FINDING A HOME FOR THE CARBON:
AERATOR (POWDERED) OR COLUMN (GRANULAR)
Brian P. Flynn, Process Engineer
E. I. du Pont de Nemours & Co., Inc.
Deepwater, New Jersey 08023
Louis T. Barry, Senior Research Supervisor
Nichols Engineering
Belle Meade, New Jersey 08502
INTRODUCTION
The fundamental processes of adsorption and biological degradation form the basis,
either separately or in combination, of many end-of-pipe water pollution control processes.   There are four readily identifiable ways to use these fundamental processes.   The
most traditional approach is the activated sludge process with its various modifications
(biodegradation).   More recent approaches include granular carbon columns (adsorption),
activated sludge preceded or followed by granular carbon columns (adsorption + biological degradation), and powdered carbon addition to activated sludge [du Pont patented
"PACT" process], which is an adsorption and biological degradation process.   The purpose of this paper is to show a methodology for choosing between these alternatives,
with an example of this methodology applied to a complex, variable industrial wastewater.   The emphasis is on the placement and physical form of the absorbent, activated
carbon.
Activated sludge [1] is a well known and flexible process [2] employed in municipal
and Industrial wastewater treatment for years.   Granular carbon column treatment is a
more recent innovation [3] which was plagued by the problem of biological adherence
and growth on the carbon [4], "biological fouling," until it was realized that, properly
used, this vice became a virtue [5].   Granular carbon columns have been shown to be a
means of treating nonbiodegradable or difficult to degrade industrial wastewaters.   The
use of granular carbon columns before or after the activated sludge process has been successfully used to upgrade the normal secondary quality effluent from biological treatment
plants [6,7].   A recent innovation, the "PACT" process, has been successfully demonstrated on a lab [8], pilot plant [9] and full scale [10,11], with considerable work being
done on defining its advantages [8,12] and effluent quality relative to input variables
[13-15].   For the purpose of this paper, it is an "add-on" to the activated sludge process
which upgrades its effluent quality.
The methodology for choosing between these alternatives consists of four basic steps
(see Figure 1):
1. Screening of processes.  This is done so as to eliminate the technically unworkable
processes without eliminating a technically workable one.   It is necessary to operate
under stressed conditions, conditions likely to cause difficulty in full-scale operation.
This will help eliminate technically unworkable processes.  The screened process must
meet required effluent quality criteria in order to be workable.   A starting point for
screening conditions is shown in Table I.
2. Based on the screening tests, an initial set of capital and operating costs is generated
for the technically feasible processes.
2a. Perhaps at this point one or more technically feasible processes may be eliminated
due to grossly unfavorable economics.
3. By proper experiment, a cost optimization is done for the remaining processes.
4. A choice is made based on the developed cost data plus any nonquantiflable factors
that enter into consideration, Le„ expected risk of failure, corporate environmental
policy, future water quality expectations, etc.
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