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

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”

Sources of Drinking Water• Groundwater

– Shallow wells– Deep wells

• Surface water– Rivers– Lakes– Reservoirs

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

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

Surface Water Treatment

Surface water from supply

Rapid Mix Flocculation

Basin

Sedimentationbasin

Sludge

Rapid Sand Filter

Disinfection

StorageTo DistributionSystem

Screen Coagulant

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.

Colloids

• Small particles (0.001 to 1 m).• Usually negatively charged.• Particles repel so suspension is

considered stable.

Coagulation and Flocculation

• Coagulation (process)

+

+

++++

Colloidal particles(0.001 - 1 m)

floc(1 - 100 m)

++

++

+ + ++

+ +++

+++

++

++

+ + +++

++

+ + + ++

++

+ + +

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.

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

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

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.

Rapid Mixing

Flocculation

• Paddle units rotate slowly, usually <1 rpm

• Velocity of water: 0.2 – 0.4 m/sec

• Detention time of at least 20 min

Flocculation

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.

Settling in Treatment Train

Circular Clarifiers

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)

Overflow rate

cAQvvs 0

wherevo = overflow

rate (m/s)Q = water flow (m3/s)Ac = surface area (m2)

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

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)

Filtration

• Single media: sand• Dual media: anthracite coal and sand• Multimedia: anthracite coal, sand and garnet

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

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

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.

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

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

Groundwater Treatment

• Primary objectives are to1.Remove hardness and other minerals2.Eliminate pathogenic organisms

• Treatment technologies largely based on precipitation

Groundwater Treatment

Ground waterfrom wells

Sedimentationbasin

SludgeRecarbonation

To District-bunionSystem

Rapid Mix

SlowBasin

Disinfection

Storage

CO2

Lime & Soda

ION EXCHANGE FOR HARDNESS REMOVAL

REVERSE OSMOSIS Total dissolved solids removal

REVERSE OSMOSIS

ConclusionEngineered Water Systems