(2).CE-212 Conventional Water Treatment-long Presentation

8/11/2019 (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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(2).CE-212 Conventional Water Treatment-long Presentation

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Transcript of (2).CE-212 Conventional Water Treatment-long Presentation