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8/14/2019 ENVI Chapter 4
1/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
8/14/2019 ENVI Chapter 4
2/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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
8/14/2019 ENVI Chapter 4
3/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Aeration
Softening
Filtration
Disinfection
Storage
Raw Water
Gases to
atmosphere
Lime
Soda Ash
CaCO3
Mg(OH)2
Chlorine
Chlorine
Typical plant treating hard groundwater.
8/14/2019 ENVI Chapter 4
4/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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
8/14/2019 ENVI Chapter 4
5/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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)
8/14/2019 ENVI Chapter 4
6/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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
8/14/2019 ENVI Chapter 4
7/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Iron and Manganese in the absence of oxidizing agents
both are stable in water
Oxidation Reaction
++
++
+++
+++
HMnOOHOMn
HOHFeOHOFe
4222
8)(4104
2222
322
2
8/14/2019 ENVI Chapter 4
8/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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)
8/14/2019 ENVI Chapter 4
9/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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)
8/14/2019 ENVI Chapter 4
10/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Devices for Liquid Gas Contact
Fountains
Cascade Towers
Tray Towers
Diffused Aerators
8/14/2019 ENVI Chapter 4
11/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
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
8/14/2019 ENVI Chapter 4
12/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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
8/14/2019 ENVI Chapter 4
13/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
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.
8/14/2019 ENVI Chapter 4
14/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Flocculation
gentle mixing to speed the agglomeration process
8/14/2019 ENVI Chapter 4
15/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Softening
Chemical Precipitation
- calcium hardness to calcium carbonate- magnesium hardness to magnesium hydroxide
Lime-soda Process
Caustic soda Process
8/14/2019 ENVI Chapter 4
16/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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
8/14/2019 ENVI Chapter 4
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UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Caustic soda Process
++
++
++
+
++++
++++++++++
+++
242
24
2
22323
2
22333
2
2232
2)(2
224)(4)(2
222)(2
22
SONaOHMgNaOHSOMg
OHCONaOHMgNaOHHCOMg
OHCONaCaCONaOHHCOCa
OHCONaNaOHCO
8/14/2019 ENVI Chapter 4
18/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Stabilization
Addition of Acid
OHSOMgSOHOHMg
SOHCOCaSOHCaCO
224
2422
243
2423
2)(
)(222
+++
+++
+
+
Recarbonation
+
+
+++++
)(22)(
)(2
32
22
32
223
HCOMgCOOHMg
HCOCaOHCOCaCO
8/14/2019 ENVI Chapter 4
19/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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
8/14/2019 ENVI Chapter 4
20/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering Ozone
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
8/14/2019 ENVI Chapter 4
21/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
Dissolved-Solids Removal
Inorganic Materials
- demineralization and desalinization
Ion exchange
Microporous Membranes
reverse osmosis
electrodialysis
Organic Materials
Adsorption
Chemical Oxidation
8/14/2019 ENVI Chapter 4
22/22
UniversityofSa
nCarlos-Departm
entofChemicalEn
gineering
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?