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Water purification

2007 Schools Wikipedia Selection. Related subjects: Drink

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   Water purification is the removal of contaminants from raw water to
   produce drinking water that is pure enough for human consumption or for
   industrial use. Substances that are removed during the process include
   parasites ( such as Giardia or Cryptosporidium) , bacteria, algae,
   viruses, fungi, minerals (including toxic metals such as Lead, Copper
   etc.), and man-made chemical pollutants. Many contaminants can be
   dangerous—but depending on the quality standards, others are removed to
   improve the water's smell, taste, and appearance. A small amount of
   disinfectant is usually intentionally left in the water at the end of
   the treatment process to reduce the risk of re-contamination in the
   distribution system.

   Many environmental and cost considerations affect the location and
   design of water purification plants. Groundwater is cheaper to treat,
   but aquifers usually have limited output and can take thousands of
   years to recharge. Surface water sources should be carefully monitored
   for the presence of unusual types or levels of microbial/disease
   causing contaminants. The treatment plant itself must be kept secure
   from vandalism and terrorism.

   It is not possible to tell whether water is safe to drink just by
   looking at it. Simple procedures such as boiling or the use of a
   household charcoal filter are not sufficient for treating water from an
   unknown source. Even natural spring water—considered safe for all
   practical purposes in the 1800s—must now be tested before determining
   what kind of treatment is needed.

Stages in typical municipal water treatment

   There are three principal stages in water purification:-
    1. Primary treatment - Collecting and screening including pumping from
       rivers and initial storage
    2. Secondary treatment - removal of fine solids and the majority of
       contaminants using filters, coagulation, flocculation and membranes
    3. Tertiary treatment - polishing, pH adjustment, carbon treatment to
       remove taste and smells, disinfection, and temporary storage to
       allow the disinfecting agent to work.

Primary Treatment

    1. Pumping and containment - The majority of water must be pumped from
       its source or directed into pipes or holding tanks. To avoid adding
       contaminants to the water, this physical infrastructure must be
       made from appropriate materials and constructed so that accidental
       contamination does not occur.
    2. Screening (see also Screen filter) - The first step in purifying
       surface water is to remove large debris such as sticks, leaves,
       trash and other large particles which may interfere with subsequent
       purification steps. Most deep Groundwater does not need screening
       before other purification steps.
    3. 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 by slow sand filters. Storage reservoirs
       also provide a buffer against short periods of drought or to allow
       water supply to be maintained during transitory pollution incidents
       in the source river.
    4. Pre-conditioning - Many waters rich in hardness salts are treated
       with soda-ash ( Sodium carbonate)to precipitate calcium carbonate
       out utilising the common ion effect.
    5. Pre-chlorination - In many plants the incoming water was
       chlorinated to minimise the growth of fouling organisms on the
       pipe-work and tanks. Because of the potential adverse quality
       effects (see Chlorine below), this has largely been discontinued.

Secondary treatment

   There are a wide range of techniques that can be used to remove the
   fine solids, micro-organisms and some dissolved inorganic and organic
   materials. The choice of method will depend on the quality of the water
   being treated, the cost of the treatment process and the quality
   standards expected of the processed water.
    1. pH adjustment - If the water is acidic, lime or soda ash is added
       to raise the pH. Lime is the more common of the two additives
       because it is cheaper, but it also adds to the resulting water
       hardness. Making the water slightly alkaline ensures that
       coagulation and flocculation processes work effectively and also
       helps to minimise the risk of lead being dissolved from lead pipes
       and lead solder in pipe fittings.
    2. Coagulation and flocculation - Together, coagulation and
       flocculation are purification methods that work by using chemicals
       which effectively "glue" small suspended particles together, so
       that they settle out of the water or stick to sand or other
       granules in a granular media filter. Many of the suspended water
       particles have a negative electrical charge. The charge keeps
       particles suspended because they repel similar particles.
       Coagulation works by eliminating the natural electrical charge of
       the suspended particles so they attract and stick to each other.
       The joining of the particles so that they will form larger
       settleable particles is called flocculation. The larger formed
       particles are called floc. The coagulation chemicals are added in a
       tank (often called a rapid mix tank or flash mixer), which
       typically has rotating paddles. In most treatment plants, the
       mixture remains in the tank for 10 to 30 seconds to ensure full
       mixing. The amount of coagulant that is added to the water varies
       widely due to the different source water quality.
       One of the more common coagulants used is aluminium sulfate,
       sometimes called filter alum. Aluminium sulfate reacts with water
       to form flocs of aluminium hydroxide.
       Coagulation with aluminium compounds may leave a residue of
       aluminium in the finished water. This is normally about 0.1 to 0.15
       mg/L. It has been established that Aluminium can be toxic to humans
       at high concentrations.
       Iron(II) sulfate or iron (III) chloride are other common
       coagulants. Iron(III) coagulants work over a larger pH range than
       aluminium sulfate but are not effective with many source waters.
       Other benefits of iron(III) are lower costs and in some cases
       slightly better removal of natural organic contaminants from some
       waters. Coagulation with iron compounds typically leaves a residue
       of iron in the finished water. This may impart a slight taste to
       the water, and may cause brownish stains on porcelain fixtures. The
       trace levels of iron are not harmful to humans, and indeed provide
       a needed trace mineral. Because the taste and stains may lead to
       customer complaints, aluminium tends to be favoured over iron for
       coagulation.
       Cationic and other polymers can also be used. They are often called
       coagulant aids used in conjunction with other inorganic coagulants.
       The long chains of positively charged polymers can help to
       strengthen the floc making it larger, faster settling and easier to
       filter out. The main advantages of polymer coagulants and aids are
       that they do not need the water to be alkaline to work and that
       they produce less settled waste than other coagulants, which can
       reduce operating costs. The drawbacks of polymers are that they are
       expensive, can blind sand filters and that they often have a very
       narrow range of effective doses.
    3. Flocculation - In flocculation coagulants are used but the
       resultant floc is settled out rather than filtered through sand
       filters. The chosen coagulant and the raw water is slowly mixed in
       a large tank called a flocculation basin. Unlike a rapid mix tank,
       the flocculation paddles turn very slowly to minimise turbulence.
       The principle involved is to allow as many particles to contact
       other particles as possible generating large and robust floc
       particles. Generally, the retention time of a flocculation basin is
       at least 30 minutes with speeds between 0.5 feet and 1.5 feet per
       minute (15 to 45 cm / minute). Flow rates less than 0.5 ft/mian
       cause undesirable floc settlement within the basin.
    4. Sedimentation -Water exiting the flocculation basin enters the
       sedimentation basin, also called a clarifier or settling basin. It
       is a large tank with slow flow, allowing floc to settle to the
       bottom. The sedimentation basin is best located close to the
       flocculation basin so the transit between does not permit
       settlement or floc break up. Sedimentation basins can be in the
       shape of a rectangle, where water flows from end to end, or
       circular where flow is from the centre outward. Sedimentation basin
       outflow is typically over a weir so only a thin top layer-furthest
       from the sediment-exits.The amount of floc that settles out of the
       water is dependent on the time the water spends in the basin and
       the depth of the basin. The retention time of the water must
       therefore be balanced against the cost of a larger basin. The
       minimum clarifier retention time is normally 4 hours. A deep basin
       will allow more floc to settle out than a shallow basin. This is
       because large particles settle faster than smaller ones, so large
       particles bump into and integrate smaller particles as they settle.
       In effect, large particles sweep vertically though the basin and
       clean out smaller particles on their way to the bottom.
       As particles settle to the bottom of the basin a layer of sludge is
       formed on the floor of the tank. This layer of sludge must be
       removed and treated. The amount of sludge that is generated is
       significant, often 3%-5% of the total volume of water that is
       treated. The cost of treating and disposing of the sludge can be a
       significant part of the operating cost of a water treatment plant.
       The tank may be equipped with mechanical cleaning devices that
       continually clean the bottom of the tank or the tank can be taken
       out of service when the bottom needs to be cleaned.
       An increasingly popular method of floc removal is by dissolved air
       flotation. A proportion of clarified water, typical 5-10% of
       throughput, is recycled and air is dissolved in it under pressure.
       This is injected into the bottom of the clarifier tank where tiny
       air bubbles are formed which attach themselves to the floc
       particles and float them to the surface. A sludge blanket is formed
       which is periodically removed using mechanical scrapers. This
       method is very efficient at floc removal and reduces loading on
       filters, however it is unsuitable for water sources with a high
       concentration of sediment.
    5. Filtration - After separating most floc, the water is filtered as
       the final step to remove remaining suspended particles and
       unsettled floc. The most common type of filter is a rapid sand
       filter. Water moves vertically through sand which often has a layer
       of activated carbon or anthracite coal above the sand. The top
       layer removes organic compounds including taste and odour. The
       space between sand particles is larger than the smallest suspended
       particles, so simple filtration is not enough. Most particles pass
       through surface layers but are trapped in pore spaces or adhere to
       sand particles. Effective filtration extends into the depth of the
       filter. This property of the filter is key to its operation: if the
       top layer of sand were to block all the particles, the filter would
       quickly clog.
       To clean the filter, water is passed quickly upward through the
       filter, opposite the normal direction (called backflushing or
       backwashing) to remove embedded particles. Prior to this,
       compressed air may be blown up through the bottom of the filter to
       break up the compacted filter media to aid the backwashing process;
       this is known as air scouring. This contaminated water can be
       disposed of, along with the sludge from the sedimentation basin, or
       it can be recycled by mixing with the raw water entering the plant.
       Some water treatment plants employ pressure filters. These work on
       the same principle as rapid gravity filters differing in that the
       filter medium is enclosed in a steel vessel and the water is forced
       through it under pressure.
    6. Slow sand filters may be used where there is sufficient land and
       space. 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 at the base convey
       treated water away for disinfection. Filtration depends on the
       development of a thin biological layer on the surface of the
       filter. 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.
    7. Ultrafiltration membranes are a relatively new development; they
       use polymer film with chemically formed microscopic pores that can
       be used in place of granular media to filter water effectively
       without coagulants. The type of membrane media determines how much
       pressure is needed to drive the water through and what sizes of
       micro-organisms can be filtered out.

Tertiary treatment

   Disinfection is normally the last step in purifying drinking water.
   Water is disinfected to destroy any pathogens which passed through the
   filters. Possible pathogens include viruses, bacteria, including
   Escherichia coli, Campylobacter and Shigella, and protozoans, including
   G. lamblia and other Cryptosporidia. In most developed countries,
   public water supplies are required to maintain a residual disinfecting
   agent throughout the distribution system, in which water may remain for
   days before reaching the consumer. Following the introduction of any
   chemical disinfecting agent, the water is usually held in temporary
   storage - often called a contact tank or clear well to allow the
   disinfecting action to complete.
    1. Chlorine- The most common disinfection method is some form of
       chlorine or its compounds such as chloramine or chlorine dioxide.
       Chlorine is a strong oxidant that kills many micro-organisms.
       Because chlorine is a toxic gas, there is a danger of a release
       associated with its use. This problem is avoided by the use of
       sodium hypochlorite, which is a relatively inexpensive solid that
       releases free chlorine when dissolved in water. Handling the solid,
       however, requires greater routine human contact through opening
       bags and pouring than the use of gas cylinders which are more
       easily automated. Both disinfectants are widely used despite their
       respective drawbacks. A major drawback to using chlorine gas or
       sodium hypochlorite is that they react with organic compounds in
       the water to form potentially harmful levels of the chemical
       by-products trihalomethanes (THMs) and haloacetic acids, both of
       which are carcinogenic and regulated by the U.S. Environmental
       Protection Agency (EPA). The formation of THMs and haloacetic acids
       is minimised by effective removal of as many organics from the
       water as possible before disinfection. Although chlorine is
       effective in killing bacteria, it has limited effectiveness against
       protozoans that form cysts in water. (Giardia lamblia and
       Cryptosporidium, both of which are pathogenic).
    2. Chlorine dioxide is another fast-acting disinfectant. It is,
       however, rarely used, because it may create excessive amounts of
       chlorate and chlorite, both of which are regulated to low allowable
       levels. Chlorine dioxide also poses extreme risks in handling: not
       only is the gas toxic, but it may spontaneously detonate upon
       release to the atmosphere in an accident.
    3. Chloramines are another chlorine-based disinfectant. Although
       chloramines are not as effective as disinfectants, compared to
       chlorine gas or sodium hypochlorite, they are less prone to form
       THMs or haloacetic acids. It is possible to convert chlorine to
       chloramine by adding ammonia to the water along with the chlorine:
       The chlorine and ammonia react to form chloramine. Water
       distribution systems disinfected with chloramines may experience
       nitrification, wherein ammonia is used a nitrogen source for
       bacterial growth, with nitrates being generated as a byproduct.
    4. Ozone (O[3]) is a relatively unstable molecule of oxygen which
       readily gives up one atom of oxygen providing a powerful oxidising
       agent which is toxic to most water borne organisms. It is a very
       strong, broad spectrum disinfectant that is widely used in Europe.
       It is an effective method to inactivate harmful protozoans that
       form cysts. It also works well against almost all other pathogens.
       Ozone is made by passing oxygen through ultraviolet light or a
       "cold" electrical discharge. To use ozone as a disinfectant, it
       must be created on site and added to the water by bubble contact.
       Some of the advantages of ozone include the production of
       relatively fewer dangerous by-products (in comparison to
       chlorination) and the lack of taste and odour produced by
       ozonation. Although fewer by-products are formed by ozonation, it
       has been discovered that the use of ozone produces a small amount
       of the suspected carcinogen Bromate. Another one of the main
       disadvantages of ozone is that it leaves no disinfectant residual
       in the water. Ozone has been used in drinking water plants since
       1906 where the first industrial ozonation plant was built in Nice,
       France. The U.S. Food and Drug Administration has accepted ozone as
       being safe; and it is applied as an anti-microbiological agent for
       the treatment, storage, and processing of foods.
    5. UV radiation is very effective at inactivating cysts, as long as
       the water has a low level of colour so the UV can pass through
       without being absorbed. The main drawback to the use of UV
       radiation is that, like ozone treatment, it leaves no residual
       disinfectant in the water.
       Because neither ozone nor UV radiation leaves a residual
       disinfectant in the water, it is sometimes necessary to add a
       residual disinfectant after they are used. This is often done
       through the addition of chloramines, discussed above as a primary
       disinfectant. When used in this manner, chloramines provide an
       effective residual disinfectant with very little of the negative
       aspects of chlorination.

Additional treatment options

    1. Fluoridation -in many areas fluoride is added to water for the
       purpose of preventing tooth decay. This process is referred to as
       water fluoridation. Fluoride is usually added after the
       disinfection process. In the United States, fluoridation is usually
       accomplished by the addition of dihydrogen hexafluorosilicate,
       which decomposes in water, yielding fluoride ions.
    2. Water conditioning: This is a method of reducing the effects of
       hard water. Hardness salts are deposited in water systems subject
       to heating because the decomposition of bicarbonate ions creates
       carbonate ions which crystalise out of the saturated solution of
       calcium or magnesium carbonate. Water with high concentrations of
       hardness salts can be treated with soda ash ( sodium carbonate)
       which precipitates out the excess salts, through the common ion
       effect, as calcium carbonate of very high purity. The preciptated
       calcium carbonate is traditionally sold to the manufacturers of
       toothpaste. Several other methods of industrial and residential
       water treatment are claimed (without general scientific acceptance)
       to include the use of magnetic or/and electrical fields reducing
       the effects of hard water.
    3. Plumbo-solvency reduction: In areas with naturally acidic waters of
       low conductivity (i.e surface rainfall in upland mountains of
       igneous rocks), the water is capable of dissolving lead from any
       lead pipes that it is carried in. The addition of small quantities
       of phosphate ion and increasing the pH slightly both assist in
       greatly reducing plumbo-solvency by creating insoluble lead salts
       on the inner surfaces of the pipes.
    4. Radium Removal: Some groundwater sources contain radium, a
       radioactive chemical element, including many groundwater sources
       north of the Illinois River in Illinois. Radium can be removed by
       ion exchange, or by water conditioning. The back flush or sludge
       that is produced is, however, a low-level radioactive waste.
    5. Fluoride Removal: Although fluoride is added to water in many
       areas, some areas of the world have excessive levels of natural
       fluoride in the source water. Excessive levels can be toxic. One
       method of reducing fluoride levels is through treatment with
       activated alumina.

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 supplies. Particularly important
   are distillation (de-salination of seawater) and reverse osmosis.
    1. Boiling: Water is heated hot enough and long enough to inactivate
       or kill microorganisms that normally live in water at room
       temperature. Near sea level, a vigorous rolling boil for at least
       one minute is sufficient. At high altitudes (greater than two
       kilometers or 5000 feet) three minutes is recommended. US EPA
       emergency disinfection recomendations In areas where the water is
       "hard" (that is, containing significant dissolved calcium salts),
       boiling decomposes the bicarbonate ions, resulting in partial
       precipitation as calcium carbonate. This is the "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). Boiling does not leave a
       residual disinfectant in the water. Therefore, water that has been
       boiled and then stored for any length of time may have acquired new
       pathogens.
    2. Carbon filtering: Charcoal, a form of carbon with a high surface
       area, absorbs many compounds including some toxic compounds. Water
       passing through activated charcoal is common in household water
       filters and fish tanks. Household filters for drinking water
       sometimes contain silver to release silver ions which have a
       bactericidal effect.
    3. Distillation involves boiling the water to produce water vapour.
       The vapour contacts a cool surface where it condenses as a liquid.
       Because the solutes are not normally vaporised, they remain in the
       boiling solution. Even distillation does not completely purify
       water, because of contaminants with similar boiling points and
       droplets of unvaporised liquid carried with the steam. However,
       99.9% pure water can be obtained by distillation. Distillation does
       not confer any residual disinfectant and the distillation apparatus
       may be the ideal place to harbour Legionnaires' disease.
    4. Reverse osmosis: Mechanical pressure is applied to an impure
       solution to force pure water through a semi-permeable membrane.
       Reverse osmosis is theoretically the most thorough method of large
       scale water purification available, although perfect semi-permeable
       membranes are difficult to create. Unless membranes are
       well-maintained, algae and other life forms can colonise the
       membranes.
    5. Ion exchange: Most common ion exchange systems use a zeolite resin
       bed to replace unwanted Ca^2+ and Mg^2+ ions with benign (soap
       friendly) Na^+ or K^+ ions. This is the common water softener.
    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 deionized water results. The water is usually passed through
       a reverse osmosis unit first to remove non-ionic organic
       contaminants.

Portable water purification

   Portable techniques for purifying water are used for hiking, camping
   etc. or for use in rural areas or emergency situations. Common
   techniques include boiling, disinfection with tablets or
   ultra-filtration using a small hand pump.

Water purification for hydrogen production

   For small scale production of hydrogen water purifiers are installed to
   prevent formation of minerals on the surface of the electrodes and to
   remove organics and chlorine from utility water. First the water passes
   through a 20 micrometre interference ( mesh or screen filter) filter to
   remove sand and dust particles, second, a charcoal filter ( activated
   carbon) to remove organics and chlorine, third stage, a de-ionizing
   filter to remove metallic ions. A test can be done before and after the
   filter for proper functioning on barium, calcium, potassium, magnesium,
   sodium and silicon.

   Another used method is reverse osmosis.
   Retrieved from " http://en.wikipedia.org/wiki/Water_purification"
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