Water purification

Water purification, or drinking water treatment, is the process of removing contaminants from surface water or groundwater to make it safe and palatable for human consumption. A wide variety of technologies may be used, depending on the raw water source, contaminants present, standards to be met, and available finances.


1 Other Water purification techniques
2 Portable water purification
3 See also
4 External links

Sources of Drinking Water

Water to be used in public or private water supplies can be drawn from a variety of sources. Different sources of raw water demand different treatment methods to render it fit for human consumption.

  1. Deep Groundwater - The water emerging from some deep groundwaters may have fallen as rain many decades or even hundreds of years ago. Soil and rock layers will have naturally filtered the groundwater to a high degree of clarity even before it is pumped to the treatment plant. Such water may emerge as springs, artesian springs, or may be extracted from boreholes or wells. Deep groundwater is generally of very high bacteriological quality but may be rich in dissolved solids especially carbonates and sulphates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be present including chloride, and bi-carbonate and there may be a requirement to reduce the iron or manganese content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection is also required. Where groundwater recharge is practised, such groundwaters should be graded as being equivalent to lowland surface waters for treatment purposes.
  2. Shallow groundwaters - water emerging from shallow groundwaters are usually abstracted from wells or boreholes and the bacteriological quality can be variable depending on the nature of the catchment. A variety of soluble materials may be present including potentially toxic metals such as copper or zinc. In parts of Bangladesh, many shallow ground water sources are contaminated with unacceptably high levels of arsenic.
  3. Upland lakes and reservoirs - typically located in the headwaters of rivers systems, upland reservoirs are usually sited above any human habitation and may be surrounded by some form of protection zone to restrict the opportunities for contamination. Bacteria and pathogen levels are usually low but some bacteria, protozoa and algae will be present. Where upland are forested or are peaty, humic acids can colour the water brown. Many upland sources have low pH which requires adjustment before the water is put into supply.
  4. Rivers , Canals and low-land reservoirs - lowland surface waters will have a significant bacterial load and may also contain algae, suspended solids and a variety of dissolved constituents

Water Treatment Methods

  1. Screening - A municipal surface water treatment plant must first screen or sieve out large objects such as trash and leaves. The tighter the mesh of the sieve, the smaller the particles must be to pass through. Filtering is not sufficient to completely purify water, but it is often a necessary first step, since such particles can interfere with the more thorough purification methods.
  2. Storage- Water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is to be by slow sand filters The filtered water is then treated to remove any microscopic organisms including protozoa and bacteria. This is generally followed by a disinfection stage to eliminate any residual bacteria and viruses. For waters that are particularly difficult to treat such as from catchments with intensive agriculture, both physical and biological treatment methods may be combined.
  3. Flocculation -The water is -treated with small volumes of appropriate chemicals which will form a chemical floc which entraps particles. The most common flocculent chemicals are aluminium salts such as aluminium sulfate which is flocculated by the small addition of lime to raise the pH.
  4. Rapid Sand Filters - The use of rapid sand filters is the most common form of physical treatment of water. Passing flocculated water through a sand filter strains out the floc and the particles trapped within it. Where taste and odour may be a problem (organo-leptic impacts), the sand filter may include a layer of activated carbon to remove the taste and odour. Sand filters become clogged with floc after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into special settling tanks so that the floc can precipitate out and is then disposed of as waste material. In some countries this may be used as a soil conditioner.
  5. Slow Sand Filters - Where land and space are available, water may be treated in slow sand filter beds. These rely on biological treatment processes for their action rather than physical filtration. Slow sand filters are carefully constructed using graded layers of sand with the coarsest at the base and the finest at the top. Drains buried at the base of the filter convey treated water away for disinfection. When a new slow sand filter bed is brought into use, raw water is carefully decanted onto the filter material until a water depth of 1 to 3m is achieved, dependant on the size of the filter bed. The water passing through the filter for the first few hours is recirculated through the filter and not put into supply. Within a few hours, a biological film comprised of bacteria, protozoa, fungi, and algae builds on the surface of the sand. This is the Schmutzdecke layer and it is this layer that removes all the impurities. An effective slow sand filter may remain in service for many weeks or even months if the pre-treatment is well designed and produces an excellent quality of water which physical methods of treatment rarely achieve.
  6. Disinfection - The finished water is then disinfected with chlorine gas, chloramine, sodium hypochlorite, chlorine dioxide, ozone, or ultraviolet light, before it is pumped into the distribution system of water mains and storage tanks on its way to consumers. Some plants also pre-chlorinate their raw water influent after the screening phase to reduce the incidence of biological films in the treatment cycle. They may also pre-chlorinate to oxidize and precipitate out dissolved iron and manganese from the water, in order to prevent unsightly iron (orange) and manganese (black) stains in the consumer's sink. Water utilities may choose to further boost chlorine levels (termed re-chlorinating) in the distribution system to counteract any pathogens that may occur. Bleach may be used for emergency disinfection at the rate of 2 drops of 5% bleach per liter or quart of clear water according to a treatment table in the following US EPA document Emergency Disinfection (http://www.epa.gov/safewater/faq/emerg.html).

Many environmental and cost considerations affect the siting and design of water purification plants. Groundwater is cheaper to treat, but aquifers once depleted can take thousands of years to recharge. Surface water sources must be carefully monitored for the presence of unusual types or levels of contaminants. The treatment plant itself must be kept secure from vandalism or terrorism and the presence of large quantities of dangerous chemicals mandates special training for workers and emergency personnel. The facility must responsibly dispose of its settled and filtered solids and prevent them from contaminating the treatment components or the source waters. All facilities disinfect finished water, but the exact method of disinfection can be controversial, and the costs and benefits of different methods must be evaluated.

Other Water purification techniques

Other popular methods for purifying water, especially for local private supplies are listed below. In some countries some of these methods are also used for large scale municipal supply. Particularly important are Distillation (de-salination of sea-water) and reverse osmosis

  1. Boiling: Water is heated to its boiling point long enough to inactivate or kill microorganisms that normally live in water at room temperature. Near sea level, bring the water to a vigorous rolling boil for at least one minute. At high altitudes (greater than a kilometer or a mile) boil for three minutes. Source: US EPA (http://www.epa.gov/safewater/faq/emerg.html) In areas where the water is "hard", (containing dissolved calcium salts), boiling decomposes the bicarbonate ion, resulting in some (but not all) of the dissolved calcium being precipitated in the form of calcium carbonate. This is the so-called "fur" that builds up on kettle elements etc. in hard water areas. With the exception of calcium, boiling does not remove solutes of higher boiling point than water, and in fact increases their concentration (due to some water being lost as vapour).
  2. Carbon filtering: Charcoal, a form of carbon with a high surface area due to its mode of preparation, adsorbs many compounds, including some toxic compounds. Water is passed through activated charcoal to remove such contaminants. This method is most commonly used in household water filters and fish tanks. Household filters for drinking water sometimes also contain silver, trace amounts of silver ions having a bactericidal effect.
  3. Distilling: Distillation involves boiling the water to produce water vapour. The water vapour then rises to a cooled surface where it can condense back into a liquid and be collected. Because the solutes are not normally vaporized, they remain in the boiling solution. Even distillation does not completely purify water, because of contaminants with similar boiling points and droplets of unvaporized liquid carried with the steam. However, 99.9% pure water can be obtained by distillation.
  4. Reverse osmosis: Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. The term is reverse osmosis, because normal osmosis would result in pure water moving in the other direction to dilute the impurities. Reverse osmosis is theoretically the most thorough method of large-scale water purification available, although perfect semi-permeable membranes are difficult to create.
  5. Ion exchange: Most common ion exchange systems use a zeolite resin bed and simply replace unwanted Ca2+ and Mg2+ ions with benign (soap friendly) Na+ or K+ ions. This is the common water softener. A more rigorous type of ion exchange swaps H+ ions for unwanted cations and hydroxide (OH-) ions for unwanted anions. The result is H+ + OH- → H2O. This system is recharged with hydrochloric acid and sodium hydroxide, respectively. The result is essentially deionized water.
  6. Electrodeionization: Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity de-ionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.

Portable water purification

Portable drinking water systems or chemical additives are available for hiking, camping, and travel in remote areas. Portable pump filters are commercially available with ceramic filters that will filter 5000 to 50,000 liters per cartridge. Some also utilize activated charcoal filtering.

Chemical additives include chlorine dioxide or iodine solutions.

Iodine, in solution, crystallized, or in tablets, is added to water. The iodine kills off many, but not all, of the most common pathogens that may be present in natural fresh water sources such as lakes, rivers, and streams. Carrying iodine for water purification is a lightweight but imperfect solution for those in need of field purification of drinking water.

Solar Disinfection (Sodis): Microbes are destroyed through temperature and UVA radiation, provided by the Sun. Water is placed in a transparent plastic bottle, which is oxygenated by shaking, followed by topping-up. It is placed on tile or metal for six hours in full sun, which raises the temperature and gives an extended dose of solar radiation, killing any microbes that may be present. The combination of the two provides a simple method of disinfection for tropical developing countries. [1] (http://www.sodis.ch/Text2002/T-Howdoesitwork.htm)

See also

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