Reverse osmosis also known as RO is used for the purification of water and it works on the principle of reversing the natural process of osmosis.
What is Osmosis?
First, we will discuss what is osmosis. Osmosis is a process in which water molecules move from an area of low concentration to an area of high concentration through a semipermeable membrane.
An example of osmosis is the uptake of water by plants from soil.
Reverse Osmosis is a process in which water molecules move from an area of high concentration to an area of low concentration through a semipermeable membrane.
To reverse the natural process of osmosis a lot of energy is required and in reverse osmosis this energy is achieved by applying pressure through a high pressure pump.
The pressure applied by the pump reverses the normal osmosis process,meaning water moves from an area of high concentration to low concentration through a semipermeable membrane.
Basic Principle Of Reverse Osmosis
Reverse osmosis works on the basic principle of cross filtration by using energy in the form of pressure.
In reverse osmosis the raw water is supplied to the reverse osmosis membrane after pre-treatment by using a high pressure pump.
The pressure applied forces the water to pass through the RO membrane and all the contaminants in water are not able to pass through the Reverse Osmosis membrane and are rejected through the drain.
Function of RO membrane is not like the conventional filter media where contaminants are collect over top of the filtration media rather it is like cross filtration where pure water also known as product water or permeate passes through the membrane is collected in storage tank and contaminated water also known as reject stream or concentrate is rejected through the drain.
Why do we Need Reverse Osmosis?
Water is an important ingredient in pharmaceutical industries and other fields and is also used in boilers for producing steam.
For boilers the water used should be free from ions and contaminants which may result in scaling.
In pharmaceutical industries water is used as a part of formulation during manufacturing and is also used for washing of equipment and in different analytical procedures so water used should be of high quality and free from contaminants to meet the regulatory requirements.
To remove contaminants from water and to increase its quality the Reverse osmosis system is usually used.
Working Of RO system
To understand the Reverse Osmosis system we will discuss from beginning to the end to have a complete concept of a Water purification system.
Following are main sources of raw water
- Deep groundwater
Water water may contain following type of contaminants
- Physical Contaminants
- Chemical Contaminants
- Biological Contaminants
Physical contaminants may include large suspended particles like sand,dirt,leaves etc.
Chemical contaminants may include different types of ions like magnesium,calcium etc.
Biological contaminants may include living organisms like bacteria,viruses etc.
During passing through the membrane the dissolved salts like chloride and sparingly soluble salts like silica,carbonates and sulphate may start depositing on the membrane and this process in which the membrane surface is covered by the contaminants is known as Fouling.
Fouling results in reduction in membrane performance.
Calcium and magnesium ions result in increased water hardness and if they are not properly removed it may result in deposition of these ions on the RO membrane and disturb its function and this process is known as scaling.
To prevent RO membrane scaling, an anti-scalant dosing system is used.
So before passing through the RO membrane the water is pretreated with anti-scalants.
Depending upon the quality of raw water, the raw water is treated with chlorine or hypochlorite to disinfect the water and to improve its quality.
Chlorine is also used to oxidize the manganese and iron if present in the raw water.Sodium hypochlorite reacts with water to form hypochlorite acid which acts as a disinfectant.
SMBS or Sodium Metabisulphite dosing system is also used to remove the free chlorine from the water because this chlorine may damage the membrane and water transfer lines.
The raw water from any source is collected or stored in a storage tank which is then supplied by a pump to the pre-filtration system.
Pre Filtration System
Before supplying raw water directly to the RO membrane it is first filters using the following type of filter media.
- Sand Filter/Sediment Filter
- Carbon Filters
- Resins/Softener Filter
Sand Filter/Sediment Filter
Sand filters or sediment filters are used to remove large suspended contaminants like dirt,leaves,debris etc.
Feed raw water may contain smell,odour,chlorine depending upon the quality of raw water.
Carbon filters are used to remove chlorine from the water.In carbon filters activated charcoal is used and it also removes smell,odour,color organic and inorganic impurities from the ram water.
Ion Exchange Resins/Softener Filter
Ion exchange Resins or softener filters are used to exchange undesirable ions from the feed raw water with desirable cations and anions.
Cations are positively charged and anions are negative charged.
Cation resins exchange undesirable cations like Magnesium,calcium,copper and lead with hydrogen ions.
Anion resin exchanges undesirable anions like chloride,phosphate,nitrate with hydroxide ions.
Combination of hydrogen and hydroxide ions form water.
High Pressure Pump
High pressure pump is used to push the water through RO membranes.
Most RO membranes are made of the following.
- Cellulose Acetate
RO water systems remove 97-99% of salts and contaminants.
When water passes through the Reverse Osmosis membrane the pure water also known as permeate has low TDS and rejected water has high TDS Value.
EDI is electrodeionization and it is the process of deionization using electricity,ion exchange membrane resin.
The pure water or permeate is collected in a storage tank.
The water is treated with UV rays to kill any microorganisms which may grow during storage.
0.2 Micron Filter
When treated water is removed from the storage tank for use it is again filtered through a very fine filter to remove any contaminant which may develop during storage.For this 0.2 micron filter is used.
Filtration Flow Process
In RO filtration plant the flow of raw water from start to end is as follow,
Raw water from any source is stored in a storage tank and then by using a transfer pump this raw water is delivered to the sand filter or multimedia filter where the suspended impurities are removed from the water.
After passing through the multimedia filter this water is passed through the carbon filter and after the carbon filter this water is passed through the water softeners.
After the above pre-treatment the treated water is passed or transferred by the high pressure pump through the RO membrane.
Depending upon the construction of the plant different Number of RO membranes can be used.
In RO membranes the process of cross filtration takes place in which the pure water also known as permeate is collected in a storage tank and the concentrated water is rejected into the drain.
From storage tank on user requirement the transfer pump delivers the water from storage tank to the UV light treatment to kill any microorganism and after that this water passes through the 0.2 micron filter.
After all the above treatment the pure water is produced and is used.
What is 1 stage and 2 Stage RO
Here we will discuss the difference between 1 stage and 2 Stage RO water systems.
1 Stage or Single Stage RO System
1 stage or single stage Reverse Osmosis System is a simplest representation of RO water system where water passes through the membrane and is collected as Permeate and rejected water is drained as concentrate.
2 Stage or Double Stage RO System
To increase the efficiency of RO water plants we can use 2 stage or double stage RO water systems where concentrate or rejected water from 1st stage is used as a feed water to the 2nd stage RO and permeate of 1st stage is combined with permeate of 2nd stage and is collected in a storage tank.
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