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Contents• Economic Importance 1• Production of Potable Water 2• Break-Point Chlorination and Ozonization 3• Flocculation and Sedimentation 4• Filtration 5• Removal of Dissolved Inorganic Impurities 5• Activated Charcoal Treatment 6• Safety Chlorination 7• Production of Soft or Deionized Water 8• Production of Freshwater from Seawater and Brackish Water 9• Production by Multistage Flash Evaporation 10• Production using Reverse Osmosis 1 1• Facts About Water 12
Water• A raw material in principle available in unlimited quantities, since usedwater is fed back into the Earths water circulation
Economic Importance• Water is not consumed since, after use, it is fed back sooner or laterinto the Earths water circulation.• The local availability of water (e.g. in arid regions), especially withthe purity necessary for the particular application, is another matter.•• Cheap high purity water is required for many applications
Production of Potable Water• Only good spring water can be used as potable water without furthertreatment.• The untreated water is more or less contaminated depending upon thesource.• In obtaining potable water some or all of the following steps have to becarried out:• Break-point chlorination (alternatives are ozone and chlorine dioxide)• Flocculation• Sedimentation• Filtration• Treatment with activated charcoal• Safety chlorination• pH adjustment
Production of Potable Water• The number of steps carried out in practice depends entirely upon thequality of the untreated water.• In the case of spring water only safety chlorination is necessary, to• prevent infection from mains water.•• In the case of strongly polluted water (e.g. water filtered through thebanks of the Rhine or Ruhr) almost all the steps are necessary.• In this way potable water can be obtained even from strongly• contaminated water.• However, industrial water with lower purity, e.g. for coolingpurposes, 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 ionexchange.
Break-Point Chlorination and Ozonization• Addition of sufficient chlorine to ensure 0.2 to 0.5 mg/L of free chlorinein the water after treatment• In the case of strongly polluted surface water, chlorination is the firstpurification 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 to0.5 mg/L in the water after treatment (break-point chlorination).• Chlorine reacts with water forming hydrochloric acid and the hypochloriteanion, depending upon the PH.
Break-Point Chlorination and OzonizationChlorination results in:• Elimination of pathogenic germs, deactivation of viruses, oxidation ofcations such as iron or manganese to higher valency states, chlorinationof ammonia to chloramines or nitrogen trichloride, chlorination ofphenols to chlorophenols, and chlorination of organicimpurities, particularly humic acid, e.g. to aliphatic chlorohydrocarbons.• OR Briefly• Elimination of pathogenic organisms• Chlorination of ammonia• Formation of undesirable organochloro compounds.
Break-Point Chlorination and Ozonization• The last two processes are undesirable:• chlorophenols have very strong taste and some of the aliphatic• chlorohydrocarbons (e.g. chloroform) are also suspected of beingcarcinogenic.• It is therefore usual to perform the chlorination only up to thechloramine stage and to carry out the further elimination ofimpurities, e.g. microbiological degradation processes, on activatedcharcoal.• The most important alternative to chlorination of water is ozonization inwhich 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 onactivated charcoal.• About 0.2 to 1.0 g of ozone is required per m’ of water, in exceptionalcases up to 3 g/m’.• A further alternative is treatment with chlorine dioxide (from sodium• chlorite and chlorine), in which there is less formation of organochloro-compounds than in the case of chlorination.
Flocculation and Sedimentation• Flocculation:• 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 waterhas a high turbidity, particularly as a result of colloidal or soluble organicimpurities.• Iron or aluminum salts are added to the water, so that iron or aluminum• hydroxide is precipitated:Al2(S04)3 + 6H20 2A1(OH)3 + 3 H2SO4FeS04CI + 3 H20 Fe(OH)3 + H2SO4 + HClFe2(S04)3 + 6 H2O 2 Fe(OH)3 + 3 H2S04
Flocculation and Sedimentation• The optimum pH for flocculation is about 6.5 to 7.5 for aluminum salts andabout 8.5 for iron salts.• If the natural alkali content of the untreated water is insufficient to• neutralize the acid formed, alkali has to be added (e.g. calcium hydroxideor sodium hydroxide).• In addition flocculation aids such as poly(acry1amide) or starch derivativesmay 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 andadsorb the negatively charged colloidal organic materials and clay particles.• A variety of industrial equipment has been used to carry out the• flocculation process and the separation of the flocculated materialsproducing a well-defined sludge suspension layer, which can be removed.• Some plant operates with sludge feedback to enable more efficient• adsorption.• Sludge flocks can also be separated by flotation.
Filtration• Separation Of Undissolved solids over a sand filter, optionally combinedwith 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 sandfilter 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 filterresistance and it is then cleaned by flushing upwards together withair, if necessary.• Alternatively, a multiple-layer filter can be used, optionally combinedwith a 0.5 m high anthracite layer
FiltrationConstruction of a two layer filter.a) Inletb) Outletc) bottomd) Sande) filter charcoalf ) water distribution
REMOVAL OF DISSOLVED INORGANIC IMPURITIESWater
Removal of Dissolved Inorganic Impurities• Hardeners, especially calcium and magnesium hydrogen carbonatesrendered an troublesome by addition of:• sulfuric acid and expulsion of carbon dioxide, calcium hydroxide andseparation of the carbonates formed.• Untreated water containing much dissolved hydrogen carbonateforms, upon heating, a precipitate consisting mainly of calciumcarbonate (carbonate hardness, boiler scale):Ca(HCO3)2 CaC03 + C02 + H20• The carbonate hardness can be removed by adding acid, whereupon themore soluble calcium sulfate is formed:Ca(HCO3)2 + H2S04 CaS04 + 2C02 + 2H20
Removal of Dissolved Inorganic Impurities• The resulting carbon dioxide has to be expelled, as carbon• dioxide-containing water is corrosive.• The hydrogen carbonate can be removed by the addition of calcium• hydroxide:Ca(HCO3)2 + Ca(OH)2 2 CaC03 + 2 H2O• In an industrial variant of this process the calcium hydroxide, as asolution or a suspension, is added to hydrogen carbonate-containingwater and the mixture passed over calcium carbonate beads, upon whichthe freshly formed calcium carbonate is deposited.• Fresh beads form on the crystal nuclei added and those beads which• become too large are separated off.
Removal of Dissolved Inorganic Impurities• Carbon dioxide must also be expelled from soft water containing a highconcentration of carbonic acid, a simultaneous hardening can be obtainedby 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 withair, and if necessary after increasing the PH. These are then filtered off.• Treatment with air expels the dissolved carbon dioxide at the same time. Ifair is an insufficiently powerful oxidation agent, e.g. when considerablequantities of humic acid (which acts as a complexing agent) ispresent, stronger oxidizing agents such as chlorine or ozone are used.• Small quantities of phosphates are desirable in household effluent toprotect household equipment from corrosion by suppressing heavy metaldissolution.• Reservoirs can contain too much phosphate due to run off from intensively• used 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 DEnitrification, the mandatory minimum concentrations being obtained bymixing.• Decomposition of ammonium salts is carried out on biologically colonized• activated charcoal filters.• Removal of iron and manganese ions by oxidation of the bivalent ionswith air, or if necessary, with chlorine and separation of the oxidehydrates formed Dissolved carbon dioxide also expelled during airoxidation.
Activated Charcoal Treatment• If after the above-mentioned treatment steps, water still containsnonionic organic impurities e.g. phenolic matter or• chloro/bromohydrocarbons from chlorination, adsorption by treatmentwith activated charcoal is advisable.• Activated charcoal provides an additional safety element for dealingwith sporadic discharges, e.g. accidental, into of organic substancese.g. mineral oil, tempering oils.• So-called absorber resins based on poly(styrene) are recommended asan alternative to activated charcoal, but have as yet found littleapplication.• Chlorohydrocarbons and phenols are efficiently adsorbed by activatedcharcoal.• Humic acid is less well adsorbed, its detection being a sign of activatedcharcoal filter exhaustion.
Activated Charcoal Treatment• If powdered charcoal is added (widely used in the USA) adsorption canbe carried out simultaneously with flocculation, but passing through abed of granular activated charcoal beds is more widely used inEurope.• Use of powdered charcoal has the advantage that the amount usedcan be easily adjusted to the impurity level of the water and that theinvestment costs are low.• Powdered charcoal is, however, not easy to regenerate, whereas• granular activated charcoal can be regenerated thermally.• Since the composition of the impurities varies from water towater, the conditions required for the treatment of water withgranular activated charcoal (e.g. number of filters, contact time)have to be established empirically.
Activated Charcoal Treatment• The release of already adsorbed compounds e.g. chloro-alkanes into theeluant 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 carbonare on average removed from water per day.• This value is higher, if the water is not break-point chlorinated or ispretreated with ozone.• Back flushing is used to remove the sludge from the activated charcoalfilter.• Thermal reactivation of the filters under similar conditions to activatedcharcoal production has to be performed periodically to avoid break-through of pollutants.
Activated Charcoal Treatment• This can be carried out either at the waterworks or by themanufacturer of the activated charcoal.• The activated charcoal treatment also has effects other than theelimination of dissolved organic impurities:• excess chlorine is decomposed• ammonia and some of the organic compounds are biologicallyoxidized.• iron and manganese oxide hydrates are removed.• Between 5O and I5O g TOC/m3 water removed by activated carbonper day
Activated Charcoal Treatment• Activated charcoal treatment also leads to:• Decomposition of excess chlorine• Biological oxidation of ammonia and organic compounds bymicrobiological processes on the activated charcoal surface.• removal of iron and manganese ions
Safety Chlorination• Avoidance of reinfection of potable water in the distributionnetwork by adding 0.1 to 0.2 mg/L chlorine.• After the water treatment is finished a safety chlorination is carried outto 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 Water• Water with a lower hardener content is required for a range of industrialprocesses.• This can be accomplished by ion exchange with solid polymeric organicacids, the “ion exchangers”.• When the sodium salt of sulfonated poly(styrene) is used as the cationexchanger, 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 Water• Regeneration of ion exchangers charged with calcium and magnesiumions (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 becomes• alkaline upon heating:2 NaHCO3 Na2C03 + CO2 + H2O• If ion exchangers are used in the acid form, then the eluant will beacidic:PS-SO3-H+ + M+ 4 PS-SO3-M+ + H+(M+: monovalent metal ion or equivalent of a multivalent ion)
Production of Soft or Deionized Water• If (weakly acidic) resins containing carboxy-groups are used, only thosehardeners present as hydrogen carbonates are removed, as only theweak 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 performanceboilers or in the electronics industry) virtually ion-free water isrequired.• 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 andbasic ion exchangers based on poly(styrene) modified with amino- or• ammonium-groups are present, e.g.PS-N(CH3)2 or PS-N(CH3)2+OH-
Production of Soft or Deionized Water• Basic ion exchangers remove anions and are regenerated with sodiumhydroxide, 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 replacedby protons and the anions by hydroxide ions. Protons and hydroxide ions togetherform water, making the resulting water virtually ion-free with an ion residue of0.02 mg/L.• The higher density of anion exchangers (than cationic exchangers) makes the• regeneration of mixed beds possible.• The mixed bed ionxchange columns are flushed from the bottom upwards withsuch a strong current of water that the resins are transported into separatezones, in which they can be regenerated independently of one another.• For the electronics industry etc. a further purification using reverse osmosis isnecessary 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 Water• Production by Multistage Flash Evaporation• Seawater contains on average 3.5% by weight of dissolved salts, forthe most part sodium chloride.• Calcium, magnesium and hydrogen carbonate ions are also present.• Potable water should not contain more than 0.05% of sodiumchloride and less than 0. I o/o of dissolved salts.• The removal of such quantities of salt from seawater using ionexchangers would be totally uneconomic.• Distillation processes are currently mainly used in the production ofpotable and irrigation water from seawater.• Distillation is carried out by multistage (vacuum) flash evaporation.
Production of Freshwater from Seawater and Brackish Water• Important process• Multistage (vacuum) flash evaporationFlowchart of a multistage distillation plant.V evaporator; K heat exchanger (preheater); E expansion valve
Production of Freshwater from Seawater and Brackish Water• Seawater freed of particulate and biological impurities is evaporated attemperatures of 90°C up to 120°C in a number - generally 18 to 24 - ofstages in series.•• The seawater feed is also the coolant for condensing the stream producedand 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 issupplied by stream using a heat exchanger.• The temperature of the ever more concentrated salt solution decreasesfrom stage to stage as does the prevailing pressure.• Additional seawater is necessary in a supplementary circuit for coolingthe steam produced in the last (coolest) stages.• This is returned directly to the sea, which represents a considerableenergy loss
• The rest of the prewarmed water is used as feed-water and is heated bythe 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 withrespect to the seawater.• Disposal of this concentrate also represents an energy loss.• The quality of the seawater has to fulfill certain requirements: inaddition to the removal of coarse foreign matter and biologicalimpurities, hardener removal or stabilization is necessary.• Calcium carbonate and magnesium hydroxide (Brucite) are depositedfrom untreated seawater onto the heat exchanger surfaces with• loss of carbon dioxide, resulting in a strong decrease in the• distillation performance of the plant.Production of Freshwater from Seawater and Brackish Water
• Hardener precipitation can be prevented by adding sulfuricacid, whereupon the fairly soluble calcium and magnesium sulfates areformed.• However, considerable quantities of acid are required and desalinationplants are often poorly accessible.• Furthermore, exact dosing is necessary, underdosing leading toencrustation and overdosing leading to corrosion.• Therefore polyphosphates are currently used for hardener stabilization inunderstoichiometric quantities in the first (hottest) stage attemperatures of up to ca. 90°C. Above 90°C polyphosphates• (sodium tripolyphosphate) hydrolyze too rapidly, thereby• losing their activity and forming precipitates.Production of Freshwater from Seawater and Brackish Water
• In plants operating above 90”C, poly(maleic acid) is almost exclusivelyused for hardener stabilization.• It is usual to use sludge balls for removing encrustation.•• Above 120°C calcium sulfate precipitates out as anhydrite (the solubility• of calcium sulfate decreases with increasing temperature), which inpractice limits the final heater temperature to 120°C.• The cost of potable water production from seawater is mainly dependentupon the cost of the energy consumed.• It is, however, considerably higher than that for potable water producedfrom 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 fromseawater is becoming established:• Reverse osmosis (RO). The RO-process is particularly suitable for• small plants.• Therefore almost 70% of all plants operate according this principle, butthey account for only 35% of the desalination capacity.• In osmosis, water permeates through a semipermeable membrane froma dilute solution to a concentrated solution resulting in a hydrostatic• pressure 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 asalt-containing solution through a semipermeable membrane under• pressure.• To produce a usable quantity of water, the pressure applied must besubstantially 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 higherthe pressure on the feed water side the higher the permeation of water.• However, the salt concentration in the water thus produced increaseswith increasing pressure, as the membrane is unable to retain the• salt completely.• A multistep process has sometimes to be used.Production of Freshwater from Seawater and Brackish Water
• The membranes are manufactured from acetylcellulose or, morepreferably, polyamide.• The technical construction is complicated and made expensive by thelarge pressure differences and the need for thin membranes.• Bundles of coiled thin hollow capillaries (external diameter 0.1mm, internal diameter 0.04 mm) are, for example, placed in a pressurecylinder (Shown in the figure in the coming slide) These capillariesprotrude from the ends of the cylinder through plastic sealing layers Ofthe (high salt content)-water fed into the cylinder from the otherside, 30% passes through the capillary walls into the capillaries and therest 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 alsonecessary:• i n addition to the removal of all colloidal and biologicalimpurities, treatment of the feed water is also necessary e.g. by acidaddition.• The use of feed water from wells in the neighborhood of beaches is• particularly favored.Production of Freshwater from Seawater and Brackish Water
In water production, reverse osmosis requires less than 50% of the energy required bymultistage 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• Water is the most common substance found on earth.• In 1989, Americans dumped 365 million gallons of motor oil or theequivalent of 27 Exxon Valdez spells.• Of all the earths water, 97 percent is salt water located in oceans andseas.• Only one percent of the earths water is available for drinking water.• About two thirds of the human body is water. Some parts of the bodycontain more water than others. For example, 70 percent of your skin iswater.• There are more than 200,000 individual water systems providing water tothe public in the United States.
Facts• Public water suppliers process 34 billion gallons of water per day fordomestic and public use.• Approximately 1 million miles of pipelines and aqueducts carry water inthe United States and Canada. Thats enough to circle the earth 40times.• About 800,000 water wells are drilled each year in the United States fordomestic, farming, commercial, and water testing purposes.• Sixty-one percent of Americans rely on lakes, rivers, and streams as theirsource 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 thatdrinking water is safe for consumption. The Act requires public watersystems to monitor and treat drinking water for safety.
Facts• More than 13 million households drink from their own private wells andare responsible for treating and pumping the water themselves.• Industries released 197 million pounds of toxic chemicals into waterwaysin 1990 alone.• The average daily requirement for fresh water in the United States isabout 338 billion gallons a day, with about 300 billion gallons used asuntreated water and for agriculture and other commercial purposes.• You can survive about a month without food, but only five to seven dayswithout 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 thesame cost as a six-pack of pop.
Facts• The average automatic dishwasher uses 9-12 gallons of water while handwashing dishes can take up to 20 gallons.• If every household in America had a faucet that dripped once eachsecond, we would waste 928 million gallons of water a day.• The five Great Lakes bordering the United States and Canada containabout 20 percent of the worlds available fresh water.• More than 39,000 gallons of water are used to manufacture a newcar, including tires.• Seventy-five percent of a tree is water.• One gallon of gasoline can contaminate approximately 750,000 gallons ofwater.