ROBERT A. TAFT WATER RESEARCH CENTER
REPORT NO. TWRC-2
A COMPARISON OF EXPANDED-BED
AND
PACKED-BED ADSORPTION SYSTEMS
ADVANCED WASTE TREATMENT LABORATORY-II
U.S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
OHIO BASIN REGION
Cincinnati, Ohio
-------
A COMPARISON OF EXPANDED-BED AND
PACKED-BED ADSORPTION SYSTEMS
by
Charles B. Hopkins, Walter J. Weber, Jr.,
and Ralph Bloom, Jr.
for
The Advanced Waste Treatment Research Laboratory
Robert A. Taft Water Research Center
This report is submitted in
fulfillment of Contract No.
14-12-76 between the Federal
Water Pollution Control Ad-
ministration and the FMC
Corporation.
U. S. Department of the Interior
Federal Water Pollution Control Administration
Cincinnati, Ohio
December, 1968
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FOREWORD
In its assigned function as the Nation's principal natural
resource agency, .the United States Department of the Interior
bears a special obligation to ensure that our expendable re-
sources are conserved, that renewable resources are managed to
produce optimum yields, and that all resources contribute their
full measure to the progress, prosperity, and security of
America — now and in the future.
This series of reports has been established to present the
results of intramural and contract research carried out under
the guidance of the technical staff of the FWPCA Robert A. Taft
Water Research Center for the purpose of developing new or im-
proved wastewater treatment methods. Included is work conducted
under cooperative and contractual agreements with Federal, state,
and local agencies, research institutions, and industrial organi-
zations. The reports are published essentially as submitted by
the investigators. The ideas and conclusions presented are,
therefore, those of the investigators and not necessarily those
of the FWPCA.
Reports in this series will be distributed as supplies per-
mit. Requests should be sent to the Office of Information, Ohio
Basin Region, Federal Water Pollution Control Administration,
4-676 Columbia Parkway, Cincinnati, Ohio 45226.
-------
CONTENTS
Page
ABSTRACT
INTRODUCTION 1
SUMMARY 3
RECOMMENDATIONS 5
PRELIMINARY LABORATORY INVESTIGATIONS 6
Hydraulic Characteristics 6
Carbon Properties 8
PILOT PLANT PROGRAM 15
Pilot Program Phase I 19
Pilot Program Phase II 24
Pilot Program Phase III 48
Analysis of Spent Carbons 60
GENERAL DISCUSSION 72
REFERENCES 74
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ABSTRACT
The overall objective of this program was to evaluate
the feasibility of the expanded-bed technique for contacting
granular activated carbon with biologically treated sewage
effluent, and to provide a rigorous comparison between this
technique and the conventional packed-bed mode of operation.
The evaluations and comparisons were to be made with pilot
scale adsorbers under actual field operating conditions.
Although secondary effluent without further pretreatment was
of primary interest, the effects of additional clarification
were also to be determined. Clarification methods included
dual media filtration and chemical treatment followed by
dual media filtration.
The packed-bed and expanded-bed adsorption systems,
operating under comparable conditions, were found essentially
equivalent in their effectiveness for removal of soluble
organic material from a secondary sewage effluent. Suspended
solids, present to some degree in all the wastewaters tested,
were more effectively removed by the packed beds. The
expanded beds did, however, remove some suspended material.
Accumulation of suspended solids caused rapid increase in
head loss in the packed beds, necessitating frequent cleaning
of the carbon in these beds. Cleaning was carried out by
air agitation and back washing of the beds. Suspended material
accumulated on individual carbon granules in the expanded beds
creating larger particles of decreased density. Although
there was no head loss associated with this behavior, there
was increased expansion of the bed. When such expansion
became excessive, the beds had to be cleaned. However, the
expanded beds required much less frequent cleaning than did
the packed beds.
Because the expanded beds are not subject to clogging
from accumulated solids, their pumping power and maintenance
requirements are lower than for a packed bed of the same
size. Where the improved suspended solids removal provided
by a packed bed is not required, the expanded-bed adsorber
may be preferred.
IV
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INTRODUCTION
The demand for more stringent protection of our national
water resources from the degradation of pollution has mounted
steadily in recent years. Concurrently, increases in population
and improvements in standard of living have placed a heavy
burden on the diminishing supply of high-quality water. As
a result, much emphasis has centered on the improvement of
the quality of wastewater discharged into streams, rivers,
lakes, and other receiving waters. Considerable effort has
been expended in research to improve conventional wastewater
treatment techniques. Cognizant of future requirements as
well as of the immediate needs, the Federal Water Pollution
Control Administration and others1 are conducting major
research, development, and demonstration programs for new
processes which will provide the higher levels of treatment
required for those situations where conventional treatment
processes are inadequate.
One area of investigation in which promising results
have been achieved has been the removal of persistent organic
materials by adsorption on activated carbon. Persistent organic
compounds are those which remain in the wastewater even after
conventional secondary treatment. Several large-scale studies
on carbon treatment of waste waters are presently underway
in this country. Notable among these are the studies at Lake
Tahoe2 and Pomona,3 California, where packed-bed carbon
contacting systems are being utilized. A packed-bed system
is also being used in Nassau County, New York, where adsorption
on granular activated carbon is one of a series of processes
being used to treat secondary sewage effluent to produce water
of satisfactory quality for recharging ground water aquifers.4
The use of powdered activated carbon for renovation of secondary
effluent has been studied at several locations,5 including
the FWPCA's Lebanon, Ohio, pilot plant operated by the Cincinnati
Water Research Laboratory.
Packed beds of granular activated carbon are well suited
for treatment of liquids that contain little or no suspended
solids, and with a clear feed can be expected to operate
effectively for extended periods without clogging or excessive
pressure loss. However, the presence of suspended solids in
municipal wastewaters presents some problems for the use of
activated carbon in packed beds. These solids lead to progressive
clogging of the beds, much as in a sand filter, with resulting
increases in head loss. At the Pomona installation the filtering
action of packed beds of activated carbon is actually used as
part of the treatment scheme. In this system the first of four
activated carbon beds operating in series serves as a filter
to provide a clear feed to the three subsequent beds. Suspended
solids are removed from the first bed daily by surface washes
and backwashing. The Lake Tahoe and Nassau County systems
include pretreatment of secondary effluent by chemical clarification
- 1 -
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and filtration to provide a highly clarified feed permitting
extended operation of the carbon beds.
The work reported here has been directed toward field
evaluation of expanded-bed adsorbers, which have certain potential
operating advantages over packed-bed adsorbers for treating
solutions which contain suspended solids. By passing water to
be treated upward through a bed of activated carbon at a velocity
sufficient to expand the bed, problems of plugging of the bed
and increasing pressure drop are eliminated. Effective operation
over longer periods of time results, as clearly demonstrated
previously in comparative laboratory studies and preliminary ..
short-term, small-scale field studies of expanded-bed adsorbers.
Another advantage of the expanded bed is the lack of dependence
of pressure drop on particle size. In an expanded bed it is
possible to use carbon of smaller particle size than is practical
in a packed bed, thus taking advantage of the higher adsorption
rates which obtain for smaller particles. The purpose of the
present work has been to extend preliminary field studies to
operationally more meaningful field evaluations and to provide
comparisons on a practical scale and for extended periods.
- 2 -
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SUMMARY
After a preliminary program of laboratory studies to provide
information required for the design of a pilot plant, field studies
of the expanded-bed and down-flow packed-bed adsorption systems were
performed. In these field studies, packed-bed and expanded-bed
adsorbers were compared in parallel using a biologically treated
secondary sewage effluent from the Ewing-Lawrence, (New Jersey,)
Sewerage Authority trickling filter treatment plant, for feed to
the systems. Although secondary effluent without further treatment
was used for some of the work, the effects of additional clarification
were also studied. Clarification methods included dual media
filtration, and chemical clarification followed by dual media
filtration. For all studies, a flow rate of 5 gpm/ft2 was employed.
Adsorption column diameters were either 6 in. or 10 in. Carbon
depths for most of the studies were either 12 ft or 24 ft. Carbon
used in the packed beds was a commercially available product
having a size range of 12x40 mesh. Carbon in the same size range,
and in a finer size range obtained by screening and crushing to
20x40 mesh, was used in the expanded beds.
The packed-bed and expanded-bed adsorption systems were found
to be essentially equivalent as far as effectiveness of removal of
soluble organic carbon from each of the differently treated effluents
was concerned. For the same carbon particle size, packed beds
theoretically should be more effective if the carbon is not mixed
during operation. Because of the need for frequent backwashing of
packed beds, this advantage is lost.
Suspended solids, remaining to some degree in all of the
differently pretreated sewage effluents, were partially removed
in the packed-bed adsorbers, leading to clogging and increased
head loss. These increases in head loss required frequent cleaning
of the packed beds by air agitation and backwashing. The expanded-
bed adsorbers also served to remove some of the suspended solids,
although to a smaller degree than did the packed beds. The suspended
solids removed in the expanded bed tended to surround the individual
particles of carbon in a relatively uniform film. This film had no
apparent effect on the adsorption process, nor was there any problem
with increases in head loss in the expanded beds as there was in the
packed beds. The only apparent adverse effect of the solids which
accumulated on individual carbon granules in the expanded bed was
a decrease in the effective density of the encapsulated carbon
particles which caused an increased expansion of the bed. Thus, it
was necessary periodically to clean the expanded bed when the
expansion became excessive. The required cleaning frequency of
the expanded beds, however, was only about 1/3 to 1/4 as great as
that for the packed beds.
- 3 -
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The films that developed around the particles in the expanded
beds had the appearance of biological slime; these films might
have functioned to trap some of the smaller suspended solids from
the wastewater moving through the beds. Carbon beds serve as a
rather favorable environment for development of biological growth,
probably because of the concentration of organics by the carbon.
The fact that this growth environment is somewhat unique to
activated carbon beds was confirmed in the present work by parallel
experiments with coal and activated carbon beds, identical in all
respects except the composition of the solid media. Abundant
growth occurred on the activated carbon particles but none on the
coal particles. The extent of the removal of soluble organics
which might have been accomplished by strictly biological action,
rather than by adsorption, is difficult to estimate. However,
it is probably very small since no real differences were noted
between the removal rate with increased accumulation of solids in
the adsorbers, and that after air scouring and cleaning of the
carbon to remove most of the attached biological growths.
The results of all of the experiments conducted under this
program point to the conclusion that expanded-bed adsorbers can
provide about the same degree of removal of organic substances
from secondary sewage as can packed-bed adsorbers, but at lower
operating pressures and with significantly less down-time and
cleaning cost. These advantages of the expanded-bed adsorber should
permit activated carbon treatment by this technique at a cost several
percent below that of the packed-bed technique.
The fact that the packed bed removed somewhat more of the
suspended solids would not appear to be a very significant factor.
On this effluent, neither system was completely satisfactory.
If a low level of suspended solids is required, it would be necessary
to add additional clarification treatment to each of the two
adsorption systems. The notable advantage of the expanded-bed
technique in this regard is that the clarification step could follow
the adsorption step, thus providing for removal of solids which
might be generated in the adsorption column. As mentioned previously,
there is a notable tendency for biological growth to develop in the
carbon adsorbers, both packed and expanded. Clarification could,
of course, be added after a packed-bed adsorber, but this would
not solve the problem of clogging and fouling of the adsorber.
- 4 -
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RECOMMENDATIONS
When adsorption on activated carbon is to be used for advanced
treatment of secondary effluent, serious consideration should be
given to the expanded-bed concept of contacting.
Observations made during this investigation suggest that the
expanded-bed method of contacting activated carbon may be very
effective for the treatment of raw sewage; therefore, further
studies should be conducted to evaluate the expanded-bed contacting
system for removal of organics from raw sewage or primary effluent.
Detailed design and cost analysis of an expanded-bed contacting
system should be made.
A program should be carried out to develop an expanded-bed
contacting system with provision for continuous addition of fresh
carbon and removal of spent carbon.
- 5 -
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PRELIMINARY LABORATORY INVESTIGATIONS
Preliminary laboratory studies were conducted to examine
the sorptive properties and physical characteristics of the carbon
to be used in the pilot field study, and to provide appropriate
design and operating information relative to the expanded and
packed-bed systems to be field tested in the pilot-scale phases of
the program. These preliminary laboratory studies were divided
into two principal categories: hydraulic characteristics of the
experimental expanded-bed adsorbers, and properties of the activated
carbon to be used in the experiments.
HYDRAULIC CHARACTERISTICS
The expanded-bed concept for contacting wastewater with
activated carbon calls for the maintenance of a semi-fluidized
bed of particles with relatively uniform distribution and motion
of the particles throughout the bed. If there is a very wide
distribution of particle sizes of carbon in such a system, it is
quite possible that the smallest particles will be carried over
in the effluent stream at flow velocities just sufficient to cause
expansion of the largest particle size fractions. This suggests
the desirability of a relatively narrow range of particle sizes.
The importance of particle size range was the factor to be determined
in the preliminary laboratory investigations. Parameters affecting
the hydraulic behavior of the bed include particle size, shape,
and density; velocity; properties of the fluid; and the presence of
suspended matter in the fluid.
The preliminary hydraulic studies were conducted with tap
water in a 9-ft high, 6-in. diameter glass column, as shown in
Figure 1. The column was suspended and adjusted to plumb by
reference to a precision level, and maintained in a vertical
position during the course of all operations and measurements.
The bottom of the column was fitted with a cone distributor which
was covered, for the major part of the test, by a 6-in. bed of
gravel. Granular activated carbon* that had been thoroughly soaked
in water for up to three days was placed in the water-filled column
to give a settled bed of about 4-ft depth. Water was then pumped
upward through the bed to expand gently the carbon and expel air
bubbles and remaining fines before any measurements were made.
During each test, water was pumped from a supply reservoir through
a rotameter into the bottom of the column, upward through t;he bed
and back to the supply reservoir. The temperature of the water in
the system was maintained relatively constant by appropriate
additions of either hot water or ice to the supply reservoir.
Three different particle-size ranges and several different
flow rates were tested at controlled water temperatures of 10°C,
*Pittsburgh CAL (Calgon Filtrasorb) activated carbon was used as
received or after particle size reduction for all studies described
in this report. This carbon is manufactured by the Pittsburgh
Activated Carbon Company, a subsidiary of the Calgon Corp.
- 6 -
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Gravel
xxxxxxxxxxxxxxx
3/8" Pipe
Distributor Detail
6" ID Glass Pice
Met a I
Flange
Tank
3-Poi nt
Co I umn
S uo oort
Carbon
Bed
FIGURE I. APPARATUS FOR GLASS COLUMN EXPERIMENTS
- 7 -
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20°C, and 25°C. All of the carbon tested was taken from the
same batch as that to be used for the pilot-scale field studies.
The appearance of the bed was noted, and the heights of various
regions of particle activity were measured. The different patterns
of particle behavior observed in the column are described as static,
when no particle motion was evident; moving, when particles were
moving slowly over short distances; and mixing, when there was
considerable random and relatively rapid particle motion. The
top of the expanded bed of carbon was very clearly defined at all
times during these tests, and remained perfectly flat. Conversely,
the lines of demarcation between the mixing and moving regions
and between the moving and static regions were rather poorly
defined, and generally irregular. Different types of particle
motion on different sides of the column were frequently observed,
and, in some cases, channeling was noted over rather limited areas.
The plot of the data, Figure 2, provides a graphical comparison
of the behavior of the different particle size fractions used
in the experiments. The 12x40* range represents the particle size
distribution in the commercial product as supplied by the manufacturer.
The 20x30* and 20x40* sizes were obtained by crushing and screening
the commercial product.
The 12x40 particles were tested with two types of fluid
distributors in the bottom of the column. The first distributor
system consisted only of the 3-in. inverted cone mentioned earlier,
which provided for the water to enter the column through holes in
the circumference of the cone. The second distributor consisted
of the same cone set in a 6-in. deep bed of gravel, as illustrated
in Figure 1. The results from the experiments with the two
different distributor systems were essentially the same, and the
gravel support was used for all subsequent tests.
As expected, a greater expansion was observed with the
mixtures containing smaller particles. The bed expansion approached
200% at about 7 gpm/ft2 for the 30x40 size range and at about
10 gpm/ft2 for the 20x40 size range. At the chosen experimental
rate of 5 gpm/ft2, the 20x40 particles gave a bed expansion of
about 130%, of which about 75% of the bed was moving. This particle
size range was selected along with the commercial 12x40 carbon
for use in the pilot study, rather than the finer 30x40 size
which might have resulted in excessive loss of carbon particles
during the experiment.
CARBON PROPERTIES
As previously noted, the activated carbon obtained from the
manufacturer consisted of particles in the size range 12x40, with
a considerable fraction of these particles being in the 12x20
category. Several methods of fractionation and size reduction
were tested in an effort to develop a procedure for producing
maximum yield of the 20x40 size carbon particles selected for use
*12x40 means particles passing a U.S. Standard Sieve No. 12 and being
retained on a U.S. Standard Sieve No. 40; 20x30 means passing a
No.20 sieve, retained on a No. 30 sieve; and, 20x40 means passing
a No. 20 sieve, retained on a No. 40 sieve.
- 8 -
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I 75
CT)
o I5°
IE
.^ 125
4-
(0
4-
co
0 I 00
4-
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(J
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m
50
25
Particle Size
I 2/40
O / Mix; ng
O Reg i on
Part i c I e
S i ze
30/40
Particle Size
20/40 °
Temperature
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ro
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CJ
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(0
4-
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l_
o.
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Water Flow Rate, gpm/ft 2
FIGURE 2. BEHAVIOR OF EXPANDED BEDS OF ACTIVATED CARBON
-------
in the pilot study. The method finally chosen made use of batch
screening and a jaw crusher which, after appropriate modifications,
was capable of converting about 60% of the 12x40 carbon to 20x40
without excessive fines. Particle-size analyses of the carbon
were conducted with 50 to 100 gram samples shaken for 10 minutes
in 8-in. U.S. Standard Sieves in a Ro-Tap machine. A small amount
of 30x40 carbon was prepared for the preliminary tests, and then
sufficient 20x40 was prepared to fill the four pilot columns,
which held about 85 pounds each. The remaining 40% of the original
carbon obtained from the manufacturer was discarded either as fines
or as over-size carbon.
For meaningful evaluation of the relative effectiveness of
the packed-bed and expanded-bed adsorption systems, the carbons
used in the two types of systems had to have very nearly identical
adsorption properties. Measurement of iodine adsorption was the
principal test used to determine the effects of the crushing and
size-separation operations on the adsorption properties of the
experimental carbon. These tests shown in Table 1 indicated that
crushing of the carbon resulted in a somewhat lower activity for
the plus-20 granular fraction remaining after crushing and sieving
and a fairly high activity in the dust produced during crushing.
Similarly, slight differences were noted between the activity of
the coarse and fine particles of the original carbon. Differences
between the activities of coarse and fine particles in a batch of
granular activated carbon are not at all uncommon. In many cases,
these differences are accentuated when the particles are further
processed by grinding, crushing, or rough sieving. The more
active particles are generally structurally weaker, and are therefore
more readily crushed or abraded to yield powder-like fines, while
the harder and more dense, but less active, particles are more
resistant to abrasion and crushing.
While the iodine-adsorption procedure does serve as a
convenient method for rapidly estimating the gross adsorption
properties of an activated carbon, a realistic evaluation of the
characteristics of a carbon with respect to sorption of organic
matter from wastewater requires a more directly related measure
of the specific property. For this reason, equilibrium adsorption
tests with a filtered secondary sewage effluent were conducted on
pulverized samples of the different particle size fractions of
the activated carbon. These tests involved the mixing of 100-ml
samples of the filtered secondary effluent with different quantities
of the pulverized activated carbon for one hour, after which the
carbon was allowed to settle. The supernatent was then filtered and
the total concentration of organic carbon* determined on the filtrate.
The organic carbon remaining was compared with the total concentration
of organic carbon in another portion of the effluent to which no
carbon had been added. The results of these experiments are
presented as Freundlich isotherms in Figure 3. The comparison
shows no consistent difference in capacity for adsorption of
organic carbon among the different size fractions of the activated
carbon prepared from the original 12x40 commercial product. These
*Determined using a Beckman Carbonaceous Analyzer
- 10 -
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TABLE 1
Iodine Numbers of Activated Carbon
Batch No. 12 34
Total 12x40 950 915 915 962
Minus 40 985 960 962 981
Plus 20 950 830 840 834
20x40 965 930 942 934
30x40 962
Iodine No. is mg I adsorbed per gm of carbon from
50 ml 0.1 N iodine solution by 0.5 gm carbon
- 11 -
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0.15 -
0. I
0.08
D)
E
\
O)
E
T3
Q)
O
in
(D
O
U
c
(O
05 -
03 -
02 -
D 20X40
Pulverized for Test
I I I I I I I I
0.03
0.02
2 3 4 6810 15 20 25
Organic Carbon in Filtrate, mg/L
FIGURE 3. ADSORPTION ISOTHERMS FOR SECONDARY
EFFLUENT ON ACTIVATED CARBON
- 12 -
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experiments, of course, would show no differences in adsorption
characteristics which might result from the physical difference
of particle size, since all samples of carbon were pulverized
to the same size before testing. The sole purpose of these tests
was to determine whether the size fractionation operations
produced fractions that exhibited significantly different activity
for removal of organic carbon from secondary effluent. The only
activity differences of the various particle size fractions which
might have been uncovered in this particular series of experiments
are those which are themselves not properties of particle size but
which might result from separation according to particle size.
A similar test was carried out with unpulverized 0.1-gram
samples of activated carbon of the several different particle size
ranges. These samples were added to 100-ml aliquots of secondary
effluent and vigorously agitated for extended periods of time.
This test was conducted on two occasions to compare the 12x40 and
the 20x40 fractions of the activated carbon. Because equilibrium
between the carbon and the liquid is attained much more slowly
with particles of these larger sizes than with the pulverized
carbon, it was necessary to extend the length of the runs to about
100 hrs. The results of these experiments are shown in Table 2.
Separate samples of the secondary effluent containing no activated
carbon were agitated for the same period as those containing the
carbon, and the concentration of organic carbon after filtration
was measured at various times during this period.
The data from this series of experiments indicate little
difference in the adsorption properties of the two size ranges
of the activated carbon (12x40 and 20x40). The difference in the
average particle size, i.e., 1.0 mm vs. 0.67 mm, is apparently not
sufficient to affect either the rate of adsorption or the total
amount that can be adsorbed. Very little adsorption was observed
in 1 hr with either size granular carbon, and the amount of organic
carbon adsorbed in 100 hr was comparable to that adsorbed by the
pulverized samples after 1 hr. The samples of secondary effluent
differ in the sense that the sample taken for the test on November 2
appears to contain organic material which is somewhat less readily
adsorbable than the sample taken on October 25.
- 13 -
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TABLE 2
Laboratory Adsorption Test Results for
Removal of Organic Carbon From Filtered Secondary Effluent
By Pittsburgh GAL Granular Activated Carbon
Contact Time
5 minutes
30 minutes
1 hour
2 hours
4 hours
24 hours
Average Blank
5 minutes
30 minutes
1 hour
2 hours
4 hour s
24 hours
100 hours
Average Blank
Pulverized
1 hour
Organic Carbon Concentration, mg/1
BLANK 12x40 mesh 20x40 mesh
(Results for 10/25/67)
28-0
31-0
30.0
29. 7
26.0
30.0
26.8
27. 6
23.5
23.0
22.8
21.0
18.0
11.0
(Results for 11/2/67)
27.0
26.0
27.0
23.5
21.8
13.2
8.5
23.5
22.0
22. 0
19.5
17.0
11. 0
24.5
23.5
27.0
24.0
17. 1
11.0
11.0
12.8
9.5
All tests in 250-ml iodine flasks with 100 ml filtered
secondary effluent. Activated carbon in 0. 100-gm
amounts added then flask shaken for time indicated.
Contents filtered through Whatman No. 42 and organic
carbon determined on filtrate.
- 14 -
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PILOT PLANT PROGRAM
The pilot plant field program for comparative evaluation
of packed-bed and expanded-bed modes of contacting activated carbon
with secondary sewage effluent was divided into three principal
phases. Phase I of the program involved field testing of 6-in.
diameter columns of activated carbon with an expanded-bed mode of
operation. These columns were constructed of Pyrex glass pipe to
facilitate observation of the behavior of the expanded bed.
Larger steel columns were used in later phases. The smaller size
of the glass columns, coupled with their transparency, allowed
greater flexibility in the test program for experimentation with
other media, and in cleaning procedures. Phase II of the program
was a 100-day comparative study of packed-bed and expanded-bed
modes of operation using 10~in. diameter steel columns, with a
total depth (settled) of carbon of 12 ft. Both unfiltered and sand-
filtered secondary effluents were tested in parallel operations
during Phase II. Phase III was a 90-day comparative study of
packed-bed and expanded-bed (10-in. diameter, 24-ft settled depth)
modes of operation with a chemically clarified and sand filtered
secondary effluent.
The experimental work was conducted at the treatment plant
of the Ewing-Lawrence Sewerage Treating Authority (ELSA) near
Trenton, N.J. This plant serves most of the residential, commercial,
and industrial areas within the two townships. The sewage is
comprised of about 25% industrial waste and 75% domestic waste.
A schematic diagram of the ELSA plant is given in Figure 4. The
original plant, consisting of two lines (primary sedimentation,
trickling filter and secondary sedimentation) went into operation
in 1953 and was expanded in 1964 by addition of two larger lines
and a larger chlorine detention tank. The plant is designed for
an average daily flow of 9 mgd with pumping capacity of 30 mgd,
the average daily flow is about 7 mgd.
Chlorinated secondary effluent for the studies was taken
from the line supplying utility requirements of the sewage treatment
plant. The experimental apparatus was set up on a poured concrete
slab installed specifically for this project next to the ELSA return-
pump building. The steel columns resting on the slab were secured
to an angle-iron frame to maintain vertical position. The filter
and other tanks were located on the slab and pumps, valves, and
controls were installed in a 10-ft x 12-ft building constructed on
the slab. Connections between columns and valves were rubber
hoses through the building wall. The smaller glass columns used
for the Phase I studies were set up next to the building.
Figure 5 is the pilot plant flow diagram and Figure 6 shows
photographs of the installation.
- 15 -
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I r
i
Pr imary
S I udge
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Primary
Tr i ckI ing
Secondary
Plant
Effluent
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Digestors
00
b I udge
Dry i ng
Beds
Clari fidrs
Secondary
S I udge
Ch lorine
Contact
Tank
Pumps
0000
F I ume
Bar
Screen
FIGURE 4. FLOW DIAGRAM EWING-LAWRENCE SEWERAGE AUTHORITY PLANT
- 16 -
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Sand
Fi I ten I
Secondary Effluent Supply 30 psi
Feed Tanks
250 ga I .
Active Carbon Columns
(Two Identifcal Systems)
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Act i ve Carbon
Columns
To Co I . I Co I . 2 Drain
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Pressure
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P rod uct Drums
D r 3 '• n
FIGURE 5. FLOW DIAGRAM OF FIELD TEST UNIT
- 17 -
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PILOT PLANT INSTALLATION
FILTER AND COLUMNS
FEED PUMPS
VALVES & METERS
FIGURE 6 PHOTOGRAPHS OF PILOT PLANT EQUIPMENT
- 18
-------
Daily samples of the secondary effluent feed and product
from the operations were taken for analysis. The pilot column
systems included timer-operated solenoid sampling valves which
were set to collect approximately 10 I/day in approximately
100-ml increments at 15-minute intervals. For weekend operation,
the increment was decreased and the interval increased to provide
a reasonable size sample for the longer period. To inhibit
biological action in the organic materials in the sample, acid
was added to the containers before sample collection. The
composited samples were thoroughly mixed before withdrawing an
aliquot for analysis in the laboratory. The product from the
glass column systems was spot sampled at the time the other samples
were taken.
The analyses performed on the column feed and effluent
included determination of total organic carbon (TOO , soluble
organic carbon (SOC), suspended solids (SS), and turbidity.
Organic carbon measurements were made in a Beckman Carbonaceous
analyzer by injection of an aliquot of acidified and nitrogen
stripped sample. The TOC was determined directly, and the SOC
determined on the filtrate from the suspended solids determination.
The value for TOC gave a measure of the carbon contribution from
both soluble and suspended organic materials.
Suspended solids concentration was measured by a procedures
which involved filtration of a portion of the sample through a
membrane filter with 0.45-micron openings, which was then dried
to constant weight. Prior to use, the membrane filter was washed
in distilled water to remove water soluble impurities, and dried
to constant tare weight with individual desiccators for each
membrane.
PILOT PROGRAM PHASE I
Apparatus and Procedure
A 6-in. diameter glass column containing an approximately
4-ft-deep bed of 20x40 activated carbon supported on gravel was set
up at the pilot plant site. Chlorinated secondary effluent was
pumped through a rotameter, upward through the expanded carbon
in the column, then through a meter to discharge. The flow rate
was maintained at about 1 gpm, corresponding to 5 gpm/ft2 of
column area. The glass column was completely covered with an
opaque wrapping, except during inspection, to prevent photosynthetic
activity.
Experimental Results of Adsorption Run
Data for the first test are presented in Figure 7. The
experiment was interrupted after about 24 hours of operation because
of poor quality of the secondary effluent resulting from a mechanical
- 19 -
-------
failure in the sewage treatment plant. This condition was
corrected in two days and the column test re-started and operated
continuously thereafter. From Figure 7 it can be observed that,
during the first five days of continuous operation, the height of
the expanded bed of carbon increased from an initial 59 in. to
completely fill the 9-ft glass column. The bed height observations
shown in Figure 7 are plotted with breaks which represent column
cleaning and restoration of the original bed height. At the
beginning of the run, the height of the static carbon bed was 46 in
The initial height of 59 in. corresponds to an expansion of about
128% which is essentially the same as observed in experiments with
tap water for the same temperature range. Visual observations of
the carbon bed during the course of the run indicated that the
relatively uniform increase in the degree of expansion was parallel)
by a relatively uniform increase in the number of particles of
carbon which became coated with sludge and which, as a result of
decreased density, accumulated in the upper region of the expanded
bed.
The carbon in the lower part of the bed was unchanged in
appearance, whereas the carbon in the middle zones of the bed took
on a greyish-brown color as individual particles became completely
surrounded by a gelatinous coating. The top 2 in. of the bed
consisted of a tan or brown flock which appeared to consist of
only biological sludge particles.
At this point, the bed was cleaned in place with a water
jet located at the top of the bed, followed by flushing at a high
rate through a jet located at the bottom of the column. All cleanip
was carried out using carbon-treated secondary effluent. It was
necessary to repeat this procedure several times until the
agglomerations of sludge and sludge-coated carbon in the middle of
the bed were broken up and dislodged. One possible cause for the
difficulty in breaking up the agglomerations near the middle of
the bed was the 4-hr delay between stopping the flow to the column
and'the beginning of the cleaning procedure. During this period
of inactivity, the bed relaxed, and, under settling forces, the
sludge-laden carbon compacted; the static height of the settled bed
was not much greater than when no sludge was present. This
problem was minimized in subsequent cleanings by avoiding delays.
After cleaning, the bed showed the same general appearance and
original settled depth, although it still contained a few small
sludge and sludge-coated carbon agglomerates.
From organic carbon data shown in Figure 7, it is readily
evident that 4 ft of carbon was not adequate for effective removal
of organic contaminants from the secondary effluent at the flow
rate utilized in these experiments. Variations in removal can
be noted which do not appear to be related to the build-up of
sludge in the carbon bed. The variable width of the band between
the TOC of the feed and product waters for a relatively constant
flow rate indicates that there were variations in the extent of
- 20 -
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Secondary
EffIuent
Co Iumn Prod uct
Bed Height in Inches
00
75
50
30
20
Secondary
Eff I uent
Co Iumn Prod uct
I 0
I
I
I
J_
20 30 40 50
Volume Treated, 1000 gal.
60
70
80
FIGURE 7. TREATMENT OF SECONDARY EFFLUENT WITH ACTIVATED
CARBON IN 6-IN. DIAMETER UPFLOW COLUMNS
- 21 -
-------
removal of TOG with time, suggesting differences in the
characteristics of the organic substances comprising the TOG.
Data on TOG removed during the course of a second 2-month
test run in the 6-in. glass column indicate that about 0.1 to 0.2
Ib per day of organic carbon was removed by about 20 Ib of activated
carbon during its active removal phases under the experimental
conditions. As in the first 2-month run/ organic removal varied
from day to day suggesting differences in composition of the
wastewater.
Very little removal of organic carbon was observed during
the latter phases of operation of the glass column, and at the
same time the need for cleaning the column because of excessive
expansion of the bed diminished. The biological growth on the
particles of carbon which caused increased expansion of the bed
decreased as the carbon became exhausted. There are several possible
explanations for the decline in biological growth as the sorptive
capacity of the activated carbon is being depleted. Organic food
and nutrients were continuously supplied in the influent to the
carbon columns, as was dissolved oxygen. Apparently, when extensive
adsorption of organic material led to saturation of the carbon
surfaces, the possibility of sufficient quantities of essential
elements being in the vicinity of the surface at the concentrations
needed for prolific growth was greatly decreased. The observation
that biological growth on the activated carbon appeared to be a
function of the organic removal activity of the carbon should not
be misconstrued to imply that removal activity was dependent upon
or even a function of biological growth. As the biological films
developed, the rates of TOG removal established in their absence
by the clean carbon remained virtually unchanged.
Inert Media Test
To further examine the role of the biological growths which
accumulated on the activated carbon during the first 2-month run
with the expanded bed in the glass column/ two additional
experimental runs were carried out. Two identical 6-in. diameter
glass columns were constructed. One was filled to a depth of 4 ft
with 20x40 activated carbon, and the other with a like charge of
20x40 bituminous coal. The coal was selected and prepared in such
manner that its density and particle shape and size were very nearly
identical to those of the carbon. Thus, while the coal should have
little or no adsorptive capacity for organic contaminants, its
physical and hydraulic behavior in the experimental column tests
would be the same as for the activated carbon. These two parallel
column tests were carried out with the same flow rate, quantity of
carbon or coal, and system geometry as used in the previous glass-
column tests. However, during the period of this run, the water
temperature was lower, averaging about 5°C. Additionally, the
average concentration of TOG in the feed was higher than for the
earlier runs. Data from these parallel runs are plotted in
Figure 8.
- 22 -
-------
CD
E
c
O
-Q
1_
ro
O
c
ro
O)
(0
+-
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CD
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(0
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(0
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50
40
30
20
20
10
P rod u ct f rom
Act i vated Ca rbon
Bed Height in Inches
00
75
50
Act i vated
Carbon
I
I
o
20 30 40 50
Volume Treated, 1000 gal.
60
FIGURE 8. TREATMENT OF SECONDARY EFFLUENT IN 6-IN.
DIAMETER UPFLOW COLUMNS CONTAINING ACTIVATED
CARBON OR COAL
- 23 -
-------
The results obtained with the column containing the activated
carbon were similar to those observed previously. The bed of
coal, however, removed little TOG, and little biological coating
of the particles occurred. A small amount of sludge floe formed
above the bed of coal. During the latter stages of these runs,
coal particles were observed to be transported out of the bed
by gas bubbles which apparently formed within the bed. This
behavior was not observed to any extent in the bed of activated
carbon operating on the same secondary effluent at the same time.
There was no evidence of septic conditions in either the coal or
activated carbon beds.
The results of the parallel experiments with activated carbon
and "inactive" bituminous coal do illustrate and confirm the
earlier observations that the development of the biological slimes
around individual carbon particles in an expanded bed is related
to the sorptive activity of that carbon.
Cleaning Operations
The glass column was useful for making a search for effective
methods for in-place cleaning of the sludge-coated carbon. The
first attempts to clean the carbon beds, using jets of water at
the surface and bottom of the bed were effective only in limited
regions of the bed. The next cleaning method tested consisted
of inserting a motor-driven 3-in. turbine propeller into the column
to a depth of about 3 ft below the top of the expanded bed and
alternately stirring (60 rpm) and flushing with water. This was
found to be a rather effective method for dislodging sludge from
the carbon particles in the very limited part of the column in the
immediate vicinity and above the propeller blades, but the procedure
did not break up the agglomerates in the middle of the bed, where
neither the bottom jet nor the stirring provided adequate scrubbing
or scouring.
A third cleaning technique, involving a high-pressure air jet
at the end of a copper tube which was moved continuously about
in the expanded bed from the top to the bottom of the column, was
found to be the most effective. This procedure required consider-
ably less time; usually two cycles of air scouring followed by
back flushing at high rate thoroughly cleaned the bed. Subsequently,
the compressed air was introduced to the column in the influent
feed line for simpler and more rapid cleaning. This procedure
also served to remove any sludge that had accumulated in the gravel
layer surrounding the distributor.
PILOT PROGRAM PHASE II
The second phase of the pilot program called for long-term
comparative studies of packed-bed and expanded-bed columns
for contacting activated carbon with secondary effluent. Treatment
was carried out in four separate experimental systems to permit
comparison of the two different contacting methods both with and
without pretreatment of the secondary effluent by simple sand
filtration.
- 24 -
-------
Apparatus and Procedure
The four carbon-contacting systems consisted of vertical
columns constructed of 10-in. diameter steel pipe* connected as
shown schematically in Figure 9. Each column, as shown in
Figure 10, was charged with 85 Ib of activated carbon, a quantity
sufficient to provide a 6-ft-deep bed. The carbon was supported
on a 6-in. layer of gravel and coarse sand over a 5 in. cone
shaped distributor similar to that used in the glass columns.
The commercial 12x40 carbon was used in the packed beds; 20x40
carbon was used in the expanded beds. The smaller size carbon
in the expanded beds was used to attempt to take advantage of the
potentially higher adsorption rate. The columns designed for
packed-bed operation were 9 ft tall to allow for a backwashing
and disengaging zone, while those designed for expanded-bed
operation were 12 ft tall to provide sufficient space for bed
expansion during operation. The top 1-ft section of each expanded-
bed system consisted of a transparent pipe to permit observation.
Two columns were connected in series to provide a 12-ft total
settled depth of activated carbon for each system. This depth of
carbon was chosen because studies by others indicated 12 ft of
carbon would be sufficient to provide a significant degree of
organic contaminant removal. A very high degree of removal would
require more carbon and was not considered necessary for this
phase of the work.
Each of the four systems being compared was fed by a separate
constant-displacement pump. The pumps were driven by electric
motors through variable speed drives to provide for adjustment of
flow rate through the system. Lines to and between the columns
consisted of 5/8-in. inside diameter rubber hose. All flow
controls, including in-line valves, pressure gauges, flow meters,
and solenoid sampling valves, were mounted on one central operating
panel in order to facilitate operation and minimize operator
errors.
Secondary effluent was fed directly to two of the four systems
(one of the packed beds and one of the expanded beds) and filtered
secondary effluent was fed to the other two systems. All systems
were operated at a flow rate of 5 gpm/ft2 of adsorber cross-section
area, or about 2.72 gpm for the 10-in. columns.
Filtration of the secondary effluent was accomplished in
a 38-in. diameter cylindrical tank containing a 9-in. layer of
anthracite coal (effective size 0.59 mm) on a 9-in. bed of filter
sand (effective size 0.62 mm). The filter sand in turn was
supported on a coarse sand and gravel base over a pipe-grid
distributor. A constant head of effluent was maintained above the
filter by means of a float-controlled valve in the line which
*The interior surfaces of the pipe were coated with 3 coats of
Sherwin Williams water tank paint.
- 25 -
-------
I
to
SECONDARY EFFLUENT
•
n
1
FIXED BEDS
10" DIA. PIPE
9'4I2' HIGH
5 GPM/SQ.FT.
FLU!DlZED BEDS
NO PRETREATMENT
COAL-SANC
FlLTER
FIXED BEDS
n
FLUIDIZED BEDS
PRETREATMENT
FIGURE 9. TREATMENT SCHEME
-------
Hose Connection for
Column Feed or Product
II in. 0.D. Aery lie Pi pe
3/8 in. wa I I
Expa nded Bed only
1/8 in. Rubber Gaskets
3/4 in. Bo Its
10 in. Steel Pipe 1/4 in.wall
I 50 I b. F I anges
Bed Dra i n
3/4 in. I .P. Coup I i ng
Hose Connection for Column
Feed or Product
PIast i c Cone 5 i n.d i a .
Covered with Gravel
FIGURE 10. PILOT ADSORBER COLUMN DETAIL
- 27 -
-------
discharged secondary effluent to the filter. The filtrate was
pumped from the filter through the pipe-grid distributor by a
centrifugal pump and delivered to a holding reservoir from which
feed was pumped to the adsorbers.
The filter was operated at a rate of about 5.5 gpm to provide
feed for two of the column systems; this rate corresponds to about
0.75 gpm/ft2of filter bed surface. Cleaning and backwashing of
the filter, which was required about twice each week, was accomplished
by injection of compressed air and water through the distributor.
Although the sand filter consistently removed large suspended-solids
particles, it did not remove the small particles that constituted
most of the turbidity. As a result, there .was little change in
the physical appearance of the sewage. The solids which were
dislodged from the filter during backwashing had the appearance
of activated sludge; a dense suspension of tan or light brown
particles which settled quite rapidly.
The activated-carbon-column systems were operated continuously
at 5 gpm/ft2 by appropriate hand adjustment of the pumping rate,
the flow being measured continuously with water meters. Interruptions
in operation occurred only when the columns were being cleaned, or
when the pretreatment system was being serviced. A packed column
was cleaned whenever the pressure drop across that.column increased
to a value of approximately 15 psi from an initial pressure drop
of 2 or 3 psi. Because there was no measurable increase in head
loss across the expanded beds during operation, the need for
cleaning was based on the increase in expansion caused by the build-up
of biological films on the carbon particles, as discussed earlier
under Phase I. For the expanded beds, cleaning was carried out
whenever the top of the bed appeared in the transparent section
at the top of each column.
The most effective procedure for cleaning the 10 in. activated-
carbon columns was the injection of compressed air into the bottom
of the bed after drawing down the water level in the column by
about 1 ft. This procedure was followed by back flushing of the
bed with water to remove the solids dislodged during the air
scouring. The injections of compressed air .provided the vigorous
scrubbing action between particles needed to dislodge solids
throughout the entire depth of the bed. After air agitation,
the sludge could be floated away with a gentle backwash of water.
Examination of the carbon on removal from the packed columns
indicated that this procedure did not cause undue particle size
classification in the bed. Details of the analyses of spent
carbons are given in a subsequent section.
Experimental Results and Discussion
In the 100-day continuous operation of the four different
adsorption systems, each system treated between 350,000 and
370,000 gal. of wastewater. Because the packed beds required more
frequent cleaning, the volume treated by these beds was less than
- 28 -
-------
the volume treated by the expanded beds. Differences in cleaning
requirements for the packed-bed and expanded-bed adsorbers were
significant. The first of the two expanded beds operating on
unfiltered feed was cleaned only 4 times and did not require
cleaning during the last 40 days of operation. The second bed
in this pair was cleaned 13 times in the 100 days. The two
expanded beds operating on filtered feed were cleaned 10 and 6
times, respectively. Cleaning of packed beds was required more
frequently, as dictated by increases in pressure drop. The
packed beds on unfiltered feed were cleaned 48 and 26 times,
respectively, and the packed beds operating on filtered feed
were cleaned 28 and 16 times, respectively.
The observed pressures for the packed-bed system operating
on unfiltered feed are plotted in Figure 11. The total pressure
drop for both beds (12 ft of carbon ) in the system was
12-14 psig when the beds were clean. The pressure drop usually
increased at about 0.25 psi per hour, but on several occasions
the rate of increase was greater.
Plots of TOG and SOC as a function of volume of wastewater
treated are presented for the four systems in Figure 12 through
19. These plots illustrate the pattern of removal of both total
and soluble organic carbon for the total 12-ft depth of each of
the four different systems, as well as the pattern for TOC
removal at the 6-ft depth in each system. Values for organic
carbon in the respective feed solutions are plotted on each
figure to provide a base reference. The bands between the feed
and effluent lines, which represent the organic carbon removed
in each stage of treatment, converge in a similar fashion for
each of the four systems. As expected, changes in the quality
of the secondary effluent fed to the adsorbers are reflected in
the effluent from these systems. The gradual decline in organic
carbon removal with increased volume treated is evident. This
decline is shown more clearly in Figure 20. The marked daily
fluctuations in percent of removal shown in this figure may be
attributable in large part to daily changes in the nature of the
organic components represented by the TOC and SOC values, as well
as in the total concentrations of these components.
The cumulative amounts of total and soluble organic carbon
applied to and removed by the activated carbon columns are plotted
in Figure 21 and 22 and summarized in Table 3. In the plots, the
slope of each line represents the efficiency of organic carbon
removal. The unit loading of TOC or SOC on the carbon in any
system at any point can be calculated by dividing the value read
from the ordinate by 170, since each of the four systems contained
a total of 170 pounds of activated carbon, half in each column.
Figure 21 and Table 3 show that, for both the filtered and
unfiltered feeds, a slightly larger amount of TOC was removed by
the packed beds than by the corresponding expanded beds. The
- 29 -
-------
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40
30
20
2 coIumns
12 ft of carbon
o o o o o o o
second column
6 ft of carbon
I I
I
I
I
0
20
30
40 50 60
Operating Time, Days
70
80
90
100
FIGURE II. PRESSURE IN PACKED BED OF ACTIVATED CARBON
DURING TREATMENT OF SECONDARY EFFLUENT
-------
0
50
I 00
150 200 250
Volume Treated, 1000 gal,
300
350
FIGURE 12. TOTAL ORGANIC REMOVAL FROM FILTERED SECONDARY
EFFLUENT IN PACKED BEDS OF ACTIVATED CARBON
-------
to
I
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S
c
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JD
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Product 12 ft Depth
I
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Volume Treated, 1000 gal.
300
350
FIGURE 13. SOLUBLE ORGANIC REMOVAL FROM FILTERED SECONDARY
EFFLUENT IN PACKED BEDS OF ACTIVATED CARBON
-------
Ul
U)
Seconda ry
EffIuent
Filtered Feed
Product 6 ft Dept
Product
2 ft Depth
50 200 250
Volume Treated, 1000 gal.
300
350
FIGURE 14. TOTAL ORGANIC REMOVAL FROM FILTERED SECONDARY
EFFLUENT IN EXPANDED BEDS OF ACTIVATED CARBON
-------
I
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C
o
,0
1_
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Secondary
Eff I uent
Product
12 ft Depth
I
I
I
100 150 200 250
Volume Treated, 1000 gai.
300
350
FIGURE 15. SOLUBLE ORGANIC REMOVAL FROM FILTERED SECONDARY
EFFLUENT IN EXPANDED BEDS OF ACTIVATED CARBON
-------
U)
un
50 -
40
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E
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6 ft Depth
Product
12 ft Depth
1
50
100 150 200 250
VoIume Treated, I 000 ga I .
300
350
FIGURE I6,TOTAL ORGANIC REMOVAL FROM SECONDARY EFFLUENT
IN PACKED BEDS OF ACTIVATED CARBON
-------
30
c
o
(0
O
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c
(0
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10
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Feed
Product 12 ft Dept
1
1
50
00
150 200 250
Volume Treated, 1000 gal
300
350
FIGURE 17. SOLUBLE ORGANIC REMOVAL FROM SECONDARY EFFLUENT
IN PACKED BEDS OF ACTIVATED CARBON
-------
I
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50 -
40 -
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E
c
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30 -
20
10 _
Prod uct
6 ft Depth
Product
12 ft Denth
50 200 250
Vo I ume Treated, I 000 gaI
300
350
FIGURE 18. TOTAL ORGANIC REMOVAL FROM SECONDARY EFFLUENT IN
EXPANDED BEDS OF ACTIVATED CARBON
-------
U»
00
30
c
O
(0
o
o 20
c
D)
O
0)
I 10
3
O
50
I
100 150 200 250
Volume Treated, 1000 gal.
300
350
FIGURE 19. SOLUBLE ORGANIC REMOVAL FROM SECONDARY EFFLUENT
IN EXPANDED BEDS OF ACTIVATED CARBON
-------
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a:
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80 _
70 -
Unfit tered Feed
40 .
30 -
50 100 150 200 250 300
Volume Treated, 1000 gal.
350
400
Figure 20. Fractional Organic Carbon Removal by Activated Carbon
- 39 -
-------
o
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o 30
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ce
20
O
JD
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Expanded Beds
Until tared
Feed
Packed Beds
• F i I tered
Feed
_L
_L
_L
_L
10
20 30 40 50
So I ub1e-Organic Carbon Applied,
60
70
I bs
FIGURE 22. EFFECTIVENESS OF ACTIVATED CARBON FOR SOLUBLE
ORGANIC REMOVAL IN A I 2 FT BED
-------
TABLE 3
Organic Carbon Removed From
Secondary Effluent by Activated Carbon in 12-ft Beds
Pretreatment
Bed Type
Volume Treated, gal.
Total Organic Carbon
TOC Applied, Ib
TOC Removed, Ib
TOC Remaining, Ib
Avg TOC in Product, mg/1
Avg TOC in Feed, mg/1
Percent TOC Removed
Soluble Organic Carbon
SOC Applied, Ib
SOC Removed, Ib
SOC Remaining, Ib
Avg SOC in Product, mg/1
Avg SOC in Feed, mg/1
Percent SOG Removed
SOC Removed per Ib
Act. Carbon
Filtered
Packed Expanded
369,669 372,465
Unfiltered
Packed Expanded
355,547 370,662
122. 93
64. 60
58.33
18. 9
39. 8
52. 6
74. 06
31. 28
42. 78
13.9
24. 0
42. 2
0. 184
123. 36
59.45
63,. 91
20. 6
39.7
48. 1
74.45
33.33
41. 12
13.3
24. 1
44.7
0. 196
137.71
78. 10
59.61
20. 1
46.5
57.0
78.88
39. 14
39. 74
13.4
26. 6
49. 6
0.230
143. 55
74. 68
68. 87
22. 3
46. 5
52. 0
81. 89
37. 39
44. 50
14. 4
26. 5
45. 7
0. 220
- 42 -
-------
curve for the expanded bed treating filtered feed appears deceptively
low as a result of some low removals early in the run. Figure 20
shows that removals for much of the run were nearly as good as for
the packed bed. There may have been analytical errors in some of
the early data points. Better removal by the packed beds were
expected, because of their filtering action. That the difference
in removal is so small is somewhat surprising. The expanded beds
also accumulate solids, automatically adjusting for these added solids
by an increase in bed height, with no significant increase in
pressure or head loss.
For the removal of SOC, no clear difference in the effectiveness
of the two bed configurations was observed. For the filtered feed,
the expanded bed appears to be superior, possibly because of the
use of the smaller carbon size and the resulting higher rate of
adsorption. Had it been possible to prevent mixing of the carbon
in the packed beds during operation, the overall adsorption
driving force and, therefore, the removal effectiveness would
have been larger in these beds. The beds were frequently disturbed,
however, by cleaning and backwashing. The superiority of the
expanded-bed configuration was not borne out by the data for
unfiltered feed. In view of incomplete knowledge of the effects of
suspended solids and biological activity on the carbon, it is
concluded that the soluble-organic removal capability of the two
types of contactors is essentially the same.
Both types of systems showed a greater removal of soluble
organic carbon on a weight basis from the unfiltered secondary
effluent than from the effluent which was passed through the dual
media filter. Figure 15 shows, however, that the SOC was measurably
higher in the wastewater before filtration. Greater removal from
the unfiltered feed would, therefore, be expected. Biological
degradation, which undoubtedly occurred during filtration, may
also have altered the composition of some of the organic materials
and, as a result, decreased their adsorbability.
As previously noted, the packed-beds were slightly more effective
than the expanded-beds for removal of suspended solids. Figures
23, 24, 25, and 26 are plots of suspended solids in the feed and
product waters as a function of volume treated for each of the
four adsorber systems. The solids-removal effectiveness of the
expanded beds was at times comparable to that of the packed beds,
but on other occasions the expanded-beds appeared to retain little
of the suspended solids. Although the dual media filter coupled
with the activated carbon columns generally accomplished removal
of a major part of the suspended solids, each of the systems
invariably allowed a significant portion of these solids to escape
in the product water. This trickling-filter effluent contains
suspended solids consisting almost entirely of finely divided
matter of which little or no fraction was settleable.
- 43 -
-------
00
80 -
1
42*
1
1
D)
e
*
-------
00
80
en
£
in
•O
C
Q.
C/l
co
60
40
20
\
1
I
I
50
00
150 200 250
Volume Treated, 1000 ga
300
350
FIGURE 24. REMOVAL OF SUSPENDED SOLIDS FROM FILTERED SECONDARY
EFFLUENT IN EXPANDED BEDS OF ACTIVATED CARBON
-------
00
80
O)
£
I/)
T3
O
CO
5 40
c
0)
d.
tn
CO
20
0
_L
J.
_L
50
I 00 I 50 200 250
Volume Treated, 1000 gaL
300
350
FIGURE 25. REMOVAL OF SUSPENDED SOLIDS FROM SECONDARY EFFLUENT
IN PACKED BEDS OF ACTIVATED CARBON
-------
Product
2 ft Depth
100
150 200 250
Volume Treated, 1000 gal.
300
350
FIGURE 26. REMOVAL OF SUSPENDED SOLIDS FROM SECONDARY EFFLUENT
IN EXPANDED BEDS OF ACTIVATED CARBON
-------
PILOT PROGRAM PHASE III
The third phase of the field comparisons of packed-bed and
expanded-bed adsorbers was designed to evaluate the effects of
more complete suspended solids removal on the sorption process.
This phase also sought to determine whether the same type of
biological films that had been noted with the expanded beds in
Phase II would develop in expanded beds operating on a more
completely clarified feed. An additional factor to be examined
in Phase III was the effect of a greater total depth of carbon
and the resulting longer contact time.
Apparatus and Procedure
After a series of preliminary laboratory jar tests with
secondary effluent, the pretreatment system chosen for solids
removal in Phase III studies was chemical clarification with
50 mg/1 of lime and 50 mg/1 of ferric chloride (FeCl3-6H20) at a
pH between 9.0 and 9.5, followed by settling and dual media
filtration.
The pretreatment system is illustrated in Figure 27. The
lime was fed from a calibrated dry feeder into a 55-gal. drum
which served as a rapid-mix tank. A solution of ferric chloride
was also fed to this tank. The chemically treated secondary
effluent was then transferred from the rapid-mix tank through
a line discharging at the center of a 250-gal. tank which served
as the clarifier. Following sedimentation, the water from the
clarifier was fed to the sand filter.
At the beginning of each day of operation, sludge was
siphoned from the bottom of the clarifier, and the filter was
air-scoured and back flushed with water to remove accumulated
solids. Since the pH of the wastewater was maintained at 9.0
to 9.5 during the pretreatment, it was readjusted to the original
level of 7.0 to 7.5 by the addition of sulfuric acid in a 250-gal.
holding tank prior to being fed to the adsorbers.
Each of the eight experimental adsorbers constructed in
Phase II was charged with 85 Ib of 12x40 mesh activated carbon
at the beginning of the Phase III study, yielding a settled depth
of 6 ft of carbon in each adsorber. The four packed-bed adsorbers
were then connected in series to give a total packed-bed depth of
24 ft. The expanded-bed adsorbers were also connected in series.
Clarified feed from the holding tank was pumped to each of
the column systems by separate pumps, and daily composite samples
were taken of the feed and of the product water from each of the
eight individual adsorbers. All analytical determinations were
performed in the same manner as described in the foregoing
discussion of the Phase II studies. The measurement and control
systems were also the same as described in Phase II. Daily
observations were made of flow rate, volumes, and pressures.
Appropriate adjustments and cleanings were made.
- 48 -
-------
Secondary
^ ^•^••a
Effluent
. .X
Float Y
Valve '
M
L
Feec
=1
1-
TE
\x 1
5rt -
9C
me
ler
fr
II
"a n k
i
1 .
u
FeC
^ Pum
LJ-
/
/
s
Clar
2C n
_3U
'3
P
L
i f
gd
i e
•
•i r~—^
\
X
r
™T T-
^^x^^"
^~~***^ ^^^^~^
^ — •
^^-^-^"><:^-^^
"*
L^.^_^_^^
Fi 1 ter
-\
Aci d
^ Pump
, , C 1 a r i f i e d
ft I uen t
Dump
FIGURE 27. DIAGRAM OF APPARATUS FOR CLARIFICATION OF SECONDARY EFFLUENT
- 49 -
-------
Experimental Results and Discussion
Clarification, while not as effective as desired, did reduce
the suspended solids below levels obtainable with filtration alone.
Suspended solids concentrations for the secondary effluent,
clarified feed and products are shown in Figure 28. An interesting
observation from the data in Figure 28 is that the 24-ft-deep packed-
bed adsorber provided very little more removal of suspended solids
than the equivalent expanded-bed system. It is also quite apparent
that further clarification would be required for any reuse
application calling for a clear water. Operating pressure in the
24-ft packed-bed system with a clarified feed are shown, in Figure
29 to follow a similar pattern to that observed in Phase II. The
fact that the solids removal accomplished by chemical clarification
was incomplete prohibited operation of the four-column packed-
bed system without frequent cleanings. None of the expanded beds
studied in Phase II was intentionally cleaned although the
accidental introduction of air with the feed accomplished some
degree of cleaning. These incidents are noted in Figure 29.
Organic removal data for the two systems are presented in
Figure 30 through 33. Cumulative removal of organic materials
by the packed-bed and expanded-bed systems for both 12 and 24-ft
settled depths of carbon are presented in Figure 34 and 35, and
are summarized in Table 4. The plots of cumulative removal
indicate that the packed beds removed somewhat more TOC and
slightly more SOC than the expanded beds. The enhanced removal
of TOC by the packed beds is in part attributable to the filtering
action of these beds. However, some of the difference in both
TOC and SOC removal can be attributed to the accidental loss of
carbon from the expanded beds during the run, which would lead to
a corresponding reduction in performance of this system. Some
carbon was carried from the expanded-bed system by inadvertent
introduction of air with the column feed. The carbon loss from
the beds amounted to about 9% of that in the four-column system,
as estimated at the end of the run. Points noted in Figure 29
indicate when the carbon losses occurred. Table 4 shows that,
despite the loss of activated carbon from the expanded-bed system,
the removal of SOC during the course of the .run was 96% of that
obtained in the packed-bed system. As in Phase II, the mode of
contacting appears to have little effect on the effectiveness of
the adsorption process.
The data presented in Table 4 indicate that about 3/4 of
the TOC and SOC removed in the four-column adsorption systems
(24 ft of carbon) was removed in the first two columns. The
average product SOC values of 5.9 mg/1 and 6.7 mg/1 were within about
2 mg/1 of the values observed throughout most of the course of the
experiment. Few analyses, even at the beginning of the run,
resulted in product SOC values of less than 4 mg/1, indicating
that there was a small initial breakthrough even for the total
settled depth of carbon of 24 ft. This initial leakage is probably
comprised of non-adsorbing or slightly adsorbing organic substances.
- 50 -
-------
I
in
Second any
Eff I uent
Expanded
Co Iumn
P r o d u c t
100 150 200
Volume Treated, 1000 gal.
250
300
FIGURE 28. REMOVAL OF SUSPENDED SOLIDS DURING CLARIFICATION
AND CARBON TREATMENT OF SECONDARY EFFLUENT
-------
40
O)
~ 30
en
o.
*
0)
in
0)
t_
Q-
-t-
o
20
10
9
T T
T 1 1 T
T 1 1 T
® 9 « l®«9»
-Packed Beds 4 @ 6' each
Expanded Beds Range 16-18 PSIG
•Points indicate accidental injection of air into columns
J L
0
20
30 40 50
Operating Time, Days
60
70
80
FIGURE 29. PRESSURE ON 24 FT OF ACTIVATED CARBON TREATING CLARIFIED FEED
-------
en
u>
Seconda ry
Eff I uent
Clarified Feed
Product 12 ft
Depth
Product 24 ft
Depth
150 200 250
Volume Treated, | 000 la'
300
FIGURE 30. TOTAL ORGANIC REMOVAL FROM CLARIFIED SECONDARY
EFFLUENT IN PACKED BEDS OF ACTIVATED CARBON
-------
O)
6
c
o
J3
L.
ID
O
30
I
en
?. 20
c
(D
cn
L.
o
0)
O
CO
ul rA IU . f\
50
100 150 200
Vo I ume Treated,
000
250
gal .
Seconda ry
EffIuent
Clarified
Feed
Product 12 ft
Depth
Product 24 ft
Depth
300
350
FIGURE 31. SOLUBLE ORGANIC REMOVAL FROM CLARIFIED SECONDARY
EFFLUENT IN PACKED BEDS OF ACTIVATED CARBON
-------
Ul
I
50
40
O)
E
c
O
JD
1_
(0
O
u
c:
ro
co
+-
O
30
20
Seconda ry
Eff I uent
Clarified
Feed
Product 12 ft
Depth
Product 24 ft
Depth
_L
_L
50
00 150 200 250
Volume Treated, 1000 gal.
300
350
FIGURE 32. TOTAL ORGANIC REMOVAL FROM CLARIFIED SECONDARY
EFFLUENT IN EXPANDED BEDS OF ACTIVATED CARBON
-------
I
tn
O)
c
O
.0
30
O
u
c
to
O)
1_
O
(U
20
10
O
co
Seconda ry
Eff I uent
Clarified
Feed
Product
Depth
Product
Depth
12 ft
24 ft
50
100
150 200 250
Volume Treated, 1000 gal
300
350
FIGURE 33.
SOLUBLT ORGANIC REMOVAL FROM CLARIFIED EFFLUENT
IN EXPANDED BEDS OF ACTIVATED CARBON
-------
0
>
O
E
0)
cr
c
o
_O
ro
4-
O
50
40
30
O
u
I 20
0
24 ft Depth
Packed Bed
Expanded Bed
12 ft Depth
Packed Beds
Expanded Beds
I
I
I
20 30 40 50 60
Total Organic Carbon Applied, Ibs
70
80
FIGURE 34. EFFECTIVENESS OF ACTIVATED CARBON FOR TOTAL ORGANIC
REMOVAL FROM CLARIFIED SECONDARY EFFLUENT
-------
01
GO
10
24 ft Depth
Packed Beds
Expanded Beds
12 ft Depth
Packed Beds
Expanded Beds
I
20 30 40 50 60 70
Soluble Organic Carbon Applied, Ibs
FIGURE 35. EFFECTIVENESS OF ACTIVATED CARBON COLUMNS FOR SOLUBLE
ORGANIC REMOVAL FROM CLARIFIED SECONDARY EFFLUENT
-------
TABLE 4
Organic Carbon Removal from Clarified Secondary
Effluent by Activated Carbon in 24-ft Beds
Volume Treated, gal.
Total Organic
TOC Applied, Ib
Column No.
TOC Removed, Ib
TOC Remaining, Ib
Avg TOC in Product,
mg/1
Avg TOC in Feed,
mg/1
Percent TOC Removal
Soluble Organic
SOC Applied, Ib
Column No.
SOC Removed, Ib
SOC Remaining, Ib
Avg SOC in Product,
mg/1
Avg SOC in Feed,
mg/1
Percent SOC Removal
Packed Columns
307, 365
Carbon
79. 550
(1,2) (3,4) (Total)
45.842 13.034 58.876
33.708 20.674
13.4 8.06
31. 03
57. 6 16.4 74.0
Carbon
52. 325
(1,2) (3,4) (Total)
27.646 9.732 37.378
24.679 14.947
9.62 5.95
20. 41
52. 8 18. 6 71.4
Expanded Columns
313, 756
81.296 81.296
(1,2) (3,4)
41. 569 13. 052
39. 727
15. 18
31. 05
51. 1 16. 1
53.334 53.334
(1,2) (3,4)
26. 122 9. 671
27. 212
10. 39
20. 38
49. 0 18. 1
(Total)
54. 621
26. 675
10. 19
67.2
(Total)
35.793
17. 541
6.70
67.1
Ib SOC Removed/lb
Carbon
0. 163
0.056
0. 154
0. 056
- 59 -
-------
The 170 Ib of activated carbon contained in the third and
fourth columns of each system removed less than 10 Ib of SOC and
about 13 Ib of TOC in treating over 300,000 gal. of effluent from
the first and second columns in each system. The effluent
from the first two columns contained an average SOC of about
10 mg/1. Therefore, the first 170 Ib of carbon in each system
removed about half of the SOC applied, while the second 170 Ib
removed somewhat less than half of the SOC remaining. Worthy of
note is the fact that the first 12-ft settled depth of carbon in
each of the two systems operating on the clarified feed had
average removal rates of about 0.10 Ib of SOC per 1,000 Ib carbon
per hour, approximately the same as obtained for each of the four
12-ft adsorption systems studied in Phase II.
A point to be noted is that SOC, as well as TOC, was removed
both by the filtration pretreatment carried out in the Phase II
studies, and by the more extensive pretreatment of the Phase III
studies. As shown in Table 3, the difference between the SOC
values of 26.5 mg/1 for the raw secondary effluent and 24.1 mg/1
for the filtered secondary effluent resulted from the pretreatment
step. Figure 33 illustrates the consistent removal of about the
same amount of SOC during coagulation-filtration pretreatment.
Some biological activity was apparent in the pretreatment units, as
it was in the activated carbon columns. This activity might have
contributed to the SOC removal observed during pretreatment.
As in Phase II, the greatest difference between the packed-
bed and expanded-bed adsorption systems was the necessity for
much more frequent cleaning of the former system due to clogging
and build up of excessive head losses. Figure 29 shows the
seriousness of this problem. The first column in the packed-bed
system required cleaning 22 times over the course of the Phase III
pilot field study, while the expanded-bed system was not cleaned
intentionally during the entire study. Some accidental cleaning
of the expanded beds was provided on several occasions when the
clarification system failed to produce sufficient volumes of
water to maintain the flow in the adsorbers. When this happened,
air was pumped into the adsorbers through the influent lines,
thus serving to clean the carbon. This accidental cleaning was,
of course, not nearly equivalent to that provided by air scouring
and backwashing. Also, because it occurred during times when
the adsorbers were unattended, some transfer of carbon from one
column to another in the four-column series took place and some
carbon was lost from the system.
ANALYSIS OF SPENT CARBON
At the conclusion of operation of the four adsorber systems
studied in Phase II and the two systems studied in Phase III, the spent
carbon in each of the eight columns was thoroughly cleaned iri situ
by air scouring, then removed and allowed to drain. Representative
1-gal. samples taken from each of the drained carbons, were analyzed
for moisture, volatile matter, and ash. Dried samples from Phase
II were checked for sieve analysis. Iodine numbers were determined on
60 -
-------
the dried and devolatilized samples from both phases of study.
In addition, samples of the spent carbons from each column in
Phase II were extracted with chloroform and methanol in a
preliminary attempt to characterize the adsorbed substances.
Moisture and Volatile Matter
Each of the columns originally had been charged with 85
Ib of fresh activated carbon. Any carbon lost from a column
during cleaning operations over the course of the Phase II
studies was recovered and returned to that column. This recovery
was accomplished by collecting and settling the backwash waters.
For all practical purposes, the activated carbon sampled at
the end of the run was the same carbon present initially. In
Phase III, there was some loss of carbon from the expanded-bed
system. The weights of the carbon removed from each of the
eight columns after operation are listed in Table 5, along with
the calculated weights after drying and devolatilizing. Moisture,
volatile matter and ash contents are presented in Table 6.
Table 6 shows that the spent carbon from all of the columns
in Phase II was generally similar. The drained carbon contained
about 35% moisture, determined as weight loss in about 2 hours
at 140°C, and all of the dried carbon samples contained from
13% to 18% volatile matter, determined as weight loss on heating
to 900°C in an inert atmosphere.
With one exception, the dried carbon from the first column
of each of the four pairs contained more volatile matter than
the second. This result was expected because the first column
was in contact with a more concentrated feed solution. The
amounts of volatile matter on the spent carbon are in the same
range as the calculated amounts of SOC removed from the secondary
effluent during the Phase II treatment operation, as discussed
previously. The amount of SOC removed was calculated to be
about 17% to 22% of the total original weight of activated carbon
charged to the two columns in each adsorber system. At the 900°C
temperature used for liberation of volatile matter, organic compounds
undergo dehydration, decomposition and vaporization to leave
various amounts of carbonaceous residue. Although the weight
loss incurred during heating tends to approach the weight of
organic material adsorbed, the unknown extent of carbon deposition
prevents the drawing of any firm conclusion in this regard. The
analyses indicate that in all cases the weight of the dry,
de-volatilized spent carbons is greater by from 5% to 17% than
the weight of the original activated carbon charged. These
increases in weight are probably due in large part to the carbonization
reaction, and in some part to the deposition of inorganic substances.
Table 6 shows that the moisture contents for the carbon
samples from Phase III were about 45%, substantially higher than
those from Phase II. Volatile content was significantly less
- 61 -
-------
TABLE 5
Weight, Moisture and Volatile Content of
Spent Carbon From Column Tests
Weight of Weight of
Drained Dried
Column Carbon Carbon Weight of Devolatilized Carbon, Ib
Designation* Ib Ib Column System
(Phase II, Z-column systems)
PCU 1 173. 5 113.4 93. 8
PCU 2 168.0 110.2 92.1 185.9
PCF 1 188. 3 118.4 100. 5
PCF 2 162.8 103.0 89.6 190.1
ECU 1 176.9 110.6 91. 5
ECU 2 178.8 114.0 96.5 188.0
ECF 1 155.4 108. 0 90. 2
ECF 2 167.9 109.3 90.0 180.2
(Phase III, 4-column systems)
PC 1 160. 3 94. 9 82. 4
PC 2 156. 2 90. 3 79. 0
PC 3 188.2 100. 3 91. 8
PC 4 172.8 90. 6 85. 5 338.7
EC 1 130. 7 78. 6 68. 3
EC 2 155. 5 88. 6 77. 9
EC 3 151. 6 85. 9 77. 6
EC 4 142. 1 74. 3 68. 7
EC Carryover 83.6 33.6 31.6 324.1
*PCU - Packed column, unfiltered feed
PCF - Packed column, filtered feed
ECU - Expanded column, unfiltered feed
ECF - Expanded column, filtered feed
PC - Packed column, chemically clarified feed
EC - Expanded column, chemically clarified feed
Each column initially charged with 85 Ib activated carbon
- 62 -
-------
TABLE 6
Analysis of Spent
Sample
Identification
Carbon from Column Tests
Volatile
Moisture Matter Ash On
on Drained on Dried Ash On Dried
Carbon Carbon Devolatilized Carbon
Wt % Wt % Carbon % Wt %
Phase II, 2 -Column
PCU 1
PCU 2
PCF 1
34.
34.
37.
6
4
1
1
1
1
1
1
Ash Calc
on Devol
Basis
Wt %
Systems
7.
5.
6.
1.
5.
14.
PCF 2
ECU 1
ECU 2
ECF 1
ECF 2
PC
PC
PC
PC
EC
EC
EC
EC
EC
Phase
1
2
3
4
1
2
3
4
Carryover
36.
37.
36.
30.
34.
Ill,
42.
43.
46.
47.
39.
43.
45.
47.
59.
7
5
2
5
8
4 -Column
4
5
7
5
8
0
5
8
8
1
1
3.
3.
17.
1
1
1
1
1
1
1
6.
5.
5.
6.
6.
7.
4.
4 7. 1
8*
5 7.5
1
1 7. 1
9
4 7.2
0
3 6. 8
2
4 7. 3
8
5
1
7 7. 6
9
5.
6.
6.
6.
6.
6.
5.
6.
6.
6.
6.
9
0
3*
0
2
0
9
5
0
0
3
7.
7.
7.
7.
6.
7.
7.
7.
7.
7.
7.
1
2
5*
1
9
2
1
7
2
2
7
Systems
1
1
1
1
3.
2.
8.
5.
3.
2.
9.
7.
6.
2
5
5
5
1
1
5
3
0
7.
6.
6.
9.
5.
6.
6.
6.
1
9
7
2
8
3
2
1
8.
7.
7.
9.
6.
7.
6.
6.
2
9
3'
7
7
1
8
6
*Resuits of duplicate analyses
- 63 -
-------
than observed in Phase II. The volatile content declined
significantly from the first to the fourth contactor in both
packed and expanded beds. This expected result is in agreement
with SOC removals observed during Phase III operation.
Ash Content
The ash contents of the spent carbons in Phase II showed a
15% to 31% increase over the value of 5.9% by weight for the
virgin carbon. Results from the packed bed in Phase III were
even higher. Part of the increased ash content may derive from
inorganic materials associated with the organic materials
adsorbed during the treatment operation. An additional part of
the ash could result from silt or other fine mineral matter in
the suspended solids which deposited on the activated carbon
particles. This is especially true for the packed bed used in
Phase III, where precipitate from chemical treatment could escape
from the filter. A large ash build-up could be a problem for any
operation in which the carbon is to be repeatedly reactivated.
If the increased ash content has any deleterious effects on the
sorptive characteristics of the activated carbon, a factor which
has not yet been completely defined, it may be necessary to provide
for acid washing of the carbon, either before or after regeneration.
Particle Size
The particle size distributions for the dried spent carbon
and fresh carbon from Phase II are given in Table 7. The spent
carbons from both packed and expanded beds underwent a slight
reduction in average particle size. For the expanded beds, the
carbon in the lead columns for the two systems remained essentially
unchanged in particle size distribution, while the carbons in
the two trailing columns showed rather significant decreases in
the plus-30 fractions, with corresponding increases in the plus-40
fractions. The reason for this is not known. The estimated average
particle sizes calculated from the sieve analyses suggest that
somewhat more particle attrition occurred in the packed beds,
with a change in average particle size of about 0.08 mm taking
place. This result is reasonable since the carbon used in the
expanded bed had been subjected to crushing and sieving operations
which would reduce the weaker particles, leaving a greater
percentage of abrasion-resistant particles in the carbon charged
to the bed. In addition, the gentle agitation of the particles
in the expanded bed during normal operation was much less severe
than the vigorous agitation required during cleaning of the'packed
beds. As has already been noted, it was necessary to subject the
packed beds to the cleaning operation much more frequently than
the expanded beds.
Iodine Adsorption
The activities of the spent carbon samples as measured by
Iodine adsorption are listed in Table 8. These values give some
- 64 -
-------
TABLE 7
Particle Size Analysis of Carbon
Before; and After Use
Percent Retained on
U. S. Sieve No.
Average size, mm
Percent Retained on
U. S. Sieve No.
Fresh Carbon
Driod Spent Carbon
From Columns
Average size, mm
Packed Re
PCU1
16
20
30
40
50
Pass
28.
36.
23.
9.
1.
0.
0.
3
1
8
5
1
2
99
18.
37.
28.
13.
1.
0.
0.
4*
8
4
6
2
6
92
20.
28.
33.
15.
1.
-------
TABLE 8
Iodine Number of Virgin and Spent Carbon
Sample Dried Devolatilized
Identification Sample Sample
PHASE II
Virgin Carbon 1050
PCU 1 551 815
PCU 2 638 845
PCF 1 526 816
PCF 2 633 850
Virgin Carbon 1053
ECU 1 440 764
ECU 2 526 792
ECF 1 446 762
ECF 2 538 797
PHASE in
PC 1 629 878
PC 2 710 883
PC 3 800 945
PC 4 888 1035
EC 1 612 860
EC 2 670 891
EC 3 782 936
EC 4 873 1020
EC Carryover 936 1085
- 66 -
-------
indication of the remaining adsorption capacity of the spent
carbons. The dried samples of spent carbon from Phase II
exhibited iodine numbers roughly half of that of the virgin
carbon. For each of the four experimental systems in Phase II,
the spent carbon from the first column had a lower iodine number
than that from the second. Further, the spent-carbon samples
from the packed beds had somewhat higher iodine numbers than
those from the expanded beds. The differences in the iodine
numbers of about 100 for the dried samples and about 50 for the
carbonized samples suggest some possible differences in the
adsorptive behavior of the carbon particle from the packed and
expanded beds.
The additional crushing and sieving operations to which
the carbon used in the expanded beds was subjected probably
reduced the percentage of more friable particles. The more
friable particles, on the other hand, generally contain larger
pores, which would be less subject to blocking by adsorbed
molecules. The particles remaining in the 20x40 range used
in the expanded beds would then consist largely of the relatively
stronger particles with a finer pore structure. It is possible
that adsorbed organic matter concentrated at the pore openings
more effectively restricts penetration of the iodine molecules
into the smaller pores of the spent carbon recovered from the
expanded beds. Carbonizing and destruction of the adsorbed
molecules would tend to reduce this restricting effect, resulting
in smaller differences in the iodine numbers for the carbons
from the packed and expanded beds. This result suggests that on
reactivation the activity of the carbons, at least as measured
by the iodine number, would again be essentially equal.
Spent-carbon samples from both the packed and expanded beds
in Phase III showed increasing iodine numbers from the first to
the fourth contactor. This expected result is in agreement with
adsorption and volatile matter data discussed earlier. Since,
in contrast to Phase II, the same carbon was used in both packed
and expanded beds, there was not a large difference in iodine
numbers of samples from the two beds. For dried samples, the expanded
bed was lower by an average value of 23; for devolatilized samples
the difference was only 9.
Extractions
Some studies were performed to provide preliminary information
on the character of the organic materials adsorbed on the spent
carbon. Determination of the precise nature of the adsorbed
materials was considered to be much beyond the scope of the project;
thus, the studies were limited for the most part to chloroform
extractions followed by preliminary separations by column
chromatography. Some tests were conducted to determine the
presence of carbonyl compounds in the extracts.
The most simple analysis was that performed on a drained
sample "of the spent carbon from the Phase I studies. The carbon
was extracted with chloroform and subsequently with methanol to
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yield fragrant oily brown residues upon evaporation of the solvents.
The total weight of the residues amounted to somewhat over 10%
of the weight of the carbon sample from which it was extracted.
More detailed analyses were performed on the carbons
recovered from the four different adsorption systems studied in
Phase II. Samples of each of these batches of carbon were first
extracted with chloroform by the procedure described in Standard
Methods.9 The extracts were then evaporated at 60°C under
reduced pressure. The weights of the extracts so obtained are
listed in Table 9.
Chromatography
All of the residues extracted from the Phase II carbon samples
were viscous, dark brown oils exhibiting a strong earthy odor,
similar to that extracted from the Phase I carbon. In an attempt
to further classify these extracts, a 7.61-gm sample of the residue
from the packed-bed unfiltered-feed carbon was placed on a column
of chromatographic grade active alumina, in chloroform. A total
of 55 fractions was taken from the column, all of 25-ml volume
except for a final acetic acid strip. The solvent systems used
for the fractionations are listed in Table 10. The amounts of
material eluted from the various solvent systems are also listed
in Table 10, along with the weight percentages of the total eluent
represented by each fraction. The total amount of extract recovered
from the chromatographic column in the 55 fractions was 7.2 gm,
or approximately 95% of the amount of the chloroform extract
originally applied to the active alumina.
All of the 55 fractions taken from the chromatographic
column were either brown or yellow-brown oils. The original strong
earthy odor of the chloroform extract was eluted from the column
with the first ten fractions; very little of this odor remained
with the subsequent fractions, all of which exhibited in varying
degree the burnt-sugar odor characteristic of lignin pigments.
All of the fractions involving acetic acid appeared to have under-
gone some reaction which reduced them to virtually insoluble resins.
These resins accounted for about 43% of the material eluted.
The first four fractions with chloroform as the solvent
showed some tendency to form crystals. The crystal-like substances
formed were not well defined, but rather had a sticky and impure
appearance. To explore this behavior further, these four fractions
(1.71 gm) were combined and chromatographed on active alumina with
chloroform. A total of 25, 10-ml fractions was taken. Three
of these fractions were combined and crystallized from a one to
one mixture of chloroform and n-heptane, resulting in 392 mg of
very sticky crystals. The crystals were then re-dissolved in
chloroform and chromatographed again as described above, with a
total of 25 more 10-ml fractions being taken. Three of these
fractions were combined and chromatographed in the same fashion,
for 25 additional 10-ml fractions. Upon combining four of the
fractions and crystallizing them, 80 mg of very sticky crystal
again resulted, with the same appearance as the crystals formed
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TABLE 9
Chloroform Extracts from the Phase II Spent Carbon
Carbon Sample Extract Extract, %
Sample Identification Weight, gms Weight, gm by Weight
PCU 807. 2 40. 273 4. 99
PCF 857. 1 43. 096 5. 03
ECU 913. 5 47. 936 5. 25
ECF 985. 8 52. 607 5. 34
Total packed bed 1664.3 83.369 5.02
Total expanded bed 1899.3 100.543 5.29
Total unfiltered feed 1842.9 95.703 5.19
Total filtered feed 1720.7 88.209 5.12
Average extract, % by weight = 5. 16
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TABLE 10
Chromatographic Fractionation of the Chloroform Extract
from the Phase II Packed-Bed Unfiltered-Feed Carbon
Fraction
Nos.
1-12
13-19
20-23
24-29
30-32
33-38
39-47
48-54
Strip
Solvent System
Chloroform
Chloroform: Ether
Ether
Ether: Ethyl Acetate
Ethyl Acetate
Ethyl Acetate: Ethan ol
Ethanol
Ethanol: Acetic Acid
Acetic Acid
Solvent
Volume
Ratio
1:1
-
1:1
-
1:1
-
9:1
-
Extract
E luted, gm
2. 391
0. 194
0.060
0.062
0.019
1.093
0.255
2.737
0.392
% of Total
Eluent
33.2
2. 7
0.8
0.9
0.3
15.2
3. 5
38. 0
5.4
Totals 7.203 100.0
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in the four original fractions. Since this procedure was unable
to sufficiently purify the crystalline material, the tests were
terminated
Carbonyl Compounds
To test for the presence of carbonyl-group-containing substances
in the extracts from the activated carbon, a sample of the extract
from the packed-bed unfiltered-feed carbon was treated with 2,
4-dinitrophenylhydrazine (DNP), which forms specific compounds with
carbonyl groups. The resulting dinitrophenylhydrazones (DNPH's)
are very useful for characterizing the carbonyl-group-containing
compounds originally present. The DNPH's which formed in treating
the extract with DNP were isolated and recrystallized. Thin-film
chromatographic measurements suggested that the DNPH's so obtained
were virtually one pure compound. Further, the spectrum obtained
for this DNPH (in ethanol) in the ultraviolet region suggested that
it was an aliphatic DNPH. From measurement of the absorbance
of this material at a wave length of 361 my (Xmax for aliphatic
DNPH), and using a range of literature values for the molar
absorptivity of several aliphatic DNPH compounds of from 21,400
1/mol-cm to 23,600 1/mol-cm, the molecular weight range for the
isolated DNPH was calculated as being 274.1 to 305.3. Because
the basic DNP moiety has a molecular weight contribution of 196.1,
it can be concluded that the remainder of the DNPH molecule has
a weight of from 78.0 to 109.2. This corresponds to a carbon
chain of from 6 to 8 carbon atoms.
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GENERAL DISCUSSION
Experimental results have shown that the expanded-bed and
packed-bed adsorption systems are very nearly equivalent with
regard to removing soluble organic material from secondary effluent.
The packed-bed system is more effective for removal of suspended
solids, but the clogging that results from these solids causes high
pumping pressures and the need for frequent cleaning of the carbon.
Neither configuration gave solids removals from trickling-filter
effluent sufficient to eliminate the need for further treatment
when a high clarity water is desired. When the feed contains
suspended materials, but a high degree of removal of these materials
is not required, the expanded-bed system appears to have definite
advantages over the packed-bed system.
It is difficult to interpret the advantage of the expanded
bed precisely in terms of capital and operating cost savings because
these costs will be affected by the amount and character of
suspended material in the feed water. Since comparison of soluble-
organic removal by the two bed configurations was considered the
most important objective of this study, experimental work was limited
to one trickling filter plant. Care must be used in extrapolating
suspended-solids and pressure-drop results to other plants, especially
activated sludge plants. Some general qualitative and rough
quantitative size and cost comparisons for the two contactor
configurations can be made, however.
An expanded-bed system can produce more treated effluent than
a packed-bed system of equivalent size because of less down-time for
the carbon cleaning operation. From the results of the present
study, it is estimated that an hour per day would be required and
about an hour's production of treated water would be utilized during
the backwash following air scrubbing. If the expanded-bed system
required no cleaning, its actual production would be 109% of a
packed bed designed for the same flow rate. The expanded bed will
require some cleaning, but at much less frequent intervals. Results
from another carbon study conducted on an activated sludge effluent
with low suspended solids substantiate the need for daily backwash
of a packed carbon-contactor.3 The backwash water requirements were
somewhat lower than an hour's production.
The elimination of frequent carbon cleaning results in a
savings in operating labor for the expanded bed. For very small
plants with little automation the labor cost saving could be
appreciable. For plants of 10 mgd (or greater) capacity the cost
reduction probably would only be about O.lC/1,000 gal.
The higher pressures resulting from packed-bed operation have
several effects on costs. The most obvious effect is an increase in
pumping power requirements. Assuming a conservatively high average
pressure difference of 20 psi for the packed bed over the expanded
bed, the cost difference would be about 0.2C/1,000 gal. This is a
very minor factor. The pumps for the packed bed would be larger,
but this also is probably not significant. The most important
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effect of pressure is on the type of contacting equipment that can
be used. For an expanded-bed system it should be possible to con-
struct open-top tanks, probably of concrete. From the experimental
results, a height about double the unexpanded height of carbon
should be sufficient. It is likely that the carbon would be divided
into several beds in series to obtain some measure of counter-
current contacting and to give a reasonable equipment height. A
similar series arrangement for a packed-bed system would also
probably be employed. With packed-bed systems there is a question
of whether open tanks could be used or whether closed pressure
vessels would be necessary. The latter would be several times the
cost of open vessels because they must be constructed of steel and
must be relatively small in diameter. For all but very small plants,
parallel contactor-systems would be necessary. From the pressure
data shown in Figure 11 and 29, open tanks do not appear practical
for this particular effluent because of the excessive freeboard
that would be required as solids accumulated on the carbon. For
a wastewater of lower suspended solids content, open tanks might
be applicable, especially if the flow rate per unit area were
decreased. This would have to be determined, however, for each
particular case. Detailed consideration of the costs of contactors
was not within the scope of this study.
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REFERENCES
1. W. J. Weber, Jr., Review of Literature, Journal of Water
Pollution Control Federation 39, 734 (1967)
2. A. F. Schlecta and G. L. Gulp, Journal of Water Pollution
Control Federation 39, 787 (1967)
3. J. D. Parkhurst, et.al., Journal of Water Pollution Control
Federation 39.,R70 (1967)
4. D. B. Stevens and J. Peters, Journal of Water Pollution
Control Federation 38, 2009 (1966)
5. D. S. Davies and R. A. Kaplan, Chemical Engineering Progress
60, (12) 46 (1964)
6. R. L. Beebe and J. I. Stevens, Water and Wastes Engineering
4_, (1) 43 (1967)
7. W. J. Weber, Jr., "Advances in Water Pollution Control
Research", Proc. 3rd Intl. Conf. on Water Pollution Research,
W.P.C.F., Washington, D. C. Vol. I, 253 (1967)
8. J. H. Winneberger, et.al., Journal of Water Pollution Control
Federation 3JLr 807 (1963)
9. "Standard Methods for the Examination of Water and Wastewater"
llth Edition, Amer. Publ. Health, Assoc., New York (1960)
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