Q. What is the status of desalination technology as a contributor to the U.S. and world drinking water quantity and quality?
A. It is substantial and growing rapidly; it is also costly compared to other sources but costs are tolerable where the alternatives are limited and need is sufficient.
Access to sufficient quantities of safe water for domestic, commercial and industrial applications is essential for public health, quality of life and economic development. Total world water is constant but a readily accessible supply is decreasing while demand is increasing, and periodic droughts are affecting more people. I won’t blame everything on climate change, which is the current fashion.
Many areas of the world including parts of the U.S. have inadequate water resources and accessibility. Population growth and population shifts to urban areas and increased consumption are concurrent with industrialization and improved economy and quality of life. The world has a lot of water; however, most of it is seawater or brackish and it isn’t always located where it is being consumed, so it is essential to utilize technology to provide more water where it is needed. Agriculture accounts for two-thirds to three-fourths of fresh water consumption. Per capita consumption of public water supplies varies widely with the mix of industrial and domestic uses, seasons and also because incentives for conservation are often absent due to governmental subsidies providing water much below cost of production — even in wealthy water short countries like the Middle East.
Per capita consumption varies widely. In countries such as Qatar, it is 430 liters per capita per day; in Germany it is about 100 liters and in the U.S. about 375 liters. However, drinking and cooking accounts for less than one percent of public water supplied to consumers’ taps. Demand from municipal supplies in the U.S. has been decreasing at the rate of about one percent per year for a variety of reasons.
“New water” is available from just three sources: Conservation, recycling and reuse and desalination. Conservation has its limits; recycling and reuse are being more widely practiced for agricultural, industrial and potable applications; and desalination offers the opportunity to access the world’s almost limitless saline waters. We will concentrate on desalination for this article.
What is saline water?
The salinity of natural water ranges from < 100 mg/l in fresh waters to approximately 35,000 mg/l in oceans to > 50,000 mg/l of total dissolved solids (TDS) in some seawaters (e.g. Arabian Gulf). A typical seawater source could contain about 19,000 mg/l chloride, 10,500 mg/l sodium, 2,600 mg/l sulfate, 1,250 mg/l magnesium, 400 mg/l calcium, 400 mg/l potassium, 150 mg/l bicarbonate and 80 mg/l bromide plus assorted lesser ions. A brackish water of about 3,500 mg/l TDS could contain about 900 mg/l chloride, 750 mg/l sodium, 1,000 mg/l sulfate, 90 mg/l magnesium, 250 mg/l calcium, 10 mg/l potassium, 380 mg/l bicarbonate and other ions.
History of desalination
Desalination applications on a large scale began in earnest about 60 years ago in the arid Middle East, where increasing oil revenues and increasing population made water essential to development as well as being financially and technologically accessible. Desalination plants were built and ultimately almost all of the water in that region is now supplied by desalination of water from the Arabian/Persian Gulf.
Introduction of desalination in the U.S. has been slow because most areas have access to natural fresh water sources at a much lower cost. However, there are some plants in operation, such as in Tampa, Florida. Southern California has had several groundwater desalting plants in operation for many years. Recently, a seawater desalination plant has been approved in Carlsbad, California, and others are planned.
As of late 2013, more than 17,200 desalination plants were in operation throughout the world with capacity for producing about 23 billion gallons per day and capacity continues to grow rapidly. Seawater desalination comprises about 59 percent, 22 percent is brackish water, 9 percent is river water and wastewater and purified water are at five percent each. The Middle East has the largest capacity with Saudi Arabia and Abu Dhabi being the leaders. Production in North America, North Africa, Europe, Australia and other parts of Asia is growing rapidly. The largest plant in the world is in Saudi Arabia at 270 mgd and sizes go down to a few hundred gallons.
There are three general types of desalination technologies: Thermal, membrane and electrodialysis variants. As implied, thermal processes involve some type of a distillation process; whereas membrane processes involve reverse osmosis (RO) or other novel developments like forward osmosis; and electrodialysis involves migration of charged ions to oppositely charged electrodes.
Thermal technologies are of several types: Multistage flash distillation (MSF), multiple effect distillation (MED) and vapor compression distillation (VCD). They all involve heat transfers between phases at lower pressures so that vaporization occurs followed by condensation in several stages, with the heat of vaporization being recovered at condensation and transferred to the liquid phase to cause additional vaporization. About 25 to 50 percent of the flow is recovered as fresh water condensate. These were the earlier desalination processes and are still dominant in the Middle East. They are energy intensive but efficiencies have been improving as the technologies are refined.
Membrane processes usually involve RO although some nanofiltration (NF) processes are used. High pressure (55 to 70 bars for seawater and 15 to 35 bars for brackish waters; one bar is 14.5 psi) is applied to the saline side of the membrane to force the reversal of the natural osmosis process, so that water flows from the concentrated side of the membrane to the low salinity side. The nominal pore size of an RO membrane is between 0.0001-0.001 µm, while a NF membrane is about 0.001 µm.
These are small enough to remove bacteria, protozoa and viruses in an intact system. In RO, salts and almost all organics as well as microbes are removed at up to approximately 99-plus percent. Organics with molecular weights less than about 100 daltons and some inorganic species like borates are not well removed. RO is more energy efficient than distillation processes, yet is still energy intensive, primarily due to the high pressure pumping requirements. Membrane technologies are more commonly used now even in the Middle East. Water recoveries can be in the 75 percent and higher ranges.
Electrodialysis is much less utilized but does have specialty applications. A direct current is applied to the water and that drives the ions through permeable membranes to electrodes of opposite charge.
Pretreatment by disinfection and filtration is required to protect the membranes and extend run time and reduce fouling that can occur from biofilms and particulate accumulation. Many of the membranes are sensitive to chlorine so it must be removed prior to contact with the membrane.
Post treatment is also required to reduce the corrosivity of the depleted water. Alkalinity and pH adjustments are necessary.
Desalination at a large scale requires intake of millions of gallons of raw water per day from the sea. That causes impingement, which occurs when aquatic organisms are entrapped against intake screens and entrainment, when smaller organisms pass through the screens and into the process equipment. Co-location with power plants is advantageous by use of a common intake pipe, waste heat transfer from the power plant to the desalination process and dilution of the brine with cooling water before discharge.
Reject brines from RO and residue concentrates from thermal processes are at least double or triple concentrated from the source water. Brine disposal is one of the most difficult problems associated with brine management. At seawater plants, the brines are returned to the sea via long pipes to reduce mixing zone problems that can have environmental consequences. Inland plants using brackish groundwaters must either mechanically concentrate the brines to solids for land disposal, or use lined evaporation ponds or deep well injection.
The costs for thermal and membrane processes are highly dependent upon the desalination method, salt concentration, fuel costs, construction costs, inflation and local conditions. Membrane processes are lower in cost than thermal and most newer plants use membranes. My rough back-of-the-envelope production cost-estimate is in the range of approximately $4 per thousand gallons for membrane plants.
Desalination produces very high quality water equivalent to rain water or distilled water. Salts residues in treated water are well below 100 mg/l, and organic chemical residues are in the ppb or ppt range for those lower molecular weight natural and synthetic chemicals that are not efficiently removed. Thus, total organic carbon levels are fractions of ppm; disinfection byproduct formation will also be very low after subsequent residual disinfection. The water will be corrosive to most transmission pipes so corrosion control processes are essential.
Home water desalination treatment devices are also readily available in the U.S.; however, these are usually very small-scale, low-pressure RO devices applied to public water as a supplemental treatment for TDS reduction or softening. The low-line pressures and membranes make them very inefficient with very high water rejection rates. POU has applications for softening, but POE is not used because of corrosivity on home plumbing. There are, however, many industrial applications where very high quality, low TDS water is required and also where scale formation must be controlled.
Because desalinated water has most of its minerals removed it is important that dietary intakes provide all of the necessary minerals. Water is an efficient source because of a higher uptake percent of minerals versus food intake. There are also many natural surface waters that are soft and very low in dissolved minerals like calcium and magnesium. Water consumption usually is only a portion of the important dietary minerals, such as calcium, magnesium and potassium, but it might provide an important baseline increment. Calcium and especially magnesium are essential elements that can impact chronic diseases like osteoporosis and diabetes. There are studies indicating that consumption of drinking water with magnesium has shown a beneficial reduction in cardiovascular disease mortality. This is consistent with well-known effects of magnesium on cardiovascular function. Israel is one of the countries that require remineralization with calcium, magnesium and other ions.
Desalination is a significant and rapidly growing source of new water where the need is sufficient and the costs are acceptable. Costs are considerably greater than drinking water produced from natural fresh waters but that is partly because natural waters are often free, require much less treatment and their costs are artificially low because of subsidies. The saline source waters are widely distributed and almost limitless. There are environmental issues that require management but their control is feasible and post treatment is essential to control corrosivity. There are some indications that low, specific mineral intake from water could be negative to overall health if dietary intakes are not sufficient.
Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.