unit iii environmental engineering i 2013

7/29/2019 Unit III Environmental Engineering I 2013

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Unit III

Environmental Engineering-I

Water Treatment


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Story so far

Water Supply Sources

Quality Criteria

Water Demand

Water Treatment

Conveyance


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Water Treatment

Application of physicaland chemicalmeansto make water fit for intendedapplication.

Process train depends on final use.


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Water Treatment Scheme


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Process flow sheet


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Conventional Water Treatment

1. Screening: removal of big objects

2. Coagulation/flocculation-Sedimentation:Gravity/chemical-aided removal of smaller particles

3. Filtration: removal of very small particles4. Disinfection: removal of pathogens

5. Aeration: removal of taste/odour, DO increase

6. Softening: removal of hardness

7. Other specific processes: fluoridation, carbonation,desalination, etc..


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Screening

Removal of large objects

Trees, branches, animals, fishes,

Coarse & Fine screens.

Coarse screen: center-to-center spacing of 2-10 cm, generally inclined

(increases surface area, decreases flow velocity ~ 0.8-1 m/s), 45-60 with horizontal

Generally, cleaned with rake (manual or mechanical).

Fine Screen fine wire/perforated metal.

opening size < 1 cm., easily clogged.


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Screens


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Sedimentation theory

Flow velocity: slower velocity, better settling.

Flow (water) Viscosity: low viscosity (at high temperature),better settling

Size, shape & specific gravity:

Higher specific gravity, better settling Small sized particles settle slowly.

Stokes Law

Vs= (g/18). (G-1). (d2/ ) d< 0.1mm

Vs: settling velocity (m/s) for spherical particle (dia, d, in m)

G: specific gravity of particle

v: kinematic viscosity (m2/sec)

v


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Settling velocity

Vs = 1.8 {gd(G-1)}1/2 d> 1 mm

Vs = 418 (G-1)d (3T+70)/100

0.1 mm < d < 1 mm


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Circular Clarifier


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Schematics: Rectangular Clarifier


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Plain Sedimentation (Type-I)

~70 % particle removal.

Flow velocity control by extended detention

period. Use of a long tank, (large area)

Sludge continuously removed by mechanical

scrapper.

Mainly horizontal flow type.


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Sedimentation (Type-I)

Rectangular Tank

Equal velocity at all points on each vertical line

Circular Tank

Uniform radial flow with decreasing velocitytowards periphery.

Particle removal is independent of tank depth. Depends on settling velocity and overflow rate (Q/As)


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Design Concept

Overflow Rate/Surface Overflow rate/SurfaceLoading:

Design velocity: theoretical time for which the

particle stays in the tank. Settling Velocity (Vs) vs. Loading Rate (Vo)

determines the particle removal.

Units: m3/d/m2

Discharge per unit surface area. Q/As

Typical Rates: 12-18 m3/d/m2


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Settling Column Analysis (Discrete

Particles)

To determine the theoretical settling/removalefficiency of a given suspension.

Column of 2 m height is used.

Samples withdrawn regularly.

Theoretical % of particles removed (100-X) + (Vs/V0).100.dX

X: fraction with Vs < V0


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Settling column

Particles with Vs > V0 are removed 100%.

Particles with Vs < V0 are removed in ratioVs/V0.

Steps:

Find particle size by sieve analysis.

Find settling velocity. Do column analysis and prepare settling curve.

Use settling equation to find theoretical removalefficiency.


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More concepts

Detention time:

theoretical time for water to flow through thetank length.

For a rectangular tank, D.T. =Volume/discharge.

Type I tank~ 4-8 hrs.

Type II tank~ 2-4 hrs.


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Tank dimensions

Depth ~ 3-4.5 m,

1.8 m (min.), 6 m (max.)

Width~ 10 m, (12 m-max.)

Length~ 4x width (1-6 x is range).

Circular Tank

Diameter ~ < 30 m.


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Sedimentation Tank Design (Type I)

Q: Design the sedimentation scheme for a treatment plantsupplying water to 10,000 residents.

Solution:Assuming daily water demand of 300 l/h/d,

Total plant capacity = 10,000 * 300 l/d = 3 MLD (million liters per day)

Assuming an overflow rate of 15 m3/d/m2

Cross-sectional area = 200 m2

Rectangular Tank: Assuming L:B of 4:1

We get L = 28.4 m and B = 7.1 m

Circular Tank: diamater ~ 16 m.

Assume depth of ~ 4m.

Check:

Detention Time = Volume/Q ~ 6.4 hrs (in acceptable limits)


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Coagulation-Flocculation (Type-II)

Smaller colloidal particles do not settle. ~ negatively charged.

Repel each other.

Coagulation Process of destabilizing the charged particles.

Flocculation: Agglomeration/aggregation of destabilized

particles during slow mixing. Floc:

Gelatinous mass of stabilized particles.

Generally, Coagulation is universally adopted over Type-I settling.


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Type-II settling analysis

Settling column analysis

Diameter~ 30 cm, depth~ depth of tank.

Samples withdrawn at different depths.

Time vs % depth plotted with isoremovallines.

Graphical method for % removal calculations.


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Coagulants

Aluminum or iron salts.

Alum, Ferric sulfate, ferric chloride, lime,ferrous sulfate,

Generally, alkaline environment is preferredfor coagulation.

Alum: (hydrated) aluminum sulfate

Al2(SO4) 3.14.3H2O (filter alum) Generally preferred.


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Coagulation Chemistry


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Coagulation Chemistry

Copperas


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Alkalinity in coagulation

Coagulants react with alkalinity to form floc.

1 mg/l of Alum (mw=600) removes 0.5 mg/lalkalinity (as CaCO3 , mw=100).

CaCO3 + H2O + CO2 Ca(HCO3)2

Removal of existing alkalinity leads to loss of buffercapacity.

May need to add lime (Ca(OH)2)or soda ash(Na2CO3) to increase alkalinity.


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Coagulation

Effective range:

Alum: 6.5-8.5

Ferric chloride: > 8.5 and 3.5-6.5 (low

pH specialist) Ferrous sulfate: > 9.5


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Alum

Cheap, forms excellent stable floc, no skilledsupervision required.

Also removes taste and colour.

Dose ~ 5-85 ppm Average dose ~ 17ppm.

May need external alkali addition.


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Alum vs. iron salts

Iron salts can cause color if ppt. notcompletely removed.

Iron salt produce heavier floc and removemore suspended matter.

Iron salts remove taste & odor (by removingH2S).

Iron salts cause staining and growth of

bacteria. Iron salts impart corrosive character to water.

Iron salts require skilled operation.


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Jar Test: Optimum coagulant Dose


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Jar Test

Determines optimal coagulant dose.

6 jars (1 lt. each) with water sample anddifferent coagulant doses.

Rapid mixing ~ 1-2 min Slow Mixing ~ 30-45 min

Settling time

Visual determination of optimal dose forsample with best floc.


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Coagulation units

Clariflocculator

Feeding device

Mixing device or basin

Flocculation tank or flocculator

Sedimentation tank

Either as a single unit or a series of unit


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Design concepts

Detention time ~ 2-4 hrs

Surface loading rate ~ 1000-1250 lt/hr/m2.

~ 24-30 m3/d/m2 (Type-II settling)

~ 12-18 m3/d/m2 (Type-I settling)

Turbidity removal up to 15-20 ppm.

Also removes bacteria.

Sludge production ~ Solids removed + floc


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Coagulation numericals

Num # 1: Quantity of alum required to treat 10 million lt/d with a dosage of15 ppm alum? Amount of CO2 released per lt. water treated?

Amount of alum = 10 * 10^6 lt/d * 15 mg/lt = 150 kg/d.

1 mole alum ~ 6 mole CO2

Mw: alum- 666 gm, CO2- 44 gm


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Numerical contd.

666 gm alum ~ 6 x 44 gm CO2

15 ppm alum ~ (6 x 44/ 666) x 15 ppm CO2

unit iii environmental engineering i 2013

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