Author: Leon Zhou
Editors: Shamsia Ahmed and Kira Tian
Artist: Aurora Chen
Water is one of the most vital substances for living organisms and is undoubtedly common in human beings’ life. Every individual needs water to survive, but here’s a cruel fact: even though water covers about 71% of the earth's surface, only 3% of the earth's water is fresh, and about 2.5% of the earth's freshwater is unavailable (locked up in glaciers, polar ice caps, atmosphere, and soil; highly polluted; or lies too far under the earth's surface to be extracted at an affordable cost)[1]. In addition to the little amount of freshwater available, a large proportion of water is polluted and can lead to health problems when drunk by people. As a result, the depletion of freshwater resources becomes one of the severest issues waiting to be solved. Researchers are now applying both physical and chemical methods for water purifications. This article will mainly discuss three prevalent chemical methods for purifying water.
1）Flocculation:
Flocculation is the collection of cells into aggregates by the addition of multivalent cations, metal salts, or polymers [2]. During flocculation, fine particles will clump together into a floc. The floc will either float at the top of the liquid or settle to the bottom, which can then be easily filtered, purifying the water.
How does flocculation exactly work in water purification?
Specific chemicals (including Al2(SO4)3, Fe2(SO4)3,NaAlO2), also called coagulants, are added to the water.
With the addition of coagulants, most unwanted waste such as dust, earth particles, and algae, is gathered as flocs.
Flocs heavier than water settle at the bottom and flocs lighter than water floats on the top. Both situations can result in an easy removal of unwanted particles from water.
The advantage of flocculation is that it makes the process of filtering out suspended solid particles very effective: about 90% of the suspended particles can be removed using flocculation.
The major disadvantages of using coagulants and flocculation for treatment are the cost and the need for complicated and frequent monitoring, as well as measuring an accurate amount of chemicals that would be added. The process of flocculation also needs elaborated equipment to function efficiently. In addition, based on various conditions of water, it usually requires a special test (jar test) to determine the choice of specific chemicals and other operating conditions. Therefore, flocculation would not be cost-effective for treating small water supplies.
2) Chlorination:
Chlorination is the process of adding chlorine to water to disinfect it, making the water safe to drink. Usually, chlorine is added to the water in the form of compressed gas (Cl2), sodium hypochlorite solution (NaOCl), or solid calcium hypochlorite (Ca(OCl)2). Those chemicals could be harmful in high doses, but when being added to water, the amount of these chemicals are controlled at low levels where germs can be killed and the water is still safe to drink [3].
Chlorine inactivates a microorganism by damaging its cell membrane. After the cell membrane is weakened, chlorine can enter the cell and disrupt DNA activity [4].
The advantage of chlorination is that, unlike flocculation, it is effective in eliminating bacteria and viruses in water. However, it cannot inactivate all microbes. Still, chlorination is a good disinfection method to use because it is inexpensive. The chlorination process is also relatively easy to implement when compared to other water treatment methods. A common example of chlorination in daily life would be the water treatment in swimming pools. Also, chlorination is one of the methods used to treat tap water.
3) Use of ultraviolet light (UV water purifier)
A UV water purifier treats biologically unsafe water with ultraviolet light. The ultraviolet light damages the DNA of living organisms in the water, including bacteria, viruses, parasites, and fungi, so the organisms can no longer reproduce.
A UV water purifier mainly includes a chamber, holding an ultraviolet light lamp inside. As water flows past the lamp, the ultraviolet light emitted by the lamp will play its role in eliminating illness-causing microorganisms.
Ultraviolet water purification is the most effective method for disinfecting bacteria from water. As one of the chemical methods for water purification, it does not add chemicals into water, which means it does not leave by-products. Compared to chlorination, ultraviolet water purification does not add tastes or odors to the water. In addition, a UV water purifier is easy to maintain, requiring merely an annual UV lamp change.[5]
However, to purify water, ultraviolet light cannot remove contaminants. It only inactivates living organisms, so it is used with other treatment methods (like flocculation).
Citation
Bureau of Reclamation: “Water Facts - Worldwide Water Supply”, https://www.usbr.gov/mp/arwec/water-facts-ww-water-sup.html
J.Masojídek, G.Torzillo “Mass Cultivation of Freshwater Microalgae”, https://www.sciencedirect.com/science/article/pii/B9780124095489093738
EPA. “Basic information about disinfectants in drinking water: Chloramine, chlorine and
chlorine dioxide.” 2013 https://www.epa.gov/dwreginfo/drinking-water-regulations
SDWF. “What is Chlorination?”
“What is a UV Water Purifier and How Does It Work?” Fresh Water Systems. John Woodard.
April 15, 2019 https://www.freshwatersystems.com/blogs/blog/what-is-a-uv-water-
purifier-and-how-does-it-work
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