Reverse osmosis is commonly used for desalination. As well, RO is commonly used for the removal of dissolved constituents from wastewater remaining after advanced treatment with microfiltration. RO excludes ions but it requires high pressures to produce deionizer water (850-7000 kPa).

                                              
        
Osmosis
Osmosis is a phenomenon where pure water flows from a dilute solution through a semi permeable membrane to a higher concentrated solution. Semi permeable means that the membrane will allow small molecules and ions to pass through it but acts as a barrier to larger molecules or dissolved substances. To illustrate this, assume that a semi permeable membrane is placed between two compartments in a tank. Assume the membrane is permeable to water, but not to salt. If we place a salt solution in one compartment and pure water solution in the other one, the system will try to reach equilibrium by having the same concentration on both sides of the membrane. The only possible way to do this is for water to pass from the pure water compartment to the saltwater compartment.

                                                

As water passes through the membrane to the salt solution, the level of liquid in the saltwater compartment will rise until enough pressure, caused by the difference in levels between the two compartments, is generated to stop the osmosis. This pressure, equivalent to a force that the osmosis seems to exert in trying to equalize concentrations on both sides of the membrane, is called osmotic pressure.

Reverse Osmosis

                                               

If pressure greater than the osmotic pressure is applied to the high concentration the direction of water flow through the membrane can be reversed. This is called reverse osmosis (abbreviated RO). Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.

How Does Reverse Osmosis Work?

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semipermeable membrane. The semipermeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, and Cl-) or larger molecules (e.g., glucose, urea, and bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane.
Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.

Membrane Separation Technologies
If pressure greater than the osmotic pressure is applied to the high concentration the direction of water flow through the membrane can be reversed. This is called reverse osmosis (abbreviated RO). Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.

        

FEATURES OF REVERSE OSMOSIS

1. Removal of dissolved salts
Reverse osmosis can stably and effectively remove dissolved salts, dissolved organic substances (trihalomethane, its precursors, agricultural chemicals, etc.), and micro fine particles (living germs, dead germs, and many other micro fine particles) from water. Thus it is ideal for a wide area of applications ranging from production of ultra pure water to desalination of seawater.

2. Energy-saving separation technique
Reverse osmosis keeps water from evaporating, making it energy-saving separation technique th/?at requires less energy consumption.

3.Utilizable as a concentration and recovery method
Reverse osmosis does not need heating, it can concentrate and recover valuable process materials dissolved in a solution without any degradation which might otherwise occur in such materials.

4. Compact equipment
Modules can be arranged in a three dimensional configuration to provide excellent space efficiency, so the space needed for installation can be minimized.

5. Simple operation and control
Reverse osmosis is a simple process, its operation and control are uncomplicated, while maintenance is easy and free from trouble.

Shape
To maximize surface area and minimize space, filter membranes usually wound in a spiral element. Water flows through the channels between the spiral windings of the element. Permeate is collected in a tube in the center of the element.

                       

Several spiral elements are connected together within a pressure vessel, and several vessels are connected together within the filter system. The vessels allow permeate to collect in a tube in the center of each spiralwound element before flowing out to a holding tank or drain depending on the application. The reject then passes through another vessel and then another, until as much pure water as possible is removed. The reject is then disposed of.
 
RO cleaning recommendation
For a consistent and reliable system performance and longevity of membrane elements, it may be helpful to use special cleaning agents, efficient and compatible with the applied membrane. However, not only the quality of the applied chemicals is important. Also application know-how and backup from the chemical manufacturer is needed for normal use and in particular for troubleshooting. Various membrane cleaners are certified by their suppliers to be compatible with all commonly used membrane elements of the leading RO membrane manufacturers for water purification. They should be designed for minimised environmental impact, efficient and easy use.