Voices From The Podium: Water Resources and Desalination

By Dr. Noam Lior, Triangle Faculty Advisor and Professor, Mechanical Engineering and Applied Mechanics (University of Pennsylvania)

Every issue of the Triangle will include a piece written by a member of Penn’s faculty on any topic of their choosing.

“…many of the wars in this century were about oil, but wars of the next century will be over water"
World Bank's vice president for Environmentally Sustained Development, 1995

The water problem
Consumption
Rising population, standards of living and water pollution are diminishing the amounts of naturally available fresh water of good quality while the demand is increasing relentlessly. Consumption is increasing at 4-8%/yr , 2.5-times the population growth.

For example, agricultural irrigation, which constitutes 70% of the fresh water use, increased 7-fold over the past century. Furthermore it is typically done wastefully, sometimes only 15% efficient, where the remainder doesn't reach the plant roots and is lost through leaks and evaporation. The amounts of water needed for agricultural products are enormous: typically production of the grain for a loaf of bread requires more than a ton of water, and a 1000-3000 weight ratio is typical of vegetables and fruits too. Worse than that, a kilogram of beef requires the use of 4 to 108 tons of water (depending on whether the cattle is range or farm raised), and an egg needs 480 kg. Not only does agriculture consume these enormous quantities of water, but the associated ground water depletion combined with the addition of fertilizer and pesticides caused 100 million acres to be sterilized by excessive salination over the past 50 years or so. It is not surprising that much (but not nearly enough) research is conducted to reduce agricultural water consumption, including areas such as improved root absorption, reducing evapotranspiration from leaves, tissue-culture research with protoplast fusion and recombinant DNA technologies to reduce plant stresses and increase photosynthesis efficiency, leaf sparing, soil waterproofing, hydroponics, and the development of plants which extract and use the water from saline water solutions. All of these serve as extremely interesting and rewarding challenges for scientists and engineers.

Industry accounts for about 21% of the water use. About 600 tons of water are used to make a ton of fertilizer, 150 - 240 tons for a ton of steel, 480 tons for a ton of gasoline, 1000 tons for a ton of acetate fiber, but little if any of this is required chemically. Recycling can thus reduce industrial requirements by a factor of 10-50. The electric power industry in the United States has passed through its heat exchangers in 1990 about 40% of the total supply of surface water, a quantity similar to that used for agriculture, and it was 48%of the combined fresh and saline water withdrawals. Single power stations producing 1000 MW may heat as much as 12 Mm3/d (3.2 x 109 gal/d) by as much as 10-15oC. Due to their much lower efficiency, water-cooled nuclear power stations use 50% more water than fossil-fueled fired ones, and they would use twice as much water as the efficiency of the latter rises now to 60%.

Water use is not only high, but is also highly inequitable: 9% of the population consumes ¾ of the world fresh water, and half a billion people are undersupplied, a number which is expected to rise to 1.5 billion in 25 years

The water problem is compounded by deforestation and poor land development practices that lead to (1) floods, resulting in 100,000 deaths per year, poor replenishment of running and ground waters, and climate degradation, (2) desert gain of 15 million acres/yr, and (3) making 1/3 of the exposed land unsuitable for agriculture.

Pollution
20 billion tons of refuse end up in oceans each year. Besides the everlasting plastic refuse which chokes sealife, the waste includes many very harmful species, bacteria, virus, heavy metals, pesticides, fertilizers, hydrocarbons, and the ingredients of acid rain produced from emissions from fossil fuel combustion in cars, power and chemical plants, home furnaces, etc. SOx and NOx emissions acidified rain the U.S. from pH 5.6 (clean), to less than pH 4, rather acidic.

95% of the world's urban areas dump raw sewage into rivers and other water bodies, killing 10 million or more people every year and causing great economical damage. In the U.S. for example, 40% of the rivers, lakes and estuaries are too polluted for even fishing or swimming. The Great Lakes contain 1/5 of the world's fresh surface water, but 97% of it is substandard.

Resources
Fresh water is not only scarce, but is also one of the most nonuniformly distributed natural resources. 15% is in the Amazon basin, which has only 0.3% of world population, while 60% of the land mass has perpetual water scarcity. The Table below demonstrates some of the ill-distribution among different countries.

Country - Annual renewable fresh water per person, m3
Iceland 666,667 (highest in the world)
Canada 108,900
Brazil 46,631
United States 9,913
Japan 4,428
France 3,262
United Arab Emirates 308
Saudia Arabia 306
Bahrain 179
Kuwait 75
Djibouti 23 (lowest in the world)
Some water distribution statistics: on average these values were twice as large just 50 years ago, and are foreseen to diminish to about half again in 20 years.

It is noteworthy that a few of the most water-deficient countries are oil-rich and thus can easily afford to produce fresh water by desalination, while most of the water deficient countries can't.

Consequences
Eighty countries with 40% of the world's population have water shortages which could cripple agriculture and industry, and the developing countries need $600 - $800 billion for water projects over the next ten years, with no identifiable available funding sources. It might be a shocking revelation that two out of five U.S. cities have inadequate water supplies, and that at least a quarter of the U.S. population faces serious water shortages. A case in point is the rapidly growing population in Southern California which essentially is a desert, and thrives only because it imports increasing amounts of water from increasing distances, as far as a 800 miles away, depleting the water from the source regions. The mighty Colorado river, for example, is thus reduced to a toxic trickle by the time it flows into Mexico.

Water shortages have a severe detrimental effect on sanitation and health: more than 2 billion people lack sanitary facilities, and water-carried illnesses kill 25 million people per year in developing countries, and sicken many more, often permanently.

In the frenzy of development, ground water was often over-pumped. Some of the consequences in our country are that in some places the water table has dropped by 1-5 m for each year of the present generation, thus exhausting a historical non-renewable treasure. To add insult to injury, this also resulted in soil subsidence, for example in Houston, Mexico City, and Florida; California San Joaquin Valley sunk by 9 m in 80 yrs, and Las Vegas sunk by ~1 meter recently (I am sure the avid gamblers never even noticed...). Overpumping of ground water in locations close to the sea or saline water aquifers results in the intrusion of saline water into the fresh water aquifers, making them unusable. This is taking place in many coastal locations, especially in Orange County, California.

The increasing consumption and desire for improvement of the standard of living, opposed by the water shortages, and the highly nonuniform and inequitable distribution of the water resources inevitably leads to both national and international conflict. Some of the hot spots are the dying Aral Sea (Uzbekistan, Turkmenistan, Afghanistan), the Indus River (Kashmir: India and Pakistan), the Jordan River (Jordan, Palestine, Israel; Syria), the Tigris and Euphrates Rivers (Turkey, Syria, Iraq; the Arabian/Persian Gulf), and the River Nile (Ethiopia, Sudan, Egypt; the Mediterranean). Many other serious disputes exist, such as those between the U.S. and Mexico over the Colorado River and the Rio Grande, that are being dealt with peacefully, and there is no doubt that some in the water-short U.S. look with some interest at the water-rich Canada (see Table above).

Remedies
Conservation
The first remedial measure to implement is always conservation. In the ongoing “orgy of water wastage”, this should be relatively easy. In the U.S. for example, unmetered urban use tops at ~1 m3/day/person, and metering reduces it about 3-fold. Legal restrictions, e.g. lawns, car washing, restaurant drinking water…, reduce consumption too. Many places in the world, and close by in the Caribbean islands, don't allow the scarse rain water to be wasted, and employ rain catchment and storage systems. In water-short countries there is also wide use of “gray” (not potable but suitable for other purposes) or saline water where clean water is not needed (dual water systems)

Remediation and reuse
The technology of water treatment for reuse is well established and employed world-wide, but not sufficiently. Cities with good water treatment systems use water that has been through sewers upstream as many as 14 times. It is noteworthy though that hard contaminants (that do not ferment or oxidize under ordinary sewage treatments) increase proportionally with reuse of the water, and some are carcinogens, and those must be removed other means, such as by desalination processes.
As an estimate for needed expansion, just in the U.S. the need for sewage treatment is larger than $100 billion.

Water reuse for manufacturing is 1.3, to rise shortly to 6.7, but the potential is up to about 50.

Desalination: water production.
"Water, water, every where, Nor any drop to drink." was the lament of the shipwrecked sailor stranded in the ocean in The Rime of the Ancient Mariner by Samuel Taylor Coleridge. On this blue planet, it is indeed frustrating to note that only about 0.06% of the total water is available for human consumption, with the oceans being an immense water resource holding about 97% of the earth's water. The oceans, as well as many surface and ground water sources contain more salt than is fit for human or agricultural consumption. Potable water should contain no more than about 500 parts per million (ppm) of dissolved salts, where typically such water are supplied at salt contents smaller that 100 ppm; while the salinity of the oceans ranges from about 20,000 to 50,000 ppm. A logical solution is to extract fresh water out of saline, a process started in antiquity and called water desalination. In fact, nature indeed has a continuous desalination process where ocean water is heated by the sun to generate vapor, which rises due to natural convection, condenses in the colder atmosphere forming clouds, which then return fresh water to earth in the form of rain, hail, and snow.

Access to seawater is rather available, for example about half of the U.S. population has direct access to ample seawater. Clearly, however, the seawater should not be overly contaminated if it is to be used for desalination.

There are many water desalination methods (for more information see some of the references at the end of this article), which may be classified by the thermodynamic force that drives them: temperature difference, which are the various distillation processes such as multi-stage flash (MSF), multi-effect (ME), vapor compression (VC), and membrane distillation (MD), and and freeze-separation desalination (FD); pressure difference, which is reverse osmosis (RO) using semi-permeable membrane that allow transfer of water but not of the salt in the solution; electric potential differences, which is electrodialysis (ED), using membranes that allow transfer of salt ions but not of water and adsorption potential differences, which is ion exchange (IE). In fact, an early reference to the scientific or miraculous conversion by Moses of bitter ground water to fresh, viz. "...and the Lord showed him a wood and he put it into the water and the water became sweet," is made in the Old Testament (Exodus, 15, 22-25), and those who don't believe in miracles opined that the "wood" was a tree or other plant with ion exchange properties, that absorbed the salt from the bitter waters.

6 x 56,800 m3/d Dual-Purpose MSF, Al Taweelah B, UAE (courtesy Italimpianti)

2 x 5,000 m3/day MED units, St. Croix, USVI (courtesy IDE Technologies, Ltd)

8 x 5,000 m3/day seawater RO stacks, Okinawa; 63 modules/stack, 6 elements/module; (courtesy Japan Water Research Center)

The choice of the method is at the end one of economic superiority, which also depends on the nature of the saline water resource. Distillation processes produce nowadays about 2/3 of the desalted water in the world, primarily by MSF, but RO is catching up because it consumes less energy.

The minimal energy requirements for ideal water desalination can be determined from thermodynamics to be between 2.55 MJ/(ton fresh water) for infinitesimally small concentration change, and up to about 3 MJ/(ton fresh water) for 3.45% seawater. The energy consumption in exiting plants is much higher, 18-33 MJ of mechanical/electric power per ton water produced in RO processes, and 120-280 MJ of heat plus 15 MJ of mechanical/electric power per ton when using MSF. For whatever it's worth, producing a barrel of water thus requires the use of about one to seven thousandths (0.001 to 0.007) of a barrel oil in energy, and while the number looks very small, it is good to recall that people in most places would hardly think twice about spilling or using a barrel of water.

Obviously, renewable energy can be used for desalination, and sometimes is. Solar heat can be applied to operate all heat-driven plants, and solar photovoltaic cells and windmills can provide electricity for driving pressure or electricity-driven process. A well-known device is the solar still, shown in the Figures below, which combines solar energy collection with water desalination, but the small solar flux typically requires 500 m2 of single-effect still for producing 1 m3 fresh water per day.

A student competition for designing and building a cost-efficient solar still in Dr. Lior's course, MEAM 100: Intro to Mechanical Engineering

Compared with some other chemical processes, water desalination faces several unique technological challenges, led by the need for an extremely low price of the product, which is down to $0.5/ton now in some of the newest plants, corrosiveness of the saline water, scale deposition of precipitates on the equipment, and organic fouling.

A relatively new and increasing concern is desalination plant security. Since ingestion into the plant of contaminated saline water feed (oil spills, heavy metals, etc.) affects both product and plant, plants should be protected not only from poor management and accidents, such as the occasional spills of oil, waste, and surfactants in the vicinity of the seawater intake, but also from war and terrorism. A sad example is the intentional dumping of oil into the Gulf by Sadam's people with an objective to incapacitate the water desalination plants of his near and far neighbors, and the sky-darkening pollution generated by their ignition of the Kuwaiti oil wells (see photo).

It is thus vitally important to design the plants with robust safeguards against ingestion of undesirably contaminated saline water, ensure that regional resources' management prevents such contamination, provide for adequate fresh water storage, and provide adequate plant security. This of course increases the cost.

Desalination has become a rather established and wide-spread industry, with nearly 11,000 plants producing at least 100m3/day in operation all over the world (including the U.S.). The largest, the Al Taweelah B MSF plant in Abu Dhabi, United Arab Emirates, produces 341,000 m3/day. As many others, t is a dual-purpose plant, which at the same time produces also 732 MWe of power. The produced water price by desalination processes is between $0.54 and $2/m3, and there are about 1,700 manufacturing companies.

Policy
Responsible policy on the local, and more importantly on the national and international levels, is of key importance in sustaining our water resources. Addressing for example agriculture, which is the main water consuming activity, poor policies make agriculture a major threat to water. For instance, the political power of farmers encourages and maintains agriculture in regions where it is unsustainable due to water scarcity. Southern California, and the high plains regions in central and western U.S. are a strong case in point. Nations are also concerned abut their food security, and encourage untenable agriculture by offering its farmers water at unrealistically low prices through subsidies, absence of water contamination penalties, and imposition of trade barriers on foreign agricultural products. An apple grown and sold in Japan costs about $5, and I don't even wish to mention the price of Japanese grown rice relative to the world market. And Japan is just one example, and not the best.

Furthermore, the “haves” who can afford such subsidization (~350 b$ in Europe and North America), thus prevent the “have nots” who do have sustainable agricultural conditions from using heir products competitively, thus not only endangering the water supplies but also enforcing poverty.

From grass-roots to political leadership, the world is rather aware of the water problem and many hand-wringing high level international water forums have been held (the major ones created by the World Water Council, http://www.worldwatercouncil.org): 1997 in Marakesh, Marocco, 2000 the Hague, the Netherlands, 2003 Kyoto, Shiga, Osaka, Japan (24,000 participants, 1000 journalists,130 ministers (as in "cabinet secretaries")!).

A key Forum Statement was “…the current levels of investment fall short of what is needed to meet the Millennium Development Goals for water and sanitation services. It is not clear whether the current rate of investment allows for even simple replacement of worn-out infrastructure.”

R&D Funding is about 0.1% of the projected capital expenditures for water projects in the next 10-15 years… Can one say more?

The Ministerial Declaration confirmed that water is important and that many things should be done to remedy the problem... I am not sure whether frequent and solemn prayer was suggested, but money wasn't committed. The G8 followed with an Action plan:

“Promoting good governance, education
Utilising all financial resources
Building infrastructure by empowering local authorities and communities
Strengthening monitoring, assessment and research
Reinforcing engagement of international organisations” (UN, World Bank)

but it remains to be seen whether adequate resources would indeed be committed.

For the rising number of people who would like to see less government involvement in the citizens' affairs, it should be pointed out that government intervention is a least in some cases (besides war) of critical importance and value. Related to the water problem, the grass roots political movements that arose from the Vietnam war era forced two of the brightest gems of our government's legislation in the last century. The 1970 Clean Air Act brought, by 1990, the SOx emissions down by 10% and the NOx emissions down by 6%, with a consequently slight decrease in rain acidity, despite the large increase in fuel use. The Clean Water Act of 1972 launched to "restore and maintain the chemical, physical and biological integrity of the Nation's waters," resulted in a significant slowdown of the pollution processes and in many cases reversed the trend. These and other related acts are under continuous attack by special interests, and ought to be defended vigorously.

Conclusions and outlook
The world needs sustainable management of water resources to meet the needs of a much larger human population, sustain the life support systems of the planet, and substantially reduce hunger and poverty

If the current trends persist, many human needs will not be met, life support systems will be dangerously degraded, and the numbers of hungry and poor will increase. (see "Our common journey" in the reference list below)

The water problem poses incredibly interesting and rewarding challenges in science and technology, and requires policies that are based on sustainability.

Water is an extremely abused natural resource, and there is increasing awareness of the problem, but actual action and financial commitments are still woefully inadequate. It seems that it will have to get much worse before the situation is stabilized, let alone improved, and we should strive to ensure that the attempt to reverse this ominous trend does not occur too late.

Some references for further reading
N. Lior and R. Bakish, "Water, Supply and Desalination", Chapter in the Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, Vol. 25, John Wiley and Sons, Inc., 1998, pp. 438-487.
N. Lior, "Water Desalination", chapter 4.1 in The CRC Handbook of Thermal Engineering, Editor: F. Kreith, pp. 4-13 - 4-19, CRC Press, Boca Raton, FL, 2000.

National Academy of Science, "Our common journey, a transition toward sustainability", National Academy Press, Washington DC, 2003.

N. Lior, "Water Desalination", chapter 20.6 in The CRC Handbook of Mechanical Engineering, Editor: F. Kreith, pp. 20-59 - 20-76, CRC Press, Boca Raton, FL, 1998, and in The CRC Handbook of Mechanical Engineering, Second Edition, 2004.

A.M. Alklaibi and Noam Lior Membrane-distillation desalination: status and potential, Desalination, 171 (2004) 111-128