Waste water treatment: Precipitation

Objective: To remove undesirable pollutants by a precipitation reaction.. Maq + Naq MNs Example removal of hardness by lime soda-ash and removal of iron and manganese by oxidation.. The

Objective: To remove undesirable pollutants by a precipitation reaction

M(aq) + N(aq) MN(s) Example removal of hardness by lime soda-ash and removal of iron and

manganese by oxidation

The focus of this lecture is on removal of hardness by lime and soda-ash, but before proceeding with the lime soda-ash method, we need to know how to express concentration in units of meq/l and mg/l as CaCO3?

#

.

.

eq

wt molecular or

Atomic wt

) /

(

) /

(

meq mg

wt Eq

L mg ion

Concentrat L

meq in

ion

) /

(

.

) /

( 50 ) /

3

meq mg

X of wt Eq

meq CaCO

mg L

mg X

of ion Concentrat CaCO

as X

of

L

Symbol At or M wt.

Lime Soda-Ash Softening

A precipitation method through which Ca is removed as

Ca(HCO3)2+Ca(OH)2 2CaCO3 +2H2O

Lime

Ca hardness

H2CO3+Ca(OH)2 CaCO3 +2H2O

Lime

The first step is to remove CO2 from water:

CO2+H2O H2CO3

Now remove Ca or Mg that has alkalinity in the water:

Mg(HCO3)2+2Ca(OH)2 2CaCO3 + Mg(OH)2+ 2H2O

Lime

Mg hardness

MgSO4+ Ca(OH)2 +Na2CO3 CaCO3 + Mg(OH)2+ Na2SO4

Lime

Ca hardness Soda ash

CaSO4+ Na2CO3 CaCO3 + Na2SO4

Soda ash is needed if there is not enough alkalinity in the water:

1 meq/l of meq/l needed of

lime Soda ash

-Ca with ions other than alkalinity - 1

Mg with ions other than alkalinity 1 1

Amount of lime and soda ash needed to remove hardness

Practically, some Ca and Mg will be left in solution after treatment:

For Ca: 30 mg/l as CaCO3 =0.6 meq/l in equilibrium with CaCO3 is left For Mg: 10 mg/l as CaCO3=0.2 meq/l in equilibrium with Mg(OH)2 is left

Component Concentration

CO2 (mg/l) 8.8

Ca2+ (mg/l) 40

Mg2+ (mg/l) 14.7

Na+ (mg/l) 13.7 HCO3- (mg/l) 164.7

SO42- (mg/l) 29

Cl- (mg/l) 17.8

Example

A groundwater sample has the following characteristics Find:

1.The total hardness

2.TDS

3.The amount of lime and soda ash needed to remove hardness

to practical limits

4.The meq/l bar graph after lime soda addition.

Component Concentration

(mg/l)

Equivalent weight (mg/meq) Concentration(meq/l)

Solution

3 To determine the amount of lime and soda ash needed, we need first to convert

the concentration to meq/l, draw a meq/l bar graph and then use the table

) /

(

) /

( 50 ) /

3

meq mg

X of wt Eq

meq CaCO

mg L

mg X

of ion Concentrat CaCO

as X

of

L

1 Hardness: Convert the concentration of Ca and Mg to mg/l as CaCO3 using

Thus Ca= 100 mg/l as CaCO3, Mg=60 mg/l as CaCO3 Total Hardness is 160 mg/l as CaCO3

2 TDS= sum of all cations and anions in mg/l = 279.9 mg/l

The meq/l bar graph of water before lime/soda-ash treatment

CO2 =0.4 0.4

-Ca(HCO3)2=2.0 2.0

-Mg(HCO3)2=0.7 1.4

MgSO4=0.51 0.51 0.51

The amount of lime and soda ash needed to remove hardness to practical limits are determined as shown in the table below

The meq/l bar graph of water after lime/soda-ash treatment but before recarbonation

Note, the pH of the treated water is high (about 11), so CO2 is added (recarbonation)

to reduce the pH