Module 4: WATER
PROCESSING
Reference: Hammer, M.J. & Hammer, M.J.J. (2014). Water and Wastewater
Technology,7th edition. London: Pearson Education Limited.
I. Chlorination
• the process of adding the element chlorine to
water as a method of water purification to make it
fit for human consumption as drinking water
(disinfection)
• oxidizes dissolved iron and manganese in water
(color removal)
Fe+2 + Mn+2 + O2
FeOx + MnO2
I. Chlorination
Chlorine
- symbol Cl, greenishyellow gaseous
element
- strong oxidizing agent,
reacting with water,
organic compounds,
and many metals
I. Chlorination
Commonly used forms:
1. Calcium Hypochlorites (Ca(OCl)2)
•
•
•
Dry form
Granular, powdered or tablet foems
Readily dissolves in water and contain 70% available chlorine
2. Sodium Hypochlorite (NaOCl)
•
•
•
Liquid form
5-15% available chlorine
Cheaper
Available chlorine – chlorine existing in water as
hypochlorous acid and hypochlorite
I. Chlorination
pH>8
Cl2 + H2O HCl + HOCl
H+ + OClpH<7
Ca(OCl)2 + H2O
Ca+2 + 2OCl + H2O
if the pH is between 4.5 – 8.5:
HOCl + NH3 H2O + NH2Cl (monochloroamine)
HOCl+NH2Cl
H2O + NHCl2 (dichloroamine)
if the pH is below 4.4:
HOCl+NH2Cl2
H2O + NCl3 (trichloroamine)
I. Chlorination
Free available residual chlorine is that residual chlorine
existing in water as hypochlorous acid or hypochlorite ion.
Combined available residual chlorine is that residual existing in
chemical combination with ammonia (chloramines) or organic
nitrogen compounds.
Chlorine demand is the difference between the amount added
to a water and the quantity of free and combined available
chlorine remaining at the end of a specified contact period.
I. Chlorination
Example 1.
I. Chlorination
Example 2.
II. Aeration
• the saturation of water with air, water is brought into
contact with air in such a manner as to produce
maximum diffusion, usually by spraying water into the
air in fountains to remove dissolved gases
• removes odors and taste caused by decomposing
organic matter, and also industrial wastes such as
phenols and volatile gases such as chlorine
• it also converts dissolved iron and manganese
compounds into insoluble hydrated oxides of the
metals which may then be readily settled out
III. Softening
• Removal of hardness
• Maximum level for public supply – 300 to 500
mg/L
• Hardness interferes with laundering and may
produce scaling in pipes
• Precipitation softening – uses lime (Ca)) and soda ash
(Na2CO3)
• Other benefits of Lime treatment:
• Bactericidal action
• Removal of iron
• Aids in clarifying turbid waters
III. Softening
• Forms of lime:
• Quicklime
• granular form
• Minimum of 90% CaO (MgO is primary
impurity)
• Hydrated lime
• Powdered form
• Approximately 68% CaO
• Lime slurry (Ca(OH)2)
• Soda ash – grayish white powder
- contains at least 98% sodium carbonate
III. Softening
Note: precipitation of Mg ion demands a high pH and the presence of
excess lime (+35 mg/L CaO)
III. Softening
Example 3. Water defined by the following analysis is to be
softened by excess lime treatment.
CO2 = 8.8 mg/L Alk (HCO3-) = 135 mg/L as CaCO3
Ca+2 = 40 mg/L
SO4 -2 = 29 mg/L
Mg+2 = 14.7 mg/L
Cl= 17.8 mg/L
Na+ = 13.7 mg/L
A) Sketch a meq/L bar graph and list the hypothetical
combinations of chemical compounds in solution.
B) Calculate the softening chemicals required, expressing lime
dosage as CaO and soda ash as Na2CO3.
III. Softening
Example 3 Solution.
a.
the figure
III. Softening
Example 3 Solution.
b.
IV. Mixing and Flocculation
Recommended detention time for rapid mixing: <30 sec
IV. Mixing and Flocculation
• causes the suspended solids to coalesce
IV Mixing and Flocculation
Recommendations for flocculation basins:
• Inlet and outlet shall prevent short-circuiting and destruction of
floc
• Min flow-through velocity: 0.5-1.5 ft/min (2.5-7.5 mm/s)
• Detetion time: > 30 min
• Agitator speed: 0.5-3.0 ft/s (0.15-0.91 m/s)
• Must be close together as possible with sedimentation basins
• Velocity through conduits: 0.5-1.5 ft/s (0.15-0.45 m/s)
• Allowances made to reduce turbulence at bends
IV. Mixing and Flocculation
Example 4. Laboratory tests were conducted on the chemical
treatment of a water using both a long, narrow flocculation
tank and a complete mixing unit. The observed rate constants
under steady state conditions were first order kinetics equal
to “6 per day” for plug flow and “18 per day” for complete
mixing. If the desired degree of removal is 90% for an influent
concentration of 60 mg/L, which process requires the shortest
retention time?
Solution:
C0 = 60 mg/L
For 90% removal, Ct =(1-0.9)60 mg/L = 6 mg/L
IV. Mixing and Flocculation
V. Coagulation
the process of adding chemicals to the wastewater
which causes the surface characteristics of the
suspended solids to be altered so that they attach to
one another and precipitate
• Commonly used coagulants:
• Aluminum based (aluminum sulfate, sodium aluminate,
potash alum, ammonia alum)
• Iron based (ferric sulfate, ferrous sulfate, chlorinated
ferrous sulfate, ferric chloride)
V. Coagulation
• Aluminum Sulfate
• Commonly known as alum, filter alum or
alumina sulfate
• Al2(SO4)3 XH2O
• Grayish white crystallized solid
• lump, ground or powdered form or
• concentrated solution
• Most widely used
V. Coagulation
Aluminum Sulfate reactions with (5) natural alkalinity, (6)
lime and (7) soda ash
1.0 mg/L of alum(MW=600) reacts with
0.5 mg/L natural alkalinity (as CaCO3)
0.39 mg/L of 95% hydrated lime as Ca(OH)2
0.33 mg/L 85% quicklime
0.53 mg/L soda ash as Na2CO3
V. Coagulation
Example 5.
V. Coagulation
Example 6
VI. Sedimentation or Clarification
• removal of particulate matter, chemical floc and
precipitates from suspension through gravity
settling
VI. Sedimentation or Clarification
t=V/Q
vo = Q / A
weir loading = Q / L
where : t = detention time
V = basin volume
Q = volumetric flow rate
vo = overflow rate
A = cross-sectional area
L = weir length
VI. Sedimentation or Clarification
vsettling > Q/A heavier flocs removed
vsettling < Q/A lighter flocs carried out in the
basin effluent
VI. Sedimentation or Clarification
Recommendations for sedimentation basins:
• Maximum horizontal velocity: 0.5 ft/min (2.5 mm/s)
• Detention time: > 4 hours
• Maximum weir loading: 20000 gpd/ft of weir length (250
m3/m-day)
• Overflow rate: 500-800 gpd/sq ft (20-33 m3/m-day)
VI. Sedimentation or Clarification
Example 7.
VI. Sedimentation or Clarification
Example 7 Solution.
VI. Sedimentation or Clarification
Example 7 Solution.
VII. Filtration
• process of separating a
suspended solid, such as a
precipitate, from the liquid in
which it is already suspended
by straining it through a
porous medium that can be
penetrated easily by liquids
• used to remove nonsettleable
floc remaining after chemical
coagulation and
sedimentation
VII. Filtration
1.
2.
3.
4.
Filtration: Valve 1,4 open ; Valves 2, 3, 5 close
Backwashing1: Valve 2 open; Valves 1, 3, 4, 5 close
Backwashing2: Valve 2,5 open; Valves 1, 3, 4 close
Before next run: Valves 1,3 open; Valves 2, 4, 5 close
VII. Filtration
Kinds of Filter Media:
1. SAND - loose, incoherent mass of mineral materials in a
finely granular condition usually consisting of quartz (silica),
with a small proportion of mica, feldspar, magnetite, and
other resistant minerals
2. COAL - a combustible organic rock composed primarily of
carbon, hydrogen, and oxygen; porous surface makes it a
good adsorbent
3. GARNET - group of related minerals, often used as
gemstones or abrasives; garnets are compound silicates
VII. Filtration
Example 8. A filter unit is 15 ft by 30 ft. After filtering 2.5 mil gal
in a 24-hr period, the filter is backwashed at a rate of 15 gpm/sq
ft for 12 min. Compute the average filtration rate and the
quantity and percentage of treated water used in backwashing.
VIII. Ion-exchange
• method of exchanging ions in a solution with ions
of the same charge in certain insoluble substances
• by this means chemicals can be removed from a
solution that contains large amounts of other
chemicals
• this is done by passing the solution through porous
solid materials, usually minerals of the zeolite
group or specially prepared synthetic resins
(plastics) containing large, complex molecules
VIII. Ion-exchange
Kinds of Cation exchanger:
1. Natural greensand (glauconite)
- dull green mineral used in water softening
- it forms in sands and sandstones as grains or pellets
- chemically, it is a hydrous iron-potassium silicate
- has a value of 2 on the Mohs hardness scale and a
specific gravity of about 2.3
VIII. Ion-exchange
Kinds of Cation exchanger:
2. Silicon gel-type zeolite
- large group of minerals composed of hydrated aluminum
-
-
silicates of alkali metals and alkaline earth metals
named because of their swelling and bubbling under
high temperature
range in hardness from 3 to 6 and have a specific gravity
of 1.9 to 2.8
are usually found in veins and cavities of basic igneous
rocks, especially basalt
are used as water-softening agents
IX. Carbon Adsorption
Taste and color removal using activated
carbon
IX. Carbon Adsorption
Activated Carbon
- prepared from hardwood charcoal, lignite, nut shells,
or other carbonaceous materials by controlled
combustion
- Each particle is honeycombed with thousands of
molecular-sized pores
- Available in powdered or granular form
X. Fluoridation
• the practice of adding fluoride compounds to water
with the intended purpose of reducing tooth decay
in the general population
• Optimum concentration between 0.6 and 1.2 ppm
• Commonly used in water treatment: sodium
fluoride, sodium silicofluoride and fluorosilicic
acids (also known as hydrofluorosilicic acid,
hexafluorosilicic or silicofluoric acid)
X. Fluoridation
Image references:
1. https://www.alibaba.com/productdetail/High-Purity-Optical-SodiumFluoride-for_50039690751.html
2. http://pureborax.sell.everychina.com/p107925065-99-disodiumhexafluorosilicate-odorless-sodiumsilicofluoride-suppliers.html
3. https://www.a2zpressrelease.com/che
micals-and-materials/fluorosilicic-acidmarket-research-report-2019-2023/
X. Fluoridation
Example 3
X. Fluoridation
Example 4
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