Desalination is the abstraction and treatment of brackish or
seawater to remove dissolved salts and organics such as pesticides,
producing freshwater for beneficial purposes including drinking
water. 97% of the world's water is seawater thus seawater
desalination can provide a plentiful and secure supply of drinking
water. Desalination is predominant in arid parts of the Middle
East, North Africa, North America, as well as Mediterranean and
Caribbean Islands with limited freshwater resources of their
own.
There are two principal methods of desalination: thermal
distillation and reverse osmosis. Distillation involves the heating
of feed water to produce steam, leaving salt molecules behind. The
steam is then condensed. Reverse Osmosis involves the passing of
highly pressurised feed water through a semi-permeable membrane so
that water molecules can pass through but molecules of impurities
(including salt) are unable to pass through and are separated as a
concentrate.
Energy Requirements
Distillation has a greater energy requirement compared with
reverse osmosis (unless there is waste heat available) because of
the energy needed to heat the feed water. Reverse osmosis does not
require energy to heat water but pumping water at high pressures
still necessitates significant energy consumption.
Energy consumption can be reduced by desalinating brackish water
rather than fully saline seawater which has a higher osmotic
pressure. Desalination of seawater also costs up to five times more
than with brackish water. Solar power is considered too costly to
supply the vast energy demands of a desalination plant except for
very small systems. The energy requirement for thermal distillation
can also be reduced if a source of pre-heated water (e.g. used
power station cooling water) can be utilised. Recently Multi Effect
Distillation which uses a vacuum to reduce the distillation
temperature to 600C, has been used in the Middle East to
produce potable water for US$ 0.70/m3. Only
300C of heat needs to be added because the summer
seawater temperature is already 300C.
Energy Recovery Devices
Although the highly saline wastewater (concentrate) will not
have the same pressurisation that was initially applied to the
intake water, energy recovery devices can convert residual pressure
in the concentrate, reusing it directly or converting it into
mechanical energy. The International Desalinisation Association
report energy usage in the range of 3 kWh/m3 for seawater reverse
osmosis plants using energy recovery devices. Energy recovery is
typically over 85%.
The Environment
The American Water Works Association suggests that 166 million
litres/day (ml/d) of seawater is needed to make 95 ml/d of drinking
water. The remaining 71 million litres is concentrate which needs
disposing of. Traditionally this highly saline wastewater has been
piped back in to the oceans, but concerns have been raised over
whether this is adversely affecting ocean water quality. Other
concerns include the impact of seawater intake pipes on marine
organisms (especially small fish) and whether desalination plants,
which for economies of scale are usually large-scale, will spoil
coastal panoramas.
The Future
The membrane technology used in reverse osmosis desalination is
a well established technology. Significant improvements have
already been made in reducing the cost of membranes and improving
their durability through reducing the fouling of the membranes but
desalination remains a costly option. There are around 15 000
desalination plants already operational globally but whether
desalination becomes more widespread, will depend on whether costs
such as energy, membranes, concentrate disposal and chemical usage
can be reduced further and what happens to the availability of
other water sources.
