Elsevier

Water Research

Volume 43, Issue 9, May 2009, Pages 2317-2348
Water Research

Review
Reverse osmosis desalination: Water sources, technology, and today's challenges

https://doi.org/10.1016/j.watres.2009.03.010Get rights and content

Abstract

Reverse osmosis membrane technology has developed over the past 40 years to a 44% share in world desalting production capacity, and an 80% share in the total number of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technology for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (concentrate) disposal options, and plant location, have created significant differences in process development, implementation, and key technical problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technology in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.

Introduction

The U.S. Geological Survey (Gleick, 1996) found that 96.5% of Earth's water is located in seas and oceans and 1.7% of Earth's water is located in the ice caps. Approximately 0.8% is considered to be fresh water. The remaining percentage is made up of brackish water, slightly salty water found as surface water in estuaries and as groundwater in salty aquifers. Water shortages have plagued many communities, and humans have long searched for a solution to Earth's meager fresh water supplies. Thus, desalination is not a new concept; the idea of turning salt water into fresh water has been developed and used for centuries.

Today, the production of potable water has become a worldwide concern; for many communities, projected population growth and demand exceed conventional available water resources. Over 1 billion people are without clean drinking water and approximately 2.3 billion people (41% of the world population) live in regions with water shortages (Service, 2006). For most, solutions such as water conservation and water transfer or dam construction are not sufficient methods to cope with increasing demand and, in many cases, decreasing supply. Traditional fresh water resources such as lakes, rivers, and groundwater are overused or misused; as a result, these resources are either diminishing or becoming saline. As countries continue to develop and cities expand, few new water resources are available to support daily fresh water needs. As a result, solutions such as water reuse and salt water desalination have emerged as the keys to sustaining future generations across the globe.

Both water reuse and desalination have been incorporated successfully to provide additional fresh water production for communities using conventional water treatment and fresh water resources (Nicot et al., 2007, Reahl, 2004, Sanz et al., 2007, Sauvet-Goichon, 2007, U.S. EPA, 2004). Water reuse has been used to provide water for uses such as irrigation, power plant cooling water, industrial process water, and groundwater recharge and has been accepted as a method for indirect drinking water production (Focazio et al., 2008, Fono et al., 2006, Sedlak et al., 2000). Desalination has become an important source of drinking water production, with thermal desalination processes developing over the past 60 years and membrane processes developing over the past 40 years (Gleick, 2006).

Desalination is a general term for the process of removing salt from water to produce fresh water. Fresh water is defined as containing less than 1000 mg/L of salts or total dissolved solids (TDS) (Sandia, 2003). Above 1000 mg/L, properties such as taste, color, corrosion propensity, and odor can be adversely affected. Many countries have adopted national drinking water standards for specific contaminants, as well as for TDS, but the standard limits vary from country to country or from region to region within the same country. For example, the World Health Organization (WHO, 1970) has a drinking water taste threshold of 250 mg/L, and the U.S. Environmental Protection Agency (EPA) has secondary (non-enforceable) standards of 250 mg/L chloride and 500 mg/L TDS (U.S. EPA, 2002). Each U.S. state can set a primary, enforceable standard. The state of Utah currently has a TDS limit of 2000 mg/L (Utah Rule R309–200, 2006), while California has a standard of 1000 mg/L TDS (California Code of Regulations, 2007), and Florida has a standard of 500 mg/L TDS (F.A.C., 2007). The WHO and the Gulf Drinking Water standards recommend a drinking water standard of 1000 mg/L TDS for drinking water (Fritzmann et al., 2007). Australia has a drinking water standard of 1000 mg/L TDS (Australian Drinking Water Guidelines, 2004). The European Union does not have a drinking water standard for TDS, although standards for other drinking water contaminants have been established (WHO, 1970). In comparison to governmental standards, most desalination facilities are designed to achieve a TDS of 500 mg/L or less (Gaid and Treal, 2007, Petry et al., 2007, Sanz et al., 2007, Xu et al., 2007). Desalinated water used for other purposes, such as crop irrigation, may have a higher TDS concentration; irrigation water standards often include concentration limits for TDS, chloride, sodium, and boron. Depending on the type of crop, the chloride standard can range from 350 mg/L to more than 2000 mg/L (Fipps, 2003).

The feed water salinity for desalination facilities ranges from approximately 1000 mg/L TDS to 60,000 mg/L TDS, although feed waters are typically labeled as one of two types: seawater or brackish water. Although most seawater sources contain 30,000–45,000 mg/L TDS, seawater reverse osmosis membranes are used to treat waters within the TDS range 10,000 – 60,000 mg/L. Brackish water reverse osmosis membranes are used to treat water sources (often groundwater sources) within a range of 1000–10,000 mg/L TDS (Mickley, 2001). The feed water type can dictate several design choices for a treatment plant, including desalination method, pretreatment steps, waste disposal method, and product recovery (the fraction of influent water that becomes product).

Desalination processes fall into two main categories, thermal processes or membrane processes. Thermal desalination (distillation) has been used for hundreds of years to produce fresh water, but large-scale municipal drinking water distillation plants began to operate during the 1950s (Gleick, 2006). Countries in the Middle East pioneered the design and implementation of seawater thermal desalination, first using a process called multi-effect distillation (MED) and later using a process called multi-stage flash (MSF) distillation (Van der Bruggen and Vandecasteele, 2002). Today, the Middle East collectively holds 50% of the world's desalination capacity (Henthorne, 2003) and primarily uses MSF technology. While thermal desalination has remained the primary technology of choice in the Middle East, membrane processes have rapidly developed since the 1960s (Loeb and Sourirajan, 1963) and now surpass thermal processes in new plant installations. Outside of the Middle East, new RO desalination installations have been steadily increasing; in 2001, 51% of new installed desalination capacity used RO desalination, and in 2003, RO desalination accounted for 75% of new production capacity (Wolfe, 2005). Countries in the Middle East continue to use thermal desalination due to easily accessible fossil fuel resources and the poor water quality of the local feed water. Water bodies such as the Persian Gulf and the Gulf of Oman have extremely high salinities, high temperatures, and high-fouling potential for membrane systems. At high salinities and high recoveries (55,000 mg/L TDS and above 35% recovery), the pressure required for membrane desalination can be greater than the maximum allowable pressure of membrane modules, and thermal desalination must be used (Kim et al., 2007, Mandil et al., 1998). High feed water temperatures and foulants can also cause problems in membrane desalination that can be avoided by using thermal desalination.

Reverse osmosis (RO), nanofiltration (NF), and electrodialysis (ED) are the three membrane processes available for desalination. ED membranes operate under an electric current that causes ions to move through parallel membranes and are typically only used for brackish water desalination (Reahl, 2004). NF membranes are a newer technology developed in the mid-1980s (Singh, 1997) and have been tested on a range of salt concentrations (Hilal et al., 2005, Tanninen et al., 2006, Wang et al., 2005, Wang et al., 2006). Research has shown that NF, as a singular process, cannot reduce seawater salinity to drinking water standards, but NF has been used successfully to treat mildly brackish feed water (Bohdziewicz et al., 1999, Lhassani et al., 2001, M'nif et al., 2007). Coupled with RO, NF can be used to treat seawater (Hamed, 2005, Hassan et al., 1998, Hilal et al., 2005). In particular, NF membranes are used to remove divalent ions, such as calcium and magnesium that contribute to water hardness, as well as dissolved organic material (Choi et al., 2001, Gorenflo et al., 2002, Wilf, 2003).

RO membranes, however, are able to reject monovalent ions, such as sodium and chloride. Today, seawater RO membranes have salt rejections greater than 99% (Bates and Cuozzo, 2000, Brehant et al., 2003, Reverter et al., 2001); some membranes, when operated under standard test conditions (32,000 mg/L NaCl, 5.5 MPa, 25 °C, pH 8, 8% recovery), can achieve as high as 99.7–99.8% salt rejection (Reverberi and Gorenflo, 2007, Hydranautics, 2007). RO membrane technology has developed for both brackish and seawater applications. Brackish water RO membranes typically have higher product water (permeate) flux, lower salt rejection, and require lower operating pressures (due to the lower osmotic pressures of less saline waters), while seawater RO membranes require maximum salt rejection. Membranes designed for higher salt rejection, have lower permeate fluxes, due to the trade-off between membrane selectivity (salt rejection) and membrane permeability (permeate flux). In addition, seawater RO membranes must operate at higher pressures to compensate for the higher osmotic pressure of seawater.

Section snippets

History of desalination

In the modern world, desalination first began to be developed for commercial use aboard ships. Distillation, the process of using a heat source to separate water from salt, was used to provide drinking water to ocean-bound ships to avoid the possibility of depleting onboard fresh water supplies (Seigal and Zelonis, 1995). Thermal desalination enabled ships to travel farther for longer periods of time because it was no longer necessary to transport all the fresh water required for the voyage. In

Reverse osmosis: basic principles

RO membranes do not have distinct pores that traverse the membrane and lie at one extreme of commercially available membranes. The polymer material of RO membranes forms a layered, web-like structure, and water must follow a tortuous pathway through the membrane to reach the permeate side. RO membranes can reject the smallest contaminants, monovalent ions, while other membranes, including nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF), are designed to remove materials of

Reverse osmosis desalination: feed waters

To illustrate key differences between brackish water and seawater, a comparison of water data is shown in Table 1 (DBHYDRO, 2001, Blavoux et al., 2004, Gaid and Treal, 2007, Jurenka and Chapman-Wilbert, 1996). The seawater source is surface water from the Mediterranean Sea, and both of the brackish water sources are groundwaters. Data for boron concentration in groundwater are limited because boron is a relatively new regulated contaminant, and concentrations are typically low for groundwater.

Seawater RO

Seawater sources often have particulate and colloidal contaminants, as well as hydrocarbons from oil contamination and biological contaminants (algal blooms and other microorganisms).

One of the most difficult seawater components to remove is boron, an inorganic molecule shown to cause adverse reproductive and developmental effects, as well as plant and crop damage (Desotelle, 2001, Magara et al., 1998, Nadav et al., 2005). In general, ions are rejected better by RO membranes than respective

Membrane fouling

Two fouling mechanisms are generally observed for membrane processes: surface fouling and fouling in pores. However, RO membranes do not have distinguishable pores and are considered to be essentially non-porous. Thus, the main fouling mechanism for RO membranes is surface fouling. Surface fouling can occur from a variety of contaminants, including suspended particulate matter (inorganic or organic), dissolved organic matter, dissolved solids, and biogenic material (Amiri and Samiei, 2007). In

Membrane cleaning

A combination of acidic and/or basic (alkaline) chemicals is used to clean RO membranes. Common acidic solutions (pH ∼2) include hydrochloric acid, phosphoric acid, sodium hydrosulfate (Na2S2O4) and sulfamic acid (NH2SO3H), while alkaline (pH ∼12) chemicals include sodium lauryl sulfate, sodium hydroxide, sodium ethylenediamine tetraacetic acid (Na4EDTA), and proprietary cleaners (e.g., Permaclean 33) (Bonné et al., 2000, Fritzmann et al., 2007, Reverberi and Gorenflo, 2007). Most cleaning

RO pretreatment for seawater and brackish water

The primary goal of any RO pretreatment system (for seawater or brackish water) is to lower the fouling propensity of the water in the RO membrane system. Surface water resources (seawater and brackish water) typically have a greater propensity for membrane fouling and require more extensive pretreatment systems than groundwater resources (Morenski, 1992). In general, seawater RO tends to use surface water sources, while brackish water RO often uses groundwater sources.

Typical operational parameter ranges

A comparison of typical operating ranges for key RO parameters is shown in Table 4 for brackish water and seawater (Afonso et al., 2004, Bonnelye et al., 2004, Gabelich et al., 2003, Glueckstern, 1999, Glueckstern and Priel, 2003, Hasson et al., 2001, Rahardianto et al., 2006, Rahardianto et al., 2007, Shih et al., 2004, Van der Bruggen and Vandecasteele, 2002, Wilf and Klinko, 2001). Due to lower feed water TDS concentration, most parameter values for brackish water RO are less constrained

Seawater RO

The permeate from seawater RO is often treated before distribution. Depending on the permeate TDS, the permeate may be blended with another water to either increase or decrease the salinity (Sanz et al., 2007, Zidouri, 2000). Lime (Ca(OH)2) or limestone contactors may be added to increase the hardness, alkalinity and pH, as well as prevent the water from causing calcium to leach from pipes in the distribution system (Khawaji et al., 2007). Hardness is necessary to achieve the typical taste of

Seawater RO

For seawater RO plants, the disposal method is usually discharge back into the same body of water; the primary concerns are only the pumping system and length of piping needed to reach the chosen discharge point underwater (Mooij, 2007, Ravizky and Nadav, 2007). The feed water intake and the concentrate discharge are positioned in separate locations, and the concentrate is diluted into the large seawater body without influencing the feed water composition.

Brackish water RO

If feasible, surface water disposal is

Seawater RO

The coupling of alternative (renewable) energy sources with RO desalination plants has had increased interest and development. The plants in operation are small-scale (<10 m3/day) plants and represent approximately 0.02% of the total world desalination capacity (Mathioulakis et al., 2007). These plants are largely demonstration or research plants and often operate non-continuously; in addition, renewable energy sources are still more expensive than traditional resources (Helal et al., 2008,

Costs

Karagiannis and Soldatos (2008) conducted a review of water desalination cost literature and found that the type of feed water (seawater or brackish water), as well as the plant size and the energy source, play major roles in the cost of desalinated water ($/m3). The investment cost per unit of production capacity of seawater RO plants is higher than that of brackish water RO plants; seawater RO capital costs tend to fall between $600/(m3/day) and $800/(m3/day) (Reddy and Ghaffour, 2007,

Technological challenges and the future of RO

An emerging application of RO membranes is in wastewater treatment and trace organic contaminant removal. A host of new organic contaminants have been identified (Richardson et al., 2007), and RO technology is a potential treatment candidate. Particularly for hydrophilic organic compounds, including many disinfection by-products and pharmaceutical compounds, traditional treatment processes (coagulation and flocculation) are not effective at removal. However, RO membranes may remove these

Conclusions

The field of RO membrane desalination has rapidly grown over the past 40 years to become the primary choice for new plant installations. Membrane technology has improved, allowing significant increases in product production and cost savings. While the basic operating principles remain the same for all RO applications, individualized applications have developed, based on feed water quality. In particular, the two key types of feed water, seawater and brackish water, have distinguishing features

Acknowledgements

The author would like to thank the National Science Foundation International Research and Education in Engineering (IREE) program (NSF Award Title: Collaborative Research: A Polymer Synthesis/Membrane Characterization Program on Fouling Resistant Membranes for Water Purification, NSF Award Number: CBET 0553957) for funding support during the preparation of this manuscript. This work was also supported by the Office of Naval Research (ONR) (Grant # N00014-05-1-0771 and Grant # N00014-05-1-0772)

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