Table of Contents
Desalination refers to salt and mineral removal in water to make it fit for human consumption and other uses like agricultural and industrial applications. Desalination has been enhanced by fresh water shortage. It has been adopted around the globe in regions where fresh water demand has increased beyond the supply due to overdrawn or fragile sources and climate change which makes reliable sources to become undependable. The rapid increase in population is also exerting pressure on the already available sources making desalination the only option left in acquiring water. Meanwhile during desalination table salt and brine is produced as the by product (Gurudeo and Mahbub 6-12).
Brine is a high concentrated solution that has to be disposed of or diluted using effluent that is treated and sprayed on open spaces as a way of disposing. Desalination can also be carried out to process waste water that is already treated. The water is used in the mining or industry processing hence reducing the water demand from the public supplies of water. It also helps to reduce treated waste water volume that is introduced to the environment. Desalination has already taken roots in countries such as North Africa, Middle East, southern Europe, china, USA, and Singapore (Everest and Murphree 102).
There are several methods that are employed in desalination. They include distillation, ion exchange, membrane processes, freezing, geothermal, and solar desalination. Multi-stage flash distillation is one of distillation processes used in desalination. It involves sea water distillation by flashing some water into steam in multiple stages referred as counter current heat exchangers. It is common as it contributes to 64% of desalinated water in the globe. The process involves stages which are series of spaces containing condensate collector and heat exchanger. The sequence includes of a cold and hot end and intermediate stages. Each stage has different pressures depending on the water boiling points. A brine heater is found at the hot end (Hoepna 12).
Cold water is pumped in the inlet through the heat exchangers making it to warm up. In the brine heater heat is added and the water flows into stages with lower temperature and pressure through the valves (Younos 11-18).
When it flows back is referred to brine and flows in each stage where it has temperatures above the boiling points. This results to some brine to flash to steam lowering the temperature till equilibrium is arrived at. The steam produced cools and condenses against the tubes of the heat exchanger. This in turn heats the cold water fed into the inlet. The brine and the condensate are pumped out as the condensate is used as fresh water. This desalination process is used in plants like Jebel Ali which is recognized as the largest plant of desalination in the world. This process is mostly applied to produce the largest percentage of desalinated water. It is costly as compared to other desalination methods (Engeler, Desalination Raises Environmental, Cost Concerns).
Meanwhile the most employed process is reverse osmosis which is a membrane process that has been employed since 1970s. It was first demonstrated by Sidney Loeb and Srinivasa Sourirajan in California. It does not require heating or changing of phases making its energy requirement to lower as compared to other desalination processes. The other membrane process is electro dialysis (ED). Reverse osmosis process allows separation of dissolved salts in pressurized saline water by flowing the water through a membrane that is permeable to water. The flow of water membrane through the cell is enhanced by the pressure differential that is developed between the product water and the pressurized inlet water. This pressure is near to the atmospheric pressure (Engeler). The water initially fed continues to flow as brine through the reactor pressurized side. In the process no phase change or heating that takes place and energy is only required in the initial application of pressure to the water fed to the system.
The water fed into the system is pumped against the water permeable membrane into a container that is closed to pressurize it. The remaining water fed and the brine becomes more concentrated hence some portion is withdrawn from the container to lower the remaining dissolved salts concentration. However, the feed water and dissolved salts concentration would rise, if there would be no discharging that would require increasing energy inputs, in order to overcome the osmotic pressure that is naturally increased (Everest and Murphree 35).
This process consists of four processes that are major; pressurization, pretreatment, post- treatment stabilization and membrane separation. Pretreatment involves the treatment of feed water in order to make it compatible with the water permeable membranes by the removal of any solids present, PH adjusting and the addition of threshold inhibitor which regulates scaling that is caused by water constituents such as Calcium Sulphate (Younos 23).
Pressurization of the pretreated water by a pump to pressure level that is appropriate for the salinity of the water fed and the membrane. Membrane separation where they inhibit dissolved salts to pass through while allowing the water that is desalinated to pass through. Since the membranes do not guarantee rejection of all dissolved salts, some are found in the product water at a lower percentage. Feed water application on the membranes results in concentrated brine stream and fresh water stream. The membranes involved in the process come with variety though hollow fine fiber and spiral wound are the most popular. They are made up of aromatic polyamides, cellulose acetate, or thin film composites of polymer used nowadays. Both membranes are employed in sea water and brackish water desalination. Meanwhile variations occur in the specific membrane and pressure vessel to the various pressures employed for the feed water types (Everest and Murphree 54).
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Stabilization involves the passage of the product water through an aeration column in order to adjust the water PH from 5 to 7 and to degasify it before it is transferred into the distribution system which is later used as drinking water. However the water is transferred into a cistern for storage in many cases to be used later.
This process is effective as polyamide and cellulose acetate membranes have been replaced with thin film composite membranes which work at high temperatures, wide range of PH and large limits of chemicals. This allows also the membranes to adjust to more conditions that are found commonly in industrial applications. Reverse osmosis desalination efficiency is generally determined by the status of the membrane (Engeler, Desalination Raises Environmental, Cost Concerns 22).
The product water after desalination by reverse osmosis is used mainly for agricultural and industrial purposes. Most of the hydroponic and greenhouse farmers employ the RO process to purify the water used in the greenhouse as it is considered to be lower in nematodes and bacteria. Industrial applications including pharmaceutical, electronic parts and food manufacturing that require water that is pure use RO as production process element. Advantages of reverse osmosis technique of water desalination include;
• The installation costs are low
• Simple processing system though a clean feed water supply is required to minimize frequent membrane cleaning.
• It has a high production capacity/space ratio that ranges between 25000 and 60000I/day/m2.
• Materials that are nonmetallic are employed in construction resulting to lower maintenance.
• They are perfect for supply of water for emergency purposes as the systems can be assembled from modules that are prepackaged and have increased mobility allowance.
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• Minimal energy requirement
• It is capable of inorganic and organic contaminants removal.
• It has minimal impact to the environment apart from the brine disposal.
• The process uses fewer chemicals.
• The membranes used are too sensitive.
• The feed water requires to be treated before the process in order to extend the membrane life as it removes any present particulates.
• Standards of high quality are needed for equipment and materials operations.
• Proper care of brine disposal is required to minimize environment impacts.
• Membranes can be contaminated with bacteria resulting to some taste or smell to the product water
• A reliable source of energy is required.
• RO technologies are complex and expensive leading to less application in regions with saline water.
Desalination of sea water offers many benefits such as socio-economic, human health and environment by its ability to provide supply of water that is constant and unlimited without interfering with the natural resources of fresh water. This however has resulted to negative results to the environment which are mainly due to brine and chemical discharges from the plants. This mostly affects the quality of the coastal water and the marine life. Brine that is emitted back into the sea mostly has increased temperatures, contain heavy metals and chemicals.
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The brine from the reverse osmosis process however has density that is higher than that of the sea water. Discharges from distillation plants float on the water surface affecting the oceanic community productivity. Due to the increased salinity and temperatures in sea water the oxygen in the water tends to be less soluble leading to its depletion (Gurudeo and Mahbub 16).
The RO brine contains mostly of sodium bisulfite which is applied as chlorine neutralizing agent resulting also to the oxygen depletion. This mostly affects the organisms found in the water sources since the dissolved oxygen may be toxic to them. This can be corrected by aeration prior to discharge into the sea water. Chlorine is an effective biocide and a strong oxidant which is a major distillation processes pollutant. Meanwhile in RO process the application of polyamide membranes which are oxidizing agents’ sensitive makes the brine to be free of chlorine (Engeler, Desalination Raises Environmental, Cost Concerns).
This is because it is neutralized in the water before it is pumped into the RO system. High levels of chlorine in the water may become toxic to the organisms in the water. Chlorine application in desalination process also results to oxidation by-products formation including halogenated organics. This product readily stays for a longer period in the sea water and some have been proved to be carcinogenic to animals. Ozone and mono chloramine biocides have been applied instead of chlorine to curb the situation and ultraviolet light used to remove microorganisms instead of biocides.
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The brine also contains some low heavy metals amounts that are as a result of corrosion of the desalination plants interior surfaces. Contamination by copper is the most common as copper- nickel alloys are mostly used as material for surfaces of the heat exchangers. Meanwhile in the reverse osmosis plants, stainless steel and equipment that are non-metallic are used hence minimal metals are found in its brine. Iron, nickel, molybdenum and chromium are metals found in the brine at very low levels (Hoepna 25).
The metal accumulates in water sediments and suspended materials affecting the habitats and some benthic organisms feed on the deposited materials enriching the metals in their bodies. This is passed to the higher trophic levels when they are consumed. Antiscalants application is common in desalination plants to prevent formation of scales such as polymaleic and polyacrylic acid. Their toxicity levels are low hence low risk to the marine environment upon their release. Aluminum chloride coagulant is applied in the reverse osmosis to facilitate the filtration of materials suspended in feed water. They pose no toxicity risks to the marine life but can lead to low primary production or sessile organisms’ death due to the increased turbidity (Gurudeo and Mahbub 66).
Environmental issues refer to negative aspects on biophysical environment by the human activities. Environmentalism is a movement formed in the 1960s and addresses the issues through education, activism and advocacy. This issues include; pollution, resource depletion, climate change and environment degradation.
Water pollution is one of the major problems facing the world today. It has been estimated to be the leading cause of diseases and deaths worldwide. Water is polluted when it contains some contaminants which makes it unfit for human consumption. Increase in the global population has resulted to depletion of resources such as water, mineral and oil. Other causes include industrial and technological development, irrigation, erosion, forestry and improper resource distribution. Oil spills in the sea has largely contributed to water pollution which results to death of marine organisms. Brine disposal from desalination plants has also contributed to the pollution.
Air pollution is also one of the environment issues that have developed due to industrial and technological advances such as nuclear weapons formation that emit radiations in the air and deforestation. Cutting down of trees has reduced the rate at which air contaminants are cleared hence leading to their accumulation in air. Climate changes resulting to smog formation also contributes to the pollution. Emission of toxins through spraying of chemicals such as pesticides, herbicides, DDT for various applications has resulted into pollution of air (Engeler, Desalination Raises Environmental, Cost Concerns).
Climate changes such as global warming are another major environment issue that has resulted to many effects such as drought. it refers to the overall increase in temperatures worldwide. The climate changes can be indicated by the decline in arctic sea ice over the last decades and the changes in coverage and distribution of vegetation. It has been greatly contributed by deforestation due to the increase in demand for land for agricultural purposes. This is due to the population increase and hence land becomes a necessity. This has resulted to water cycle changes, levels of carbon dioxide in the atmosphere, soil erosion increase and biodiversity decrease.
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Environment degradation refers to environment deterioration due to resource depletion such as water, soil and air; ecosystem destruction and wildlife extinction. This has been mainly contributed by overpopulation. The world population has gradually increased from the 19th century. This has added great pressure on resources such as water supplies and land resulting to deterioration of the resources. Degradation of natural water supplies has led to the development of desalination plants which in turn has led to the pollution of sea water (Engeler, Desalination Raises Environmental, Cost Concerns).
Desalination of water has developed due to environmental issues facing the globe today especially resources degradation which has led to water shortage. However having developed to address the water shortage issue it has caused other problems especially water pollution. This has led to lower production in marine life and even death to some organisms. This has also led to other problems such as species extinction. Environmental issues have been contributed mainly by overpopulation in the globe and rapid technology expansion including nuclear weapons development (Hoepna 33). These issues have also had impact on the environmental health and this has contributed to human diseases such as asthma due to air pollution. Proper measures therefore to control the environmental issues need to be put in place to avoid the development of other issues which will be more complex.