Seawater Desalination

Seawater desalination to convert seawater into potable water is being used in many parts of the world. Reverse Osmosis process using thin-film composite membranes has evolved over the last 20 years and has brought down the cost of desalination. Major improvements in the membranes, energy recovery, pumps and pressure vessels have brought down the cost of desalinated water significantly.

The key technology in the desalination process is Reverse Osmosis. In this process sea water is forced against semi-permeable membranes under pressure in a continuous flow condition. The high salt content of sea water requires that the operating pressure for Reverse Osmosis must be between 60-70 bar. As the water permeates through the membrane most of the dissolved impurities removed and 99.5% of the total salt is removed. The impurities are left behind in the flowing water and the concentrated stream from the membranes is discharged to the ocean. The design of the complete system must optimize the flows, the area of the membranes and other conditions to keep the system operation at the highest efficiency possible.

Applied Membranes has installed a number of Reverse Osmosis Sytems for seawater desalination. A typical system consists of filtration, UV, chemical injection followed by reverse osmosis Membranes. The Table below gives typical performance of a sea water system:

Water Quality of Seawater RO
Recovery: 45%
Operating Pressure: 900 PSI

	Seawater (ppm)	Concentrate (ppm)	Permeate (ppm)
Sodium, Na	10,967	19,888	64
Potassium, K	406	736	3
Magnesium, Mg	1,306	2,372	2
Calcium, Ca	419	761	0.5
Bicarbonate, HCO3	109	194	0.9
Chloride, Cl	19,682	35,771	105
Sulfate, SO4	2,759	5,014	1.5
TDS	35,666	64,771	176
pH	7.8	7.7	6.1

A successful desalination system requires proper understanding and design to overcome the high salt content and large number of micro-organisms present in the seawater. The high degree of turbidity and corrosiveness of the seawater also requires measures to overcome these. Experience in seawater desalination and recent improvements in energy recovery and low energy membranes has brought the cost of seawater desalination down as shown below:

Operating Costs – Desalination

	Per Cubic Meter of Permeate (US $)
Power at 10¢/KWH	$ 0.33
Membrane (3 Year Life)	$ 0.05
Chemicals	$ 0.05
Misc.	$ 0.03
Total	$ 0.46/Cubic Meter

Not Included in the Cost:
• Intake Water
• Concentration Discharge
• Building Cost
• Amortization of Equipment
• Labor to Monitor & Maintain

The future of sea water desalination looks very good. The problem of a lack of potable water and increase in drought in many parts of the world in coastal areas can be solved by sea water desalination. Hundreds of seawater systems are producing drinking water or process water for municipalities, resorts, hotels, off-shore drills, ships, yachts and military use. The size of these systems varies from 100 gallons per day to millions of gallons per day.

Conclusions:

RO Desalination of seawater will find more applications because of the lower cost of desalination.
Proper design, operation and maintenance are essential to reduce these costs further.
Careful consideration must be given to both the intake of sea water to the system and the discharge of the concentrate from the RO membranes.