By Dr. Joseph A. Cotruvo
More than 11,000 desalination plants are in operation throughout the world, producing more than 20 million cubic meters (m3)—roughly six billion gallons—of water per day. About 60 percent of the capacity exists in West Asia and the Middle East. North America has about 11 percent and North Africa and Europe account for about seven percent each. South and Central America together account for about four percent of desalination capacity. Desalination is also a significant and growing source of fresh water in the Caribbean Islands. Plant sizes and designs range from more than 500,000 m3/day (13,200,000 gallons per day or 13.2 mgd) down to 20 to 100 m3/day (5,280 to 26,400 gallons per day). Due to rapid advances in technology and improved efficiencies, the cost of producing desalinated water is now approaching $0.50 USD per cubic meter in large plants ($1.89 per thousand gallons or Kgal), so it is becoming much more accessible in areas where alternative fresh water supplies are not available.
Most desalination plants use seawater and/or brackish water as their sources for drinking water production, however; membrane technologies are also used for wastewater treatment and for removing salts from processed wastewaters for recycling applications including prior-to-aquifer recharge. Performance, operating and product quality specifications have evolved virtually on a site-by-site basis relative to source and the specific end product water use. Most drinking water applications outside of North America use World Health Organization Drinking Water Quality Guidelines (WHO GDWQ) as water quality specifications. WHOGDWQ cover a broad spectrum of contaminants from inorganic and synthetic organic chemicals, disinfection byproducts, microbial indicators and radionuclides. They are aimed at typical drinking water sources and technologies. Because desalination is applied to non-typical source waters and often uses non-typical technologies, the existing WHO GDWQ may not fully cover the unique circumstances that can be encountered during production and distribution of desalinated drinking water.
Drinking water production
Drinking water production chains can be divided into three broad categories, each of which will impact the quality of the finished water received by the consumer: source water quality, treatment processes and distribution. Some issues that distinguish desalination processes from typical drinking water operations include:
- Total Dissolved Solids (TDS) in the range of 40,000 ppm for seawater and about 10,000 ppm for brackish water.
- High levels of metal salts including sodium, calcium, magnesium, bromides, iodides, sulfates and chlorides.
- Total Organic Carbon (TOC) type.
- Petroleum contamination potential.
- Microbial contaminants and other organisms.
- Reverse osmosis membranes and thermal distillation.
- Leachates from system components.
- Pretreatment and antifouling additives.
- Possible unique disinfection byprod-ucts.
- Post treatment blending with source waters.
- Corrosion control additives and corrosion products.
- Bacterial regrowth in distribution systems.
There are other issues of interest and/or concern as well. System components that can contribute chemicals to the water as direct additives or indirectly from surface contact. Health risks can be imparted from consumption of reconstituted or restabilized water from general reduced or selective mineralization, or from reduced intake of specific minerals like calcium and magnesium. The environmental impacts of desalination facility siting, operations and brine disposal can be significant. Also, some microorganisms unique to saline waters may not be removed by the desalination process or post disinfection. Thus, monitoring of source water, process performance, finished water and distributed water must be rigorous to assure consistent quality at the consumer’s tap. Moreover, additional water quality or process guidelines specific to desalination are needed to assure water quality, safety and environmental protection.
Several common desalination technologies are described herein. Desalination processes remove dissolved salts and other chemicals and materials from seawater and brackish water. Related processes are also used for water softening and wastewater reclamation. The principal desalination technologies are reverse osmosis (RO) and thermal (distillation). Electrodialysis and electrodialysis reversal are also in use.
Reverse osmosis (RO)
These systems reverse the natural solvent transport process driven by osmotic pressure across a semi-permeable membrane from a region of lower solute concentration into one of higher solute concentration to equalize the free energies. In RO, external pressure is applied to the high solute (concentrated) water to cause solvent (water) to migrate through the membrane pores leaving the salts and other nonpermeates behind in a more concentrated brine. Some membranes will reject over 99 percent of all ionic solids and organics and can have molecular weight separation as low as 50 to 100 Daltons. Mechanisms of salt removal by RO membranes are not fully understood and some ions like borate and arsenite are not removed with high efficiency. Increasing the operating pressure also increases the rate of water permeation, however the fouling rate will also increase.
Saline feedwater is pretreated to protect the membranes, reduce fouling and extend membrane operation cycles. Suspended solids are removed by filtration; pH adjustments (lowering) are made to limit precipitation of salts; scale inhibitors are added to reduce formation of calcium carbonates and sulfates. Inorganic oxides such as those from iron or manganese, along with organic products, contribute to membrane fouling. Disinfection helps to control bacterial biofouling of the membrane; chlorine species, ozone or UV light are commonly used. Other marine organisms such as algae must also be eliminated. Excess ozone or chlorine must be neutralized prior to contact with the membrane.
Common polymeric membranes include cellulose triacetate or more recently polyamides and polysulfones. Selection factors for membranes include pH and oxidant stability, working life, mechanical strength, pressurization capacity and selectivity for solutes. Membranes are packed into a module and they can be configured as spiral, plate and tubular. Each has its own characteristics that affect performance in particular cases. Spiral configurations generally have more favorable operating characteristics of performance relative to cost and they are most commonly used. Operating pressures are in the range of 250-1000 psi (17 to 68 atm).
Membranes are typically layered or thin film composites. The contact layer (rejection layer) is adhered to a porous support, which can be produced from the same material as the surface. Thin film membranes can be made by polymerization of the rejection layer to the surface of the porous support. Membrane thicknesses are on the order of 0.05 mm.
Product water must be treated to stabilize it, reduce its corrosivity and make it compatible with the distribution system. Adjusting pH to approximately 8 is required along with the addition of corrosion inhibitors and increasing the alkalinity. Lime, limestone or bases such as sodium carbonate or sodium hydroxide may be added to the product water; alternately, the product water might be blended with the source water to increase TDS and stabilize the water. Post disinfection is also necessary to control microorganisms during distribution, as well as to eliminate pathogens introduced by blending. Degasification may also be necessary.
The principal distillation (vaporization → condensation) systems include Multistage Flash (MSF) Distillation, Multi-effect Distillation (MED) and Vapor Compression Distillation (VCD). They can produce water in the range of 1 to 50 ppm TDS.
Source water is heated and vaporized and the condensed vapor has very low TDS, while a concentrated brine is produced as a residual. Simple distillation processes can be applicable to desalination because significant amounts of volatile chemicals are usually not present in seawater and brackish waters. Salts and high molecular weight natural organics are non-volatile and easily separated; however, there are circumstances where volatile petroleum chemicals are present due to spills and other contamination. Even though their vapor pressures can range from low to very high, many of them of higher molecular weight could be steam distilled in a process. In addition, some physical entrainment may also allow low volatility substances to be carried over into the distillate. Periodic cleaning is required to remove scale and salts deposits from pipes, tubing and membranes. Alkaline cleaners remove organic fouling and acid cleaners are used to remove scale and salts (See Figure 2).
The boiling point of water (where the vapor pressure of the liquid is the same as the external pressure) is 100°C (212°F) at 1 atmosphere (760 mm Hg or 14.7 pounds per square inch). As the pressure is decreased, the boiling temperature decreases. The amount of energy required to vaporize a liquid at its boiling point is called the heat of vaporization. For water, this amounts to 2,256 kilojoules per kilogram at 100°C (970 Btu per pound at 212°F). The same amount of heat must be removed from the vapor to condense it back to liquid at the boiling point. In desalination processes, heat generated from vapor condensation is transferred to the feed water to raise its temperature and cause vaporization and thus improve thermal efficiency and reduce fuel consumption and cost.
Multistage Flash Distillation (MSF)
MSF plants are major contributors to desalting capacity. The principle of MSF distillation is that heated water will boil rapidly (flash) when the pressure of the vapor is rapidly reduced below the vapor pressure of the liquid at that temperature. The vapor generated is condensed on one side of surfaces in contact with feed water on the other side, thus preheating the water prior to its introduction into the flash chamber. This will recover most of the heat of vaporization. Approximately 25 to 50 percent of the flow is recovered as fresh water in multistage plants. MSF plants typically have high feed water volume and flow, incur corrosion and scaling in the plant and have high rates of use of treatment chemicals.
Multiple Effect Distillation (MEF)
Several configurations of MEF plants exist, including vertical and horizontal tubes. In all cases, steam is condensed on one side of a tube causing evaporation of saline water on the other side. Pressure is reduced sequentially in each effect (stage) as the temperature declines and additional heat is provided in each stage to increase vaporization.
Vapor Compression Distillation (VCD)
VCD systems function by mechanically compressing water vapor causing condensation on a heat transfer surface (tube) that allows the heat of condensation to be transported to salt water on the other side of the surface resulting in its partial vaporization. The principal energy requirement is in the operation of the compressor. The compressor functions to increase the pressure on the vapor side and lower the pressure on the feed water brine side to lower its boiling temperature.
Electrodialysis processes utilize selective membranes that contain cation and anion exchange groups. Under a direct current electric field, cations and anions migrate to the respective chambers so that ion-rich and ion-depleted streams form in alternate spaces between membranes. Reversal of electric fields reduces scaling and flushes the membranes. Pretreatment is required to control scale and extend membrane life and to prevent migration of non-ionized substances such as bacteria, organics and silica.
Potential technical, health and environmental issues associated with desalination
In general, public drinking water supplies are not relied upon as contributors of significant trace nutrients to daily intake, but rather as a serendipitous supplementation whenever it occurs. The geographic distribution of the nutrients in conventional drinking water will be varied and inconsistent so an appropriate diet should be the principal source.
Dietary supplementation is, however, widely practiced for general benefit; e.g., vitamin D in milk, vitamin C in drinks, iron, B vitamins and folic acid in bread.
The only therapeutic substance commonly added to drinking water is fluoride to strengthen dental enamel and reduce the incidence of tooth decay (dental caries). Seawater is low in fluoride and fluoride is depleted by desalination, so desalinated water does not contribute fluoride to daily intake unless it is present in blending waters or added post treatment.
Water can be a source of beneficial dietary substances, as well as harmful contaminants such as microorganisms and chemicals that can mitigate dietary components. The presence or absence of beneficial ions can affect public health in the population over the long term, just as the presence or absence of toxicants. Water components can supplement dietary intake of trace micronutrients and macronutrients or contribute undesirable contaminants. In both toxicology and nutrition, the line between health and illness in a population is not a single line, but rather a matter of optimal intake versus adequate intake, versus intake that is inadequate to maintain good health versus a toxic intake that will lead to frank illness in some segment of the population. Some parts of the population such as young children, pregnant women, the aged and infirm and immune compromised can be more sensitive than the typical healthy adult to essential and hazardous dietary components. Often the specific requirements for optimal health states and minimal risk are not understood for these high-risk segments of the population.
Some of the beneficial chemicals of interest in drinking water include calcium, magnesium, sodium, chloride, selenium, potassium, bromide, iodide, fluoride, chromium, zinc, copper and manganese. Seawater is rich in ions such as calcium, magnesium, sodium, chloride and iodine, but low in other essential ions like zinc, copper, chromium and manganese. Desalting processes significantly reduce virtually all of the ions in drinking water to the point where people who traditionally consume unreconstituted desalted water may be consistently receiving smaller amounts of some important nutrients relative to people who consume water from more traditional sources. Thus, they may be disadvantaged if their diets do not provide sufficient mineral intake. Since desalinated water is stabilized by blending and chemical addition, some of these ions can be replenished in that process if the appropriate treatment chemicals, such as lime or limestone, are selected.
Over about 50 years, a body of epidemiological work involving more than 80 studies of varying quality (e.g. in U.K., U.S., Canada and Scandinavia), has fairly consistently suggested that cardiovascular disease mortality rates in many communities are inversely proportional to the hardness of the water supply. Calcium and magnesium are significant components of hard water, so many researchers have concluded that calcium and magnesium may have a protective effect. There are biochemical arguments that support this hypothesis, however the issue is not resolved with certainty. More recent studies seem to be finding greater positive benefits from magnesium rather than calcium intake, particularly in regard to reduced risk from stroke or ischemic heart disease.
Some researchers have argued that water softening by ion exchange or RO also reduces trace nutrients and increases sodium (ion exchange) in drinking water, or that there is a negative health effect from general decreased mineralization (total of dissolved salts and electrical conductivity changes). Ion exchange softening replaces each calcium or magnesium ion with two sodium ions. Other researchers argue that low mineralized water is more aggressive to piping and metal surfaces and thus increased risks could be caused by exposure to trace elements like lead and cadmium that are extracted. Cooking foods in low mineralized water increases the depletion of essential minerals from the foods. The health significance of these hypothesized relationships with drinking water in any given population would be dependent upon many factors including diet, lifestyle, age, smoking, population genetics, occupation and other confounders.
Sodium can be present in desalinated water depending upon the efficiency of salts removal and the type of post treatment blending or stabilization. Typical daily dietary intake of sodium can be in the range of 2,000 to 10,000 mg or more and is a function of personal taste and cultural factors. Some segment of the population is salt sensitive (hypertensinogenic) which means that blood pressure elevation and its commensurate adverse effects occur to a greater degree in those individuals associated with their total salt consumption. Salt sensitivity is estimated in the range of 15 percent of some populations. Water is usually not a significant contributor to total daily sodium intake except for persons who are required to be on highly restricted diets of less than 400 mg sodium per day.
Raw and refined petroleum contains a very large number of toxic chemicals and substances that impart undesirable taste to finished water. Crude oil contains hydrocarbons and other chemicals that contain nitrogen and sulfur. Aliphatic hydrocarbons can range from gaseous methane (C1) and other small molecules, to midrange liquids (C5 to C16 approximately) like heptane (C7) and cetane (C13) and to high molecular weight solids that are dissolved or suspended in the mixture. Aromatic hydrocarbons range from benzene and toluene to polynuclear aromatic naphthalene (2 rings) to benzopyrene (6 rings) and above. Benzene is known to cause leukemia and benzopyrene causes skin and other cancers.
Numerous sulfur compounds are present both as heterocycles and as thiols and other forms including sulfur and hydrogen sulfide that is highly toxic and particularly malodorous at very low concentrations. Refined petrochemicals and gasoline products challenge the treatment processes with high concentrations of mobile, volatile and often more toxic lower and mid-range fractions.
The lower molecular weight cut off for RO rejection is typically in the range of 100-300 Daltons. Very large molecules are removed by RO membranes, however significant fouling can impede operations by reducing flux. Small non-polar molecules pass through the membrane. Some molecules, although rejected due to their size, may, depending on their solubility characteristics and the chemistry of the polymeric membrane, dissolve in the membrane polymer and diffuse through to the finished water.
Distillation processes can theoretically separate any substance by fractionation based upon boiling point differences, however desalination distillation is not designed to be a fractionating system, thus substances with boiling points lower than water’s would easily be carried over in the vapors and even higher boiling substances could “steam” distill and be carried into the distillate.
Pretreatments are used to avoid contamination of finished water by certain organics and these can involve an adsorption process using granular activated carbon, or more frequently, powdered activated carbon for intermittent contamination. Pretreatment of blending water may also be required for the same reason.
Wastes from desalination plants include concentrated brines, backwash liquids containing scale and corrosion salts and antifouling chemicals and pretreatment chemicals in filter waste sludges. Depending upon the location and other circumstances, including access to the ocean and sensitive aquifers, concentrations of toxic substances etc., wastes could be discharged directly to the sea, mixed with other waste streams before discharge, discharged to sewers or treated at a sewage treatment plant, lagooned, or dried and disposed in landfills.
Desalination plants require significant amounts of electricity and heat depending upon the process and type of source water. It has been estimated that one plant producing about seven million gallons per day could require about 50 million kWh/yr., which would be similar to the energy demands of an oil refinery or a small steel mill. Co-generation facilities that produce both electric power (or produce excess heat) and water provide significant opportunities for efficiencies by using waste heat from power generation in the desalination process.
Installation and operation of a desalination facility will have the potential for adverse impacts on air, seawater, groundwater and possibly other aspects. These impacts should be considered and their acceptability and mitigation requirements would usually be matters of national and local regulation and policies. Studies to examine these effects would usually be conducted at each candidate site and post installation monitoring programs should be instituted. A brief partial listing of issues follows:
Construction: Coastal zone and sea floor ecology, birds and mammals habitat, erosion and non-point source pollution.
Energy: Fuel source and fuel transportation, cooling water discharges, air emissions from electrical power generation and fuel combustion. Air Quality: Energy production related.
Marine Environment: Constituents in waste discharges, thermal effects, feed water intake process, effects of biocides in discharge water and toxic metals, oxygen levels, turbidity, salinity, mixing zones, commercial fishing impacts, recreation and many others.
Ground Water: Seepage from unlined drying lagoons causing increased salinity and possibly toxic metals deposition.
Disinfection and microbe control
Sea and brine waters contain microorganisms including bacteria, protozoa and viruses that could be pathogenic, especially if impacted by sewage discharges or urban runoff. Disinfection occurs at several points during the treatment process. The question is: what is adequate disinfection to protect public health from exposure to pathogenic microbes and are there any unique risks that may be associated with desalination practices? During pretreatment, a disinfectant, often chlorine, will be added to reduce biofouling and protect the membrane from degradation. Chlorination of seawater would be expected to produce substantial amounts of brominated organic byproducts. Membranes also have the capacity to separate microorganisms by preventing their passage through the membranes. So long as the membrane is intact, complete removal of microorganisms can occur; however, some bacteria can grow through the membrane. Leakage can also occur in membranes and through seals, so surveillance to assure continued membrane integrity is an important part of the process management system.
Even ultrafiltration (UF) membranes, which have pores (~0.001 to 0.1 microns), have been demonstrated to achieve significant reductions of virus and protozoa. Better performance would be expected from RO membranes. Several challenge tests employing giardia lamblia and cryptosporidia oocysts and MS2 bacteriophage with an ultrafiltration membrane of nominal pore size of 0.035 microns and absolute 0.1 micron have demonstrated very effective removals. Giardia oocysts can vary from 4 to 14 microns in length and 5 to 10 microns in width; cryptosporidia oocysts range from about 4 to 6 microns. Intact microfiltra-tion (0.1 micron nominal) and RO membranes should completely remove the cysts. MS2 bacteriophage size is approximately 0.027 micron, which is smaller than the pore size of the membrane. Substantial virus removal is achieved probably due to adsorption of the virus on suspended particles, adsorption on the membrane, or from the secondary filtration due to fouling of the membrane surface. (Curiously, RO is noted for passing microbes and microscopic particles, usually assumed to be around end-seals and the like. In some instances, particle counts from RO exceeds particle counts from UF, depending on the configuration—spiral, dead end, etc. End seal leakage does not effect salt rejection by much.)
Distillation at high temperatures close to the normal boiling point of water would likely eliminate all pathogens. Reduced pressures are used in some desalination systems to reduce the boiling point and reduce energy demands. Temperatures as low as 50° C may be utilized. Several pathogenic organisms are denatured or killed in a few seconds to minutes at temperatures in the 60° to 80°C range, but spores require higher temperatures and longer times.
Since desalinated waters are lower in TOC than most natural waters it would be expected that the disinfectant demand and also disinfectant byproduct formation would be relatively low. This has been indicated in some studies of trihalomethane production that have been reported. However, this could be significantly affected by the type of blending water that is used post treatment to stabilize the water. One of the factors to consider would be the amounts of brominated organic byproducts that could be formed from predisinfection of salt waters containing bromide and from disinfection of blending waters. Some of these may reach the finished water. This is a concern since data is accumulating that some brominated disinfection byproducts have greater carcinogenic or toxic potential than many chlorinated byproducts. Indeed chloroform is not considered to be carcinogenic at levels typically found in drinking water. Since the TOC found in seawater could be different than TOC in fresh waters, it is also possible that there could be some differences in the chemistry of the byproduct formation reactions that could lead to some different byproducts or different distribution of byproducts.
Desalination of seawater and brackish waters offers the opportunity to significantly increase the world’s supply of fresh water for drinking and other purposes. Costs of production have been declining rapidly and further economies are expected. Due to the saline source waters and treatment processes involved, several issues in finished water composition and process arise that are not typically dealt with in conventional drinking water supplies. The World Health Organization is currently engaged in developing guidance aimed at the health and environmental issues associated with desalination technologies and applications so as to facilitate broader usage of desalination technologies.
- Desalination and Wastewater Treatment to Augment Water Resources; H.K. Sadhukhan et al, in Water Management Purification and Conservation in Arid Climates, Vol. 2, Matthews F.A. Goosen and Walid H. Shayya eds. Technomic Publishing Co., 2000.
- Immersed Membrane Technology for Parasite and Microorganism Removal in Surface Water Supplies; M. Singh et al, in Proceedings of the International Symposium on Small Drinking Water and Wastewater Systems, NSF International, Jan. 12-15, 2000.
- Membrane Separation Technologies; Peter S. Cartwright in Proceedings of the International Symposium on Small Drinking Water and Wastewater Systems, January 12-15, 2000.
- Membrane Techniques in Water Treatment and Renovation; Michal Bodzek, in Water Management Purification and Conservation in Arid Climates, Vol. 2, Matthews F.A. Goosen and Walid H. Shayya eds. Technomic Publishing Co., 2000.
- Monitoring of Trace Metals in Desalinated Drinking Water and their Permissible Levels; P.C. Mayan Kutty et al, Proceedings of the International Desalination Association, Vol. V, pp. 219-230, 1995.
- New Developments in Desalination; David H. Furukawa, in Providing Safe Drinking Water in Small Systems, J.A. Cotruvo, G. Craun and N. Hearne eds., pp. 257-264, Lewis Publishers, 1999.
- Seawater Desalination in California; California Coastal Commission, 1999.
- Studies in THMs Formation by Various Disinfectants in Seawater Desalination Plants; P.C. Mayan Kutty, Proceedings of the International Desalination Association, Vol. VII, pp. 367-399, 1995.
- The USAID Desalination Manual, PN-AAJ-122, USAID; 1980.
- Desalting Handbook for Planners, Report #72. US Bureau of Reclamation, 3rd Edition, July 2003
- Trace Metals in Ground Water RO Brine Water; A.I. Alabdula’aly, M.A. Khan in Proceedings of the International Desalination Association, Vol. V, pp. 573-596, 1997.
- Worldwide Desalination Research and Technology Survey; Ministry of Foreign Affairs, Sultanate of Oman, April 1994.
- World Health Organization Guidelines for Drinking Water; World Health Organization, Geneva, 3rd edition, 2004. www.who.int/water_sanitation_health/dwq/guidelines/en
- World Health Organization, Nutrient Minerals in Drinking Water and Potential Health Consequences of Long-Term Consumption of Demineralized and Remineralized and Altered Mineral Content Drinking Waters. WHO/SDE/WSH/04.01, 2004. www.who. int/water_sanitation_health/dwq/nutrients/en/
This assessment was derived in part from an assessment prepared for the World Health Organization’s Eastern Mediterranean Regional Office to investigate the appropriateness of developing WHO guidance for the health and environmental impacts of desalination practices. It was also derived in part from a contribution to a WHO Expert Workshop investigating nutrients in drinking water.
About the author
Dr. Joseph Cotruvo is President of Joseph Cotruvo & Associates, Environmental and Public Health Consultants, Washington, D.C. USA. His Ph.D is in Physical Organic Chemistry. He was Director of the Drinking Water Standards Division and the Toxic Substances Risk Assessment Division during his almost 25 years of service with U.S. EPA. He currently works extensively on drinking water quality, desalination, water reuse and water delivery systems and health science issues. Cortruvo also serves on several World Health Organization panels and on research panels with the National Water Research Institute and the WaterReuse Foundation. He is a member of the Agua Latinoamerica Comite Consultivo de Asesores Technicos and Vice President of AIDIS USA. Contact information: phone/fax: 1 202 362 3076, email [email protected].