United States
Environmental Protection f

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

 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
 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
! i
                                      - Boll Ring
                                    T1>-^< Influent
                                    _t*-J> Backwash
                                     j-* -* Carbon
                                       - Surface Wash
                                       , Carbon Bed
Figure 1  Downflow Type Granular Activated Carbon

  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
  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
  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
 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

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.

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.

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.


         Carbon Contactor

         9  Hydrogen sulfide generation in the carbon
          Corrosion of the carbon contactor.
          Media clogging.
        Carbon Transport System

         Clogging of the carbon slurry transport
         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<

                 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

     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