07 - b - water treatment

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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



Module 7


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.


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)



Module 7


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



Module 7


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


SP7

From the previous water analysis, construct a bar chart

to determine the speciation of the hardness.



Module 7


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=



Module 7


CH – lime

NCH – soda ash

Lime-Soda Softening Reactions

Softening Treatment


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



Module 7


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




Module 7


Flocculation

physical process of producing differential velocities so

that the particles can come into contact




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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


Table 9.3 Typical Settling Rates (P226)



Module 7


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


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?



Module 7


Filtration


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.



Module 7


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


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?



Module 7


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




Module 7


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.


07 - b - water treatment

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