HOW DESALINATION OF SEAWATER FOR USE AS DRINKING WATER RELATES TO THE PRINCIPLES OF OSMOSIS

Introduction

Advancement in technology has made it possible for desalination of seawater to clean domestic water for domestic purposes. This procedure is possible through a process known as reverse osmosis. In reverse osmosis, water pollutants and dissolved salts that make seawater saline pass through a semi permeable membrane. The membrane selectively allows passage of pure water while blocking the inorganic salts and pollutants. Reverse osmosis is gaining popularity as the best method of desalination of seawater for drinking purposes. With major industries already applying the procedure in large scale, this report provides an in-depth analysis of osmosis applicability to desalination of seawater for domestic use.

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Discussion

Water flows through a semi permeable membrane in a process known as osmosis (Chen ET al.2011). The process is plants to absorb water and nutrients from the ground and in the body cells of animals in absorbing water and excreting waste. The procedure also has a wide application especially for industrial purposes. Of the major applications of the process is in the desalination of seawater for domestic purposes (Elimelech, 2012).

The osmosis process must involve a semi permeable membrane and two aqueous solution but of different concentrations. Water flows from low concentration region to the one that has higher concentration. The process can be reversed if external pressure is applied. This is the principle behind the desalination process and is referred to as reverse osmosis. The pressure that is applied is referred to as the osmotic pressure. To determine the pressure, an equation known as Van’t Hoff equation is used (Voutchkov & BCEE, 2007).

Using the osmosis principles, seawater is desalinated to clean water that is first for human consumption and domestic use. However, the procedure involves the state of the art technology when applied in the large scale. Sources of seawater may be an open ocean or a subsurface intake. Once the water is obtained from the source, it goes through the pretreatment process. In this process, it undergoes the convectional coagulation and filtration using the membranes. The process then continues to the reverse osmosis stage where it is passed through the spiral wound modules that contain high- permeability membranes. Lastly, it is taken through the post treatment process where it is cleaned from contaminating chemicals like boron and chloride. It is also disinfected to ensure that its safe human use (Fritzmann.c et al.2006).there were over 15233 desalting plants through the world in the year 2001. Currently the number is estimated to be twice as much. These plants had a desalting capacity of 32.4 million meters cubed of water in a day (Elimelech, 2012).

With the high shortage of clean water due to adverse climatic changes, attention is being shifted to desalinating seawater. This is because it has several advantages. First, it available in plenty from the oceans therefore offers a more steady supply. The other advantage is desalinating seawater does not impair natural ecosystems. However, the method has several disadvantages that include use of more energy during the treatment process as compared to the treatment of fresh water sources. There is also a growing concern that the large-scale desalination of seawater may have an impact in the environment (Chen at al., 2011).

There are several factors considered while designing   plants. These factors have an impact on lives of the membrane creating a need to optimize the membranes for a long life span. In addition, seawater has variations in temperatures, salinity, turbidity and organic matters, which have an affect osmosis. Therefore, treatment plants must put into considerations such factors while building effective desalination plants. The desalination technology developed   from the 1950 when these plants began desalinating seawater for industrial services. Since then, technology has come a long.

Reverse osmosis is applicable in removing a number of ions and molecules. The basic principle is maintaining different concentration between the two sides of the semi permeable membrane. These differing concentrations naturally generate the osmotic pressure. This process has been replicated in the desalinating plants where pressure is applied on the artificial membrane leading to the release of clean water at the side with low pressure.

Desalination process requires het energy that is provided by the power plants hence known as thermal desalination. The process is comparable to a natural cycle that is undergone by water in the natural cycle of evaporation of water from the water body. Condensation of the evaporated paper and as rain or snow after it accumulates in the atmosphere. This process can be subdivided further to different types of desalination. This includes vapor comprehension, multi effect distillation and multi stage flash distillation. The processes are used differently in desalination plants.

The vapor compression type of thermal desolation, heat is used to produce compressed vapor that is collected and recycled throughout the process. The method is the most preferred method of water desalination used by major plants since it is easy to operate. The plants also rely on the use of electrically driven mechanical process, which makes the method preferable due to its ease in operation (Chen et al, 2011).As the demand, for fresh water and shortage of the tradition sources have led technology to be pushed behold its limits creating a need for innovation for more developed desalination methods. However, reverse osmosis still remains as the most popular method of desalinating seawater to clean water for domestic purposes.

There are several challenges that this method of water desalination. The environmental impact of the filtered materials and salts is one of the most challenging issues on the technology. As the demand for more fresh water drives the desalination of more seawater, the waste also increases. Without proper disposal, the waste becomes a challenge and a hazard to the environment. In addition, the more water is desalinated, the higher the energy use considering the energy is a scarce resource. Therefore, a wide production of the process is bound to increase more costs that are commercially viable today. However, this has not stopped developing advanced technology that is sustainable as the world moves towards more crises on the sources of flesh water (Al-Karaghouli & Kazmerski, 2012).

The needed for efficiency in commercial efficiency of desalinating of seawater had led to the need for more research on the type of membranes used for reverse osmosis process. The polypropylene membranes polytetraflouroethylene and polyvinylidene fluoride membranes are developed for their efficiency in the reverse osmosis technology. They are available in tubular, capillary of flat sheet forms. The main advantage s of the method is their low conductivity due to their thickness, and ability for high flux permeability, which increases their efficiencies (Rayan, & Khaled, 2003).

The other challenge of the reverse osmosis technology in desalinating seawater is this produced fresh water would still require to be mineralized. Dissolved minerals are art of the human body requirements from fresh water and are available in the naturally existing fresh water. Desalinated water can cause some problem unless it is first treated because of being hype pure. The mineralization process is essential in order in reintroduce the necessary ions that were eliminated during the process. In addition, minerals contained in seawater are different from those existing in the naturally existing fresh water. To balance the se minerals pushes the cost even higher for the desalinated water. Important minerals that have to be reintroduced in the desalinated water include calcium, magnesium, sodium, chlorine and sulphate in the quantities as existence in fresh water. Several mineralization procedures are involved. These procedures include lime dissolution of carbon dioxide or injection of chemical solutions dosages in the desalinated water that contains the minerals in their respective required quantities (Voutchkov & BCEE, 2007).

Conclusion

 Desalination of seawater to fresh drinking water for domestic purposes relies on the principles of osmosis. Specifically, the process uses the reverse osmosis principle where the external pressure is applied to the membrane forcing it to allow the flow of only pure water from the high concentration to low concentration. This process is already in use in various parts of the world but stiff faces numerous challenges that must be addresses to make embraced as a commercial means of producing fresh water. These challenges include the disposal of the waste concentrate arising from desalinated seawater especially with the proven dangerous impact the environment. The other challenge is the high amount of energy required for the whole process. Additional research is ongoing to provide alternative solutions to these challenges which may make the method more economically and environmentally viable.

References:

Al-Karaghouli, A., & Kazmerski, L. (2012). Economic and Technical Analysis of a Reverse-Osmosis Water Desalination Plant Using DEEP-3.2 Software.Journal of Environmental Science and Engineering. A1(3A).

Chen, J. P., Chian, E. S., Sheng, P. X., Nanayakkara, K. N., Wang, L. K., & Ting, Y. P. (2011). Desalination of seawater by reverse osmosis. InMembrane and Desalination Technologies (pp. 559-601). Humana Press.

Elimelech ,M.(2012). seawater desalination. Department of chemical and envirometal engeneeering. Yale university. Newport Beach, California: prize conference presentation

Fritzmann.c et al.(2006). States of the art reverse osmosis desalination. Germany: Aechen

Rayan, M. A., & Khaled, I. (2003). Seawater desalination by reverse osmosis.  (Case study). Desalination153(1), 245-251.

Voutchkov,N.& BCEE(2007). Advanced membrane technologies: Seawater reverses osmosis and design optimization. Stanford University