration. The water, however, would be able
to move. It would go from where the relative
amount of salts is low to where the relative
amount is high. This process is known as
Rogers, P. Nation’s Largest Ocean Desalination
Plant Goes Up Near San Diego; Future of
the California Coast?, The Mercury News,
May 29, 2014: https://www.mercurynews.
Skilhagen, S. E. The Power of Osmosis, Water
Power and Dam Construction, April 13, 2010:
Habboush, M. Abu Dhabi’s Masdar Seeks Partners
for Clean-Energy Desalination, Bloomberg News,
Sept 8, 2016: http://www.bloomberg.com/news/
Scauzillo, S. Is Desalination Part of the Future of
Water in California?, San Gabriel Valley Tribune,
Feb 21, 2016: http://www.sgvtribune.com/envi-
Greenmeier, L. A Fine Brine: New Desalination
Technique Yields More Drinkable Water, Scientific
American, May 22, 2012: http://www.scientificam-
Talbot, D. Desalination Out of Desperation. MIT
Technology Review, Dec 16, 2014: https://www.
Matt Ruppel is a science writer who lives in San
Diego, Calif. This is his first article in ChemMatters.
Reverse osmosis operates using the same
principles, but water flows in the opposite
direction. High pressure is applied to a salty
solution that is in contact with a semipermeable
membrane. To relieve the pressure, some water
must cross the membrane to the other side. The
membrane blocks the salts, resulting in freshwater on one side of the membrane and extra-concentrated salty water on the other.
For industrial desalination, large quantities of seawater must be treated quickly and
efficiently. First, it’s pretreated to remove large
components such as algae. The seawater is
then pushed through a tightly
of semipermeable membranes.
of fresh water
emerges on the
other side and
passes over a rock
bed. This allows
small amounts of
rock minerals (con-
taining ions, such as
magnesium, calcium, and sulfate) to dissolve
into the water at low concentrations, about a
few milligrams per liter. It is then disinfected
and mixed with the rest of a municipality’s
Although desalination technology has
improved, it’s no panacea. Reverse-osmosis
desalination plants use far less energy than
distillation plants, but the start-up costs for a
new facility are exorbitant, making it cheaper
for many communities to import water from
regions hundreds of miles away.
Beyond direct costs, many environmental
groups caution that desalination plants can
lead to environmental damage. The method
produces unnaturally salty water that is often
returned to the oceans,
where the extra-briny
solution could harm
nearby marine life.
But because water
scarcity is already
affecting so many lives
and is expected to
worsen, it’s important
to create a strategy
to provide enough
fresh water for
everyone. As popula-
tions grow and rainfall patterns shift due to
climate change, desalination’s role in supplying
parched areas with fresh water could grow
more important in coming years.
The 3. 2 million residents of San Diego County have traditionally relied on imported water from
Northern California and the Colorado River for their water supplies
(accounting for 20% and 64% of
total water use, respectively). But
a new option in town promises to
change the game.
The Carlsbad Desalination
Plant, which uses reverse
osmosis technology, opened
in 2015 and has made a sizable dent in where San Diego gets
its water. At full capacity, this plant could produce enough water for 400,000
residents and to meet about 12% of Southern California’s freshwater needs.
While desalination has yet to take hold as a water source in much of the
United States, the Carlsbad facility could be a prototype for other plants. In the
past few years, 18 desalination plants have been approved in the Golden State.
For drought-stricken California, this creative use of science and engineering is
becoming an important strategy for keeping the faucets running.
Water Desalination in Southern California
A Drop in the Bucket
By the end of 2016, nearly
19,000 desalination plants were expected to be up-and-running worldwide, producing 23.4
billion gallons of fresh water per day.
Sound like a lot of water? Desalination
only provides 1% of the world’s
An estimated 44% of desalinated water
is produced in the Middle East and North
Source: International Desalination Association,
International Water Association