Abstract:
Water is crucial in our daily lives for consumption, industry and agricultural purposes. Many parts of the world, particularly in developing countries, do not have access to clean water. This is due to the lack of importance in providing clean water as well as lack in funds and other reasons. A method that produces clean water from wastewater, seawater or brackish water is desalination technology. Desalination technology has many processes such as reverse osmosis, electrodialysis, ion exchange, freeze desalination and distillation. This literature review deals with the different methods of desalination technology in producing clean, quality water.
Keywords: Desalination; Wastewater treatment; Reverse osmosis; Electrodialysis; Ion exchange; Freeze desalination
Introduction:
Two thirds of the Earth consists of water, yet many areas are facing water problems. Water problem is a very severe crisis as it leads to many diseases causing a deep impact in society. Although water treatment is extremely important to public health and other viewpoints, it is not given enough priority in places where funds are limited such as in developing countries (Shiny et al, 2004).
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In some parts of the world, such as the Gulf Region, the population is suffering from water scarcity, however, in some regions where water is abundant, the water quality is poor (Al-Handhaly et al. 2003).
It has been stated by the World Health Organization tha 80% of all known diseases are related to water borne diseases. In addition, more than 1 billion people worldwide do not have access to safe drinking water. (Raluy et al. 2006).
This reflects the importance of clean, quality water. Raluy et al (2006) states that not only is clean water needed for human consumption, it is also needed for industrial and agricultural use. Due to the increasing population of the world, and industrial and agricultural activities, available water resources have been exploited and fresh water sources have been polluted. As such, other sources such as treatment of wastewater, and seawater should be looked into to provide alternate means of clean, quality water.
According to Sonune and Ghate (2004), wastewater represents the wastes of community life and consists of waterborne solids and liquids discharged into sewers. The term wastewater treatment refers to the partial removal of solids in wastewater and the partial change via decomposition from highly complex, putrescible, organic solids to mineral or relatively stable organic solids (Sonune & Ghate 2004).
Conventional methods of wastewater treatments are not sufficient and advanced wastewater treatments have become the focus for many nations as each persevere to keep water resources available and suitable for use (Sonune & Ghate 2004).
The area of focus of advanced wastewater treatment methods in this literature review is desalination technology. Desalination technology has many techniques and each will be explored in greater detail and compared with regards to their methods, applications and their benefits and drawbacks.
1.0 Desalination Technology
Among the many methods of wastewater treatment, in many parts of the world, desalination technology is becoming a very important �non-conventional� growing source for drinking water. Apart from this, in areas where fresh water is scarce, this technology solves the problem where other alternatives to water supply are unavailable (Raluy et al. 2005).
Desalination is a water treatment process that involves removing salts from water. This technology gives rise to the possibility of providing fresh water from saline groundwater (Al-Agha & Mortaja 2005), seawater or brackish water (Raluy et al. 2005).
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I. INTRODUCTION Fairchild Water Technologies was founded in 1980 by Eugene Fairchild. The company’s first product was a desalinator used by mobile home parks in Florida to remove salt from well water supplied to residents. As the desalinator became a huge success, the company expanded into the coastal region’s adjacent to the company’s headquarters in Tampa, Florida, and then to desert areas in ...
Presently, with the awareness and importance of desalination technology on the rise, there are over 11,000 desalination plants in 120 countries worldwide. These plants have a combined capacity of 13.25 Mm3/d (Dore 2005).
There are 5 basic techniques in desalination technology. These techniques are namely distillation, reverse osmosis (RO), electrodialysis (ED), ion exchange (IX) and lastly freeze distillation. Distillation and freeze desalination has the same principle in that it extracts pure water, in the form of ice or water vapor from salty brine. The principle of RO and ED is based on the use of membranes to separate dissolved salts and minerals from water. The principle of IX is based on the exchange of dissolved mineral ions in the water for other dissolved ions through resins (Sonune & Ghate 2004).
Among the many methods of desalination technology mentioned above, the most common methods used today are distillation and membrane processes (reverse osmosis and electrodialysis).
The reason behind the popularity of distillation method in seawater desalination is due to the co-generation of water and power and also due to the earlier membrane technology being less reliable. Membrane processes are very common now due to its many advantages over other methods such as their improvements in reliability and their low costs (Schiffler 2004).
Desalination technology has much importance in many areas. Some of these areas are in industries such as papermaking, printing, dyeing, pharmaceutical, biotechnical, mining, oil fields, chemical (Chen 1995), and many others.
Although desalination technology has showed its importance, there are also several undesirable impacts caused by this technology. According to Meerganz con Medeazza (2005), one of its main impacts is on the environment, causing pollution due to its by-products, such as brine. Brine is by-product of desalination and is most commonly discharged into the marine environment which will consequently have adverse effects. Brine discharge makes up a hypersaline layer that has a tendency to sink towards the seabed as it has a greater density. This causes the increased risk in marine biota (Meerganz von Medeazza 2005).
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... solving part of the water shortage problems. Currently, there are two methods of desalination; reverse osmosis and multi-stage flash distillation. Reverse osmosis water is produced much in ... 000 times smaller than a pinhole. The water then passes through reverse osmosis, which consists of membranes with pores that are 5 million times ...
Another impact on the environment is due to the emission of greenhouse gasses caused by the production of electricity and steam to power the desalination plants (Schiffler 2004).
Another source of the emitted greenhouse gasses is from fossil fuel burning, in which most of its energy of the plant is derived presently (Meerganz von Medeazza 2005).
Even though 5 techniques of desalination technology have been mentioned, only 4 of the techniques will be explored in greater detail in this literature review. These are RO, ED, IX and freeze desalination.
2.0 Reverse osmosis
2.1 Method of reverse osmosis
Reverse osmosis (RO) makes use of permeable membranes in the process and is based upon physical-chemical filtration (Meerganz von Medeazza, 2005).
It involves pumping feed water at high pressures through permeable membrane, causing the separation of salts from the water (Sonune & Ghate 2004).
The high pressure applied, which is higher than the osmotic pressure, causes the solvent to move from a concentrated solution to a weaker solution (Al-Agha & Mortaja 2005).
Due to the presence of the membranes, the salts are effectively separated from the water based on the particle size and the molecular weights. The water produced is also of high quality and this depends on the pressure applied, salt concentration in the water as well as the type of membrane (Sonune & Ghate 2004).
Below is a diagram describing the method of RO.
Fig 1: A simplified diagram of a reverse osmosis system (Source: Sonune & Ghate 2004)
2.2 Applications of reverse osmosis
Reverse osmosis method has many applications, which is the reason why it is one of the most widely used methods of desalination. This method has been proven to be useful in treating water effluents from many industries such as chemical, textile, pulp, paper (Bódalo-Santoyo et al. 2003) and also in dye manufacturing industry to treat dye wastewater (Kim et al. 2005)
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2.3 Benefits and drawbacks of reverse osmosis
Reverse osmosis method is one of the most common desalination technologies as it has many benefits over other methods. Such benefits include reduced pollution and save in costs. It is economically beneficial as the processed water and concentrate steam are both valuable products, which can be recycled, and thus the waste does not need to be diverted to a municipal wastewater facility. (Bódalo-Santoyo et al. 2003).
Apart from that, it does not use chemicals in the treatment and is a well-arranged process conduction (Sonune & Ghate 2004).
In comparison with other methods of desalination technologies, RO also produces higher recovery rates for seawater and it is able to produce a large amount of drinking water for a lower price (Dore 2005), and the energy consumption is also reduced (Bódalo-Santoyo et al. 2003).
Although the benefits of RO are plenty, this method also has drawbacks. One of the major drawbacks is to the environment whereby the reverse osmosis plants causes the by-product of concentrated brine and sludges (Sommariva et al. 2004).
The discharge of brine into the environment causes adverse effects on the environment. Apart from that, another drawback to this method is that it has severe limitations to removing organics from chemical effluents from industries. These organic compounds are of low molecular weight and generally it must be oxidized to easily ionizable products prior to this method (Bódalo-Santoyo et al. 2003).
Although RO has a few drawbacks, the benefits of RO far outweigh the drawbacks. Not only is it beneficial economically, it also does not use chemicals and has low energy consumption.
3.0 Electrodialysis
3.1 Method of electrodialysis
Electrodialysis (ED) was introduced commercially during 1960s, which was about 10 years before reverse osmosis (Nagarale et al. 2006) ED is an old membrane technique that is based on charge transfer field. The driving force is via an electric field (Güvenç & Karabacakoğlu 2005).
It consists of several hundred flat, parallel, ion-permeable membranes that are assembled in a stack and brackish water is pumped through them under low pressures. Membranes that allow cations to pass through them are alternated with anion-permeable membranes. Electrodes are positioned at both ends of the stack and this causes an electric current that attracts theses ions through the membranes and concentrates them between the alternate pair of membranes (Sonune & Ghate 2004).
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The electric potential achieved between the two faces of the membranes are permeable to either anions or cations (Güvenç & Karabacakoğlu 2005) and consequently Partially desalted water is left between each adjacent set of membrane pairs (Sonune & Ghate, 2004).
A diagram representing this method is shown below.
Fig 2: A simplified diagram illustrating the principle of electrodialysis. Cation ( ); Anion ( ).
(Source Nagarale et al. 2006).
3.2 Applications of electrodialysis
Electrodialysis has many applications. Such applications are in brackish water desalination, wastewater treatment, heavy metal recovery such as gold, platinum, silver, copper, nickel, tin, palladium, cadmium, tin, led and zinc. (Güvenç & Karabacakoğlu 2005).
Other applications include removing nitrate from ground water as nitrate contamination is a serious problem due to the widespread use of fertilizers. ED is also applied in food and biotechnology industries whereby they are developed in the demineralization of cane sugar juice, desalination of cheese whey used in ice cream, bread, cakes, sauces etc. In addition, ED was recently applied in the extraction of cytoplasmic proteins from alfalfa, to coagulate whey proteins and also reduce disulfide bonds in whey proteins (Nagarale et al. 2006).
In addition, other applications can be extended to the production of boiler feed water (Mohammadi et al. 2005).and acid pickling before electroplating (Paquay et al. 2000).
3.3 Benefits and Drawbacks of electrodialysis.
Electrodialysis has many benefits. Such benefits are its ability to purify and eliminate wastes as it is able to separate ionic chemicals from non-ionic chemicals. ED also allow the reuse of chemicals as it can concentrate the separate chemicals relative to concentrations in the initial process or waste streams. It also has the ability to convert chemicals to other more desirable at high efficiency (Mohammadi et al. 2005).
In addition, ED is beneficial as it enables the recovery of valuable metals such as silver (Güvenç & Karabacakoğlu 2005).
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Overall, ED is complete, flexible and effective (Paquay et al. 2000) and most importantly, compared to some methods of desalination technology, ED is more environmentally friendly (Greiter et al. 2004; Mohammadi et al. 2005)
4.0 Ion exchange
4.1 Method of ion exchange
Ion exchange (IX) is based upon a fixed-bed separation technology that uses ion exchange resins. The resins consist of many firmly attached bonds on their surfaces that can adsorb anions or cations (Greiter et al. 2004).
Feed water contains many undesirable ions such as nickel, copper, sodium etc. These ions are exchanged for counter ions on the surface of the bead via electric force. As water flows through the column, ion exchange resins use cation resins that are able to selectively exchange cation ion with hydrogen in wastewater. Cations such as nickel, copper or sodium are exchanged with hydrogen ions and anions such as sulphates, chromates and chlorides are exchanged with hydroxyl ions. After absorbing the cations, the resin is regenerated by flushing with acid (Eom et al. 2005).
4.2 Applications of ion exchange
The main application of ion exchange is in pharmaceutical industries. It is also applied in food industries (Greiter et al. 2004; Eom et al. 2005), in water softening processes, and also in catalyst and manufacture of ultra-pure water used in semiconductor processes (Eom et al. 2005).
In addition, IE has also been used satisfactorily to treat fertilizer wastewater (Leaković et al. 2000).
4.3 Benefits and drawbacks of ion exchange
Ion exchange method has many benefits. One of the benefits is that a large volume can be treated at once and the metals can be directly recycled in a plating bath. This also gives a recovery of up to 97% (Eom et al. 2005).
Apart from that there are many benefits in treating fertilizer wastewater. These are the production of own chemicals for the regeneration of the ion resins, the treated wastewater can be reused, and the product regeneration can also be used for fertilizer production and most importantly, there is no other pollutant (Leaković et al. 2000).
Although the benefits of IX method are many, there are also drawbacks. The drawbacks include the unsatisfactory anion resin condition and vacuum evaporation condition in fertilizer wastewater (Leaković et al. 2000).
However the major drawback is that this method is comparatively more expensive. This is because the resins have to be replaced more often if the concentration of the dissolved salts in the water is high. It is rather expensive to replace the resins and dispose the regeneration solution (Sonune & Ghate 2004).
5.0 Freeze desalination
5.1 Method of freeze desalination
In freeze desalination, the method is comparatively simple. This method involves mixing seawater with cold liquid refrigerant in a freezer unit. This causes the heat to be absorbed from the seawater, and hence it freezes forming ice crystals distributed in water. These crystals are salt free and the impurities are excluded from the crystals. The crystals are then separated from the remaining brine and melted, resulting in purified water (Rice & Chau 1997)
5.2 Applications of freeze desalination
This method is not practiced widely due to practical difficulties and developmental mishaps. However, as it has many advantages over other methods, efforts are on the way to make reconsideration of freeze desalination worthwhile (Rice & Chau 1997).
5.3 Benefits and drawbacks of freeze desalination.
Compared to other methods of desalination technology, freeze desalination has an advantage as it is more energy efficient, and more cost-saving, a higher ease of maintenance (Rice & Chau 1997).
Apart from that, freeze desalination can also produce very pure potable water and can also produce water for irrigation at less cost (Rice & Chau 1997).
The drawbacks that have been recorded are the problems with the conventional refrigerant compressors required. These compressors need lubrication and some of this lubricant interferes with the saltwater freezing process. The lubricant contaminates the ice as the refrigerant is in direct contact with the water and this contamination causes a major operational difficulty.
Conclusion
There is not enough awareness made worldwide regarding the proper treatment of water. This is evident in the uprising number of diseases caused by water problems in developing countries. It is extremely crucial that wastewater treatment be given more priority in every part of the world. Without proper treatment, much of the developing countries do not have access to clean water, and this causes environmental hazards as well as health problems such as disease outbreak. Conventional methods of wastewater treatment are not sufficient enough and more advanced treatment must be employed. Desalination technology is one of the most efficient advanced treatment of wastewater and there are 5 major processes. Of all the methods, reverse osmosis and electrodialysis, which are membrane technologies, have more advantages in terms of costs and efficiency over the rest of the other methods. Freeze desalination method has much potential and many advantages and hence should be given more consideration.
(2628 Words)
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