Water Treatment Overview [Read-Only]

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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Water Treatment Overview [Read-Only]