Download Quality of Water

Source of Water
Sources of Water
• Hydrological Cycle
- Rain – Surface runoff – water storage infiltration – ground water – evaporation –
transpiration • Rain
– Surface Water
– Groundwater
• Aquifer
Choice of Source
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Location of source
Quantity of water available
Quality of water
Cost of the entire scheme
Surface water
• Surface water source may be further
classified as:
• lakes
• Impounding reservoirs
• River, streams, irrigation canals
• The area where water is collected for water
supply purpose is called catchment or
watershed.
Ground Water
• Aquifer
• An aquifer is a layer of pervious material beneath
the ground surface and underlied by impervious
stratum.
• Water table
• The free surface of water in the aquifer is called
water table
• Sources
- Spring
- Wells
Characteristics of Ground
Water
• Generally ground water is clear and
colorless. When water seeps down into
ground, it dissolves inorganic salts.
• Therefore, sometimes harmful
Sources of Water in Hong Kong
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natural catchment
Guangdong Province of China.
Administration by Water Authority
Responsible for supply, treatment and
distribution
Quality of Water
Quality Monitoring
• conform both chemically and bacteriologically
to the Guidelines for Drinking Water Quality
(WHO, 1984).
• Parameters:
– Physical
– Chemical : a) inorganics
–
b) organic
– Biological
– Bacterialogical
– Radiological
• Physical : e.g. pH, conductivity, turbidity,
colour, etc.
• Chemical :
– a) inorganics e.g. nurients, ions, heavy metals etc.
– b) organic e.g. trihalomethane, pesticides etc.
• Biological : e.g. algae, zooplankton,
macroinvertebrate, fish,
• Bacterialogical : total coliform, E. Coliform,
plate count
• Radiological : e.g. gamma emitters.
Consumer Complaints
• Consumers can file complaints regarding
the quality of water. Typical complaints on
the quality of water in Hong Kong are :
– discoloured water
– tastes and odours
– cloudy, milky or chalky water
Introduction to Water
Treatment
Why water need treatment
• The water is polluted by the people
inhabiting the catchment.
• Water carries bacteria, some of which are
pathogenic, and can cause water-borne
diseases.
• Factories let off water into rivers and
pollute the water.
• Surface run-off dissolve and minerals.
Impurities
- Physical impurities
- Chemical impurities
- Bacteriological impurities
• Physical impurities can affect the color, taste,
odour and turbidity to the water.
• Chemical impurities cause hardness in water.
Excess quantities of metals and dissolved gases
cause corrosion to pipes and fittings.
• Bacteriological impurities are due to pathogenic
bacteria which spread diseases.
Objectives of Water Treatment
 To remove colour, objectionable taste and odour
 To remove dissolved gases, dissolved and
suspended impurities and harmful minerals
 To remove suspended and dissolves organic
impurities
 To remove harmful bacteria
 To make the water safe and attractive for
drinking and domestic purposes
Components of a Treatment Plant
• Intake work including pumping plant
 Plain sedimentation tank
 Clairifier-flocculator:
(coagulation
&
flocculation)
 Filter
 Disinfecting plant
 Water storage reservoir.
 Pumping plant
 Distribution system
 Other processes Hardness (Colour, Iron and
manganese, Fluoride, )
Treatment processes
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Source
Pre-chlorination
Flocculation
Sedimentation
Filtration
Post-chlorination
Storage and pumping
Distribution system
Plain Sedimentation
• Suspended and colloidal impurities that are
settleable are separated in the sedimentation
tanks by gravitation.
Principle of Sedimentation
• The main principle of sedimentation is to
allow water to rest or flow at a very slow
velocity so that the heavier particles settle
down due to gravity.
Types of Sedimentation Tanks
• Sedimentation tanks can be
• Horizontal flow tanks
˙Vertical flow tanks
Horizontal Flow Tanks
• The direction of flow in the tank is
essentially horizontal.
• (a) Rectangular tanks
• (b) Circular tanks
– Radial flow tanks
– Circumferential flow tanks
Vertical Flow Tanks
Sedimentation with coagulation
• Colloidal particles which are fine particles
of size finer than 0.0001 mm carry electrical
charges on them. These are continuously in
motion and will never settle down under
gravity.
Coagulation
• The process of adding certain chemicals to
water in order to form an insoluble,
gelatinous, flocculent precipitate for
absorbing and entraining suspended and
colloidal matter is called coagulation.
Purpose of Coagulation
• The fine particles are removed rapidly and
turbidity is reduced to about 20 ppm. The
bacterial load also is reduced by 5%.
Principle of Coagulation
 Floc Formation
 Electric Charges
 Floc Formation
– forming of a thick gelatinous precipitate which
is insoluble in water and settle.
• Electric Charges
– Colloidal particle possess negatively charged
ions. The floc thus attracts the colloidal
particles and makes settle down.
Coagulants
• The common coagulants used are:
• Salts of aluminium:
 alum (明礬), filter alum, Sodium aluminate
• Salts of iron:
 Ferrous sulphate, Ferric sulphate, Ferric chloride
Chemistry of Coagulation
• When alum is added to water, Al3+ ion will
hydrolyze to different charged hydrolysis
products depending on the pH :
• Al(H2O)6 3+ < == > [Al(H2O)5OH] 2+ < ==
> [Al(H2O)3 (OH)3] (s) < ==> Al(H2O)
(OH)
]
2
4
Polyelectrolytes
 Polyelectrolytes are increasingly used in
water treatment as primary coagulant,
flocculant coagulant aid or filter aid.
 They are synthetic long-chain organic
compounds which contain different
monomers.
 They can exhibit either positive (cationic),
negative (anionic) or nonionic charge.
Advantages of polyelectrolytes
 increase process stability and efficiency
 improve water quality
 improve sludge quality and reduce sludge
quantity
Limitations of polyelectrolytes
 expensive products
 require careful control
 improper dosing can cause serious filter
problems filter blockage
 require efficient mixing
Process of Coagulation and
Sedimentation
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Feeding
Mixing
Flocculation
Clarification.
Flocculation
• Agglomeration of particles
separate mechanisms :
• 1. Brownian motion
• 2. stirring/agitating
• 3. Differential settling
involve
3
Clarifying
• After flocculation, the floc is allowed to
settle down in basins called clarifiers. The
clear water is drained out for further
purification. The detention period is about
1*1/2 - 3 hours.
Filtration
• The process of filtration involves allowing
the water to pass through a layers of sand
called media which acts as strainers. The
water is then collected by means of drains
placed under the sand bed.
Objectives of Filtration
• Remove colloidal and suspended matter
remaining after sedimentation
• Remove bacterial load
Theory of Filtration
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Mechanical straining
Sedimentation or adsorption
Biological Metabolism
Electrolytic changes
Filter Sand
The characteristics of the sand
• Effective size : The effective size of a sand is the
size of sieve, in mm, through which 10% of the
sand (by weight) passes. It is usually denoted by
D10.
• Uniformity coefficient : This is the ratio of the
size of sieve through which 60% of the sample of
sand passes to the effective size of sand.
Classification of Filters
• Gravity filters
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Slow sand filters
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Rapid sand filters
• Pressure filters
Types of Filter Media
configuration in Hong Kong
In Hong Kong, only rapid sand filter is used.
Filtering materials can be single media
filter or dual media filter.
Disinfecting
• The process by which harmful bacteria are
destroyed to make it safe for drinking is
called Disinfecting.
• Chemicals used for this purpose are called
disinfectants.
Criteria for a Good
Disinfectant
• Capable of destroying pathogenic organisms
within the contact time available.
 Should NOT change the physical and chemical
characteristics
• Should not be toxic
• Available at reasonable cost
 Should have residual concentration
• Detectable in the water by simple tests
• Easy in transportation and application
Methods of Disinfecting
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Boiling water
Ozone gas treatment
Excess lime treatment
Iodine and bromine treatment
Ultraviolet rays treatment
Potassium permanganate treatment
Chlorination
Chlorination
• The process is economical and cheap
• It is harmless to human beings
• It is reliable and effective
 Residual chlorine can be maintained in the water
• Easy to apply and ease to measure
Disadvantages of Chlorination
• Formation of trihalomethanes (THMs) such as
carcinogen chloroform (CHCl3)
• To avoid the formation of THMs, water should be
removed of organic impurities as much as possible
before chorination.
Chemistry of Chlorination
• Cl2 + H2O < == > HOCL + H+ + Cl• The hypochlorous acid is the prime
disinfecting agent.
• Depending on the pH, the following
reaction can also take place
• HOCl < == > H+ + OCl• Together, HOCl and OCl- are called free
available chlorine.
Chlorine Demand
• Chlorine and its compounds are consumed by a
variety of organic and inorganic materials present
in water due to its oxidising power before
disinfecting is achieved.
• The difference between the amount of chlorine
added to the water and the residual chlorine is
called chlorine demand.
• This depends on the amount of chlorine added,
time of contact, pH and temperature.
Breakpoint Chlorination
• Chlorine acts as oxidising agent to remove
bacterial and organic matters
• When chlorine is applied, it first kills the bacteria
Any further addition will appear as residual
chlorine increases.
• After a certain point, the residual chlorine
suddenly decreases with emanation of bad smell
and objectionable taste.
This indicates that
chlorine is being used for oxidising the organic
impurities.
Breakpoint Chlorination
• Again, after some time, there is a sudden increase
in residual chlorine indicating that oxidation of
impurities is over.
• The point at which both the demands are satisfied
and residual chlorine increases is called breakpoint.
• Beyond the breakpoint, any further dose of
chlorine only gets accumulated. The process of
adding chlorine beyond the breakpoint is called
breakpoint chlorination.
• Hardness of Water
Problem with Hardness in water
˙ Corrosion and incrustations in pipes
˙ Consumption of more soap
˙ Scales in boiler
˙ Bad taste in water
˙ Difficulties in dyeing industry
˙Creates bubbles in the manufacturing posses
of paper making, textile finishing, ice
making, etc.
Types of Hardness
˙Temporary hardness
˙Permanent hardness
Temporary hardness
˙is caused by the presence of
bicarbonates
of
calcium
and
magnesium.
Permanent hardness
is caused by the presence of sulphates,
chlorides and nitrates of calcium and
magnesium
Removal of Hardness
Temporary hardness is removed by:
˙Boiling the water
˙Adding lime to the water
Removal of Hardness
Permanent Hardnes
˙Lime-soda process
Zeolite process
Lime-soda Process
Ca(HCO3) 2+Ca(OH)2 2CaCO3 + 2 H2O
Zeolite Process
• Ca(HCO3) 2 + Na2Z  CaZ + 2NaHCO3
• Reverse Process
• CaZ + 2NaCl  Na2Z + CaCl
Miscellaneous Treatments
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Removal of Iron and Manganese
Removal of Fluorides
Taste and Odour
Colour