Reverse Osmosis is a method of water purification, which uses a semi permeable membrane and pressure application to remove most contaminants in water.
This article discusses the following issues with respect to reverse osmosis;
Reverse Osmosis Definition
Reverse Osmosis (RO) is a method of water purification that is used to remove ions, particles, and dissolved matter, from water .
RO is in fact a form of filtration, and is often referred to hyper-filtration. This term is used because reverse osmosis, unlike the more conventional forms of filtration, helps to remove the smallest particles present in water, such as ions
Due to its relatively high efficiency, the method is also used to purify some volatile fluids like glycol and ethanol.
Origin and Development of RO
A formal discovery and observation of the process of osmosis was first made by the French Physicist Jean-Antonne Nollet, in the year 1748 .
Using a pig’s bladder as the semi-permeable membrane, Nollet showed that the molecules of the solvent could flow through the bladder and into a more concentrated solute which contained alcohol. The observation proved that a solvent could naturally pass through a semi-permeable membrane, into another solvent with higher concentration, till a state of equilibrium is reached on both sides of the semi-permeable membrane.
In the year 1950, osmosis again became relevant, in the course of investigations on the desalination of seawater using semi-permeable membranes, at the University of California, Ls Angeles (UCLA). In collaboration with the University of Florida, the process of osmosis was again replicated, however, at a very low efficiency which was unusable tor commercial or industrial application.
Following these outcomes, Sidney Loeb and Srinivasa Sourirajan in the year 1959, were able to produce a reverse osmosis system at the National Research Council of Canada, Ottawa, by using an external pressure to force the solute through the semi-permeable membrane . The membrane was itself designed so as to allow only pure solute to pass through, thereby leaving the contaminants on the other side.
By rejecting contaminants like Total Dissolved Solids (TDS) and Sodium Chloride (NaCl), reverse osmosis functions in a manner that is opposed to the natural process of osmosis, whereby the fluid passes through the membrane to create an equilibrium state of concentration on both sides. This is the reason why it was named Reverse Osmosis.
Under the guidance of Joseph W. McCutchan and Sidney Loeb, the first commercial RO plant was built in Coalinga, California, commencing its operations in the year 1965 . This plant proved that the desalination of seawater could be carried out at a large scale.
In the United States, the RO process was first practically and actively utilized at a large scale at Cape Coral, Florida, in the year 1977 . The plant began with an operating capacity of 11.35 million liters per day, and grew to 56.8 million liters by the year 1985.
These and other developments formed the foundation of the RO technology as it is known today. Through innovation and further studies, membrane designs have been made to be more efficient and effective, while becoming more affordable and relevant for use in several different industries.
The developments of RO technology have led to several different forms of RO systems, including RO trains; countertop systems; potable systems; and decentralized water purification units.
Facts about Reverse Osmosis
1). RO helps to eliminate heavy metals like lead from water.
Such metallic contaminants are very hazardous and can lead to increase in blood pressure, and renal dysfunction among other health problems.
Considering the fact that heavy metal pollution is a common problem in the world today, RO is crucial for treatment of borehole water in the United States and several other regions
2). RO is effective in the removal of resilient parasites like Cryptosporidium, from water.
Such parasites are known to cause serious gastrointestinal problems, among other issues
3). For cancer patients undergoing chemotherapy, water which has been treated by reverse osmosis is often the best option
4). Reverse Osmosis treatment does not involve the use of any chemical additives
5). Impurities or contaminants in water, are removed by RO, through the use of a semi permeable membrane, through which the impure water is passed under the influence of high pressure
6). One of the cheapest and yet most effective methods of water purification is reverse osmosis.
While reducing the need to rely on bottled water, the installation of a reverse osmosis system in your household provides you with a much safer and cleaner supply of water. It also reduces the energy cost involved in using a distillation system
7). Reverse Osmosis eliminates sodium from water.
While this may be done by some other water purification methods as well, RO makes it possible in a cost-effective manner, for the user. The removal of sodium from water is very important because sodium consumption can lead to hypertension and other cardiovascular ailments
8). Because of the highly effective purifying capacity of the RO process, it is the best purification method for water which will be consumed by individuals who are ill or have low immunity
9). RO is one of the most effective ways to improve the taste and eliminate the smell of water.
Unlike other methods that often require chemical additives, reverse osmosis simply removes contaminants in water. It also removes halogens like fluorine and chlorine which are usually added as disinfectants, and may produce a slightly pungent odor.
How RO Works
The principle behind reverse osmosis is a simple and yet effective one.
In reverse osmosis, a semi-permeable membrane is used to purify the fluid. This is usually driven by high pressure, which forces the fluid through the membrane . The semi-permeable membrane is designed to permit the fluid to pass through it, while preventing contaminants.
A technology known as crossflow technology is usually utilized, which enables the semi-permeable membrane to regenerate or decontaminate itself.
To supply pressure to force the fluid through the membrane, a pump is usually utilized. High pressure pumps are generally more efficient to provide high pressure for this process.
The contaminants which are eliminated by reverse osmosis include low molecular weight constituents like dyes, proteins, and sugars .
In order to remove ions from water, reverse osmosis makes use of charged particles. These particles are embedded into the semi-permeable membrane, and they serve to prevent ions (which are also charged particles) from passing through.
In RO, the water which passes through the semi-permeable membrane is referred to as the permeate, while the water which is rejected by the membrane is called concentrate or brine.
The filtration apparatus of a RO can be differentiated into pre-filter and post-filter. The pre-filter includes filter bed(s) that help to remove impurities like chlorine and suspended, insoluble materials from the water, before it passes through the semi-permeable membrane.
The post-filter acts on water after it has passed through the semi-permeable membrane . It helps to further purify the water before it flows out through a faucet.
In order to simplify our understanding of RO, we can arrange the entire process into a definite sequence of stages or steps. Although the semi-permeable membrane is the main tool for purification of water in RO, there are also other forms of filtration that are usually employed in the process.
As we have stated earlier, the filtration apparatus in a RO system can be categorized into pre-filter and post-filter. The pre-filter apparatus treats water in the system before it reaches the semi-permeable membrane; while the post-filter apparatus treats water after it has passed through the semi-permeable membrane.
The types of filter beds in a RO system include a sediment filter which helps to remove insoluble particles like dust and dirt; and a carbon filter which helps to remove materials like Chlorine, and Volatile Organic Compounds (VOCs), from water.
Steps in RO
This stage involves the use of the pre-filter to remove insoluble impurities like silt, as well as chemical substances like chlorine.
The importance of pre-filtration is mainly to protect the semi-permeable membrane from damage by these relatively heavy pollutants
2). Semi-Permeable Membrane Passage
After the pre-filtration stage, the water is passed through the semi-permeable membrane.
As earlier stated, this passage is facilitated by the use of pressure which is supplied by a pump, in most cases.
The importance of a semi-permeable membrane is to eliminate inorganic contaminants, and resilient, microscopic organic contaminants.
3). Water Storage
The water which passes through the semi-permeable membrane is subsequently stored for later use. For this purpose, a storage tank is often utilized.
Post-Filtration is carried out for the purpose of removing all contaminants that have not been eliminated by the semi-permeable membrane. It usually involves the use of a carbon post-filter which removes organic and gaseous contaminants.
The end result of RO is the production of pure permeate which passes through the semi-permeable membrane. This permeate is usually free of most of its ionic and particulate content, and is therefore safe for use, while meeting standards for purity.
Difference between Reverse Osmosis, and Osmosis
‘Reverse Osmosis,’ is a term used to signify that the process differs from the conventional form of Osmosis. In fact, using the word Reverse implies that it is an inverted form of Osmosis.
The following table differentiates RO from Osmosis;
|Water increases in concentration of contaminants as it passes the semi-permeable membrane
|Water decreases in contaminant concentration, becoming purer as it passes the semi-permeable membrane
|The final outcome is a state of equilibrium concentration on both sides of the semi-permeable membrane
|The final outcome is an increase in contaminant concentration on one side of the semi-permeable membrane
|Does not require any external pressure
|Usually involves and requires the application of external pressure
|Must be driven by artificial means
Difference between Reverse Osmosis and Filtration
It is not difficult to see that RO is a very similar concept to Filtration.
The difference between RO and Filtration is summarized briefly in the following table;
|Is a physicochemical process
|Is a purely physical process
|Occurs as a result of differences in concentration of contaminants in a solute
|Occurs as a result of the presence of a size-exclusion apparatus like a filter bed, which permits particles of a certain size class to pass through, while preventing others
|Is driven mainly by pressure and concentration
|Is not dependent on pressure or concentration
Factors that Affect the Process of Reverse Osmosis
On the above table, we have mentioned some of the factors that drive the process of RO These and a few others are discussed in this section of the article;
Increased concentration of salts or total dissolved solids (TDS) affects the process of RO, by creating a higher gradient in concentration between both sides of the semi-permeable membrane.
The effects of this may include a deterioration of the RO apparatus, fouling of the semi-permeable membrane, and reduction of the efficiency of the entire process.
Pressure is usually supplied from an external source in order to drive the process of reverse osmosis.
The effects of pressure may however be negative, especially if the pressure applied is excessive. Such effects include the deformation of the semi-permeable membrane, and possible damage of the apparatus.
Excessive pressure may also force some of the soluble contaminants through the membrane, thereby reducing the efficiency of the process.
With increase in temperature, the rate of passage of soluble contaminants through the semi-permeable membrane will increase.
RO is best conducted in a well-organized apparatus called the Reverse Osmosis system.
This system is typically composed of a number of distinct components, including;
– A Pre-filter apparatus consisting of one or more filter beds
This component helps to filter the water before it passes through the semi-permeable membrane. Pre-filtration removes contaminants like chlorine, iron, and particulate matter
-A Semi-Permeable Membrane
This is the main component of a RO system. Its role is to remove the major impurities in water after it has passed through the pre-filtration apparatus
-A Pressure Valve
The role of the valve is mainly to supply pressure that will help to drive water from the pre-filtration apparatus to the membrane
The storage tank helps to retain water which has been treated or purified, i.e.; water which has passed through the pre-filter and the semi-permeable membrane
-A Post-filter Apparatus
This component plays a secondary role in the RO system.
The post-filter simply ensures that the purified water from the storage tank is polished before it is released through the faucet for use. This helps remove any contaminants which may still be in the water at this point, and ensures that the water has a good taste and smell before its consumption.
Benefits of RO: Why you need a Reverse Osmosis Water Purification System
The benefits of RO, which have already been mentioned in this article, include;
1). Removes sodium from water, thereby reducing the risk of heart disease and hypertension
2). Eliminates lead from water. This helps to prevent some of the health problems which are caused by lead consumption, such as high blood pressure, and retarded neurological development
3). RO removes parasites and microorganisms from water
4). It is a cost-effective purification method, and helps to reduce the overall cost for water treatment
5). RO produces water with a very high quality and purity compared to other purification methods
6). Dissolved contaminants can be removed very efficiently using this method
Disadvantages of Reverse Osmosis
1). It often causes the wastage of a significant amount of water .
Because RO separates the solute into a purified portion and a highly contaminated portion, it leads to the production of large amounts of waste water, compared to other methods of purification like distillation and filtration
2). The semi-permeable membrane may become deteriorated.
Because of its interaction with contaminants, the semi-permeable membrane in a reverse osmosis system is vulnerable to deterioration and damage.
Also, the pressure which is supplied by the valves. may cause deformation of the semi-permeable membrane. As a result of this; the effectiveness of the entire process will be reduced.
3). The RO system may be expensive.
Purchase and installation of a reverse osmosis system may involve high expenses. However, this can be addressed by choosing the size, type, and quality of reverse osmosis system to install, based on the available resources
4). RO is not always effective for treating water that contains microorganisms
Applications and Uses
1). Desalination of Water
This is applied especially when there is need to purify seawater. Reverse Osmosis provides an effective means by which the salt content of seawater can be removed
2). Purification of Fluoride-Contaminated Water
Water containing high concentrations of fluoride can be purified through reverse osmosis. This is a very similar concept to the desalination of ocean water
3). For purification of feed-water in power stations
Water which is fed into the boilers in power stations, can be treated through RO. This helps to remove insoluble and soluble impurities in the feed-water, ad, as a result, improves the entire process of energy generation
4). Used in the Food and Beverage Industry
RO is effective for purifying water that is used to manufacture food products. In the manufacture of beverages, RO is used to concentrate the food content of the beverage
5). Pharmaceutical Manufacture
Water which has been purified by RO is particularly effective for manufacturing pharmaceutical products.
This is because such water is usually free of ionic contaminants that may alter the chemical composition of the products.
6). Waste-water Treatment
In several commercial industries, the wastewater which has been produced in the process of manufacturing, is treated by reverse osmosis.
This helps preserve the environment from contamination, while conserving water that could be used for other purposes
7). Metal Treatment
For semiconductors and other metallic materials, water which has been purified by reverse osmosis is often utilized.
Metal finishing operations like electroplating or galvanization, also make use of reverse osmosis water.
This article has discussed the importance of the process of RO, and has shown that it is applied in the following areas;
-Desalination of seawater
-Purification of drinking water
We have also discussed the importance of RO in water purification, including its low cost and efficiency of contaminant removal, which make it a most suitable option for such purposes.
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