National Drinking Water Clearinghouse
West Virginia University
P.O. Box 6064
Morgantown, WV
26506-6064


Q&A:
What is desalination?
Desalination is a process that removes dissolved minerals including salts from saline water and produces potable water.

Desalination is a natural and continuous process and a part of the natural water cycle.Seawater evaporates and returns to earth as desalinated rainwater.

by Babu Srinivas Madabhushi
URS Corporation

Why desalinate?
Desalination has become increasingly important in the last four decades due to the tendency, in recent years, for the world’s swelling population to dwell in areas where supplies of high quality fresh water sources are less than adequate. Moreover, as there are a considerable number of saline sources available, desalination is becoming an attractive possibility. More than three quarters of the earth’s surface and more than 95 percent of the world’s water is either salty or brackish, and therefore not potable. Desalination increases the range of water resources available for use by communities.

According to Ron Linsky of the National Water Research Institute in California, “the country needs to improve the reliability of its supply. Water could be on the commodities market in the next century. We may ultimately face rationing or importing.”

Can I drink desalinated water?
Yes, desalinated water can be used for drinking purposes, groundwater recharge, and also for irrigation. During the Persian Gulf War, the U.S. Army used mobile units that could produce 3,000 gallons of potable water per hour from brackish water sources. If the desalinated water is to be used for domestic use, post treatment should be done to ensure that treated water meets the health and anti-corrosive standards.

How do I desalinate?
A number of technologies, such as the membrane process, distillation, and vacuum freezing, have been developed to perform desalination. Distillation and the membrane process are the techniques used the most. Nearly 60 percent of the world’s desalination systems use distillation, heating thesalty water to produce water vapor that is then condensed to form fresh water.
What are the distillation techniques?

The most commonly used distillation methods are multistage flash distillation (MSF), multiple effect distillation (MED), and vapor compression (VC). In MSF, the raw water is heated and pressure is lowered, so that the water flashes into steam. This process is carried out in a number of stages, in which the pressure is gradually reduced. In the MED process, the raw water passes through a number of evaporators connected in a series. Interestingly, the vapor in the earlier stage is used to evaporate the water in the later stages. In the VC process, the raw water is evaporated, the vapor produced is compressed, and the compressed vapor is used again for heating the additional raw water. VC is generally used for small and medium scale desalination units.

Can RO be used to desalinate?
Yes, there are several membrane processes, such as reverse osmosis (RO) and electrodialysis, which can be used for efficient desalination. In RO, the water is pumped through semipermeable membranes to separate and remove the minerals from the saline water. Before being pumped through the membranes, the water is pretreated to remove any particles that might clog membrane pores. Suspended solids in the raw water can be removed by using coagulation and filtration.

What is the desalinated water quality?
Distillation plants produce water with total dissolved solids (TDS) concentrations ranging from 1 to 50 milligrams per liter (mg/l) and RO plants produce water with TDS concentrations ranging from 10 to 500 mg/l. The quality of the treated water depends on the concentration of salts in the raw water, and, in the case of RO, the membranes selected and the pressure applied. In distillation, the quality depends upon the heating system, the pressures applied, etc. Quality of treated water can be improved by repumping the water through the membranes.

Why is pretreatment required?
Pretreatment is required to avoid substances that could interfere with the process and reduce desalination efficiency. Bacteria can grow in RO and distillation plants, hence disinfection may be required. Metals should be removed from the raw water as they may contribute to corrosion problems in distillation plants.

Comparison of Distillation and RO
According to some sources, the distillation process produces better quality water more economically than does RO. There is no break in operation for cleaning and replacement. Pretreatment requirements are fewer for distillation, as coagulation and filtration have to be carried out to prevent clogging of membrane pores in RO. Moreover, distillation plants do not produce secondary wastes from backwash of filters used for pretreatment. But, in the case of RO, no heating is required, and hence, less energy is required. No corrosion problem is faced. Simultaneously, RO can remove other contaminants and requires less space for installation of the equipment.

Is any secondary waste produced?
Desalination plants produce liquid wastes with high concentrations of salts, metals, etc. Liquid wastes may be discharged in the ocean after making sure that they satisfy the discharge requirements. Otherwise, they have to be treated and disposed.

What is the energy requirement?
The desalination process uses mainly electricity and heat. The energy required depends on the raw water quality, temperature and the treated water quality, and on the process used.

How much does desalination cost?
For brackish water, the treatment costs may range from approximately $1 to $2.40 per 1,000 gallons. The cost for sea water desalting is estimated at $4 to $16 per 1,000 gallons.

References
Buros, O.K. 1990. “The Desalting ABCs.” Saline Water Conservation Research Department. Riyadh, Saudi Arabia.

Porteous, Andrew. 1983. Desalination Technology, Development, and Practice. Applied Science Publishers.

Howe, Everette D. 1974. Fundamentals of Desalination. Marcel Dekker Inc.

Bates, Lincoln. 1995. American City and County. “Water Issues Prompt New Look at Desalination.”

Babu Srinivas Madabhushi was previously a technical assistance specialist with the National Drinking Water Clearinghouse. This article was reprinted from the Winter 1998 On Tap.