Water

WaterAhmad Sameer NawabKardan UniversityKabul, Afghanistan

Rippling Water(Basic)

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ContentsEconomic Importance 1Production of Potable Water 2Break-Point Chlorination and Ozonization 3Flocculation and Sedimentation 4Filtration 5Removal of Dissolved Inorganic Impurities 5Activated Charcoal Treatment 6Safety Chlorination 7Production of Soft or Deionized Water 8Production of Freshwater from Seawater and Brackish Water 9 Production by Multistage Flash Evaporation 10Production using Reverse Osmosis 1 1Facts About Water 12

WaterA raw material in principle available in unlimited quantities, since used water is fed back into the Earth's water circulation

Economical ImportanceWater

Economic ImportanceWater is not consumed since, after use, it is fed back sooner or later into the Earth's water circulation.

The local availability of water (e.g. in arid regions), especially with the purity necessary for the particular application, is another matter. Cheap high purity water is required for many applications

Production OF potable waterWater

Production of Potable Water

Only good spring water can be used as potable water without further treatment.

The untreated water is more or less contaminated depending upon the source.In obtaining potable water some or all of the following steps have to be carried out:

Break-point chlorination (alternatives are ozone and chlorine dioxide)FlocculationSedimentationFiltrationTreatment with activated charcoalSafety chlorinationpH adjustment

Production of Potable Water

The number of steps carried out in practice depends entirely upon the quality of the untreated water.

In the case of spring water only safety chlorination is necessary, toprevent infection from mains water. In the case of strongly polluted water (e.g. water filtered through the banks of the Rhine or Ruhr) almost all the steps are necessary.

In this way potable water can be obtained even from stronglycontaminated water.

However, industrial water with lower purity, e.g. for cooling purposes, requires fewer purification steps.

Production of Potable Water

Further purification steps may also be necessary to:

reduce the concentration of water hardeners (calcium and magnesium ions)remove free carbon dioxide, iron and manganese ions

Certain applications require deionized water. This can be obtained by ion exchange.

Break-Point Chlorination and OzonizationWater

Break-Point Chlorination and Ozonization

Addition of sufficient chlorine to ensure 0.2 to 0.5 mg/L of free chlorine in the water after treatment

In the case of strongly polluted surface water, chlorination is the first purification step and is carried out after removal of any coarse foreign matter.

Sufficient chlorine is added to ensure a free chlorine concentration of (ca) 0.2 to 0.5 mg/L in the water after treatment (break-point chlorination).

Chlorine reacts with water forming hydrochloric acid and the hypochlorite anion, depending upon the PH.

Break-Point Chlorination and OzonizationChlorination results in:

Elimination of pathogenic germs, deactivation of viruses, oxidation of cations such as iron or manganese to higher valency states, chlorination of ammonia to chloramines or nitrogen trichloride, chlorination of phenols to chlorophenols, and chlorination of organic impurities, particularly humic acid, e.g. to aliphatic chlorohydrocarbons.

OR Briefly

Elimination of pathogenic organismsChlorination of ammoniaFormation of undesirable organochloro compounds.

Break-Point Chlorination and OzonizationThe last two processes are undesirable:chlorophenols have very strong taste and some of the aliphaticchlorohydrocarbons (e.g. chloroform) are also suspected of being carcinogenic.It is therefore usual to perform the chlorination only up to the chloramine stage and to carry out the further elimination of impurities, e.g. microbiological degradation processes, on activated charcoal.The most important alternative to chlorination of water is ozonization in which the above-mentioned disadvantages occur to a much lesser extent. However, the higher cost of ozonization is a problem. Ozonization helps subsequent flocculation and biological degradation on activated charcoal. About 0.2 to 1.0 g of ozone is required per m of water, in exceptional cases up to 3 g/m. A further alternative is treatment with chlorine dioxide (from sodiumchlorite and chlorine), in which there is less formation of organochloro-compounds than in the case of chlorination.

Flocculation and SedimentationWater

Flocculation and SedimentationFlocculation:removal of inorganic and organic colloids by adsorption on (in situ produced)aluminum and iron hydroxide flakes.If necessary flocculation aids are added

Preliminary purification by flocculation is necessary, if the untreated water has a high turbidity, particularly as a result of colloidal or soluble organic impurities. Iron or aluminum salts are added to the water, so that iron or aluminumhydroxide is precipitated:

Al2(S04)3 + 6H20 2A1(OH)3 + 3 H2SO4

FeS04CI + 3 H20 Fe(OH)3 + H2SO4 + HCl

Fe2(S04)3 + 6 H2O 2 Fe(OH)3 + 3 H2S04

Flocculation and SedimentationThe optimum pH for flocculation is about 6.5 to 7.5 for aluminum salts and about 8.5 for iron salts. If the natural alkali content of the untreated water is insufficient toneutralize the acid formed, alkali has to be added (e.g. calcium hydroxide or sodium hydroxide). In addition flocculation aids such as poly(acry1amide) or starch derivatives may be added (not in the case of potable water production). When aluminum sulfate Al2(S04)3 .18H20 is used 10 to 30 g/m3 is added. The very fine hydroxide flakes which precipitate are positively charged and adsorb the negatively charged colloidal organic materials and clay particles.A variety of industrial equipment has been used to carry out theflocculation process and the separation of the flocculated materials producing a well-defined sludge suspension layer, which can be removed. Some plant operates with sludge feedback to enable more efficientadsorption. Sludge flocks can also be separated by flotation.

FiltrationWater

FiltrationSeparation Of Undissolved solids over a sand filter, optionally combined with an anthracite filter. Flushing with water or water/air when the filter is covered.

Water having undergone flocculation then has to be filtered.The water is generally filtered downwards through a 1 to 2 m high sand filter with 0.2 to 2 mm sand particles at a rate of 3 to 5 mm/s.

When the filter is covered with impurities this increases the filter resistance and it is then cleaned by flushing upwards together with air, if necessary.Alternatively, a multiple-layer filter can be used, optionally combined with a 0.5 m high anthracite layer

Filtration

Construction of a two layer filter.InletOutletbottomSandfilter charcoalf ) water distribution

Removal of Dissolved Inorganic ImpuritiesWater

Removal of Dissolved Inorganic ImpuritiesHardeners, especially calcium and magnesium hydrogen carbonates rendered an troublesome by addition of:sulfuric acid and expulsion of carbon dioxide, calcium hydroxide and separation of the carbonates formed.

Untreated water containing much dissolved hydrogen carbonate forms, upon heating, a precipitate consisting mainly of calcium carbonate (carbonate hardness, boiler scale):

Ca(HCO3)2 CaC03 + C02 + H20

The carbonate hardness can be removed by adding acid, whereupon the more soluble calcium sulfate is formed:

Ca(HCO3)2 + H2S04 CaS04 + 2C02 + 2H20

Removal of Dissolved Inorganic ImpuritiesThe resulting carbon dioxide has to be expelled, as carbon dioxide-containing water is corrosive. The hydrogen carbonate can be removed by the addition of calciumhydroxide:

Ca(HCO3)2 + Ca(OH)2 2 CaC03 + 2 H2O

In an industrial variant of this process the calcium hydroxide, as a solution or a suspension, is added to hydrogen carbonate-containing water and the mixture passed over calcium carbonate beads, upon which the freshly formed calcium carbonate is deposited. Fresh beads form on the crystal nuclei added and those beads whichbecome too large are separated off.

Removal of Dissolved Inorganic ImpuritiesCarbon dioxide must also be expelled from soft water containing a high concentration of carbonic acid, a simultaneous hardening can be obtained by filtering over semi-calcined dolomite.

Iron and manganese are present as bivalent ions in many waters. They are removed by oxidation to their oxide hydrates, preferably with air, and if necessary after increasing the PH. These are then filtered off. Treatment with air expels the dissolved carbon dioxide at the same time. If air is an insufficiently powerful oxidation agent, e.g. when considerable quantities of humic acid (which acts as a complexing agent) is present, stronger oxidizing agents such as chlorine or ozone are used.Small quantities of phosphates are desirable in household effluent to protect household equipment from corrosion by suppressing heavy metal dissolution. Reservoirs can contain too much phosphate due to run off from intensivelyused agricultural areas.

Removal of Dissolved Inorganic Impurities

This is then precipitated by flocculation with iron or aluminum salts.Dedicated nitrate removal is hardly used despite known processes for DE nitrification, the mandatory minimum concentrations being obtained by mixing. Decomposition of ammonium salts is carried out on biologically colonizedactivated charcoal filters.

Removal of iron and manganese ions by oxidation of the bivalent ions with air, or if necessary, with chlorine and separation of the oxide hydrates formed Dissolved carbon dioxide also expelled during air oxidation.

Activated Charcoal TreatmentWater

Activated Charcoal TreatmentIf after the above-mentioned treatment steps, water still contains nonionic organic impurities e.g. phenolic matter orchloro/bromohydrocarbons from chlorination, adsorption by treatment with activated charcoal is advisable.

Activated charcoal provides an additional safety element for dealing with sporadic discharges, e.g. accidental, into of organic substances e.g. mineral oil, tempering oils.

So-called absorber resins based on poly(styrene) are recommended as an alternative to activated charcoal, but have as yet found little application. Chlorohydrocarbons and phenols are efficiently adsorbed by activated charcoal.

Humic acid is less well adsorbed, its detection being a sign of activated charcoal filter exhaustion.

Activated Charcoal TreatmentIf powdered charcoal is added (widely used in the USA) adsorption can be carried out simultaneously with flocculation, but passing through a bed of granular activated charcoal beds is more widely used in Europe.

Use of powdered charcoal has the advantage that the amount used can be easily adjusted to the impurity level of the water and that the investment costs are low.

Powdered charcoal is, however, not easy to regenerate, whereasgranular activated charcoal can be regenerated thermally.

Since the composition of the impurities varies from water to water, the conditions required for the treatment of water with granular activated charcoal (e.g. number of filters, contact time) have to be established empirically.

Activated Charcoal TreatmentThe release of already adsorbed compounds e.g. chloro-alkanes into the eluant due to displacement by more easily adsorbed compounds (chromatographic effect) has, however, to be avoided.

About 50 to 150 g TOC/m3 (TOC = total organic carbon) of organic carbon are on average removed from water per day.

This value is higher, if the water is not break-point chlorinated or is pretreated with ozone.

Back flushing is used to remove the sludge from the activated charcoal filter.

Thermal reactivation of the filters under similar conditions to activated charcoal production has to be performed periodically to avoid break-through of pollutants.

Activated Charcoal TreatmentThis can be carried out either at the waterworks or by the manufacturer of the activated charcoal.

The activated charcoal treatment also has effects other than the elimination of dissolved organic impurities:

excess chlorine is decomposedammonia and some of the organic compounds are biologically oxidized.iron and manganese oxide hydrates are removed.

Between 5O and I5O g TOC/m3 water removed by activated carbon per day

Activated Charcoal TreatmentActivated charcoal treatment also leads to:

Decomposition of excess chlorineBiological oxidation of ammonia and organic compounds by microbiological processes on the activated charcoal surface.removal of iron and manganese ions

Safety ChlorinationWater

Safety ChlorinationAvoidance of reinfection of potable water in the distribution network by adding 0.1 to 0.2 mg/L chlorine.

After the water treatment is finished a safety chlorination is carried out to prevent reinfection of the potable water in the distribution network.

This is also necessary after prior ozonization.

Potable water contains about 0.1 to 0.2 mg/L chlorine.

Production of Soft or Deionized WaterWater

Production of Soft or Deionized WaterWater with a lower hardener content is required for a range of industrial processes.

This can be accomplished by ion exchange with solid polymeric organic acids, the ion exchangers.

When the sodium salt of sulfonated poly(styrene) is used as the cation exchanger, calcium and magnesium ions are exchanged for sodium ions:

PS-SO3-Na+ + 0.5 Ca2+ PS-S03-Ca2+o.5 + Na+ [PS poly(styrene)]

Production of Soft or Deionized WaterRegeneration of ion exchangers charged with calcium and magnesium ions (1 L of ion exchange material can be charged with ca. 40 g of CaO) can be accomplished by reversing the above equation by (countercurrent) elution with 5 to 10% sodium chloride solution.

If the hardeners are present as hydrogen carbonate, the eluant becomesalkaline upon heating:

2 NaHCO3 Na2C03 + CO2 + H2O

If ion exchangers are used in the acid form, then the eluant will be acidic:

PS-SO3-H+ + M+ 4 PS-SO3-M+ + H+(M+: monovalent metal ion or equivalent of a multivalent ion)

Production of Soft or Deionized WaterIf (weakly acidic) resins containing carboxy-groups are used, only those hardeners present as hydrogen carbonates are removed, as only the weak carbonic acid can be released:

PS-(COOH)2+CA(HCO3)2 PS-(COO-)2CA2++2CO2 +2H2O

For very high purity water (for applications such as high performance boilers or in the electronics industry) virtually ion-free water is required. This is achieved in alternate layers of cation and anion exchangers or so-called mixed bed exchangers. In these, both strongly acid cationic exchangers in the proton form and basic ion exchangers based on poly(styrene) modified with amino- orammonium-groups are present, e.g.

PS-N(CH3)2 or PS-N(CH3)2+OH-

Production of Soft or Deionized WaterBasic ion exchangers remove anions and are regenerated with sodium hydroxide, e.g.

PS-N(CH3)3+OH- + CI- + PS-N(CH3)3+Cl- + OH-

Upon passing salt-containing water through a mixed bed, the cations are replaced by protons and the anions by hydroxide ions. Protons and hydroxide ions together form water, making the resulting water virtually ion-free with an ion residue of 0.02 mg/L. The higher density of anion exchangers (than cationic exchangers) makes theregeneration of mixed beds possible. The mixed bed ionxchange columns are flushed from the bottom upwards with such a strong current of water that the resins are transported into separate zones, in which they can be regenerated independently of one another.For the electronics industry etc. a further purification using reverse osmosis is necessary to remove dissolved nonionic organic compounds. Distillation (distilled water) is no longer economic.

Production of Freshwater from Seawater and Brackish WaterWater

Production of Freshwater from Seawater and Brackish WaterProduction by Multistage Flash EvaporationSeawater contains on average 3.5% by weight of dissolved salts, for the most part sodium chloride.

Calcium, magnesium and hydrogen carbonate ions are also present.Potable water should not contain more than 0.05% of sodium chloride and less than 0. I o/o of dissolved salts.

The removal of such quantities of salt from seawater using ion exchangers would be totally uneconomic.

Distillation processes are currently mainly used in the production of potable and irrigation water from seawater.

Distillation is carried out by multistage (vacuum) flash evaporation.

Production of Freshwater from Seawater and Brackish WaterImportant processMultistage (vacuum) flash evaporation

Flowchart of a multistage distillation plant.V evaporator; K heat exchanger (preheater); E expansion valve

Production of Freshwater from Seawater and Brackish WaterSeawater freed of particulate and biological impurities is evaporated at temperatures of 90C up to 120C in a number - generally 18 to 24 - of stages in series. The seawater feed is also the coolant for condensing the stream produced and in so doing is heated up as it proceeds from stage to stage.

In the first (hottest) stage the energy required for the complete system is supplied by stream using a heat exchanger.

The temperature of the ever more concentrated salt solution decreases from stage to stage as does the prevailing pressure.

Additional seawater is necessary in a supplementary circuit for cooling the steam produced in the last (coolest) stages.

This is returned directly to the sea, which represents a considerable energy loss

The rest of the prewarmed water is used as feed-water and is heated by the final heater and subjected to evaporation.

The concentrate, which is not recycled to the final heater, is run off. The concentration factor of the run off concentrate is about I .6 with respect to the seawater.

Disposal of this concentrate also represents an energy loss.The quality of the seawater has to fulfill certain requirements: in addition to the removal of coarse foreign matter and biological impurities, hardener removal or stabilization is necessary.

Calcium carbonate and magnesium hydroxide (Brucite) are deposited from untreated seawater onto the heat exchanger surfaces withloss of carbon dioxide, resulting in a strong decrease in thedistillation performance of the plant. Production of Freshwater from Seawater and Brackish Water

Hardener precipitation can be prevented by adding sulfuric acid, whereupon the fairly soluble calcium and magnesium sulfates are formed.

However, considerable quantities of acid are required and desalination plants are often poorly accessible.

Furthermore, exact dosing is necessary, underdosing leading to encrustation and overdosing leading to corrosion.

Therefore polyphosphates are currently used for hardener stabilization in understoichiometric quantities in the first (hottest) stage at temperatures of up to ca. 90C. Above 90C polyphosphates(sodium tripolyphosphate) hydrolyze too rapidly, therebylosing their activity and forming precipitates. Production of Freshwater from Seawater and Brackish Water

In plants operating above 90C, poly(maleic acid) is almost exclusively used for hardener stabilization.

It is usual to use sludge balls for removing encrustation. Above 120C calcium sulfate precipitates out as anhydrite (the solubilityof calcium sulfate decreases with increasing temperature), which in practice limits the final heater temperature to 120C.

The cost of potable water production from seawater is mainly dependent upon the cost of the energy consumed. It is, however, considerably higher than that for potable water produced from freshwater, a factor of 4 in Europe. Production of Freshwater from Seawater and Brackish Water

Production using Reverse Osmosis

Currently another process for the production of potable water from seawater is becoming established:

Reverse osmosis (RO). The RO-process is particularly suitable forsmall plants. Therefore almost 70% of all plants operate according this principle, but they account for only 35% of the desalination capacity.

In osmosis, water permeates through a semipermeable membrane from a dilute solution to a concentrated solution resulting in a hydrostaticpressure increase in the concentrated solution.

This process proceeds spontaneously. Production of Freshwater from Seawater and Brackish Water

In reverse osmosis, water with a low salt content is produced by forcing a salt-containing solution through a semipermeable membrane underpressure.

To produce a usable quantity of water, the pressure applied must be substantially higher than the equilibrium osmotic pressure.

This is 3.5 bar for a 0.5% by weight salt solution.

Pressures of 40 to 70 bar are necessary for water production, the higher the pressure on the feed water side the higher the permeation of water.

However, the salt concentration in the water thus produced increases with increasing pressure, as the membrane is unable to retain thesalt completely.

A multistep process has sometimes to be used.

Production of Freshwater from Seawater and Brackish Water

The membranes are manufactured from acetylcellulose or, more preferably, polyamide.

The technical construction is complicated and made expensive by the large pressure differences and the need for thin membranes.

Bundles of coiled thin hollow capillaries (external diameter 0.1 mm, internal diameter 0.04 mm) are, for example, placed in a pressure cylinder (Shown in the figure in the coming slide) These capillaries protrude from the ends of the cylinder through plastic sealing layers Of the (high salt content)-water fed into the cylinder from the other side, 30% passes through the capillary walls into the capillaries and the rest is run off as concentrate and disposed of. Production of Freshwater from Seawater and Brackish Water

An intensive and expensive pretreatment of the feed water is also necessary:i n addition to the removal of all colloidal and biological impurities, treatment of the feed water is also necessary e.g. by acid addition.

The use of feed water from wells in the neighborhood of beaches isparticularly favored.

Production of Freshwater from Seawater and Brackish Water

In water production, reverse osmosis requires less than 50% of the energy required by multistage flash distillation (8 to 10.6 kWh for freshwater for a capacity of 19. lo3 m3/d).Schematic lay-out of a RO-module. Production of Freshwater from Seawater and Brackish Water

Facts About WaterWater

FactsWater is the most common substance found on earth.

In 1989, Americans dumped 365 million gallons of motor oil or the equivalent of 27 Exxon Valdez spells.

Of all the earth's water, 97 percent is salt water located in oceans and seas.

Only one percent of the earth's water is available for drinking water.

About two thirds of the human body is water. Some parts of the body contain more water than others. For example, 70 percent of your skin is water.

There are more than 200,000 individual water systems providing water to the public in the United States.

FactsPublic water suppliers process 34 billion gallons of water per day for domestic and public use.

Approximately 1 million miles of pipelines and aqueducts carry water in the United States and Canada. That's enough to circle the earth 40 times.

About 800,000 water wells are drilled each year in the United States for domestic, farming, commercial, and water testing purposes.

Sixty-one percent of Americans rely on lakes, rivers, and streams as their source of drinking water. The other 39 percent rely on ground water -- water located underground in aquifers and wells.

In 1974, Congress passed the Safe Drinking Water Act to ensure that drinking water is safe for consumption. The Act requires public water systems to monitor and treat drinking water for safety.

Facts

More than 13 million households drink from their own private wells and are responsible for treating and pumping the water themselves.

Industries released 197 million pounds of toxic chemicals into waterways in 1990 alone.

The average daily requirement for fresh water in the United States is about 338 billion gallons a day, with about 300 billion gallons used as untreated water and for agriculture and other commercial purposes.

You can survive about a month without food, but only five to seven days without water.

Each person uses about 100 gallons of water a day at home. The average five-minute shower takes between 25-50 gallons of water.

You can refill an 8 oz. glass of water approximately 15,000 times for the same cost as a six-pack of pop.

Facts

The average automatic dishwasher uses 9-12 gallons of water while hand washing dishes can take up to 20 gallons.

If every household in America had a faucet that dripped once each second, we would waste 928 million gallons of water a day.

The five Great Lakes bordering the United States and Canada contain about 20 percent of the world's available fresh water.

More than 39,000 gallons of water are used to manufacture a new car, including tires.

Seventy-five percent of a tree is water. One gallon of gasoline can contaminate approximately 750,000 gallons of water.

ThanksEngr. Ahmad Sameer Nawab