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Types of Desalination

Thermal
Multi-stage Flash
Multi-effect Distillation
Vapor Compression Distillation

Membrane
Reverse Osmosis
Electrodialysis
Electrodialysis Reversal

Middle East Desalination

Huntington Beach Proposal

Sources Cited

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Reverse Osmosis


To understand reverse osmosis, it is probably best to start with normal osmosis. Osmosis is the movement of a solvent through a semi-permeable membrane into a solution of higher solute concentration. This action tends to equalize the concentrations of solute on the two sides of the membrane.
The reverse osmosis process uses pressure as the driving force to overcome the osmotic pressure of the salt solution.

History of use
Relatively new process that was commercialized in the 1970s.
Currently is the most widely used method for desalination in the United States.

A reverse osmosis plant consists of four major systems:
Pretreatment system
High pressure pumps
Membrane systems
Post-treatment

Mechanism
Pretreatment is very important because membrane surfaces need to remain clean.
o All suspended solids must be removed
o Microbial bacterial must be removed
o Processes include coagulation/flocculation/sedimentation
High pressure pumps supply pressure from about 150 psi for brackish water to 800-1,000psi for seawater
Membrane materials consist of cellulose acetate or of other composite polymers.
o Pressure applied to feed water causes clean water to permeate across membrane into central colleting tube.
o Salts are rejected from the membrane and separation is complete.

Post-treatment consists of stabilizing the water by adjusting the pH and disinfection.
 

image source:http://www.iwra.siu.edu/win/win2000/win03-00/semiat.pdf

 


Benefits
Continuously operating process, 24 hours a day
New membranes have high rate of water flow per unit area (flux)
High overall water recovery rates
o Salt removal, up to 99.8%
Low power consumption
o Development of energy recovery devices (turbines)
Minimal cleaning required, longer service


Cons
Pesticides, herbicides, and chlorine are molecularly smaller than water and can pass through membrane if not pretreated correctly.
Removal of healthy, naturally occurring minerals in water.
Wastes a portion of the water that runs through its system.
Slower process than other water treatment alternatives.
 

Sources


http://www.historyofwaterfilters.com/reverse-osmosis-pc.html

 

http://www.ionics.com/technologies/ro/index.htm

 

http://www.iwra.siu.edu/win/win2000/win03-00/semiat.pdf