Download - CE-102 Water Treatment
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WATER TREATMENT
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Engineered Systems
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Sources of Drinking Water Groundwater
shallow wells, extract water from unconfined aquifers,generally not used for drinking water source.
deep wells, extract water from confined aquifers, generally
used for drinking water source.
Groundwater is normally potable. Only chlorination ordisinfection is done for water supply.
Surface water
Rivers
Lakes Reservoirs
Surface waters are generally not potable. They need to be
properly treated before supplying for drinking purpose.
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Characteristics Groundwater River Lake/Reservoir
Safety(bacteriological) Generally safe Not safe Better than river
Composition Constant Varying withseason
Relativelyconstant
Hardness Present Variable Variable
Turbidity/SS Nil Present Very Less
Minerals Present Variable(Generallyoptimum)
Variable
(Generallyoptimum)
Iron & Manganese Sometimes Nil VariableColor Low /nil Less/Nil Present
Nitrate Sometimes Less Less
Hydrogen Sulphide Sometimes Nil Nil
Sulfates & Carbonate Present Variable Variable
Taste & Odor Sometimes Less Summer
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Water Treatment objectives
Wholesome water potable as well aspalatable.
Fit for metabolism and health
The availability of reliable supply of
wholesome water is one of the most
important determinants of our health
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Steps Surface Water Hard Groundwater
1 Screening/Microstrainer Aeration
2 Pre-Sedimentation/Pre-
Chlorination
Oxidation/Prechlorination
3 Rapid Mixing (CoagulationTank)
Rapid Mixing (Lime & SodaAddition
4 Slow Mixing (Flocculation
Tank)
Precipitation
5 Sedimentation Sedimentation
6 Filtration Filtration
7 Adsorption Optional fororganics removal for better
taste/color/odor)
Recarbonation
8 Disinfection Disinfection
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Surface Water Treatment
Primary objectives are to
1. Remove suspended material (turbidity)
and color
2. Eliminate pathogenic organisms
Treatment technologies largely based on
coagulation and flocculation
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Type I Settling -- Stokes Law
18
)(2dg
v ss
where
s=settling velocity
s = density of particle (kg/m3)
= density of fluid (kg/m3)
g = gravitational constant (m/s2)
d = particle diameter (m)
= dynamic viscosity (Pas)
Settling ofdiscrete
particles
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Overflow rate
sA
Qv
where
v = overflow
rate (m/s)
Q = water flow
(m3/s)
As= surface
area (m2)
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Surface Water Treatment
Surface water
from supplyRapidMix
FlocculationBasin
Sedimentationbasin
Sludge
RapidSand Filter
Disinfection
StorageToDistribution
System
Screen Coagulant
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Surface Water Treatment
Surface water
from supplyRapidMix
Clariflocculator
Sludge
RapidSand Filter
Disinfection
StorageToDistribution
System
Screen
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Coagulation and Flocculation
Goal: To alter the surface charge of the
particles that contribute to color and
turbidity so that the particles adhere to one
another to form bigger particles that settleby gravity in reasonable time.
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Colloids
Small particles (0.001 to 1 m)
Usually negatively charged
Particles repel each other; they are calledstable particles.
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Coagulation and Flocculation
Coagulation (process)
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+
Colloidal particles(0.001 - 1 m)
floc(1 - 100 m)
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+ +
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+ + +
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Coagulant
Non-toxic and relatively inexpensive
Alum: Al2(SO4)3.14H2O, Ferric chloride: FeCl3, Ferric
sulfate: FeSO4, Polyelectrolyte
Insoluble in neutral pH range we do not want high
concentrations of metals left in treated water.
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How does alum work?
Al2(SO4)314H2O 2Al3++ 3SO4
2-+ 14H2O
2Al3+ + colloids neutralize surface charge
2Al3+ + 6HCO3- 2Al(OH)3(s) + 6CO2
If insufficient bicarbonate is available:
Al2(SO4)314H2O 2Al(OH)3(s) + 3H2SO4-
+14H2O
Optimum pH: 5.5 to 6.5
Operating pH: 5 to 8
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Aquometallic ions, great
affinity to surfaces
Neutralization of
surface charge on
colloids
Al3+ + H2O -------- Al(OH)2+ + H+
Al3+ + H2O -------- Al(OH)2+ + H+
Al 3+ + H2O -------- Al(OH)3 + H+
Aquometallic ions
Aluminum Hydroxide Flocs
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Rapid Mixing
Used to blend chemicals and water being
treated
Retention time of rapid mixers range from 10 to
30 second. Mechanical mixing using vertical-shaft impeller
in tank with baffles.
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Rapid Mixing
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Flocculation
Paddle units
rotate slowly,
usually
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Flocculation
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Sedimentation/Settling
Following flocculation, the water then flows into
the settling basins
Water in settling basins is nearly quiescent
low flow with little turbulence.
Water resides for at least 2 hours in the tank
and the flocs settle out at the bottom, from
where they are removed. This type of settling of
particle aggregates is called type-II settling.
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Circular Clarifiers
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Filtration
The final step in removing particles isfiltration. It removes those particles thatare too small to be effectively removed
during sedimentation Multiple removal mechanisms depending
on design
Sedimentation effluent: 1 - 10 NTU
Desired effluent level:
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Filtration
Depending the flow rate through filters, they may beclassified as slow sand filter and rapid sand filters.
Sometimes pressure is applied in rapid sand filters to
increase flow rate, then they are called pressure filters.
Depending on the media used, filters are classified as
below.
Single media: sand
Dual media: anthracite coal and sand
Multimedia: anthracite coal, sand and garnet
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Removal Mechanism of filtration
Mechanical Straining of particles. When size of
particle is larger than void space in the media,
particles do not pass through media. (slow sand
filter)
Biological mechanism-Impurities removal my
microorganisms on the top filter layer. (slow
sand filter)
Adsorption to filter media.
Sedimentation on filter media
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Filter Design
whereva=face velocity(m/day) or loadingrate (m3/daym2)
Q = flow rate (m3/day)
As= filter surface area(m2)
Slow sand filters:
va = 2.9 7.6 m3/daym2
Rapid sand filters:
va= 120 m3/daym2
Removal mechanisms
are different
Rapid sand widely used,
s
aA
Qv
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Rapid Sand Filtration
As particles are removed - filter becomes
clogged - headloss increases, turbidity
increases
Must be backwashed which takes about
10-15 min and is done about once per day
Must be designed to handle flow with one
filter out of service
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Rapid Sand Filtration
Backwashing is accomplished by forcing water(and sometimes air) up from the clearwell back
through the filter.
The particles in the filter media become
suspended, releasing the trapped particles.
Backwash water is retreated or disposed of.
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Head Loss
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Groundwater Treatment
Primary objectives are to
1.Remove hardness and other minerals
2.Eliminate pathogenic organisms Treatment technologies largely based on
precipitation
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Groundwater Treatment
Ground waterfrom wells
Sedimentationbasin
Sludge
Recarbo-nation
To Distri-butionSystem
RapidMix
SlowBasin
Disinfection
Storage
CO2
Lime & Soda
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TYPICAL TREATMENT SCHEME FOR RIVER WATER
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ADVANCED WATER TREATMENT FACILITYTYPICAL TREATMENT SCHEME FOR RIVER WATER
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REVERSE OSMOSIS
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REVERSE OSMOSIS
DISINFECTION
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DISINFECTION It is a process of killing pathogens in water.
Some contamination may occur during transporting water to
consumers via pipelines. Some amount of chemicals are left inwater in the disinfection process, which provide residual
protection.
A large number of chemicals are used for disinfection they
include halogen group, i.e. chlorine, bromine and iodine.
Fluorine oxidizes water and therefore it is not used. Chlorine is
frequently used in public water supplies, bromine for pond
waters and iodine often has military applications in remote
areas. Ammonia added with chlorine for disinfection forms
chloramines in water which provide residual protection,because chlorine gas escapes from water very soon after
application. Chlorine forms trihalomethane (THM, e.g.
chloroform) in water which are carcinogens. Good water
supplies therefore use ozone for disinfection, which has the
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DISINFECTION
The factors that affect disinfection are
Characteristics of disinfectant, i.e. its intrinsic
nature
Characteristics of water, i.e. pH, temperature
and presence of extraneous matter
Contact time, i.e. the duration for which the
disinfectant remains in contact with water to be
disinfected
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DISINFECTION
Killing of microorganisms in disinfection process follows firstorder kinetics:
This is called Chicks law.
The amount of chemicals need to be added for achieving a
certain percent kill of microorganism follows the following
relationshipCn t = constant. Where C is concentration of disinfectant, i.e.
chemical that does disinfaction, in mg/L, t is contact time in
min and n is an exponent for a particular species, e.g. 0.88 for
E-Coli.
kteNNor
kNdt
dN
0
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