Unit III Environmental Engineering I 2013
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Transcript of 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