While the major part of the earth is covered with water in the form of oceans, seas, rivers, and lakes, such water is raw and not fit for human consumption. It has to be purified by removing chemicals and other impurities that contaminate it. In nearly all parts of the world, raw water from lakes and rivers is purified by Water Purification Plants. In a few global areas where there are no rivers, ocean and sea water is purified by Desalination Plants.
River/Lake Water Purification Process
Distillation
This technology involves heating water to boiling point. The water vapor which is given out is directed to a condenser where cooling water acts on it to bring down the temperature and cause it to condense, whereupon it is gathered and stored. A majority of the contaminants stay behind. Distillation is beneficial as it is reusable and separates a large variety of contaminants from pure water. Its drawbacks include heavy consumption of electricity, need of painstaking maintenance to guarantee water purity and the possibility of a few contaminants being transferred into the water (H2ro.com).
Ion Exchange
This technology involves passing water through bead-shaped resin materials during which ions in the water are substituted for other ions attached to the beads. The two ion exchange technologies frequently employed are softening and deionization. The former is utilized largely as a pre-treatment technique to decrease water hardness wherein 2 sodium ions are substituted for each magnesium or calcium ion taken from the water. The latter involves the use of cation and anion exchangers; in the cation exchanger, beads made of styrene and divinylbenzene with sulfonic acid content substitute hydrogen ions for any cations found in the raw water, while in the anion exchanger, beads made of styrene with quaternary ammonium content substitute hydroxyl ions for any anions found in raw water. The hydrogen and hydroxyl from both exchangers then join to yield pure water. The benefits of ion exchange deionization include effective separation of soluble inorganic contaminants, ability to revitalize and low capital investment. Its drawbacks include inability to properly separate contaminants such as bacteria and pyrogens, heavy long-term operating costs and possibility of cation and anion exchange beds producing culture bacteria and tiny pieces of resin (H2ro.com).
Carbon Adsorption
During the ion exchange technology process which separates cations and anions from raw water, a third type, called nonionic contaminant, covers the surface of the bead-shaped resins. This action, called ‘resin-fouling,’ reduces the longevity of the resins and undermines their function. Carbon filters are put in an upstream position to safeguard the ion exchange resins by separating nonionic contaminants through a process called carbon adsorption. This process is regulated by the diameter of the tiny holes in the carbon filter, and by the pace of organic molecules passing through the pores. The benefits of carbon adsorption include lengthy life span characterized by high capacity and effective separation of chlorine and soluble organic contaminants. It is disadvantageous because it can produce carbon fines (H2ro.com).
Micro-porous Membrane Filtration
This technology involves usage of 3 kinds of micro-porous filters. Depth filters, made of compressed matted fibers that entrap particles by irregular adsorption, are employed as pre-filters due to their ability to separate 98% of floating particles. Surface filters, made of several media layers that separate contaminants, are also employed as pre-filters due to their ability to separate 99.99% of floating particles. Screen filters, patterned like a sieve, separate all particles bigger than the specifically regulated pore size on the top layer. The benefits of micro-porous membranes are their need for very little maintenance while efficiently separating all contaminants larger than the pore dimensions. Their drawbacks include the inability to separate dissolved contaminants, their relatively high cost and their inability to regenerate (H2ro.com).
Ultra-filtration
This technology separates dissolved contaminants which micro-porous membrane filtration is unable to remove. Ultra-filters are strong, slim, discerning permeable membranes that entrap macromolecules bigger than a specific size; those smaller than that are permitted to enter the filtrate. The benefits of ultra-filtration are its ability to regenerate, its capability to separate almost all particles, pyrogens and colloids larger than their rated size and its ability to yield best quality water with minimum amount of energy. Its only drawback is its inability to separate dissolved inorganic contaminants (H2ro.com).
Reverse Osmosis
Reverse Osmosis involves the use of membranes with a tighter pore structure as compared to ultra-filtration membranes, which enable the removal of nearly all particles, organisms, pyrogens and organics less than 300 Daltons molecular weight. Natural osmosis takes place when solutions having 2 differing amounts of dissolved substances are separated by a membrane. Osmotic pressure then pushes water through the membrane, making it dilute the more concentrated solution (H2ro.com). By then employing reverse osmosis, pressure is applied on the side having the concentrated solution to push the water molecules through the membrane to the side containing the fresh water (Helmenstine). The benefits of RO include the need for minimum maintenance and its ability to separate all kinds of contaminants to a certain degree. Its only drawback is its restricted flow rates (H2ro.com).
Electro Deionization
This relatively new technology involves a module having several cells which contain polypropylene frames having cation and anion permeable membranes on both sides with ion exchange resins in the middle. As the feed water enters the module, the ion-exchange resins entrap dissolved cations and anions which are dragged through the membrane in the direction of the cathode and anode respectively when current is passed through the module. 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. This precludes further merging of ions which are instead made to gather in the middle of the cell from where they are forced out into the drain. As water travels down the channel that runs through the middle of ion exchange resin bed it is subjected to gradual deionization which ultimately splits H2O into H+ and OH- which revitalizes the ion exchange resins. 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 (H2ro.com).
Ultraviolet Radiation
This technology involves sanitization of water by 254 nm UV light produced by mercury low pressure lamps which cause inactivation of microorganisms present in water. A latest UV technology has come up with lamps that produce both 185 nm as well as 254 nm UV light which is able to decrease the Total Organic Carbon content in highly purified water to just 5 ppb. UV radiation benefits include efficient sanitization of water and oxidation of 185 nm and 254 nm organic compounds to 5 ppb TOC. Its drawbacks include its inability to separate contaminants like colloids and its tendency to reduce resistivity (H2ro.com).
Merging them all in a Water Purification Plant
Since each of the preceding 8 water purification technologies separates a distinctive kind of contaminant, none can be depended upon to separate all contaminants. An efficient water purification plant employs a series of purification technologies in proper sequence to produce the best water quality (H2ro.com).
The first stage includes pre-treatment equipment to separate contaminants in feed water. The second stage is RO – the nucleus of the Plant – which separates between 90 to 99% of contaminants. The third stage is the storage container into which water is gathered. The fourth stage is the purifying systems which purify the water by separating traces of any contaminant residue (H2ro.com).
The proper merger of purification technologies and effective pre-treatment serves to yield pure water that no longer contains ionic, organic or microbial contaminants (H2ro.com).
Ocean/Sea Water Purification Process
The predominant content of ocean or sea water along with other contaminants is removed by the Desalination process.
Desalination Plants
While desalination of water can be done by using the RO technology (Helmenstine), RO plants have become outdated and replaced by the much more productive Multi-Storage Flash Distillation desalination plants. MSF plants presently produce more than 85% of global desalinated water (Net Resources Int.).
The MSF process involves utilizing steam to heat tubs inside brine heaters. This causes the incoming ocean or sea water to be heated, after which it travels into a low-pressure container where it quickly boils and turns into steam. Heat exchanger tubes cooled by inflowing feed water then condense the steam into water, which is then re-mineralized to render it palatable (Net Resources Int.).
The Kingdom of Saudi Arabia is the largest global producer of desalinated water. The desalinated water, which supplies 70% of the country’s national water needs, is produced by 30 plants including the Shoaiba Desalination Plant which is the biggest in the world (Net Resources Int.).
References
Helmenstine, Anne Marie. “Reverse Osmosis.” About.com. 2009.
“Purification Technologies.” H2ro.com. (N.d). Web.
“Shoaiba Desalination Plant, Saudi Arabia.” Net Resources International. 2009. Web.