Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
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PART B. TECHNOLOGY PROFILES
2. WATER QUALITY IMPROVEMENT TECHNOLOGIES
2.1 Desalination by Reverse Osmosis
Desalination is a separation process used to reduce the dissolved salt
content of saline water to a usable level. All desalination processes
involve three liquid streams: the saline feedwater (brackish water or
seawater), low-salinity product water, and very saline concentrate (brine
or reject water).
The saline feedwater is drawn from oceanic or underground sources. It is
separated by the desalination process into the two output streams: the
low-salinity product water and very saline concentrate streams. The use of
desalination overcomes the paradox faced by many coastal communities, that
of having access to a practically inexhaustible supply of saline water but
having no way to use it. Although some substances dissolved in water, such
as calcium carbonate, can be removed by chemical treatment, other common
constituents, like sodium chloride, require more technically sophisticated
methods, collectively known as desalination. In the past, the difficulty
and expense of removing various dissolved salts from water made saline
waters an impractical source of potable water. However, starting in the
l950s, desalination began to appear to be economically practical for
ordinary use, under certain circumstances.
The product water of the desalination process is generally water with
less than 500 mg/l dissolved solids, which is suitable for most domestic,
industrial, and agricultural uses.
A by-product of desalination is brine. Brine is a concentrated salt
solution (with more than 35 000 mg/l dissolved solids) that must be
disposed of, generally by discharge into deep saline aquifers or surface
waters with a higher salt content. Brine can also be diluted with treated
effluent and disposed of by spraying on golf courses and/or other open
There are two types of membrane process used for desalination: reverse
osmosis (RO) and electrodialysis (ED). The latter is not generally used in
Latin America and the Caribbean. In the RO process, water from a
pressurized saline solution is separated from the dissolved salts by
flowing through a water-permeable membrane. The permeate (the liquid
flowing through the membrane) is encouraged to flow through the membrane
by the pressure differential created between the pressurized feedwater and
the product water, which is at near-atmospheric pressure. The remaining
feedwater continues through the pressurized side of the reactor as brine.
No heating or phase change takes place. The major energy requirement is
for the initial pressurization of the feedwater. For brackish water
desalination the operating pressures range from 250 to 400 psi, and for
seawater desalination from 800 to 1 000 psi.
In practice, the feedwater is pumped into a closed container, against
the membrane, to pressurize it. As the product water passes through the
membrane, the remaining feedwater and brine solution becomes more and more
concentrated. To reduce the concentration of dissolved salts remaining, a
portion of this concentrated feedwater-brine solution is withdrawn from
the container. Without this discharge, the concentration of dissolved
salts in the feedwater would continue to increase, requiring
ever-increasing energy inputs to overcome the naturally increased osmotic
A reverse osmosis system consists of four major components/processes:
(1) pretreatment, (2) pressurization, (3) membrane separation, and (4)
post-treatment stabilization. Figure 16 illustrates the basic components
of a reverse osmosis system.
Pretreatment: The incoming feedwater is pretreated to be
compatible with the membranes by removing suspended solids, adjusting the
pH, and adding a threshold inhibitor to control scaling caused by
constituents such as calcium sulphate.
Pressurization: The pump raises the pressure of the
pretreated feedwater to an operating pressure appropriate for the membrane
and the salinity of the feedwater.
Separation: The permeable membranes inhibit the passage
of dissolved salts while permitting the desalinated product water to pass
through. Applying feedwater to the membrane assembly results in a
freshwater product stream and a concentrated brine reject stream. Because
no membrane is perfect in its rejection of dissolved salts, a small
percentage of salt passes through the membrane and remains in the product
water. Reverse osmosis membranes come in a variety of configurations. Two
of the most popular are spiral wound and hollow fine fiber membranes (see
Figure 17). They are generally made of cellulose acetate, aromatic
polyamides, or, nowadays, thin film polymer composites. Both types are
used for brackish water and seawater desalination, although the specific
membrane and the construction of the pressure vessel vary according to the
different operating pressures used for the two types of feedwater.
Stabilization: The prod-uct water from the membrane
assembly usually requires pH adjustment and degasification before being
transferred to the distribution system for use as drinking water. The
product passes through an aeration column in which the pH is elevated from
a value of approximately 5 to a value close to 7. In many cases, this
water is discharged to a storage cistern for later use.
Figure 16: Elements of the Reverse Os-mosis Desalination
Source: O.K. Buros, et. al., The USAID
Desalination Manual, Englewood, N.J., U.S.A., IDEA Publications.
Extent of Use
The capacity of reverse osmosis desalination plants sold or installed
during the 20-year period between 1960 and 1980 was 1 050 600 m3/day.
During the last 15 years, this capacity has continued to increase as a
result of cost reductions and technological advances. RO-desalinated water
has been used as potable water and for industrial and agricultural
Potable Water Use: RO technology is currently being used
in Argentina and the northeast region of Brazil to desalinate groundwater.
New membranes are being designed to operate at higher pressures (7 to 8.5
atm) and with greater efficiencies (removing 60% to 75% of the salt plus
nearly all organics, viruses, bacteria, and other chemical pollutants).
Industrial Use: Industrial applications that require
pure water, such as the manufacture of electronic parts, speciality foods,
and pharmaceuticals, use reverse osmosis as an element of the production
process, where the concentration and/or fractionating of a wet process
stream is needed.
Agricultural Use: Greenhouse and hydroponic farmers are
beginning to use reverse osmosis to desalinate and purify irrigation water
for greenhouse use (the RO product water tends to be lower in bacteria and
nematodes, which also helps to control plant diseases). Reverse osmosis
technology has been used for this type of application by a farmer in the
State of Florida, U.S.A., whose production of European cucumbers in a 22
ac. greenhouse increased from about 4 000 dozen cucumbers/day to 7 000
dozen when the farmer changed the irrigation water supply from a
contaminated surface water canal source to an RO-desalinated brackish
groundwater source. A 300 l/d reverse osmosis system, producing water with
less than 15 mg/l of sodium, was used.
In some Caribbean islands like Antigua, the Bahamas, and the British
Virgin Islands (see case study in Part C, Chapter 5), reverse osmosis
technology has been used to provide public water supplies with moderate
In Antigua, there are five reverse osmosis units which provide water to
the Antigua Public Utilities Authority, Water Division. Each RO unit has a
capacity of 750 000 l/d. During the eighteen-month period between January
1994 and June 1995, the Antigua plant produced between 6.1 million l/d and
9.7 million l/d. In addition, the major resort hotels and a bottling
company have desalination plants.
In the British Virgin Islands, all water used on the island of Tortola,
and approximately 90% of the water used on the island of Virgin Gorda, is
supplied by desalination. On Tortola, there are about 4 000 water
connections serving a population of 13 500 year-round residents and
approximately 256 000 visitors annually. In 1994, the government water
utility bought 950 million liters of desalinated water for distribution on
Tortola. On Virgin Gorda, there are two seawater desalination plants. Both
have open seawater intakes extending about 450 m offshore. These plants
serve a population of 2 500 year-round residents and a visitor population
of 49 000, annually. There are 675 connections to the public water system
on Virgin Gorda. In 1994, the government water utility purchased 80
million liters of water for distribution on Virgin Gorda.
In South America, particularly in the rural areas of Argentina, Brazil,
and northern Chile, reverse osmosis desalination has been used on a
Figure 17: Two Types of Reverse Osmosis Membranes.
Source: O.K. Buros, et. al.The USAID Desalination
Manual, Englewood, N.J., U.S.A., IDEA Publications
Operation and Maintenance
Operating experience with reverse osmosis technology has
improved over the past 15 years. Fewer plants have had long-term
operational problems. Assuming that a properly designed and constructed
unit is installed, the major operational elements associated with the use
of RO technology will be the day-to-day monitoring of the system and a
systematic program of preventive maintenance. Preventive maintenance
includes instrument calibration, pump adjustment, chemical feed inspection
and adjustment, leak detection and repair, and structural repair of the
system on a planned schedule.
The main operational concern related to the use of reverse
osmosis units is fouling. Fouling is caused when membrane pores are
clogged by salts or obstructed by suspended particulates. It limits the
amount of water that can be treated before cleaning is required. Membrane
fouling can be corrected by backwashing or cleaning (about every 4
months), and by replacement of the cartridge filter elements (about every
8 weeks). The lifetime of a membrane in Argentina has been reported to be
2 to 3 years, although, in the literature, higher lifespans have been
Operation, maintenance, and monitoring of RO plants require
trained engineering staff. Staffing levels are approximately one person
for a 200 m³/day plant, increasing to three persons for a 4 000 m³/day
Level of Involvement
The cost and scale of RO plants are so large that only
public water supply companies with a large number of consumers, and
industries or resort hotels, have considered this technology as an option.
Small RO plants have been built in rural areas where there is no other
water supply option. In some cases, such as the British Virgin Islands,
the government provides the land and tax and customs exemptions, pays for
the bulk water received, and monitors the product quality. The government
also distributes the water and in some cases provides assistance for the
operation of the plants.
The most significant costs associated with reverse osmosis
plants, aside from the capital cost, are the costs of electricity,
membrane replacement, and labor. All desalination techniques are
energy-intensive relative to conventional technologies. Table 6 presents
generalized capital and operation and maintenance costs for a 5 mgd
reverse osmosis desalination in the United States. Reported cost estimates
for RO installations in Latin American and the Caribbean are shown in
Table 7. The variation in these costs reflects site-specific factors such
as plant capacity and the salt content of the feedwater.
The International Desalination Association (IDA) has
designed a Seawater Desalting Costs Software Program to provide the
mathematical tools necessary to estimate comparative capital and total
costs for each of the seawater desalination processes.
TABLE 6. U.S. Army Corps of Engineers Cost Estimates
for RO Desalination Plants in Florida.
||Capital Cost per Unit of Daily Capacity ($/m3/day)
||Operation and Maintenance per Unit of Production
||1 341-2 379
TABLE 7. Comparative Costs of RO Desalination for
Several Latin American and Caribbean Developing Countries.
a Includes amortization of capital,
operation and maintenance, and membrane replacement.
b Values of
$2.30 - $3.60 were reported in February 1994.
Effectiveness of the Technology
Twenty-five years ago, researchers were struggling to
separate product waters from 90% of the salt in feedwater at total
dissolved solids (TDS) levels of 1 500 mg/l, using pressures of 600 psi
and a flux through the membrane of 18 l/m2/day. Today, typical
brackish installations can separate 98% of the salt from feedwater at TDS
levels of 2 500 to 3 000 mg/l, using pressures of 13.6 to 17 atm and a
flux of 24 l/m2/day -and guaranteeing to do it for 5 years
without having to replace the membrane. Today's state-of-the-art
technology uses thin film composite membranes in place of the older
cellulose acetate and polyamide membranes. The composite membranes work
over a wider range of pH, at higher temperatures, and within broader
chemical limits, enabling them to withstand more operational abuse and
conditions more commonly found in most industrial applications. In
general, the recovery efficiency of RO desalination plants increases with
time as long as there is no fouling of the membrane.
This technology is suitable for use in regions where
seawater or brackish groundwater is readily available.
- The processing system is simple; the only complicating factor is
finding or producing a clean supply of feedwater to minimize the need
for frequent cleaning of the membrane.
- Systems may be assembled from prepackaged modules to produce a supply
of product water ranging from a few liters per day to 750 000 l/day for
brackish water, and to 400 000 l/day for seawater; the modular system
allows for high mobility, making RO plants ideal for emergency water
- Installation costs are low.
- RO plants have a very high space/production capacity ratio, ranging
from 25 000 to 60 000 l/day/m2.
- Low maintenance, nonmetallic materials are used in construction.
- Energy use to process brackish water ranges from 1 to 3 kWh per 1 000
l of product water.
- RO technologies can make use of use an almost unlimited and reliable
water source, the sea.
- RO technologies can be used to remove organic and inorganic
- Aside from the need to dispose of the brine, RO has a negligible
- The technology makes minimal use of chemicals.
- The membranes are sensitive to abuse.
- The feedwater usually needs to be pretreated to remove particulates
(in order to prolong membrane life).
- There may be interruptions of service during stormy weather (which
may increase particulate resuspension and the amount of suspended solids
in the feedwater) for plants that use seawater.
- Operation of a RO plant requires a high quality standard for
materials and equipment.
- There is often a need for foreign assistance to design, construct,
and operate plants.
- An extensive spare parts inventory must be maintained, especially if
the plants are of foreign manufacture.
- Brine must be carefully disposed of to avoid deleterious
- There is a risk of bacterial contamination of the membranes; while
bacteria are retained in the brine stream, bacterial growth on the
membrane itself can introduce tastes and odors into the product water.
- RO technologies require a reliable energy source.
- Desalination technologies have a high cost when compared to other
methods, such as groundwater extraction or rainwater harvesting.
RO technologies are perceived to be expensive and complex, a
perception that restricts them to high-value coastal areas and limited use
in areas with saline groundwater that lack access to more conventional
technologies. At this time, use of RO technologies is not widespread.
Further Development of the Technology
The seawater and brackish water reverse osmosis process would be further
improved with the following advances:
- Development of membranes that are less prone to fouling, operate at
lower pressures, and require less pretreatment of the feedwater.
- Development of more energy-efficient technologies that are simpler to
operate than the existing technology; alternatively, development of
energy recovery methodologies that will make better use of the energy
inputs to the systems.
- Commercialization of the prototype centrifugal reverse osmosis
desalination plant developed by the Canadian Department of National
Defense; this process appears to be more reliable and efficient than
existing technologies and to be economically attractive.
John Bradshaw, Engineer and Water Manager,
Antigua Public Utilities Authority, Post Office Box 416, Thames Street,
St. Johns, Antigua. Tel./Fax (809)462-2761.
Chief Executive Officer, Crystal Palace Resort &
Casino, Marriot Hotel , Post Office Box N 8306, Cable Beach, Nassau,
Bahamas. Tel. (809)32- 6200. Fax (809)327-6818.
General Manager, Water and Sewerage Corporation, Post
Office Box N3905 , Nassau, Bahamas . Tel. (809)323-3944. Fax
Chief Executive Officer, Atlantis Hotel, Sun
International, Post Office Box N4777, Paradise Island, Nassau, Bahamas.
Tel. (809)363-3000. Fax (809)363-3703.
Vincent Sweeney, Sanitary Engineer, c/o Caribbean
Environmental Health Institute (CEHI), Post Office Box 1111, Castries,
Saint Lucia. Tel. (809)452-2501. Fax (809)453-2721. E-mail:
Guillermo Navas Brule, Ingeniero Especialista Asuntos
Ambientales, Codelco Chile Div. Chuquicamata Fono, Calama, Chile. Tel.
(56-56)32-2207. Fax (56-56)32-2207.
William T. Andrews, Managing Director, Ocean Conversion
(BVI) Ltd, Post Office Box 122, Road Town, Tortola, British Virgin
Roberto Espejo Guasp, Facultad de Ciencias, Universidad
Católica del Norte, Departamento Física, Av. Angamos 0610,
Casilla de Correo 1280, Antofagasta, Chile. Tel. (56-55)24-1148 anexo
211-312-287. Fax (56-55)24-1724 / 24-1756. E-mail:
María Teresa Ramírez, Ingeniero de
Proyectos, Aguas Industriales , Ltda., Williams Rebolledo 1976, Santiago,
Chile. Tel. (562)238-175S. Fax (562)238-1199.
Claudison Rodríguez, Economista, Instituto ACQUA,
Rua de Rumel 300/401, 22210-010 Rio de Janeiro, Rio de Janeiro, Brasil.
Tel. (55-21)205-5103. Fax (55-51)205-5544. E-mail: email@example.com.
Joseph E. Williams, Chief Environmental Health Officer,
Environmental Health Department, Ministry of Health and Social Security,
Duncombe Alley, Grand Turk, Turks and Caicos Islands, BWI. Tel
(809)946-2152/946-1335. Fax (809)946-2411.
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