The need for clean water has greatly increased in the past couple of years due to the increasing population in the metropolitan and suburban areas and the reduction of annual rainfall year by year and diminishing water supply due to climate change which has substantially affected the agriculture industry as it has led to an inflation of prices for food staples and livestock. Desalination plants bring out problems as well as their operating costs are substantially high. Operating a billion dollar plant requires a lot of energy to operate and using resources like coal releases greenhouse gases in the air. Australia has invested billions of dollars in the last 20 years in constructing the desalination plants but due to their operating costs being sky high they are not being used to their intended constructed purpose.
Review the use of desalination plants around the world and analyse how those countries are utilising these multi-billion dollar plants to help with the shortage of water and make a case as to the benefits Australia would receive by heavy use of these desalination plants. Analyse plant running requirements and weigh them against how beneficial the plants will be if they are utilised to their maximum potential rather than leaving them on ‘Hot Stand-by’. Find solution to the energy problem which helps in incrementally increasing the production of desalinated water which in turn will help the farmers who have suffered due to the reduction in rainfall over the last couple of years. Utilising an energy source which is not only renewable but also does not affect the output of desalinated water which was being gained when sources like were being used. Another area is to deduce areas where to construct plants which not only are accessible to a widespread area than constructing smaller plants which will entail higher operating costs.
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The earth contains about 1.4x109 km3 of water, which encompasses around 705 of the total area of the planet, and a vast majority of it which is 97.5% is contains salt. The remaining 2.5% is fresh water where 80% of this is frozen in icecaps or together as soil moisture. Use of both of these resources of water are not easily available for human consumption. The rest of the 0.5% is believed to be supplement all life on earth. These water resources are not evenly spread out around the globe as well as the annual rainfall which is 2x1011m3 and it is also poorly distributed around the globe. Table 1 gives a summary of the distribution of these water resources around the globe.
The different forms of water are categorised on their purpose of usage. First grade water is ideal for safe drinking, household use, and various industrial applications. This particular category of water has a salinity gauge of 5 to 1000 ppm. This form of water is located in rivers and lakes and is attainable by industrial desalination processes. In metropolitan cities varying levels of water salinity are utilised, where salinity below 150 ppm is utilised for drinking while salinity level of 1000 ppm is utilised various degree of household applications. This proved to be increasingly compelling in light of the fact that the normal per capita utilization of the low salinity drinking water (150 ppm) is constrained to 2 litres per day. Then again, the per capita utilization rate for other household intentions is 200-400 litres per day, which is utilized for cooking, washing, cleaning, cultivating and different purposes. On industrial scale, the most stringent water quality is set by the cosmetics water for boilers and application identified with the electronic industry and pharmaceuticals. The water quality for this application is restricted to a maximum salinity of 5 ppm. This high level of immaculateness is accomplished through the utilization of ion exchangers, which works on low salinity river water or industrially desalinated water. Other industrial applications call for less stringent water quality than those utilized for boilers. Application incorporate chemical reactions, dairy and nourishment, washing and cleaning and cooling.
The second water category has a salinity gauge of 1000-3000 ppm. This category of water is reasonable for irrigation applications and industrial cooling. This applies for high salinity water, which incorporates saline and seawater. The salinity range for brackish water is 3000-10000 ppm. Seawater salinity is 34,000 ppm. Water with salinity over 1000 ppm is regarded as high salinity water. The salinity of seawater is subjected to local conditions, where it is influenced by surrounding and geological conditions. For instance, encased oceans have higher salinity than untamed oceans and seas. Additionally, oceans which are found in regions of high temperature or that get high waste pace of saline water, would positively have higher level of salinity. For instance, the salinity of the bay water close to the shore lines of Kuwait, Saudia Arabia, and the United Arab Emirates may arrive at maximum values near 50,000 ppm. Then again, the salinity of the inlet water close to the Western shores of Florida, USA, may arrive at reduced values of 30,000 ppm. This is a direct result of the enormous measure of fresh water attained from rivers and springs around there.
The amount of fresh water resources has been nearly constant since the inception of life on earth. On the other side, the populace of the world has seen a rise in the span of 200 years. Figure 1 shows the rise of the population and the prediction for the 50 years ahead. The figure describes the following:
- In 1804 the population was 1 billion
- The population reached 2 billion in 123 years
- The population increase to 3 billion in 1960
- Another increment rise to 5 billion till 1987
- The population reached 6 billion by 1999
- It is expected that a population of 7.5 billion will be reached in 2020 and about 9 billion in 2050.
Inspection of the international map indicates the extent of barren region and arid zones, which covers main parts of all continents. The most famous of these deserts is the Great Sahara that encompasses all of the Arabian Peninsula and north Africa. The Great Sahara runs from the eastern shores of Saudi Arabia and for a distance of greater than 400 km to the western shores of Morocco. In particular, the Arabian Peninsula that includes Saudi Arabia, Kuwait, Qatar, Bahrain, UAE and Oman does not possess a single fresh water river. Other large barren regions are found in China, the south west of the US, in most of the Australian continent and in South America.
Desertification occurs around the world and at a fast pace and in return it is having an adverse effect on the weather pattern, rainfall and the whole environment. The main cause of it is unregulated human activities which is a result of the extermination of woodlands, forests, swamps and Savannah. Lands used for farming purposes are being ripped off from their nutrients due to over farming and poor management. Many of the rainforests in the equatorial regions are being turned into farming and mining land which harms the habitats from which they are never able to recover from and form new barren regions. The increase of the populace, development of lifestyle and the reduction of fresh water resources has made industrial desalination a necessity as we head towards the future. As 70% of the world population resides in radius of 70 km from the seas and oceans so moving forward, desalination of seawater is proving to be the most viable way to provide consumable water to the masses and, many countries around the globe see desalination as most economical source of fresh water for the future.
Desalination was practices on ship board till 1800. The technique includes the usage of single stage stills operated in the batch mode. Energy is resourced from cock stoves or furnaces without convalescing the warmness of condensation. The equipment and product effectiveness varied appreciably and were structured on the manufacturer and operator. Mist carryover was always a consistent problem. The enactment of the sugar industry in the 1800 resulted in huge growth of the evaporation process. This involved development of greater efficient and larger scale stills for production of syrup and sugar. The start of desalination industry dates lower back to the early part of the twentieth century. In 1912, a six effect desalination plant with a capability of 75 m3/d was once constructed in Egypt. Production of desalinated water increase due to the oil industry between 1929-1937 but an increment rise in production was seen between 1937 to 1960 at an annual rate 17%.
The process of eliminating salt from sea or brackish water, where the salts are concentrated in the rejected brine stream, Fig 2. The positive and negative ions ae separated during electrodialysis. Thermal and membrane separation methods are used in the desalination process, Fig 3. The thermal separation techniques include two featured categories; evaporation observed by the condensation of the formed water vapour and the other one which involves freezing accompanied via melting of the shaped water ice crystals. The former system is the most common in desalination and almost at all cases it is coupled with power generations unit, which might also be based on steam or gas turbine systems. The evaporation system may also take place over a warmth transfer region and is termed as boiling or inside the liquid bulk and is defined as flashing.
The evaporation method includes the multistage flash desalination (MSF), the multiple effect evaporation (MEE), the single effect vapour compression (SEE), humidification-dehumidification (HDH), and solar stills. The HSH and solar still are different from the evaporation methods as water is evaporated at lower than the boiling temperature and the main driving force for evaporation is the concentration difference of water vapour in the air stream. The single effect vapour compression includes mechanical vapour compression (MVC), thermal vapour compression (TVC), absorption vapour compression (ABVC), adsorption vapour compression (ADVC), and chemical vapour compression (CVC). Vapour compression is mixed together the single and multiple effect desalination units to enhance the efficiency of the thermal process. Solar energy is utilised to desalinate water directly in solars stills which in return can be used as a power source for other thermal processes. The main membrane desalination process is reverse osmosis (RO), where fresh water permeates under high pressure through semi-permeable membranes leaving behind highly concentrated brine solution. Desalination process can also be categorised as energy used to activate the process, Fig 4. This includes the MSF, MEE, HDH, and the processes which combine the thermal, chemical, adsorption or absorption heat pumps. The RO and MVC are categorised as mechanical energy. The last category in Fig shoes the electrical energy to distinguish water and salt. The electrodialysis process where the electric energy helps electrically charged ions to pass through selective membrane.
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