ENVI Chapter 4

8/14/2019 ENVI Chapter 4

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2000 B.C. in India: water was to be heated, boiled,or filtered to remove impurities

1450 B.C. in Egypt: drawings depicts people

siphoning liquid from a canister

Hippocrates (460 to 354 B.C.)

whosoever wishes to

investigate medicine properly

should consider the water that

the inhabitants use for water

contributes much to health


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Aeration

Softening

Filtration

Disinfection

Storage

Raw Water

Gases to

atmosphere

Lime

Soda Ash

CaCO3

Mg(OH)2

Chlorine

Chlorine

Typical plant treating hard groundwater.


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Pre-sedimentation

Mixing,

flocculation,

settling

Filtration

Adsorption

Storage

Raw Water

Gases to

atmosphere

Chlorine

Ammonia

Chlorine

Disinfection

Alum

Polymers

Chlorine

Typical plant treating turbid

surface water with organics


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Characteristics

Major sources of Public Drinking Water

Surface Water

Groundwater

- include streams and rivers, natural lakes, and constructed lakes

- exposed to plant and animal life and to human influences from land

- contains a wide variety of microorganisms and natural organics

- free of significant levels of organics

- low levels of microbial contamination

- contain significant levels of dissolved inorganics

(e.g. carbonates, iron and manganese)


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Treatment Processes

Objective: to produce a safe, aesthetically pleasing water

Gas Transfer (Aeration)

used to remove dissolved gases in water or to add oxygen to water

to convert undesirable substances to a more manageable form

CO2 results in a corrosive water

may interfere with other treatment process

H2S imparts an unpleasant taste and odor to water


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Iron and Manganese in the absence of oxidizing agents

both are stable in water

Oxidation Reaction

++

++

+++

+++

HMnOOHOMn

HOHFeOHOFe

4222

8)(4104

2222

322

2


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Liquid Gas Contact Systems

Bulk

Liquid

Liquid film

Gas film

Bulk Liquid

Cs < CtBulk

Liquid

Liquid film

Gas film

Bulk Liquid

Cs > Ct

-designed to drive the water gas mixture toward equilibrium-provide supersaturation or oxygen for oxidation purposes

Accomplished by:

Dispersing the water into the air

Dispersing the air into the water

Figure 1. Water Dispersed in air: (a) desorption and (b) absorption.

(a) (b)


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Bulk

Gas

Gas film

Liquid film

Bulk Liquid

Cs < CtBulk

Gas

Gas film

Liquid film

Bulk Liquid

Cs > Ct

Figure 2. Air Dispersed in water: (a) desorption and (b) absorption.

(a) (b)


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Devices for Liquid Gas Contact

Fountains

Cascade Towers

Tray Towers

Diffused Aerators


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gineering Solids Separation

Sedimentation

Clarification

Discrete Particles whose size, shape and specific gravitydo not change with time

Flocculating Particles whose surface properties are such

they aggregate with other particles

Dilute Suspensions the concentration of particles is not sufficient

to cause significant displacement of water

as they settle

Concentrated Suspensions there is velocity field interference


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Coagulation

Particle diameter

(mm)

Size typical of Settling Velocity

10 Pebble 0.73 m/s

1 Coarse Sand 0.23 m/s

0.1 Fine Sand 1.0 x 10-2 m/s (0.6m/min)

0.01 Silt 1.0 x 10-4 m/s (8.6m/day)

0.0001 Large Colloid 1.0 x 10-8 m/s (0.3m/yr)

0.000001 Small Colloid 1.0 x 10-13 m/s (3 m/million yr)

Table 1. Settling velocities of various size particles*

* Spheres with specific gravity of 2.6 in water at 20oC

Stable colloidal suspensions that do not agglomerate naturally

Large surface to volume ratio

most important factor contributing to the stability of colloidal suspension

Coagulants induces agglomeration


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gineering Major Forces Acting on Colloids

Electrostatic potential

Van der Waals Force

Figure 3. Reduction of collloidal electrostatic repulsion by

addition of trivalent aluminum ions.


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Flocculation

gentle mixing to speed the agglomeration process


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Softening

Chemical Precipitation

- calcium hardness to calcium carbonate- magnesium hardness to magnesium hydroxide

Lime-soda Process

Caustic soda Process


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Lime-soda Process

2

3

24

22

3

24

2

23223

2

23

2323

2

23232

)(

2

2

2

2

)(

2)(2

22)(2

OHMg

NO

Cl

SO

CaOHCaO

NO

Cl

SO

Mg

OHMgCaCOOHCaOCOMg

COMgCaCOOHCaOHCOMg

OHCaCOOHCaOHCOCa

+

+++

+

++++

+++++

++++

+

+

+

++

+

+++

+

+

+

3

24

332

3

24

2

2

22

2

2

NO

Cl

SO

NaCaCOCONa

NO

Cl

SO

Ca


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Caustic soda Process

++

++

++

+

++++

++++++++++

+++

242

24

2

22323

2

22333

2

2232

2)(2

224)(4)(2

222)(2

22

SONaOHMgNaOHSOMg

OHCONaOHMgNaOHHCOMg

OHCONaCaCONaOHHCOCa

OHCONaNaOHCO


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Stabilization

Addition of Acid

OHSOMgSOHOHMg

SOHCOCaSOHCaCO

224

2422

243

2423

2)(

)(222

+++

+++

+

+

Recarbonation

+

+

+++++

)(22)(

)(2

32

22

32

223

HCOMgCOOHMg

HCOCaOHCOCaCO


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Disinfection

Disinfection operations aimed at killing or rendering

harmless, pathogenic microorganisms

Sterilization the complete destruction of all living matter

Chlorination

+

+

+

+

++

OClNaNaOCl

OClCaOClCa

HOClHOHCl

2)( 22

22


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32

2

OOO

OOO voltagehigh

+

+

Chlorine Dioxide

Effective in oxidizing phenolic compounds

Generated on-site in aqueous form by the

chlorination of sodium chlorite at low pH

Irradiation with ultraviolet light


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Dissolved-Solids Removal

Inorganic Materials

- demineralization and desalinization

Ion exchange

Microporous Membranes

reverse osmosis

electrodialysis

Organic Materials

Adsorption

Chemical Oxidation


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Activity for the Day.

4. What are the characteristics of a good disinfectant?

5. Why is aeration used in water-treatment plants?

Is it more commonly used in groundwater or surface water? Why?

Documents

ENVI Chapter 4