Waste water treatment: Coagulation

Chemical Treatment ProcessesCoagulation-flocculation-sedimentation... Chemical Treatment ProcessesProcess Purpose Coagulation To remove turbidity or suspended solids.. The process is usu

Chemical Treatment Processes

Coagulation-flocculation-sedimentation

Chemical Treatment Processes

Process Purpose

Coagulation To remove turbidity or suspended solids The process

is usually employed for treatment of river or lake water for domestic purposes It may also be used in treating industrial wastewater

Precipitation A chemical reaction is involved in this process through

which the pollutant reacts with an induced chemical to form a precipitate that settles down Examples:

removal of hardness by lime and soda ash and removal

of iron and Mn by oxidation

Ion exchange Removal of undesirable ions (cations or anions) by

replacing them with other ions

Activated carbon Removal of trace organic compounds by adsorption Disinfection Inactivation of microorganisms using chlorine, ozone,

etc

Objective: To remove suspended solids (turbidity).

Coagulation

Suspended solids are negatively charged and repel each

other in solution When a coagulant is added the particles are neutralized and agglomerate.

½ minute rapid mixing

Flocculation

½ hr

slow mixing

Sedimentation

1-10 hr

Suspended

solids

Coagulant

Setup in Treatment Plants

Flocculation Sedimentation

Optimum removal of

colloidal particles

from water and

wastewater

depends, among

other factors on,

•type of coagulant

•dose

•pH

Schematic of the Jar Test Apparatus

Dose

pH

Coagulants:  Al2(SO4)3.18H2O Alum

 FeSO4

 FeCl3

Coagulants need alkalinity to form a precipitate of the metal hydroxide [Al(OH)3 or Fe(OH)3] that precipitates with the suspended solids Thus, if turbid water lacks natural alkalinity, then lime Ca(OH)2 or soda ash

Na2CO3 are added Without the alkalinity, flocs will be poorly formed.

Al2(SO4)3.14.3H2O+3Ca(HCO3)2= 2Al(OH)3+3CaSO4+14.3H2O+6CO2 According to the above reaction, 1 mole of alum (m wt 600 g/mole)

requires 3 moles of natural alkalinity (m wt 100 g/mole as CaCO3)

Thus, 1 mg/l alum requires 0.5 mg/l alkalinity as CaCO3

Mixing and Sedimentation Tanks

The Great Lake Upper Mississippi River Board

GLUMRB recommended the following values

for the design of rapid mixing, flocculation and

sedimentation tanks in water treatment plants

Example:

coagulation-flocculation-sedimentation, or softening units

Rapid mixing: Detention time= V/Q ≤ 30 sec

Flocculation: Detention time > 30 min

Velocity through tank= Q/Ax=0.5-1.5 ft/min Paddle velocity= 0.5-3 ft/sec

Sedimentation: Detention time ≥ 4 hrs

Velocity through tank ≤ 0.5 ft/min Over-flow rate= vo= Q/As=500-800 gpd/ft2

Weir loading= Q/L weir ≤ 20,000 gpd/ft

Q

d or h Ax

Q

As

W L

A clarifier 27 m long, 5 m wide, and 3.8

m deep settles 3000 m3/d The effluent

weir length is 50 m Calculate the

detention time, flow-through velocity,

over-flow rate, and weir loading

hr d

hr d

m

m Q

V

/ 3000

8 3 5

27

3

3

=

×

×

×

=

=

min / 11

0 min 60 24 8

3 5

/

3000

2

3

m

d m

d m A

Q

v

x

=

×

×

×

=

=

2 3

2

3

/ / 2

22 5

27

/ 3000

m d

m m

d

m A

Q

v

s

×

=

=

m d

m m

d

m L

Q loading

weir

weir

/ /

60 50

/

=

=

=

Solution

Q=3000 m 3 /d

3.8

Q

27m

5m