Reverse Osmosis is a method of desalination which employs a semi-permeable membrane (usually a
thin plastic film) to physically separate high quality water from water
the contains salts and other types of impurities including
micro-organisms. Pressure is applied to the saline or feedwater side of
the membrane and purified water passes through and is collected on the
other side of the membrane. The unwanted impurities on the salty side are
then flushed away. Seawater can be used as a source of feedwater, and
therefore this means of producing high quality drinking water can be used
almost anywhere in the world. The drawback is that the feedwater usually
has to be cleaned up, or filtered, before it can be passed though the
membrane and energy is required to create the pressure needed to push the
water molecules through the membrane.
The principle of reverse osmosis is very simple but in practical terms it
requires high maintenance and a sound technical understanding of the
processes involved. Reverse osmosis has many applications. It is used to
desalinate seawater or brackish water for drinking and it is used to
produce high quality water for kidney dialysis, and many other chemical
processes. Even wastewater can be treated by this process.
A
basic reverse osmosis plant comprises a feedwater supply, a pretreatment
filter system, a high pressure pump, and membrane, facilities for product
storage, and a brine disposal system.
In order to
protect the membranes which are expensive and vulnerable items, the
pretreatment system can involve a number of different procedures to make
the water safe for drinking other procedures are employed on the
downstream side of the membranes.
Small plants
usually come in the form of a prefabricated package, but the individual
components can be purchased and assembled to meet specific
requirements.
In the Bahamas the use of reverse osmosis is
widespread. Hundreds of small plants are used in boats to produce water
and plants are now common in many households. Most of the bottled
water supplies in the Bahamas is produced by reverse osmosis. Persons who
live where there is no piped supply of water, and the groundwater quality
is poor, have a choice of collecting rainwater of purifying their
groundwater by means of reverse osmosis. Where the rainfall is low or very
seasonal reverse osmosis is a more reliable option and those that do not
want to deal with and understand the technology involved can pay a person
or a company to operate and maintain their plants. Hotels, marinas, and
restaurants use reverse osmosis throughout the Bahamas. The biggest plant
in use is the 2 m million gallon per day plant operated by Waterfields in
New Providence. There are a large number of companies and individuals
selling reverse osmosis equipment in the Bahamas and it is easy to procure
and install a plant. What is not so simple is operating and maintaining
the plant successfully and the cost of the water is considerably higher
than that of traditional sources such as groundwater.
The reality
of reverse osmosis in this island environment is that it is a godsend for
the Bahamas. This is because there are many inhabited islands that do not
have adequate water resources to meet the needs of the residents, and
there are many situations where water cannot be piped in from other nearby
source areas. In some locations rainwater and groundwater sources did meet
water demands for many years but development and particularly the growth
of tourism has changed this situation. Reverse osmosis already provides
water for the residents of Grand Cay, Moores Island, Black Point and
Farmers Cay, and plans are being finalized for plants to be installed in
Bimini, Long Island, San Salvador, Inagua, and Ragged Island. Other
islands with Bahamian communities that will require the use of reverse
osmosis include Great Harbour Cay, Staniel Cay, Acklins, and Long Cay.
Some islands like the off-shore Abaco cays that do not have any freshwater
resources could obtain their water from the mainland by means of
underwater pipelines, like Harbour Island and Spanish Wells in Eleuthera,
but until the mains are laid small reverse osmosis plants are used by the
commercial enterprises on these cays. Little Exuma and Barreterre do not
have good freshwater resources but water could be supplied from Great
Exuma by pipelines. Alternatively,
local reverse osmosis plants could be utilized.
In early
days New Providence could meet its water demand from groundwater sources,
but this situation has changed because of the growth of the population and
tourism, and changing habits of water use, and there are islands where
this scenario will probably be duplicated in the future. Examples are
Eleuthera, Great Exuma, and Rum Cay. Acklins has good water resources but
unfortunately these are located well away from the population centers and
there is a need for the use of small reverse osmosis plants in sectors of
the island.
In the foreseeable future approximately twenty (20)
populated islands will require water produced by desalination, but this
number could easily increase with development or growth in population.
Presently the best means of desalination on the scale required is reverse
osmosis, and because of various technical considerations the best method
of getting the required plants installed and operated is by means of
private companies with the appropriate experience. All Bahamians can, and
probably do, benefit from the use of reverse osmosis, and what is now
needed is a true appreciation of the value of good quality water, because
this water is not cheap. Technology is constantly changing, however, and
the situation described above could quickly change. What will never change
is the growing demand for water.
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