Activated Carbon
• Process –dissolved organics removal
• Define adsorption –process for the removal of
one phase of material, and its concentration on
the surface of another phase. Accumulation at an
interface.
Activated Carbon
• Widely used in water and waste water treatment
for the removal of taste and odor
• Adsorbent –the solid (e.g, activation carbon)
• Adsorbate –surbate (substance being removed)
• Results in physical or Van Der Waals adsorption
• Physical adsorption – can also have chemical
adsorption
• Van Der Walls force attraction between two atoms
or molecules from dipole moment
Non balanced forces
Activated Carbon
• Activated carbon can also result in mechanical
filtration of large particles
• Brita water filter system
GAC
Preparation
Raw materials: -Coal – most common
-Nut shells, wood
-Bone
-Resins
3 steps
Characteristics
1) Powered – cheaper to use, low dosage PAC division @
200 mesh (0.074 mm)
2) Granular – used in bed like ion x resins GAC (fixed
bed)
3) Surface chemistry
– carbons may differ somewhat, chemically so different
selectivities
– pore size distribution also affects selectivity
3 A.C. s
Raw material
Granular
Granular
Powered
Bituminous
Lignite
Lignite
Surface area, m2/g
1000
650
475
Mesh size
8 X 30
8 x 30
––––––––
Ash %
7.5
12
––––––––
Effective size (mm)
Apparent density
lb/ft3
0.9
30
0.9
27
0.002
49
Water Treatment – PAC
• taste and odor control
• dosage set by jar test
• 5-50 mg/l usual range
Important Considerations:
• Good mixing
• Adequate contact time
• Method to remove carbon
Points of addition
1) Rapid mix, good mixing, adequate contact time,
removed in sedimentation tank
2) Dump onto top of filter –might be ok but:
a. Contract time may be too short
b. Carbon may penetrate a bad filter
3) add to sediment tank –contract time ok, but poor
mixing –removed in sedimentation tank and on filter
Water Treatment – GAC
1) Taste and odor control ( U.S. )
2) Removal of potentially harmful trace organics
(Germany)
Taste and Odor Control
• Replace top part, or all, of sand in filter, this reduces taste
and odor sufficiently for up to as long as 1-3 years.
• However, Carbon Oxygen Demand (COD) or Carbon
Chloroform Extract (CCE) removal may be quite poor
after only a few weeks
• So the plants are not really designed well in terms of
organics removal
• Typical bed depth in U.S. is 24 in, probably less than 2
dozen plants in U.S. have carbon filters
Trace Organics Removal
• Practiced in Germany
• Use deeper beds, longer contact time
Wastewater Treatment – PAC
1) May be used in tertiary treatment for taste and odor
reduction –same techniques as in water treatment –
mixes before clarifier
2) Add to activated sludge aeration tank to improve
performance, as the carbon
a) Removes non-biodegradable organics
b) Buffers against toxic organics
c) Provides a support on which biological growth can occur
d) Improves nitrification
Wastewater Treatment – GAC
• Used in beds or columns in a series or parallel operation
use as a polishing step –treatment of testing effluent
• Want low turbidity NOT GAC as a filter
Regeneration –chemical or steam oxidation–
• GAC is regenerated, PAC is generally not
• Regeneration required since AC has finite reactionation
capacity
• Regenerate thermally –such as multi-hearth furnace
• Bed more efficient than PAC
Adsorption Capacity
• Theoretical adsorption –adsorption at equilibrium
• Quantity of adsorbate
– f of temperature
– f of concentration
Most Common Isotherms
mg of X / mg of carbon
Constants determined graphically
But, hard to determine constants
from curved line
Recognize as point slope form
C
• GAC in a bed is more “efficient” than PAC dosed into the
WW high X/M
WHY??????????????
• With bed mode, GAC comes into equilibrium with
influent concentration
• With feed or batch mode, PAC comes into equilibrium
with the effluent concentration
C o = starting or influent concentration
Ce = final or effluent concentration
Bed Model
ideal
Reality: kinetics &
dispersion
In Series
Regeneration in staggered fashion
Q gal / hr
PAC
X lb/ hr
Sedimentation Tank