Water treatment 2013 14 (2)
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SEMINAR ON WATER TREATMENT
By ANURAG CHANDRA SHEKHAR(M.TECH. FIRST YEAR )ROLL No. 06
In The Expert Guidance OfDr. SONAL DIXIT
ASST. PROFESSOR(GUEST)DEPT. ENVIROMENTAL SCIENCE
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Water Treatment objectives
• Safe & Clean Water• For Proper Health & Body Metabolism
“The availability of reliable supply of
clean and safe water is one of the most
important determinants of our health”
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Sources of Drinking Water• Groundwater
– Shallow wells– Deep wells
• Surface water– Rivers– Lakes– Reservoirs
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Characteristics Groundwater River Lake/Reservoir
Safety (bacteriological)
Generally safe Not safe Better than river
Composition Constant Varying with season
Relatively constant
Hardness Present Variable Variable
Turbidity/SS Nil Present Very LessMinerals Present Variable VariableIron & Manganese Sometimes Nil Variable
Colour Low /nil Less/Nil PresentNitrate Sometimes Less Less
Hydrogen Sulphide Sometimes Nil Nil
Sulfates & Carbonate Present Variable Variable
Taste & Odor Sometimes Less Summer
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Surface Water Treatment
• Primary objectives are to1. Remove suspended material (turbidity)
and colour2. Eliminate pathogenic organisms3.3. Under extreme cases- Total Dissolved Under extreme cases- Total Dissolved
Solids RemovalSolids Removal• Treatment technologies largely based on
coagulation and flocculation
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Surface Water Treatment
Surface water from supply
Rapid Mix Flocculation
Basin
Sedimentationbasin
Sludge
Rapid Sand Filter
Disinfection
StorageTo DistributionSystem
Screen Coagulant
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Coagulation and Flocculation
• Goal: To alter the surface charge of the particles that contribute to colour and turbidity so that the particles adhere to one another and are capable of settling by gravity.
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Colloids
• Small particles (0.001 to 1 m).• Usually negatively charged.• Particles repel so suspension is
considered stable.
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Coagulation and Flocculation
• Coagulation (process)
+
+
++++
Colloidal particles(0.001 - 1 m)
floc(1 - 100 m)
++
++
+ + ++
+ +++
+++
++
++
+ + +++
++
+ + + ++
++
+ + +
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Coagulant• Non-toxic and relatively inexpensive.• Insoluble in neutral pH range - do not want high
concentrations of metals left in treated water. • Alum: Al2(SO4)3
.14H2O, Ferric chloride: FeCl3, Ferric sulfate: FeSO4, Polyelectrolyte.
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How does alum work?
• Al2(SO4)314H2O 2Al3++ 3SO42-+ 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 from – How much time the water is retained in the reactor- Volume/Flow = 10 - 30 sec.
• 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 <1 rpm
• Velocity of water: 0.2 – 0.4 m/sec
• Detention time of at least 20 min
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Flocculation
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Sedimentation/Settling
• Following flocculation, the water then flows into the settling basins
• Water is nearly quiescent – low flow with little turbulence
• Water resides for at least 2 hours ( Hydraulic Retention Time) and the flocks settle out and collect at the bottom.
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Settling in Treatment Train
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Circular Clarifiers
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Type I Settling -- Stokes’ Law
18)( 2dgv s
s
whereνs = settling velocity
ρs = density of particle (kg/m3)
ρ = density of fluid (kg/m3)g = gravitational constant (m/s2)
d = particle diameter (m)μ = dynamic viscosity (Pa·s)
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Overflow rate
cAQvvs 0
wherevo = overflow
rate (m/s)Q = water flow (m3/s)Ac = surface area (m2)
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Filtration
• The final step in removing particles is filtration.
• Removal of those particles that are too small to be effectively removed during sedimentation
• Multiple removal mechanisms depending on design
• Sedimentation effluent: 1 - 10 NTU• Desired effluent level: <0.3 NTU
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Removal Mechanism of filtration
• Biological mechanism-Impurities removal my microorganisms on the top filter layer. (slow sand filter)
• Mechanical Straining of particles. Size of particle> void space. (slow and Rapid sand filter)
• Adsorption to filter media. (Rapid Sand Filter)• Sedimentation on filter media (Rapid Sand
Filter)
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Filtration
• Single media: sand• Dual media: anthracite coal and sand• Multimedia: anthracite coal, sand and garnet
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Filter Design
whereva = face velocity (m/day) or loading rate (m3/day·m2)Q = flow rate (m3/day)As = filter surface area
(m2)
• Slow sand filters: va = 2.9 – 7.6 m3/day·m2
• Rapid sand filters: va = ≥ 120 m3/day·m2
• Removal mechanisms are different
• Rapid sand widely used,
sa A
Qv
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Rapid Sand Filtration
• As particles are removed - filter becomes clogged – head loss increases, turbidity increases
• Must backwash (takes about 10-15 min) done about once per day
• Must design 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 clear well back through the filter.
• The particles in the filter become suspended, releasing the trapped particles.
• Backwash water retreated or disposed of.
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Gravel
Inlet Chamber
Wash water
troughs
Water level while filtering
Main drain
Sand
Wash water storage tank
Under drainage
Wash water gutter
Typical Rapid Sand Filter Operation
1. Open valve 1 (This allows influent to flow to filter)2. Open Valve 2 (This allows water to flow through
filter)3. During filter operation all other valves are closed
1
3
2
4
5
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Gravel
Inlet Chamber
Wash water
troughs
Water level while filtering
Main drain
Sand
Wash water storage tank
Under drainage
Wash water gutter
Typical Rapid Sand Filter Backwash Operation
Close valve 1Open valve 3Open valve 4
1
4
3
2
5
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Groundwater Treatment
• Primary objectives are to1.Remove hardness and other minerals2.Eliminate pathogenic organisms
• Treatment technologies largely based on precipitation
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Groundwater Treatment
Ground waterfrom wells
Sedimentationbasin
SludgeRecarbonation
To District-bunionSystem
Rapid Mix
SlowBasin
Disinfection
Storage
CO2
Lime & Soda
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ION EXCHANGE FOR HARDNESS REMOVAL
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REVERSE OSMOSIS Total dissolved solids removal
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REVERSE OSMOSIS
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ConclusionEngineered Water Systems
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