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7/27/2019 Water Treatment Overview [Read-Only]
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Water Treatmentater TreatmentProf.(Mrs.) Niranjanie RatnayakeDepartment of Civil Engineering
U i it f M tniversity of Moratuwa
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a a uwawa eservo r
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Need for Treatment
Water needs to be treated if the quality ofthe water does not satisfy the objectivewater quality of the intended use.
The treatment process should bedesigned taking into consideration theesigned taking into consideration theraw water quality and the objective waterquality.y If the water is to be used for domesticwater supply, the objective water quality
h ld ti f th N ti l St d d fhould satisfy the National Standards forDrinking Water Quality, which arestipulated based on the WHO Guidelines.29/05/02 3
stipulated based on the WHO Guidelines.
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Quality Objectives of Domesticuality Objectives of DomesticWater Supply To protect the health of the community
Treated water must be free from Pathogenic micro organisms Harmful organic and inorganic chemical substances
To supply a product which is aestheticallyo supply a product which is aestheticallydesirable Treated water must havereated water must have
Pleasant taste, odour, colour Low turbidity
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(Plain Sedimentation)
Flocculation
Filtration
Stabilization
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water
Purpose: Addition of O
Oxidize Fe and Mn
Removal of CO2
Make water palatable
Reduce corrosion
Removal of H2S, NH3,CH4
so en ng
Control of taste,
odour, corrosion,
Removal of volatile gaseseffects on cementand concrete etc
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T es of Aerators
Gravit Aerators
Cascades
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Types of Aerators(contd.)
Spray Aerators
Stationary Moving
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Plain Sedimentation This process is necessary when water
contains much heavy suspended
particles, that would settle underquiescent conditions.
This step can be omitted if the source is a
reservoir or groundwater.
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Coagulation and FlocculationPurpose: To destabilize the colloidal particles
particles which would be heavy enough tosettle under uiescent conditions
Coagulation: addition of chemicals to destabilizethe colloids. Rapid mixing is important.
Common Coagulants:
um n um a s Ferric Salts
Polyelectrolytes
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oagu a on an occu a on con .
the destabilized suspension toallow particles to adhere to
each other. It is important that
a velocity gradient is created
between articles in the water.
This is achieved by slow
stirring or creating a head loss
.
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Sedimentation
Particles having high settling velocity willbe removed.
In sludge blanket clarifiers, particles are
removed b tra in within the slud eblanket.
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Sedimentation Tanks Clarifiers
HorizontalFlow Tank
Circular Tank
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Coagulation, Flocculation and
Sedimentation
oagu an s a e o e rap m xer a e n e
end Paddle flocculators provide slow mixing
The floc particles are settled in the sedimentationtank.
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Kalatuwawa
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Purpose is to improve
turbidity by removingsuspended and colloidal
par c es.
Granular Bed Filters Rapid Sand Filters
Pressure Filters
Water to be filtered
Gravel - support
an - er me um
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Under drainage
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-Filter
Filter tray
Filter Sand
Graded
gravel
Filter floorCast iron
Perforatedlaterals
Filter floor
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Slow Sand Filters First type of filter in UK
Cleaning: by scraping top 5-8 cm of sandafter 4 6 weeks
unc on ng: stratified bed
Formation of schmutzdecke Removes suspended matter by straining
and dissolved organics by biological activity
Some removal of bacteria
Limitations: Low rate of flow requires large area
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Slow Sand Rapid Sand
Flow rate (m/h) 0.1 to 0.3 5 to 10
Depth of supernatant
water (m)
1 to 1.5 1 to 1.5
(m)
. . .
Effective size of sand 0.15 to 0.35 0.45 to 0.55
Uniformity coefficient 1.8 to >2.5 1.2 to 1.7
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Effective Size= sieve size of 10% passing; Uniformity Coeff. =size
size
%10
%60
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s n ec on
Process of destroying or inactivationof atho enic microor anisms.
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Ability to destroy pathogens at
concentrations likely to occur in water Not toxic to man and other animals
Have a residual disinfectant capacity
Safe, easy to handle, economical Easy to control and detect in water
Chlorine gas NaOCl
Ca(OCl) 2ClO2
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2
Powder)
zone
UV Radiation
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Chlorine Disinfection
Chlorine gas is available as liquid chlorine
in pressurized cylinders.
Bleaching powder should be dissolved in
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Ozone Disinfection Advantages:
No taste or odour problems Produced from air, which is in abundant supply
n epen en o p
No known toxic products
Limitations Higher cost
Cannot be applied directly to existing plants
Efficient Ozone roduction and a lication
methods need to be developed Disinfection affected by competing impurities
No lon term rotection needs ost-chlorination
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Advantages: Simple to use
Minimum space requirement
No changes in characteristics of the water
No residuals
Limitations Water should not be turbid, coloured or contain
dissolved organics
No residuals to fight any subsequent contamination
PHOTO REACTIVATION reactivation of pathogens atlow energy doses.
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Adjustment of the pH and alkalinity of the
water, so that
It would not dissolve calcium carbonate
coa ngs rom p pes
Nor precipitate out calcium carbonate and
la down de osits that clo i es.
This is the final ste in Conventional
Water Treatment.
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These are done for specific purposes Water Softening
Iron and Manganese Removal
e.g. softening
Membrane processes Reverse Osmosis
Ultrafiltration
Adsorption - for removal of colour, taste,odour etc.
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