Reverse Osmosis

Category: Environment & Green Technology Published: Friday, 21 October 2016

Rainwater, though purest form of natural water, cannot be harvested before it falls on the ground. Once it falls on the soil, it gets polluted due to mixing of soil, organic matter and mineral salts. Therefore, water from  surface water sources like rivers and nallas is turbid and needs to be filtered before use. The groundwater looks clean and colorless but it contains dissolved mineral salts. The conventional water treatment plants  are designed to remove colloidal and suspended impurities but cannot remove dissolved salts. If water is brackish or salty ( Like sea water)) it cannot be used for drinking purpose.

 

Saline water – Seawater contains about 35000 mg/l of total dissolved solids ( TDS). Out of total salts, 30,000 mg/l is common salt ( sodium  chloride). Drinking water should have less than 500mg/l of total dissolved solids and less than 300mg/l of chlorides. Hence, the seawater cannot be used for drinking unless the salts are removed. The groundwater in wells near sea coast, contains  5000 to 10000 mg/l of dissolved solids and is not potable. Use of excess water and chemical fertilizers on land for irrigation increases salt content in wells and it is found to be in the range of 2000 to 5000mg/l, which is not suitable for drinking and even for irrigation.

 

Conversion of saline water into fresh potable water is very complex and costly process. 2/3rd part of earth’s surface is covered with sea and contains huge quantity of water, but due to it’s excess salt contents it is not potable. Hence people travelling in ship may find water everywhere but not a drop to drink . One can get potable water by distillation of seawater, but it requires energy and costly equipment. Now this can be achieved by a new technology developed for conversion salt water to potable water. This technology is called as Reverse Osmosis.

 

Osmosis – If we put raisin in water, in a short time it’s size and volume increases. This happens due to entry of water in raisin. However, if we drop a grape in sugar syrup, it contracts. In one case water moves in raisin, whereas water from grape moves out leading to it’s contraction. You may have a doubt as to what determines the direction of water flow. Osmosis is the answer to  this question. It depends on concentration level of water in and out of raisin or grape. Water flows through the skin of grape or raisin from high concentration side to low concentration side. The action is similar to flow of  water from high level to low level. The structure of grape or raisin skin is responsible for such action. Plant roots also have similar structure. The cells inside root skin contain high salt concentration ( low water concentration) hence water from soil ( high concentration) gets absorbed in the root. This process is called as osmosis.

Salts can easily pass through root or grape skin, hence salts can be absorbed by plants but sugar molecules being larger in size cannot pass the skin and water concentration governs the process. As saline water contains salts hence to prevent flow of salt,  a special semi permeable membrane is used in reverse osmosis. 

 

 Reverse Osmosis – The process of getting potable water from saline water is based on above osmosis 

principle. If we keep saltwater and potable water in two compartments separated by a salt flow preventing membrane, the water from potable ( less salt) side will flow to saltwater ( high conc. of salt) side.

 

 

 

Thus as shown in Fig. 1, if the side contains salt water, the water level on that side will increase due to osmotic pressure. The osmotic pressure will be indicated by the difference in water levels. If however, external pressure is applied on salt water side which is greater than osmotic pressure( Fig.2), the water from salt water side will flow to potable (less salt conc.) water side against osmotic pressure. This is reverse osmosis. As the salts cannot pass through the membrane, the potable water will be generated in the process and salt concentration on salt water side will go on increasing. Such high concentrated salt water will have to be drained and replaced by saline water.  In this way,  can use  reverse osmosis process for converting saline water to potable water. 

 

 

Microfilter membranes with micron size holes can be used to separate fine particles  and large molecules from water. But membranes  used in reverese osmosis are different than such microfilters and are called semipermeable membranes as they allow only flow of water from one side to another.

 

Research – In 1950, scientists Reid &  Braton from Florida University in USA proved that  cellulose acetate  effectively prevents flow of salt through the membrane . Later technocrats Loeb and Sorairajan developed such membrane technology  and showed that potable water can be generated by  such membrane at rate of 500 to 1000 liter per square meter area under the gauge pressure of 40 to 50 bar. It was observed that 90 % salts are retained by such membrane.

 

India is fortunate tio have a vast sea coast. There is acute scarcity of potable water in these coastal areas. Central Salt and Marine Institute is established at Bhavnagar in Gujarath.  Research on reverse osmosis technology  is being  conducted in this institute . As of now, the institute has become successful in converting brackish water from wells to potable quality water by using reverse osmosis system developed in the institute. This will greatly help in solving drinking water problem in rural area having groundwater with high salinity. NEERI, Nagpur has also developed such technology of producing cellulose membranes.

 

The osmotic pressure of salt water in sea is 20 bar i.e. 20 times that of air pressure. During reverse osmosis, the salt concentration of water goes on increasing leading to corresponding increase in osmotic pressure. Hence reverse osmosis process needs to operated at 60 to 70 bar pressure. For desalination of well water, a pressure of 15 to 30 bar is sufficient.

 

Membranes -

The membrane used for reverse osmosis is very delicate and to prevent its rupture, it is supported by plastic screen sheet. As the flow is very small, large surface has to be used to get required water quantity. Hence various types of membranes are used.

 

  1. Micro fibers – These fabrics are smaller in size than human      hair and tubular in nature. Salt water is outside of such fibers  and good quality water is inside the  tube. These membranes are prepared from cellulose acetate, polyvinyl alcohol and polyamide.
  2. Plain sheet or rolled sheet- Alternative layers composed of membrane and supporting plastic screen are used for preparing plain sheet or wound type sheets for reverse osmosis. The pipes for carrying salt water and good quality water are provided in the system. 
  3. Tubular type – The membranes are in the form of 12mm to 25 mm dia tubes. This type of system is suitable for turbid water as these tubes can be cleaned by passing water through the tubes. Of course, as these tubes provide very small area of membrane, the potable water generation is also in small quantity.

Pre Treatment – In order to design reverse osmosis system, good knowledge of chemistry, microbiology is required to control accumulation of salt layers on the surface of membranes and possibility of decomposition of membranes due to microbial action. Moreover, water quality information as regards, turbidity, dissolved gases, temperature, pH, chlorine, salt concentration and filtration capacity of membrane should be known. This information is useful to decide the type of tretreatment necessary before reverse osmosis process. Water filtration, aeration, passing through specific resin columns  may be necessary to have effective functioning of reverse osmosis and protection of membranes from damage.In order to prevent formation of calcium carbonate layer on the membrane surface, acid and phosphates may have to be added to water to be treated. Water subjected to reverse osmosis process should have following quality 

 

Iron – Less than 0.1mg/l

Manganese – Less than 0.05 mg/l

Clorine – Less than 0.1 mg/l.

 Turbidity – nil

 

Large scale reverse osmosis plants have been erected in middle east countries as potable water sources are less.  These plants are used for converting sea water  to potable quality water. Considering the potential of reverse osmosis technology in future, many foreign companies are investing in research and development of this technology. India also should take a leading role in this aspect.

 

Other uses – Now reverse osmosis is being used in various fields. As this technology can effectively remove pollutants from industrial wastes, the waste water can be recycled back in the process , avoiding environmental  pollution. Use of reverse osmosis process is increasing  in pharmaceutical industries, canning industry, distillery,, milk and food processing industry. Thus reverse osmosis process has become a boon to society.