United States
Environmental Protection f
Agency
August
1984
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EPA Offices:
EPA-OWPO (WH-547) EPA-MERL (443)
401 M Street SW 26 West St. Clair Street
Washington, DC 20460 Cincinnati, OH 45268
(202)382-7365/7368 (513) 684-7613
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The granular activated carbon (GAG) system is
generally utilized for the removal of soluble organics
in wastewater, including refractory organics. GAG
can be used either as a tertiary treatment process in
advanced wastewater treatment plants, or as a
secondary treatment process. It may be used in
conjunction with biological treatment processes, or
in independent physical/chemical (IPC) treatment
plants.
A comprehensive evaluation of selected advanced
treatment (AT) facilities was recently completed with
the objective of identifying common problems with
GAG systems related to design deficiencies, equip-
ment performance, and operation/maintenance.
Based on the information obtained from wastewater
treatment plant visits and other experiences,
remedial measures for minimizing the problems are
offered.
Wash
Water
;-\
! i
CD:
- Boll Ring
T1>-^<— Influent
_t*-J—>• Backwash
j-*— -* Carbon
— '
Charge
- Surface Wash
, Carbon Bed
Surface
Figure 1 Downflow Type Granular Activated Carbon
System
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Wastewater treatment with GAC consists of the
carbon contact system and the carbon regeneration
system. Activated carbon removes soluble organics
from water in three steps. The first step is the
transport of the dissolved substances to be removed
(solute) through a surface film to the exterior surface
of the carbon. The next step is the diffusion of the
solute within the pores of the activated carbon. The
third step is the adsorption of the solute on the
interior surfaces bounding the pore and capillary
spaces of the activated carbon. Alternative config-
urations for carbon contacting systems include the
following:
9 Downflow or upflow of the wastewater through the
carbon bed.
0 Parallel or series operation (single or multistage).
® Pressure or gravity operation in downflow systems.
® Packed or expanded bed operation in .upflow
systems.
Figure 1 presents a schematic of a typical downflow
GAC process^unit. As the carbon is exposed to
organics in solution it gradually loses its adsorptive
capacity because the available adsorption sites
become exhausted. The carbon must then be
regenerated either at the treatment plant or off-site.
Granular carbon is typically regenerated in a
furnace by oxidizing the adsorbed organic matter,
thus removing it from the carbon surfaces. Fresh
carbon is added to the system to replace any that is
lost during regeneration and hydraulic transport.
The following discussion of the problems and
remedial measures of the GAC system is subdivided
according to the different components of the system.
The specific areas discussed are: (1) carbon
contactor, (2) backwash system, (3) carbon regener-
ation system, and (4) instrumentation and control
system.
Carbon contactors constructed of mild steel tend to
become pitted and corroded from exposure to wet
granular activated carbon. Corrosion is also caused
by hydrogen sulfide that is generated when sulfates
present in the wastewater are biochemically
...ceduced by bacteria under anaerobic conditions in
the carbon column. The carbon contactors should
have protective coatings, such as coal tar epoxy, to
prevent corrosion. Potential remedies for controlling
hydrogen sulfide generation include the addition of
chemicals to the influent, such as sodium nitrate or
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chlorine, and maintaining aerobic conditions in the
column. Preaerating the influent and reducing the
detention time (if possible) are other methods for
controlling the generation of hydrogen sulfide.
Microbial growth in the carbon bed creates media
clogging problems. Media clogging problems could
be minimized by increasing the backwash frequency
and using a surface wash system.
Carbon Transport System
Clogging of the carbon slurry transport pipes occurs
at many plants. The problem is caused by
undersized piping, short radius bends, insufficient
velocity, and lack of cleanouts in the carbon
transport system. Abrasion wear of slurry transport
pipes is also a common problem in unlined mild
steel and fiberglass reinforced plastic (FRP) piping,
particularly at sharp bends. Increasing the size of
the piping (a minimum pipe diameter of 2 inches is
recommended), transporting a more dilute carbon
slurry, using.long radius piping, and providing a
sufficient number of cleanouts would help to
minimize the clogging problem. •- •
Abrasion of the pipes could be reduced significantly
by using glass or rubber lined steel piping or coated
cast iron piping for carbon slurry transport. The use
of long radius piping and extra-heavy elbows and
tees is recommended.
Backwash System
Clogging of backwash and surface wash nozzles is
a common problem. This is caused by migration of
carbon and solids to the underdrains where they are
picked up by the incoming backwash water and
clog the distribution nozzles. Screens installed at the
bottom of the carbon bed prevent media migration to
the underdrains. Frequent backwashing, especially
after loading the carbon, removes the fines from the
bed, thus decreasing the clogging of the nozzles.
Carbon Regeneration System
The regeneration system is a source of carbon loss
due to incorrect furnace operation. Preventing
excess furnace operating temperatures and timely
removal of the regenerated carbon from the furnace
are essential in order to minimize carbon loss during
regeneration. An adequate quantity of spent carbon
should be stored to permit continuous operation of
the regeneration furnace. Plant operators should
carefully follow operating instructions offered by the
equipment manufacturer and design engineer.
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Instrumentation and Control System
The maintenance of instrumentation and control
equipment at many treatment plants is not adequate,
resulting in ineffective automatic process control
systems, and consequently, the discharge of poor
quality effluent from the GAC process unit. It is
critical that certain operating parameters be
accurately monitored. These include wastewater
flow, pH of influent, head Loss across the carbon
columns, and effluent BOD, TOG, and COD.
Conclusions
The performance of GAC systems in wastewater
treatment plants indicates that many of the plants
have problems in operation, and also in achieving
the required quality of effluent from the GAC unit.
The causes of these problems are varied and relate
to design deficiencies, improper operation, influent
characteristics, and the efficiency of the carbon
adsorption process itself. It is possible to rectify
many of these deficiencies at existing facilities, but
the cost effectiveness of incorporating remedial
measures should be considered on a case-by-case
basis. Some of the remedial measures could be
incorporated in the design of new GAC systems at a
reasonable cost. A summary of major problems
experienced with GAC systems and suggested
recommendations for improvement are given in
Table 1.
The overall performance of the GAC systems could
be improved by implementing the suggested
remedial measures. However, in certain applications
some compounds may not be removed by the GAC
process. This points out the importance of
conducting extensive treatability studies prior to
utilizing GAC in a particular application. Specific
design parameters (i.e., type of carbon, wastewater
temperature and pH) should also be determined on
the basis of treatability studies. Such studies should
also demonstrate the overall effectiveness of the
proposed treatment system, including processes
preceding and following the GAC unit.
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Problem
Carbon Contactor
9 Hydrogen sulfide generation in the carbon
contactor.
Corrosion of the carbon contactor.
Media clogging.
Carbon Transport System
• Clogging of the carbon slurry transport
pipeline.
® Abrasion of the carbon slurry pipeline.
Backwash System
© Clogging of backwash nozzles.
Carbon Regeneration System
^ Excessive carbon loss.
Instrumentation and Control System
*» Nonfunctioning instrumentation and control
- systems.
Table 1 Granular Activated Carbon System: Problems and Sug<
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Suggested Remedy
© Maintain aerobic conditions in the carbon
contactor; aerating the influent; adding
chemicals such as sodium nitrate to influent;
and increasing the frequency of backwashing.
e Carbon contactors should have protective
coatings; (e.g., coai tar epoxy); use nonmetallic
connectors within the contactor; eliminate the
potential for hydrogen sulfide generation.
• Use a surface wash system; increase
backwash frequency.
Increase transport line size (minimum
suggested diameter is 2 inches); decrease
carbon slurry concentration; use long radius
piping.
Use black steel or lined steel pipe; use long
radius piping, along with extra-heavy elbows
and tees.
• Install screens at the bottom of the carbon bed
to prevent media migration; backwash
frequently, especially after loading the carbon
to remove carbon fines.
Operate the carbon regeneration furnace at the
specified conditions; store enough spent
carbon to permit more continuous operation
of the regeneration furnace.
9 An adequate maintenance program should be
established and followed.
ted Remedies
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