• 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
A reverse osmosis plant consists of four major systems:
• Pretreatment system
• High pressure pumps
• Membrane systems
• Pretreatment is very important because membrane surfaces need to remain
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
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
• 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
• 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.