(2).ce-212 conventional water treatment-long presentation
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POLLUTANTS:LIKELYTOBEPRESENTINNATURALWATERS
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Shortcut to 9.jpg.lnkShortcut to 9.jpg.lnk
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REVISION:
50
m Plain sedimentation
50 m Coagulation
up to 0.001
m)
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COLLOIDAL STABILITY
Why colloids are stable ?
Excessively large surface
to volume ratio
Surface phenomena predominate
over mass phenomena.
Electrical charge at surface.
Negatively charged.
Bound layer
(Cations)
(Travel with it ) Negatively charged colloid with apossible configuration of ions around it
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MEANS OF OVERCOMING THE ENERGY BARRIER:
(For agglomeration of particles)
Brownian movement/random movement of colloids
Mechanical agitation /momentum
Too slow/inefficientNeither results in collisions of medium-sized colloids
Other means of agglomeration required
Accomplished by:
Chemically coagulating colloids into clusters/flocs
Large to settle/gravity
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COAGULATION THEORY
4 mechanisms
Ionic layer compression Quantity of ions:
affect decay of electrostatic potential
High ionic conc.: compresses layer towards
surface
van der Waals force predominates
Net force: attractive No energy barrier
Example:
Deltas
Aluminum/ferric salts:
Ionize
Ionic conc. - does not increase much
No ionic layer compression
Reduction of thickness
in diffused layer
Reduction of net force
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Interparticle bridging
Synthetic polymers:
linear/branched highly surface reactive
Colloids attach to polymer
Polymer-colloid groups enmeshes
Formation of settleable mass
Fig. Interparticle bridging with polymers
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Mechanisms of coagulation
Fig. Results of jar tests at increasing turbidities
(S= surface area/L)
:Low
:Higher
:Still greater
:Extremely large
Z-2: Enough colloids/ adsorption and
charge neutralization/re-stabilization
seldom/sweep coagulation too
Z-2: Collision-more chances, better
coagulation over wider conc.
Z-2: Early destabilization/ adsorptionand charge neutralization
Z-3: Overdosing, charge reversal/re-
stabilization
Z-1: Insufficient no. of colloids/no
settleable mass even if chargeneutralize/Large dose
Z-4: Predominant-Sweep coagulation
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Fig. Coagulant dosage as a function of turbidity
OAD
OAD
OAD
OAD
Summary/Observations:
1. Ads.+charge neut.-impractical
Unless dose carefully controlled
Slight overdose:
re-stabilization (S2)
2. Dosage
High turbidity (S3, S4)->
S1: add bentonite/turbidity
Saving-alum
OAD
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Coagulation Practice
Alum dosage: 5-50 mg/L, depending turbidity/nature of water
Some generalizations:
At low turbidity/high dosage Al(OH) 3 certain
Predominant mechanism-sweep coagulation
At high turbidity/lower dosage
Predominant mechanism-ads+charge neutralization
Surface waters: 4 general categories
G-1: High turbidity-low alkalinity: Small dose, ads.+charge neu.
effective as pH depresses, prevent
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OAD-Jar test
Rapid mixing
Coagulant added
Violently agitated/uniform mixing
FlocculationKeep solids in suspension
Collisions between destabilized particles
particles flocs
Settling
Flocculated solids settle
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Revision:
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Rapid-Mix Tank
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Large G, short t: small, dense flocs
Low G, long t: larger, lighter flocs
Need: large, dense flocs (easy to settle)
Vary G: large
lowSmall, dense flocs (>G)-----larger flocs (
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Upflow solids contact unit
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Sedimentation
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Horizontal-flow clarifier
Up-flow clarifier
Sedimentation
Shape:
rectangularcircular
Flow pattern:
linear
radial
upward
Zones:
inletsettling
outlet
sludge storage
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Circular sedimentation basin
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Circular sedimentation basin
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Rectangular sedimentation basin
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Sedimentation zones few cm to 6m/greaterIndependent of depth
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Ideal horizontal settling tank: assumptions
1. Particles/velocity vectors-evenly distributed across tank x-section(inlet zone)
2. Water moves-as an ideal slug down the length of tank
3. Any particle hitting bottom is removed
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Independent of depth/strange/why is this so?
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Tutorial 1: Question 4
Tutorial 1: Question 5
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Q
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Cl ifi ti
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Classification
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Rapid sand filter
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Another criterion
90th percentile largest particlesfluidized
Dual-media-P90/anthracite(considered largest-diameter particle)
Not remain at bottom, re-stratify
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DISINFECTION
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Typhoid fever cases per 100000 population from 1890-1935, Philadelphia.
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