REVERSE OSMOSIS

Desalting techniques are primarily intended for the removal of dissolved salts that generally cannot be removed by conventional treatment process. Dislillation units have been used on some American ships for more than lOO years. Desalling was used on limited scale for multiple water treatment in the late 1960's.

'I‘he past four decades can be divided in to three phases of desalting. The l950 were a time of disovery, the 1960's were concerned with research and the 1970's, l980's have been the time of commercialization. Beginning in the 1970's, the industry began to concentrate on commercially viable desalination applications and processes. 

World desalination The arid region, with its very limited fresh-water potential. has generally used high-salinity waters such as seawater as major water supply sources. More than two-thirds at the wmld's desalting capacity is located in the arid, oil-rich areas at North Africa and Western Asia, or the Middle Past. 

The major desalting technologies used today are distillation, reverse-osmosis (R0), electro dialysis ( ED), electm dialysis reversal (EDR). and ion-exchange demineralization. The typical concentration ranges of total dissolved solids (TDS) in the feed water for distillation, R0, ED, and EDR demineralization are:

• 1. Between 10,000 and 100,000 mg/l distillation and other thermal processes. 
• 2. 1,500-45,000 mg/l (seawater concentrations)-RO membrane. 
• 3. 10,000 mg/l -ED and EDR membrane. 
• 4. Up to 1200 mg/l -Ion Exchange.

Ion exchange, in which anion and cation resins are used to exchange ions for hydrogen and hydroxide, is prinmily used in industrial applications for which very pure water is required end.

the feed-sister T03 is relatively low. Distillation and other themial processes are used primarily for seawater conversant and speciad imhtstrial mlications. such as brine concentration. 

if you know how to handle it by yourself and have enough time to take care of it.

The cost of desalination hm generally decreased {mm more than Rs. 1 SO/m’ to as low as Rs.50/m’ over time as a result of both technological advances and market processes.

Improvements in R0 membranes have been the main technological change in desalination in recent years. 'Ihe United States and Japan are the world ‘s leading countries in innovative research to develop the membrane industry. A high level of competition on both a national and international basis has also pllyed a significant role in containing prices for capital.

major water supply sources. More than two-thirds at the wmld's desalting capacity is located in the arid, oil-rich areas at North Africa and Western Asia, or the Middle Past. 

0 Western Asia (Middle East), 63%, 

0 North America, 1 1 “/o, 

0 North Africa, 7%,

0 Europe, 7° 0. 

0 Pacific. 4%. 

0 Caribbean, 2%, 

0 (Former) USSR, 2%, 

0 Others, 4° 0. 

7.2.1. Desalting technology and processes 

The major desalting technologies used today are distillation, reverse-osmosis (R0), electro dialysis ( ED), electm dialysis reversal (EDR). and ion-exchange demineralization. 

The typical concentration ranges of total dissolved solids (TDS) in the feed water for distillation, R0, ED, and EDR demineralization are: W 

1. Between 10,000 and 100,000 mg/l distillation and other thermal processes.

2. 1,500-45,000 mg/l (seawater concentrations)-RO membrane. 

3. 10,000 mg/l -ED and EDR membrane. 

4. Up to 1200 mg/l -Ion Exchange. 

Ion exchange, in which anion and cation resins are used to exchange ions for hydrogen and hydroxide, is prinmily used in industrial applications for which very pure water is required end 

the feed-sister TDS is relatively low. Distillation and other themial processes are used primarily for seawater conversant and speciad imhtstrial mlications. such as brine concentration. 

Desalination capacity by process 

70% of the world‘s desalination capacity is dependent on the distilling process. In the Middle Fast and North Africa. distillation ol‘seaxtater is the main pmcess being used. while the pmcesscs limited in the United States and other countries are quite different. retlecting the numerous applications for the desalination ol‘bracltish water.

Distribution of desalination capacity by process 

Share of capacity (°/o) Worid United States Destillat‘an 70 20 Reverse osmosis 25 74 Elecundialysis S 6 Economics and environment 

The cost of desalination hm generally decreased {mm more than Rs. 1 SO/m’ to as low as Rs.50/m’ over time as a result of both technological advances and market processes 

Improvements in R0 membranes have been the main technological change in desalination in recent years. 'Ihe United States and Japan are the world ‘s leading countries in innovative research to develop the membrane industry. A high level of competition on both a national and international basis has also pllyed a significant role in containing prices for capital 

3. Process of reverse Osmosis 

i) When two solutions of different concentrations are separated from one another by a membrane which is permeable to the solvent but impermeable to the solute; 

ii) Solvent flows from the dilute to the concentrated solutions. until; the chemical potential of the solvent is equal on both sides of the membrane. 

iii) Osmotic pressure is directly proportional to the concentration. ol‘solute. (For l000 ppm, solution the osmotic pressure is 0.75 bars). 

For example. the osmotic pressure of brackish water containing about 2.000 ppm TDS at a typical water temperature of 25 0C is only about 1 .6 kg7em’. whereas it is 27.7 kg/cm2 for standard seawater of 35.000 ppm TDS at 25°C. 

iv) If a pressure greater than the osmotic pressure is applied to the concentrated solution, the solvent can be forced through the membrane, leaving the dissolved substance behind. 

The pressure of the feed water,( pro-treated to meet certain established RO membrane feed-water quality guidelines). is boosted before the water enters the R0 membranes. Two flows exit the membranes: 

1) The combined product (permeate),

2) The combined concentrate (reject). 

The fraction of the feed water that results as permeate is called the recovery and is usually expressed as a percentage. The maximum allowable ratio of penneate to reject depends on the water’s scaling potential. which is a function of the feed-water quality. This ratio is maintained by the use of a control valve on the reject piping. which controls the flow rate of the reject, thus forcing the permeate flow rate to the desired value.

R0 membranes 

Membrane is a thin sheet of plastic material of about l 00 mm thickness. When the membrane allows the solvent to flow from dilute to the concentrated solution that is semi permeable membrane. 

Three main types of membrane are used 

1) Cellulose acetate.

2). Polyamide (Nylon type) 

3). Composite membrane comprising of polyamide and polysulphone

The first commercially available membranes, developed in the mid 1960's. Were made of cellulose acetate (CA) manufactured in flat sheets. Modern CA membranes are modifications of the cellulose acetate structure, including blends and different surface treatments, and are called cellulosic or Symmetric membrane. Cellulose acetate and composite membranes are used in sheet form. 

Non-cellulosic membranes, called thin-film composite membranes. have been developed since the 1970's. These include polyamide membranes with relatively thick asymmetric polyamide support structures and composite membranes with thin-film polyamide or other membrane materials on a porous support structure. This system is known a module. 

Each membrane material has advantages and disadvantages. The CA-based membranes are now generally the least expensive per liter of installed capcity. The price difference between CA and composites is decreasing.

Use of CA membranes generally requires chlorinated fecd water and higher operating pressures than those needed by the composite membranes. Composite membrane generally operate over wider the PH and temperature ranges than CA membranes. In some cases these operating characteristics composite membranes result in savings in electric power and chemicnl costs. Their greater PH tolerance provides additional advantages in cleaning for some application. 

Sensitivity to chlorine and other strong oxidants in the feed water is a disadvantage of polynmide-based membranes. New developments in membrane research to produce chlorine-tolerant composite membranes are overcoming this limitation.