Definition
Water
treatment is the process of removing contaminants from water to make it safe for drinking, cooking, bathing, and
swimming. Without water treatment, waterborne pathogens
such as Cryptosporidium
species, Escherichia coli
, hepatitis A virus, and Giardia intestinalis
(also known as G. lamblia) can proliferate and
cause illness and death, often from the dehydration that follows diarrhea.
Clean water is expected to be clear, colorless, odorless, and tasteless. This requires that water be free of particulates (minute substances). Treating water involves the killing of microbes such as bacteria, viruses, and parasites, and the binding and removal of minerals such as iron, calcium, magnesium, manganese, and sulphur.
To clean water, a series of specific processes must be performed: physical separation of solids by settling and filtration; chemical reactions of coagulation and disinfection; and biological methods such as aeration, bacterial digestion of sludge, and filtration through natural materials. The choice of processes depends on the nature and volume of the water to be purified. An analytical survey must be performed initially.
Two original sources of water exist: surface water and ground water. Surface water comprises rivers, lakes, streams, and ponds. Ground water is accessible by digging wells. Ground water generally requires less water treatment than surface water, which contains more debris and pollutants.
Treatment Processes
Coagulation. When water is first received at a water-treatment plant, large pieces of solid material, such as sewage, are removed by a coarse screen and then discarded. Smaller solid particles are then induced to bind together so that they will form into larger particles through coagulation. Ions with multiple charges (polyelectrolytes) change the pH (acidity) of the water and trigger chemical reactions that cause aggregation. Alum is frequently added to attract dirt particles, which may contain herbicides and pesticides. Lime and soda ash cause calcium and magnesium to precipitate, thus softening the water.
Sedimentation. The material resulting from coagulation, called floc, has sufficient weight that it sinks to the bottom of settling tanks. This separation of solids by sedimentation is time-consuming. Algae eventually rise to the surface, where they may be skimmed. The clearer water on the surface is then slowly siphoned for filtration. Aerobic and anaerobic bacteria may be added to the withheld solids (sludge) to digest organic waste matter and to neutralize pollutants. Carbon dioxide, ammonia, and methane gases are generated. The digested sludge may then be used as a fertilizer supplement in farming.
Filtration
. Remaining particles in the water may be removed by filters made of artificial membranes, nets, or natural materials. Water may be filtered by passing it through beds of sand, gravel, or pulverized coal. Activated charcoal may be added to the water first to remove color, odor, taste, and radioactivity. In another method of removing calcium and magnesium, water may be passed through ion exchange columns, in which sodium ions compete with these cations for binding to porous material.
Aeration is used to remove dissolved elements such as iron, sulphur, and manganese. Air is forced into the water to remove carbon dioxide, hydrogen sulfide, and other gases. In diffused aeration, air is bubbled through the water. In spray aeration, water is sprayed through the air.
The process of removing salt from the water, called desalination,
is often used to make ocean water drinkable in places where fresh water is scarce.
The salt is removed by microfiltration and by reverse osmosis.
Disinfection. Disinfection is the general method of killing pathogens (bacteria, viruses, and parasites). The most common method of water disinfection is chlorination with sodium hypochlorite bleach. Used less frequently are ultraviolet light and ozone aeration. (To disinfect water in one’s home in an emergency, one should boil the water to kill microbes.)
Storage
Treated water must then be stored and delivered under clean conditions to prevent recontamination. The water is stored in closed tanks or reservoirs; from there, it is piped to homes, businesses, and other facilities. Minimal chlorine may be added at this stage to maintain cleanliness. Fluoride also may be added to the treated water as a method to prevent tooth decay.
Impact
The U.S. Safe
Drinking Water Act of 1974 established national drinking-water standards, which includes
maximum acceptable contaminant levels. The act was amended in 1986 and 1996 to
protect natural water sources. The original act was intended to address drinking
water as it flowed in homes, businesses, and public drinking fountains. The
amendments address water safety as it flows from the original water source to the
faucet.
Most water-treatment plants are not prepared to remove pharmaceuticals, including natural and synthetic hormones, that are flushed down the sink or toilet. Those treatment plants that use chemical oxidative processes to remove estrogen and other medications generate disinfection by-products in the water supply that pose potential risks to human health. Communities are organizing collections of unused and unwanted over-the-counter and prescription medications for disposal by authorized incineration.
Bibliography
Amjad, Zahid, ed. Science and Technology of Industrial Water Treatment. Boca Raton, Fla.: CRC Press, 2010. This text by the International Water Association features discussion of the “fundamental and practical aspects of industrial water treatment.”
Binnie, Chris, and Martin Kimber. Basic Water Treatment. 4th ed. Cambridge, England: Royal Society of Chemistry, 2009. A comprehensive textbook on water quality standards and practices in the United States and in Europe.
Brettar, Ingrid, and Manfred G. Hofle. “Molecular Assessment of Bacterial Pathogens: A Contribution to Drinking Water Safety.” Current Opinion in Biotechnology 19 (2008): 274-280. A summary of detection methods for bacterial pathogens in drinking water.
Centers for Disease Control and Prevention. “Safe Water System: A Low-Cost Technology for Safe Drinking Water.” Available at http://www.cdc.gov/safewater/publications_pages/fact_sheets/WW4.pdf.
Edzwald, James K., ed. Water Quality and Treatment: A Handbook on Drinking Water. 6th ed. New York: McGraw-Hill, 2011. This text by the American Water Works Association discusses “state-of-the-art technologies, water quality from source to tap, conventional and advanced methods and processes in water treatment, and drinking water standards and regulations.”
Morris, Robert D. Blue Death: True Tales of Disease, Disaster, and the Water We Drink. New York: HarperCollins, 2007. An epidemiologist who specializes in waterborne diseases discusses the history of water purification and the drinking water industry without avoiding controversy.
Symons, James M. Plain Talk About Drinking Water. 5th ed. Denver, Colo.: American Water Works Association, 2010. Provides consumers with clear information about drinking water in a question-and-answer format written in easily understood language.
No comments:
Post a Comment