This document discusses various methods for producing potable water, including break-point chlorination, flocculation, sedimentation, filtration, and activated charcoal treatment. It also covers production of soft water through ion exchange and production of freshwater from seawater using processes like multistage flash evaporation and reverse osmosis. The key steps in water treatment are described at a high level, along with some alternative purification methods and technologies.

1. Water
Ahmad Sameer Nawab
Kardan University
Kabul, Afghanistan

2. 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

3. Water
• A raw material in principle available in unlimited quantities, since used
water is fed back into the Earth's water circulation

4. ECONOMICAL IMPORTANCE
Water

5. Economic Importance
• Water 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

6. PRODUCTION OF POTABLE WATER
Water

7. 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)
• Flocculation
• Sedimentation
• Filtration
• Treatment with activated charcoal
• Safety chlorination
• pH adjustment

8. 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, to
• prevent 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 strongly
• contaminated water.
• However, industrial water with lower purity, e.g. for cooling
purposes, requires fewer purification steps.

9. 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.

10. BREAK-POINT CHLORINATION AND OZONIZATION
Water

11. 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.

12. Break-Point Chlorination and Ozonization
Chlorination 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 organisms
• Chlorination of ammonia
• Formation of undesirable organochloro compounds.

13. 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 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 sodium
• chlorite and chlorine), in which there is less formation of organochloro-
compounds than in the case of chlorination.

14. FLOCCULATION AND SEDIMENTATION
Water

15. 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 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 aluminum
• hydroxide 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

16. Flocculation and Sedimentation
• The 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 to
• neutralize 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 the
• flocculation 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 efficient
• adsorption.
• Sludge flocks can also be separated by flotation.

17. FILTRATION
Water

18. Filtration
• Separation 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

19. Filtration
Construction of a two layer filter.
a) Inlet
b) Outlet
c) bottom
d) Sand
e) filter charcoal
f ) water distribution

20. REMOVAL OF DISSOLVED INORGANIC IMPURITIES
Water

21. Removal of Dissolved Inorganic Impurities
• Hardeners, 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

22. 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 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 which
• become too large are separated off.

23. Removal of Dissolved Inorganic Impurities
• Carbon 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 intensively
• used agricultural areas.

24. 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 colonized
• activated 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.

25. ACTIVATED CHARCOAL TREATMENT
Water

26. Activated Charcoal Treatment
• If after the above-mentioned treatment steps, water still contains
nonionic organic impurities e.g. phenolic matter or
• chloro/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.

27. Activated Charcoal Treatment
• If 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, whereas
• granular 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.

28. Activated Charcoal Treatment
• The 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.

29. Activated Charcoal Treatment
• This 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 decomposed
• ammonia 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

30. Activated Charcoal Treatment
• Activated charcoal treatment also leads to:
• Decomposition of excess chlorine
• Biological oxidation of ammonia and organic compounds by
microbiological processes on the activated charcoal surface.
• removal of iron and manganese ions

31. SAFETY CHLORINATION
Water

32. Safety Chlorination
• Avoidance 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.

33. PRODUCTION OF SOFT OR DEIONIZED WATER
Water

34. Production of Soft or Deionized Water
• Water 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)]

35. Production of Soft or Deionized Water
• Regeneration 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 becomes
• alkaline 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)

36. Production of Soft or Deionized Water
• If (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- or
• ammonium-groups are present, e.g.
PS-N(CH3)2 or PS-N(CH3)2+OH-

37. Production of Soft or Deionized Water
• Basic 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 the
• regeneration 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.

38. PRODUCTION OF FRESHWATER FROM SEAWATER AND BRACKISH WATER
Water

39. Production of Freshwater from Seawater and Brackish Water
• Production by Multistage Flash Evaporation
• Seawater 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.

40. Production of Freshwater from Seawater and Brackish Water
• Important process
• Multistage (vacuum) flash evaporation
Flowchart of a multistage distillation plant.
V evaporator; K heat exchanger (preheater); E expansion valve

41. Production of Freshwater from Seawater and Brackish Water
• Seawater freed of particulate and biological impurities is evaporated at
temperatures of 90°C up to 120°C 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

42. • 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 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

43. • 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. 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

44. • In plants operating above 90”C, poly(maleic acid) is almost exclusively
used 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 in
practice limits the final heater temperature to 120°C.
• 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

45. • 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 for
• small 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 hydrostatic
• pressure increase in the concentrated solution.
• This process proceeds spontaneously.
Production of Freshwater from Seawater and Brackish Water

46. • In reverse osmosis, water with a low salt content is produced by forcing a
salt-containing solution through a semipermeable membrane under
• pressure.
• 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 the
• salt completely.
• A multistep process has sometimes to be used.
Production of Freshwater from Seawater and Brackish Water

47. • 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

48. • 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 is
• particularly favored.
Production of Freshwater from Seawater and Brackish Water

49. 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

50. FACTS ABOUT WATER
Water

51. Facts
• Water 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.

52. Facts
• Public 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.

53. 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.

54. 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.

55. THANKS
Engr. Ahmad Sameer Nawab