This report has (1) missing figure and (2) volatile references. Author must fix. - RBL

Fig. 1: The phase diagram for water. The y-axis is pressure measured in Pascals, the x-axis is temperature measured in Kelvin. Source: Wikimedia Commons.

Americans on average consume ~150 gallons of freshwater from municipal sources per person per day. Factor in industrial and agricultural use and the amount of freshwater used per capita in the US rises to 1,110 gallons per day [1]. Despite its tremendous importance to our lives freshwater makes up only 2.75% of all the water on earth; 25% of that amount being found as groundwater, 0.3% as surface water, and the majority of the remainder being contained in the ice caps and glaciers. We can define freshwater for human consumption as based on the World Health Organization's guidelines for the level of Total Dissolved Solids (TDS) that should be allowed in potable drinking water as no more than 1,000 parts per million (ppm). [2] For agricultural use the level of TDS needs to be much lower at ~50 ppm. [3] TDS is a measure of grams per liter of dissolved solids present in a sample of water and is most often inferred by taking a measurement of the water's electrical conductivity (EC) and extrapolating for TDS. Beyond 1,000 ppm freshwater begins to become saline, with the water in the ocean having a TDS content of ~35,000 ppm.

The agricultural need for fresh water is imperative in order to maintain crop yields and preserve soil quality. Many vegetable crops begin to experience diminished yields as TDS levels begin to exceed ~300 ppm. [4] Retention of salts in soils diminishes crop performance by creating ion toxicities, nutrient lockup, and desiccating osmotic effects. As modern farming practices are additionally dependent upon chemical fertilizers in order to maintain large crop yields, copious amounts of freshwater are required in order to leech these excess salts from the soil to prevent toxicities.

Desalination is the process by which salt is removed from saline water in order to purify it either for human consumption, or agro-industrial use. Given that the oceans make up 97.25% of all water on earth, and cover approximately 70% of the surface, for arid coastal regions such as California and the Middle East a vast source of fresh water could become available through the desalination of seawater. As of 2005, only about 1.5 of the 1,110 gallons of fresh water consumed per capita, per day in the United States comes from desalination. [1] We will examine two of the primary methods of desalination - distillation and reverse osmosis.

Distillation

Distillation is the oldest and most commonly used method of desalination. In distillation a substance is first heated to its vaporization point where the vapor is then collected and passed through a cooling system where it recondenses as the purified substance. The main method of desalination distillation is the multistage "flash" evaporation (MSF) technique. Converting water from its liquid phase into its gaseous water-vapor phase can be done by either increasing its temperature or reducing its pressure, or both, as can be seen in Fig 1.

In MSF systems, saltwater is vaporized through both the application of heat (boiling) and the reduction in pressure (flashing). The saltwater is passed through a series of chambers, each with a lower pressure and temperature than the previous, where it is flash vaporized. The water vapor carries heat with it cooling the saltwater and is collected and removed from the chamber. The subsequent chambers then repeat the process on the cooler water, now using lower pressures to induce flashing. The hot water vapor is then cooled and condensed using a coolant system and passed for further treatment. As the vapor from earlier stages is hotter than the water at later stages it can be used in heat exchange processes to both cool the vapor and heat the saltwater. Coolant water is then either discharged or if saltwater was used the now warmer coolant can then be heated and passed through the MSF system itself. The leftover brine concentrate is then typically pumped back into the ocean or into evaporation ponds as determined by the proximity to the ocean. The distilled water from a MSF style desalination plant typically has a TDS level of < 25 ppm making it ideal for both human consumption and agricultural use.

Reverse Osmosis

A modern method for water purification is through reverse osmosis (RO). Osmosis is the process which occurs when a purer solvent (in this case freshwater) is separated from a less pure solvent (seawater) containing a solute (salt) by a selectively permeable membrane. The selective membrane is porous enough to allow the smaller molecules of the solvent to pass freely through the membrane but is impermeable to the larger molecules of the solute. Both the solvent and solute seek to balance their respective concentrations across the membrane, though only the solvent is able to pass freely from one side to the other. This results in an osmotic pressure gradient in which only the purer solvent moves across the barrier into the less pure solute as the overall concentration (purity) tries to equalize. In the process of reverse osmosis pressure in excess of the osmotic pressure (~350 psi for seawater) is instead applied on the impure side of the membrane to reverse the normal osmotic flow and increase the quantity of pure water on the other side of the membrane. In desalination systems the now more concentrated brine is discarded as waste water and replaced with "fresh" seawater, while the purified freshwater is passed on to further treatment.

A commercial RO plant requires first a pretreatment phase in which solids are filtered out of the system, the pH is adjusted, and compounds are added to prevent scale build up in the system. The incoming water must then be pressurized to the proper levels in order to maintain the reverse osmotic flow; 250-400 psi for brackish water (~10,000 ppm), 800-1000 psi for seawater. RO systems typically have output TDS levels of ~500 ppm from saltwater, and < 100 ppm from brackish water. This means that saltwater RO systems are suitable for human consumption but not for agricultural uses, while many brackish water RO systems can be used for either.

Comparison of the Two Methods

The energy required for MSF systems is ~90 kWh/kgal and is nearly all in the form of heat. [5] This makes MSF style plants ideal for cogeneration in which the desalination plant is integrated with a power plant. This can be done as either a stand alone unit where the power plant only generates heat for the MSF plant to desalinate water, or as a combination desalination and power generation facility. As many industries produce copious amounts of waste heat, distillation style desalination plants can be incorporated into many different industrial facilities.

As the osmotic pressure increases with the TDS of the water to be desalinated, RO systems purifying seawater require greater incoming pressures than those treating only brackish water. As such the operating costs of RO systems increase with the salinity of the water being treated. The Singapore-Tuas Seawater Desalination plant is a seawater reverse osmosis plant finished in 2005 with a capacity of ~29 million gallons per day. The plant achieves an energy rate of ~16 kWh/kgal and was able to produce water in its first year at the rate of ~$600/acre foot, comparable with quoted prices for coastal communities in California. [6,7] The Ashkelon Sea Water Reverse Osmosis plant in Israel is able to produce ~80 million gallons per day for approximately the same cost per acre foot.

Despite the higher cost of MSF desalination compared to modern RO plants per unit water produced, the quality of the MSF water is upwards of an order of magnitude more pure than those from RO. While RO is still able to produce water fit for human consumption, MSF facilities are required in order to provide desalinated water fit for agriculture.

© Jesse Sherer. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Country Fact Sheet: United States of America," Food and Agriculture Organization of the United Nations, 1 Dec 10.

[2] "Guidelines for Drinking-water Quality, 3rd Edition," Volume 1: Recommendations, World Health Organization, 2004, pp. 444-445.

[3] L. J. Summers, "Desalination Processes and Performance," Lawrence Livermore National Laboratory UCRL-ID-120367, June 1995.

[4] S. R. Grattan and J. D. Oster, "Water Quality Guidelines for Vegetable and Row Crops," California Department of Water Resources, Drought Tips Number 92-17, 1993.

[5] A. R. Hoffman, "The Connection: Water and Energy Security," Energy Security, 13 Aug 04.

[6] "Tuas Seawater Desalination Plant - Seawater Reverse Osmosis (SWRO), Singapore," water-technology.net.

[7] "Ashkelon Seawater Reverse Osmosis (SWRO) Plant, Israel, water-technology.net.