Introduction
Water covers the major part of the earth surface in form of oceans, seas, rivers, and lakes. However, such water is raw and therefore not fit for direct human consumption. Raw water has to be purified through various techniques in order to remove chemicals and other impurities. In most countries where raw water is from lakes and rivers, water purification plants are used for purification processes. On the other hand, the Gulf-countries such as Saudi Arabia and Kuwait with oceans, and sea, water purification is by Desalination Plants (Purification Technologies).
River/Lake Water Purification Process
There are 8 methods used in the process of raw water purification.
Distillation
Distillation is a process that involves heating water to boiling point, followed by cooling the water vapor through a condenser. The cooled water vapor is gathered and stored. The contaminants remain in the distiller. The principle of distillation has been applied for thousands of years. Distillation is a reusable technique of purification and large varieties of contaminants are separated. However, this method has few drawbacks which include heavy consumption of electricity, skilled and painstaking maintenance to guarantee water purity and possibility of few contaminants being transferred into the water vapor (Purification Technologies).
Ion Exchange
The technique involves two ion exchange technologies namely, softening, and deionization. Raw water is passed through bead-shaped resin materials during which ions are substituted for other ion attached to the beads. Water softening is a pretreatment method used to decrease water hardness whereas water deionization is where sodium ions are replaced for each magnesium or calcium ion taken from the water. This method involves use of cation and anion exchangers. For cation exchangers, beads made of styrene and divinylbenzene with sulfonic acid are used to substitute hydrogen ions in raw water while in anion exchange, styrene beads with quaternary ammonium content substitute hydroxide ions found in raw water. The hydrogen ions and hydroxide ions then combine to form pure water. Ion exchange deionization is beneficial in the following ways; it’s an effective method of separating soluble inorganic contaminants, is reusable, and a low capital investment. Its inability to separate bacteria and pyrogens, heavy operating cost, and possibility of the exchangers culturing bacteria counts for its drawback.
Carbon Adsorption
This process involves water purification using cations and anions bead shaped resins where a third type of resin called nonionic contaminant covers the surface. The action is referred to as resin fouling and it reduces the life of the resins and weakens their function. Carbon filters are then put in an upstream position through a process called carbon adsorption. The effectiveness of this technique is determined by the diameter of the carbon filters and organic filters passing through the pores. Carbon adsorption technique has long life and a key method in separating chlorine and soluble organic contaminants. However, it has a disadvantage in that it can produce carbon fines (Purification Technologies).
Micro-porous Membrane Filtration
In this technique, three kinds of micro-porous filters are used. Depth filters are used as pre-filters because they separate 98% of floating particles. These filters are made up of compressed matted fibers. Surface filters are made up of several media layers that separate 99% of floating particles. Screen filters are sieve-like and they separate all particles bigger than regulated pore size on the top layer. This method requires little or no maintenance but it has the inability to regenerate as well as high cost (Purification Technologies).
Ultra-filtration
Some contaminants are dissolved in water and micro-porous membrane filtration is unable to remove them. Such contaminants are removed by ultra-filtration technique. Macromolecules are entrapped by ultra-filters which are slim, strong and permeable. Ultra-filtration has a number of advantages in that it has the ability to regenerate, can separate almost all particles, and yields the best results with minimum amount of energy. However, this method fails to separate dissolved inorganic particles.
Reverse Osmosis
In reverse membrane, tighter pore membranes are used compared to ultra-filtration. This enables removal of nearly all particles, bacteria, pyrogens, and organic contaminants less than 300 Daltons molecular weight. Unlike natural osmosis where the osmotic pressure pushes the lower concentrated solution to more concentrated solution, the pressure is applied such that solution moves from higher concentration to lower concentration (more dilute part). Reverse osmosis has a capability of separating all kinds of contaminants and needs minimum maintenance, but the flow rates are restricted (Helmenstine).
Electro Deionization
This is a new technique involving a module with several cells located between two electrodes with polypropylene frames having cation and anion permeable membranes on both sides with ion exchange resins in the middle. When the current is passed through the module, water enters and dissolved anions and cations are entrapped by the ion-exchange resins. When they reach the adjacent ion-selective membrane, the existent opposite charges prevent the cations and anions from going all the way to their respective electrodes. The ions gather in the middle of the cell from where they are forced to drain. As water drains, gradual deionization occurs which splits H2O into H+ and OH. Electro Deionization is beneficial due to its low operating costs, ability to efficiently separate dissolved organic contaminants and its non-polluting and safe features. Its only drawback is its pre-purification requirement.
Ultraviolet {UV} Radiation
Purification of water using UV radiation involves subjecting water to a radiation of 254 nm UV light. Such light is produced by mercury pressure lamps which cause inactivation of microorganisms. Lamps of 185 nm are nowadays employed to decrease the Organic Carbon {TOC} content in highly purified water to just 5 ppb. Its drawbacks include its inability to separate contaminants like colloids and its tendency to reduce resistivity.
Merging them all in a Water Purification Plant
Here, almost all the 8 techniques are employed in water purification to produce the best water quality. This is done in several stages; the first stage involves pre-treatment of raw water using carbon filters and softening agents. The second stage is equipped with reverse osmosis technique which separates 90-99% of impurities. Water is gathered in the third stage. The final stage is the purifying systems which purify the water.
Ocean/Sea Water Purification Process
Oceanic and sea water contains dense contaminants which are removed by Desalination process.
Desalination Plants
Desalination process is done using the new technology of Multi-Storage Flash Distillation {MSF} desalination plants. These plants purify more than 85% of global desalinated water. In MSF plants, water is heat to boil and then the steam is cooled. Cooled water is then re-mineralized to make it pleasant. Saudi Arabia has 30 desalination plants including Shoaiba, which is the world’s biggest plant. (Shoaiba Desalination Plant, Saudi Arabia).
Recommendation
The cost of the method and its effectiveness are the key factors when it comes to selecting which method to apply. Also the type of contaminants in different raw water should also be considered when selecting a purification method. The Gulf countries are the largest global producer of desalinated water. Such countries should use MSF plants to purify their water. MSF process utilizes steam to heat inside brine heaters. This method is effective for desalinated water and supplies approximately 70% of the country’s water needs.
Conclusion
Water is important to both mankind and plants. The body requires purified water and therefore different methods discussed above have been employed in water purification. Proper water treatment makes it fit for human consumption. Different methods have been devised to purify different water contaminants. The methods have different advantages and disadvantages and therefore, the chemist is entitled to define which method to use.
Works cited
Helmenstine, Anne M. “Reverse Osmosis.” About.com. 2009.
“Purification Technologies.” H2ro.com. Web.
“Shoaiba Desalination Plant, Saudi Arabia.” Net Resources International. Web.