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8/8/2019 07 - B - Water Treatment
http://slidepdf.com/reader/full/07-b-water-treatment 1/14
Module 7
WATER SUPPLY AND TREATMENT
Water Treatment
Softening
Hardness
caused by multivalent cations – Ca2+, Mg2+, Fe2+, Fe3+,Mn2+, Sr2+, Al3+
does not cause health problems but reduces theeffectiveness of soaps and causes scale formation
determined using atomic absorption and ion-specific
electrodes or titration (titrant – EDTAethylendediaminetetraacetic acid; indicator – eriochrome
black)
units – mg/L or meq/L
Engr. Elisa G. Eleazar 1 of 14
∑= cationst multivalenTH
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Module 7
WATER SUPPLY AND TREATMENT
Cq concentration in meq/L
C concentration in mg/LEW equivalent weight in g/eq
MW molar weightN ionic charge
CCaCO3 concentration in mg/L as CaCO3
Cq concentration in meq/L
Total Hardness
Carbonate (Temporary) Hardness
associated with the anions carbonate, CO32- and
bicarbonate, HCO3-
forms scale
equal to the smaller of alkalinity or total hardness
Noncarbonate (Permanent) Hardness
associated with the other anions, SO42- etc.
Engr. Elisa G. Eleazar 2 of 14
EW
C C q =
SP4
The concentration of calcium in a water sample is 100mg/L. What is the concentration in meq/L? in mg/L as
CaCO3?
SP5
A water sample contains 60 mg/L of calcium, 60 mg/L of
magnesium and 25 mg/L of sodium. What is the totalhardness in meq/L and in mg/L as calcium carbonate?
NCH CN TH +=
50 xC C qCaCO3=
n
MW EW =
Table 9.2 Water Hardness Classifications (P211)
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Module 7
WATER SUPPLY AND TREATMENT
ClassificationHardness
meq/L mg/L CaCO3
Extremely soft to soft 0 – 0.9 0 – 45
Soft to moderately hard 0.9 – 1.8 46 – 90
Moderately hard to hard 1.8 – 2.6 91 – 130
Hard to very hard 2.6 – 3.4 131 – 170
Very hard to excessively hard 3.4 – 5.0 171 – 250
Too hard for domestic use > 5.0 > 250
Alkalinity measure of the buffering capacity of water
Hardness SpeciationEngr. Elisa G. Eleazar 3 of 14
+−−−−++= H OH CO2 HCO L / mg , Alkalinity 2
33
+−−−+++= H OH CO HCO L / meq , Alkalinity 2
33
SP6
From the following water analysis, determine the totalhardness, carbonate hardness and noncarbonate
hardness in mg/L as CaCO3.
CO2
6.0 mg/L
Ca2+ 50.0 mg/LMg2+ 20.0 mg/LNa+ 5.0 mg/L
Alkalinity 120 mg/L as CaCO3
SO4
2- 94.0 mg/L
pH 7.3
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Module 7
WATER SUPPLY AND TREATMENT
TH (sum of multivalent cations)
ALK (typically bicarbonate concentration)
CH (if ALK<TH, CH=ALK; else CH=TH)
NCH = TH – CH
CCH (if Ca2+<CH, CCH=Ca2+; else CCH=CH)
CNCH (Ca2+ - CCH)
MCH (CH – CCH)
MNCH (Mg2+ - MCH)
Check
CH = CCH + MCH NCH = CNCH + MNCH
TH = CCH + CNCH + MCH + MNCH
Ion Exchange
Engr. Elisa G. Eleazar 4 of 14
SP7
From the previous water analysis, construct a bar chart
to determine the speciation of the hardness.
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Module 7
WATER SUPPLY AND TREATMENT
appropriate for waters with high noncarbonate
hardness and total hardness less than 350 mg/L as CaCO3
column containing a resin
100% hardness removal thus the need for bypass
Breakthrough – occurs when the effluent concentration equalsthe influent concentration
Chemical Precipitation
pH increaseEngr. Elisa G. Eleazar 5 of 14
SP8
A residential water softener has 0.07 m3 of ion exchangeresin with an exchange capacity of 46 kg/m3. Theoccupants use 1500 L of water daily. If the water
contains 245 mg/L of hardness as CaCO3
and they want
to soften it to 100 mg/L as CaCO3, how much water
should bypass the softener and what is the timebetween regeneration cycles?
( ) ( )
( ) ( )TH Q
vcapacity
gh Breakthrou IX
sinre=
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Module 7
WATER SUPPLY AND TREATMENT
CH – lime
NCH – soda ash
Lime-Soda Softening Reactions
Softening Treatment
Engr. Elisa G. Eleazar 6 of 14
SP9
Using the following data, determine the stoichiometricamount of chemicals required to soften the water to thesolubility limits if the flow rate is 5 mgd and 95% pure
quicklime and 95% pure soda ash are used.CO
26.0 mg/L
Ca2+ 50.0 mg/L
Mg2+ 20.0 mg/LNa+ 5.0 mg/LAlkalinity 120 mg/L as CaCO
3
SO4
2- 94.0 mg/L
H 7.3
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Module 7
WATER SUPPLY AND TREATMENT
Coagulation and Flocculation
Turbidity – caused by tiny clay and silt particles
Coagulation
chemical alteration of the colloidal particles to makethem stick together to form large particles (flocs)
coagulant: alum (aluminum sulfate)
coagulant aids: lime and polymers mechanism:
• charge neutralization – coagulant is used to
counter the charges on the colloidal particles
• bridging – colloidal particles stick together by
virtue of the macromolecules formed by the coagulant
Engr. Elisa G. Eleazar 7 of 14
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Module 7
WATER SUPPLY AND TREATMENT
Flocculation
physical process of producing differential velocities so
that the particles can come into contact
Engr. Elisa G. Eleazar 8 of 14
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Module 7
WATER SUPPLY AND TREATMENT
Settling
Variables:
• particle size
• particle shape
• particle density
• fluid density
• fluid viscosity
Particle Dia, mm Typical Particle Settling Vel, m/s
1.0 Sand 2 x 10-1
0.1 Fine Sand 1 x 10-2
0.01 Silt 1 x 10-4
0.001 Clay 1 x 10
-6
Ideal Settling Tank
Engr. Elisa G. Eleazar 9 of 14
Table 9.3 Typical Settling Rates (P226)
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Module 7
WATER SUPPLY AND TREATMENT
Assumptions:
Uniform flow occurs within the settling tank
All particles settling to the bottom are removed
Particles are evenly distributed in the flow as they
enter the settling tank
All particles still suspended in the water when thecolumn of water reaches the far side of the tank are not
removed and escape the tank
Critical Particles – particles with lower settling velocities are not
all removed and particles with higher settling velocities are allremoved
t hydraulic retention, detention time
V volumeQ volumetric flow rate
Engr. Elisa G. Eleazar 10 of 14
Q
V t =
t H v
A
Qv
o
s
o
=
=
SP10
A wastewater treatment plant settling tank has anoverflow rate of 600 gal/day-ft2 and a depth of 6 ft.
What is the retention time?
SP11
A small water plant has a raw water inflow rate of 0.6m3 /s. Laboratory studies have shown that the
flocculated slurry can be expected to have a uniformparticle size and it has been found through
experimentation that all the particles settle at a rate of 0.004 m/s. A proposed rectangular settling tank has an
effective settling zone of L=20 m, H=3 m and W=6 m.
Could 100% removal be expected? What fraction of theparticles will be removed?
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Module 7
WATER SUPPLY AND TREATMENT
Filtration
Engr. Elisa G. Eleazar 11 of 14
SP12
What is the filtration rate for a 25-ft by 20-ft filter if itreceives 2 mgd?
SP13
How much backwash water is required to clean the filter
in SP12? Assume 20 gpm/ft2 will be used and the filterswill be cleaned for 15 minutes.
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Module 7
WATER SUPPLY AND TREATMENT
Disinfection
Objective
to destroy the remaining pathogens
Chlorination
addition of chlorine in water
formaton of HOCl
formation of chloramines due to reaction with ammonia
or organic nitrogen
Engr. Elisa G. Eleazar 12 of 14
SP14
A 4.5 mgd water treatment plant uses 21 lb/day of
chlorine for disinfection. If the daily chlorine demand is0.5 mg/L, what is the daily chlorine residual?
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Module 7
WATER SUPPLY AND TREATMENT
Other Treatment Processes
Water Stability
Marble Test and Langelier Index – calcium carbonatesaturation
Stabilization:
• Recarbonation
• Acid addition
• Phosphate addition
• Alkali addition
• Aeration
Taste and Odor
Aeration
Fluoridation
Distribution of Water
Engr. Elisa G. Eleazar 13 of 14
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Module 7
WATER SUPPLY AND TREATMENT
Exercise No. 7
1. An unconfined aquifer is 10 m thick and is being pumpedso that one observation well placed at a distance of 76 mshows a drawdown of 0.5 m. On the opposite side of theextraction well is another observation well, 100 m from the
extraction well, and this well shows a drawdown of 0.3 m.Assume the coefficient of permeability is 50 m/day. What is
the discharge of the extraction well?
2. A settling tank is 20 m long, 10 m deep and 10 m wide.The flow rate to the tank is 10 m3 /minute. The particles to be
removed all have a settling velocity of 0.1 m/minute. What isthe hydraulic retention time? Will all the particles be
removed?
3. Calculate the alkalinity, total hardness, carbonatehardness, and noncarbonate hardness for the following water
in mg/L as CaCO3.
Cations mg/L Anions mg/L
Ca2+ 12 HCO3- 75
Mg2+ 15 SO42- 41
Sr2+ 3 Cl- 25
Na+ 15 NO3- 10
K+
15 pH 7.8
4. Using the data in No. 3, determine the speciation and thestoichiometric amount of chemicals required to soften the
water to the solubility limits if the flow rate is 5 mgd and 95%pure quicklime and 95% pure soda ash are used.
Engr. Elisa G. Eleazar 14 of 14