:LEAI
VAII
WATER POLLUTION CONTROL RESEARCH SERIES • ORD- 17O2OFKBO7/7O
ADVANCED WASTEWATER TREATMENT
USING POWDERED ACTIVATED CARBON IN
RECIRCULATING SLURRY CONTACTOR-CLARIFIERS
U.S. DEPARTMENT OF THE INTERIOR • FEDERAL WATER QUALITY ADMINISTRATION
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe
the results and progress in the control and abatement
of pollution in our Nation's waters. They provide a
central source of information on the research, develop-
ment, and demonstration activities in the Federal Water
Quality Administration, in the U. S. Department of the
Interior, through inhouse research and grants and con-
tracts with Federal, State, and local agencies, research
institutions, and industrial organizations.
A triplicate abstract card sheet is included in the
report to facilitate information retrieval. Space is
provided on the card for the user's accession number and
for additional uniterms.
Inquiries pertaining to Water Pollution Control Research
Reports should be directed to the Head, Project Reports
System, Planning and Resources Office, Office of Research
and Development, Department of the Interior, Federal Water
Quality Administration, Room 1108, Washington, D. C. 20242.
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ADVANCED WASTEWATER TREATMENT USING
POWDERED ACTIVATED CARBON IN RECIRCULATING
SLURRY CONTACTOR-CLARIFIERS
by
C. F. Garland
R. L. Beebe
Infilco
Tucson, Arizona 85703
for the
FEDERAL WATER QUALITY ADMINISTRATION
DEPARTMENT OF THE INTERIOR
Program #17020 FKB
Contract #14-12-400
FWQA Project Officer, E. F. Harris
Advanced Waste Treatment Research Laboratory
Cincinnati, Ohio
July, 1970
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.0.20402 - Price 75 cents
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FWQA Review Notice
This report has been reviewed by the Federal
Water Quality Administration and approved for
publication. Approval does not signify that the
contents necessarily reflect the views and policies
of the Federal Water Quality Administration, nor
does mention of trade names or commercial products
constitute endorsement or recommendation for use.
ii
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CONTENTS
Page
ABSTRACT v
INTRODUCTION 1
Background 1
Process Description 2
Objectives of this Investigation 2
Experimental Approach 2
Analytical Procedures 3
PREPARATORY LABORATORY INVESTIGATION 4
Objectives 4
Sewage Plant Effluent Quality 4
Adsorption Uptake Rate Study 6
Adsorption Isotherm Study 6
Coagulation Studies 11
PILOT PLANT PROGRAM 18
Apparatus and Procedure 18
Experimental Results 23
Phase 1 23
Phase 2 24
Phase 3 28
Phase 4 39
Phase 5 40
Phase 6 50
ill
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Page
Supplemental Investigations 55
Temperature 55
Adsorption Theory 60
Nutrients 62
Color and Surfactants 62
Turbidity Measurement 65
DISCUSSION 67
General Considerations 67
Process Economics 69
Plant Design 69
Treatment Cost 71
SUMMARY 74
REFERENCES 76
APPENDIX 77
iv
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ABSTRACT
In this investigation, high-rate solids-contact treatment units
embodying internal slurry recirculation were operated singly and in
series as powdered activated carbon adsorption systems. Secondary
(activated sludge) sewage treatment plant effluent was treated in a
30,000-gpd pilot plant using a slurry of activated carbon and a cationic
polyelectrolyte flocculation agent.
Practical operating parameters of the system are developed for
different activated carbon dosage levels for two contact-clarifiers
operated in series with countercurrent carbon advance. Similar parameters
are developed for a single unit and one carbon dosage. Adsorption
system results, expressed as COD reduction, are evaluated by comparison
with laboratory generated single-stage adsorption isotherms.
In a practical sense, effluent COD for series adsorption was equiva-
lent to that expected from consideration of two-stage adsorption theory.
Possible modification of the process to improve efficiency is discussed.
Contactor-clarifier effluent was filtered through a gravity sand
unit and development of operational difficulties with respect to biologi-
cal growth and polyelectrolyte-flocculated carbon-sand agglomerates are
noted. At a 3-gpm/ft^ operating rate, no carbon passed through the
filter. Backwash water requirements increased to 5 per cent of the
filtered water volume as the filter condition deteriorated.
Supplemental investigations conducted during the pilot plant program
include evaluation of the influence of temperature variation on system
performance and its analysis; consideration of adsorption theory; the
influence of the treatment process on nutrients, color, and surfactants;
and comparison of the results of two methods of turbidity measurement.
The total operating cost of a 10-mgd system based on the pilot plant
process and 90 per cent reuse of carbon recovered by regeneration is
estimated as 15C/1000 gal. at a carbon dosage of 2 lb/1000 gal., or
9.7C/1000 gal. if in terms of effluent quality objectives the carbon
requirement can be reduced to 1 lb/1000 gal. A similar system rated at
100 mgd is estimated to produce treated wastewater for 12.76/1000 gal.
at a 2-lb/1000 gal. carbon requirement, or 7.40/1000 gal. at half this
dosage.
This report was submitted in fulfillment of Contract No. 14-12-400,
Program No. 17020 FKB, between the Federal Water Quality Administration
and Infilco Company.
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INTRODUCTION
Background
Adsorption is a proven process for separating relatively small
quantities of organic impurities or valuable products from dilute
solutions. Activated carbon is one of the more efficient and better
known adsorbents.
Activated carbon adsorption has long been applied to industrial
purification processes, many times successfully competing with other
purification systems. Its use in water purification dates back to the
late 1920*s.l It is only natural that activated carbon adsorption is
receiving serious attention as a candidate for advanced wastewater treat-
ment as water pollution and water shortage problems augment.
Performance of traditional single-stage powdered activated carbon
contacting systems is controlled by two different aspects of adsorption:
kinetics and adsorption equilibrium. Carbon feed rates sufficient to
improve the quality of sewage plant effluents to acceptable levels for
reuse may be very high because of the equilibrium adsorption aspect. As
a result, treatment cost is high. Until now, cost factors have favored
the use of granular activated carbon in columnar systems. Practical granu-
lar carbon regeneration systems are in operation now, so that carbon reuse
results in further treatment coat reduction.^
Powdered activated carbon should be more efficient and thus more
economical than granular carbon by virtue of its greater surface area.
However, powdered carbon is unsuitable for columnar flow schemes. Regenera-
tion of this type of carbon on an economical basis is still under study.
As powdered activated carbon is readily slurried, hydraulic transfer to and
from process equipment is practicable.
The authors' company sponsors continuing advanced waste treatment
research and a few years ago reported the results of a brief investigation
of the use of powdered activated carbon in a model ACCELATOR clarifier. a
high-rate solids-contact unit embodying internal slurry recirculation.^
The model test was conducted on chemically treated sewage plant
effluent and the recirculating slurry was developed from feeds of powdered
activated carbon and coagulant. The results provided evidence that the
single ACCELATOR contactor-clarifier functioned as a compound contact
system producing lower residual COD and more efficient carbon utilization
than theoretically predictable for a continuously operating single ad-
sorption stage.
-------
If a single treatment unit could be shown to produce some of the
process advantages of a multiple-stage adsorption system, the cost of
treating wastewater with finely divided activated carbon would be reduced.
Two such process stages could markedly lower treatment cost.
Process Description
A two-stage countercurrent system is visualized. Application of
the process involves series operation of two solids-contact clarifiers
of a type used widely for water treatment. Carbon is fed to the second
unit and first-stage slurry is developed from carbon advanced from the
second contact-clarifier. Spent carbon is withdrawn from the system by
blowdown from the first unit. To protect the receiving stream from carbon
lost during process disruption, post filtration is indicated.
Objectives of this Investigation
The primary objective of the present investigation was a performance
and cost evaluation of such a system for treatment of municipal sewage
activated-sludge plant effluent. Brief performance evaluation of single-
stage treatment was a corollary objective.
Experimental Approach
Initially, a commercially available powdered activated carbon and a
polymer flocculation agent to improve slurry settleability were selected
on the basis of laboratory study.
These materials were then used during six consecutive pilot plant
studies under various conditions. The first and second studies were in-
tended to determine physical and hydraulic capabilities of the system.
The third, fourth and fifth experiments utilized different activated
carbon feed dosages in the complete system. For the sixth study, one of
the solids-contact clarifiers was operated alone to evaluate single-stage
treatment.
During all experiments, data were collected from the pilot plant and
single-stage adsorption isotherms were developed for the various feed
streams. These data were then compared by application of adsorption
theory.-*
Peripheral laboratory analyses of 24-hr composite samples plus
auxiliary grab samples were used to disclose operational parameters other
than those directly associated with the adsorption process. These data,
together with the pilot plant control records, reveal important practical
operating requirements and results in other areas of interest.
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Analytical Procedures
With certain exceptions, methods used in this study conformed to
those in "Standard Methods."6
A modification to "Standard Methods" developed especially for low-
level COD values was used. The complete procedure appears in the Appendix.
Except as noted, all samples for which COD is reported were filtered
through 0.45-micron Millipore filter discs prior to the COD determination.
This was necessary in the case of powdered activated carbon treated
samples as residual carbon particles contribute to the COD value. In
order to provide a usable COD relationship between carbon treated samples
and untreated samples, most of the untreated samples were filtered in a
like manner.
Certain ACCELATOR clarifier control tests performed at the pilot
plant site are discussed hereinafter.
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PREPARATORY LABORATORY INVESTIGATION
Objectives
A suitable adsorption system had to be developed if the pilot plant
was to produce data of sufficient scope to develop the objectives. The
feed stream was to be effluent from a secondary sewage treatment plant.
The characteristics of the available stream had to be determined. The
adsorbent, activated carbon in powdered form, is available from different
sources. It is manufactured from various raw materials by a number of
different processes and may be specialized in its application.7 Activated
carbons were tested with aliquots of feed stream to assure selection of
a material that would perform within the time limitations and hydraulic
conditions imposed by the pilot plant. A flocculation system using this
carbon in the feed stream had to be found. Inorganic coagulation agents
were ruled out as they could be cumulative in the carbon where reactiva-
tion and reuse are concerned. The flocculated slurry must settle rapidly,
leaving little if any residual carbon particles in the supernatant; and
it would be advantageous if the slurry dewatered well on standing.
For the purposes of this study, the COD of the feed stream is the
adsorbate concentration indicator. Some impurities present may be ad-
sorbed which do not exhibit a COD while others having a COD may not be
readily adsorbed. Sewage plant effluent contains an everchanging combina-
tion of these materials, many of them not identified; so the COD determina-
tion is a useful but incomplete pollution indicator.
Sewage Plant Effluent Quality
The City of Tucson commissioned a new activated sludge sewage treat-
ment plant in 1968 parallel to two existing treatment systems. Secondary
clarifier overflow from this plant was selected as the feed stream source
for the pilot plant.
The new plant was observed to experience infrequent difficulties as
the result of the usual new equipment shakedown process. It was still
considered the most reliable source of feed stream, however, as the two
older installations were undergoing various repair and maintenance opera-
tions. Sections of them would be down from time to time. When in diffi-
culty, the new plant effluent contained appreciable amounts of activated
sludge. During these periods of several hours duration, it was observed
that the solids appeared first, occurred in heavier concentration, and
persisted longer in the outer of the two concentric take off launders in
the final clarifiers. Therefore, the inner launder was selected as the
take off point that would best minimize solids input to the pilot plant.
Physical and chemical characteristics of several of the grab samples
collected at random from this location are presented in Table 1.
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TABLE 1
Final-clarifier Effluent Characteristics, mg/1
Tucson Sewage Treatment Plant
Date
Time
Calcium (as Ca)
Magnesium (as Mg)
Sulfate (as 804)
Chloride (as Cl)
Nitrate (as NOs)
Iron (as Fe)
Manganese (as Mn)
Silica (as Si02)
Alkalinity (as CaCOa)
Hardness (as CaCOs)
Color (SU)
Turbidity (JTU)
Suspended Solids
COD (unfiltered)
COD (filtered)
BOD5
PH
7/17/68
7:00 A.M.
68
3
138
84
1
0.1
0.05
41
220
184
16
9
4
-
28
4
7.4
7/18/68
3:00 P.M.
62
8
179
74
6
0.2
0.05
42
216
190
14
11
6
33
26
-
7.2
8/12/68
7:00 A.M.
48
8
131
58
1
-
0.05
34
224
150
15
10
3
33
28
-
7.6
8/26/68
3:00 P.M.
65
8
150
81
10
0.1
0.05
48
198
195
15
9
10
35
27
-
6.5
10/14/68
3:00 P.M.
54
10
153
69
-
0.3
0.05
47
254
178
17
90
65
125
25
-
7.5
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Adsorption Uptake Rate Study
Three powdered activated carbon products were selected for evalua-
tion against project requirements. These were Darco S-51, a product of
Atlas Chemical Industries, Inc., and Aqua Nuchar A and Filtchar, both
products of the Westvaco Corporation.
Darco S-51 was used in our earlier experiments. Aqua Nuchar A was
utilized in an FWQA pilot plant at Lebanon, Ohio. Filtchar is not a
standard commercial grade product, but was furnished by the manufacturer
for experimentation.* Carload quantities could be produced for certain
tonage situations.
For each uptake-rate comparison study, sufficient effluent sample
was prefiltered through 0.45-micron Millipore filter discs for the entire
procedure. Five aliquots of one grade of dry powdered activated carbon
were weighed and wetted with demineralized water. One was added to each
of five 250-ml aliquots of effluent while mixing with a gang stirrer at
preselected speeds. A sixth 250-ml effluent aliquot was stirred as a
control. The five treated jars were stirred for 5, 10, 20, 45 and 60 min,
respectively. At the end of its stirring time each aliquot was immediately
filtered through a 0.45-micron filter disc and filtrate COD was determined.
The process was repeated for the remaining two grades of carbon.
The foregoing procedure was repeated three times at stirring speeds
of 50, 150 and 400 rpm. At the highest speed, the activated carbon dosage
was changed from 200 mg/1 to 400 mg/1 to increase the range of COD results
and to provide a more definitive picture of uptake-rate differences
between carbon grades.
All three activated carbons exhibit a rapid initial adsorbate uptake
rate (Figure 1). Equilibrium had not been reached after 60 minutes of
contact, but the rate at which adsorption was continuing at this time
was very low. At 400-rpm stirring speed, all three grades achieved essen-
tially 95% of their respective 60-min COD removals at 30 min of contact.
Aqua Nuchar A approached the 95% level after only 10 min compared with
87% for the Darco S-51 and 85% for the Filtchar.
Adsorption Isotherm Study
Equilibrium adsorption isotherms were used to study the relative
adsorptive capacities of the three activated carbons under consideration.
Sufficient sewage plant final clarifier effluent was prefiltered
through 0.45-micron filter discs to perform each comparison study. A
stock solution of each grade of carbon was prepared with special low-COD
distilled water (Appendix). The activated carbons were added to sets of
five 250-ml aliquots of the prepared effluent at concentrations of 40,
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50-RPM Stirring Speed
200 mg/1 Carbon
150-RPM Stirring Speed
200 mg/1 Carbon
-a-
-8
400-RPM Stirring Speed
400 mg/1 Carbon
A Filtchar
© Darco S-51
Q Aqua Nuchar A
10
20 30 40
Elapsed Stirring Time, Minutes
FIGURE 1: Uptake Rate Study
7
50
60
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200, 400, 800, and 1600 mg/1. A sixth aliquot in each set was left un-
treated as a control. The jars were stirred at 100 rpm for 60 min, their
contents were then filtered through 0.45-micron filter discs, and filtrate
COD determined.
Preliminary work had shown the feasibility of handling the carbons
as stock suspensions rather than individually weighed dry aliquots. The
stock suspensions provided greater handling ease by minimizing problems
associated with wetting the carbon. Additional work had shown that low
stirring speed influenced loading achieved and the shape of the isotherm
plot. A stirring speed of 100 rpm was selected as the result of this work.
Freundlich curves of the data (Figures 2 and 3) were developed. The
Freundich equation is empirical but widely used because of its simplicity.
It is generally written: X/M = KCt1/11, where X is the mg/1 of adsorbate
taken up by M mg/1 of adsorbent applied. Ct is the mg/1 of adsorbate re-
maining unadsorbed at equilibrium and K and 1/n are constants specific
for the materials and conditions of test. When X/M is plotted against Ct
on logarithmic paper, a straight line is commonly obtained which has a
slope of 1/n and an intercept K at Ct = 1.
Slight variations in the original concentration of COD (C0) , perhaps
caused by the passage of time, between the individual carbon test sets
were retained in plotting the Freundich curves. However, for positioning
curves of constant carbon dosage, an average Co value was utilized.
For the activated carbons tested, the plots show Darco S-51 to be
superior. Higher loading (X/M) and lower residual COD (Ct) are evident
at each carbon dosage.
The manufacturer states Darco S-51 is made from lignite. After
activation by heat and steam, it is thoroughly washed with mineral acids
and then with water to remove extractable inorganic constituents. The
ash content, being generally inert and insoluble, has no adverse influence
on the adsorptive capacity. The carbon wets readily with a minimum of
dusting and goes into suspension quickly.
According to the manufacturer, Darco S-51 has the following properties:
Moisture content (Max., as packed) ....................... 12%
Water-solubles (Max. , determined by 4 leachings with
boiling water) ........................................... 1%
Acid-solubles (approx. , determined by leaching with 1:1
hydrochloric acid) ....................................... 3%
Ash (normal range) ....................................... 17-24%
pH of water extract (normal range) ....................... 5.0-7.0
8
-------
-i r
T r
60
40
20
10
W)
c
•H
T3
OJ
O
C
O
cfl c
u 5
0)
j-t
(0
>
-H
4->
O
Effluent Sample 26147-T
60 min contact
27°C
100 RPM
Filtered Co = 27.7 tng/1 COD
40 mg/1 carbon
200 mg/1
400 mg/1
O Darco S-51
Q Aqua Nuchar A
A Filtchar
800 mg/1
1600 mg/1
I I I L
4 6 8 10 15
Filtered COD Remaining, mg/1
20
30 40
FIGURE 2: Adsorption Isotherm Carbon Comparison
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60-
40-
20
T2
X~\
a
t>o
d
10
o
J3
i-i
CO
U
0) c
4J 5
4J
U
i—r
Effluent Sample 26157-1
60 Min contact
27°C
100 RPM
Filtered Co = 22.4 mg/1 COD
40 mg/1 carbon
200 mg/1
400 mg/1
O Darco S-51
Q Aqua Nuchar A
Filtchar
800 mg/1
/I
8 10 15
Filtered COD Remaining, mg/1
20
30
40
FIGURE 3: Adsorption Isotherm Carbon Comparison
10
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Bulk density, lb/ft3 (determined by 30-minute tamping
test) 27-33
Mesh size (approx.) % through 100-mesh screen 95
% through 325-mesh screen 70
Filterability (max.) 1.0 sec./ml/cm, depth (water)
Storage space needed (ft3/ton, approx.) 80
Coagulation Studies
For successful operation of the high-rate clarification units to be
used in the pilot plant, it was essential that an efficient and effective
coagulation system be developed. The floe particles had to settle rapidly,
be strong enough to withstand the repeated cycling through the flocculation
paddles, entrap a high percentage of the carbon particles, and dewater
readily. In addition, inorganic chemicals were to be avoided as the re-
sulting carbon slurry had to be amenable to thermal regeneration. Repeated
regeneration and reuse of carbon flocculated with the common iron or
aluminum salts or clay-coagulant mixtures would result in progressive
contamination of the carbon with inert solids.
Organic polyelectrolytes were used, with rare exception, in all of
the laboratory coagulation studies. These materials would contribute
carbon residues, along with the organic materials flocculated and adsorbed
from solution, to the activated carbon undergoing thermal reactivation.
The emphasis for much of the coagulation investigation was directed
at cationic polyelectrolytes. When those on hand failed to produce the
results desired, anionic and nonionic products were tested individually
and in combination with cationic products and each other. Additional
cationic products were secured and some products subject to deterioration
with time were replaced with fresh material. In all, 26 different products
were involved. Those few which are weighting agents or contain weighting
agents or inorganic primary coagulants improved flocculation, but could
not be applied in the pilot plant for reasons mentioned earlier. Supple-
mentary use of a quaternary ammonia compound and two wetting agents did
not improve the effectiveness of the polyelectrolytes tested.
The products tested and their respective manufacturers were as follows;
1. Aquafloc 412 - Dearborn Chemical
2. Cab-0-Sil - Cabot Corporation
3. Cat-Floe - Calgon Corporation
4. CMC - Hercules Powder Company
5. Claracel CO-980 - North American Mogul Products Co.
6. Guar Gum
7. Hyamine 3500 - Rohm & Haas
11
-------
8. Mud Gel - Baroid Div., National Lead Co.
9. Nalco 600 - Nalco Chemical Company
10. Nalco 650 - "
11. Nalcolyte 110 - Nalco Chemical Company
12. Polyfloc 4D - Betz Laboratories
13. Primafloc XA-10 - Rohm & Haas
14. " C-3 - "
15. " C-7 - "
16. Purifloc A21 - The Dow Chemical Company
17. " A22 - " "
18. " C31 - " " " "
19. " C32 - " " " "
20. Reten 205 - Hercules Powder Company
21. Separan AP30 - The Dow Chemical Company
22. " NP20 - " " " "
23. Superfloc 16 - " "
24. Triton QS-15 - Rohm & Haas
25. " X-40 - "
26. Wisprofloc 20 - lonac Chemical (Permutit)
A total of 180 individual jar tests were conducted on unfiltered
secondary clarifier effluent with variations in mixing speed, mixing time,
carbon dosage, coagulating agent(s), order of coagulating agent and carbon
addition, and the use of floe from previous tests.
Aliquots, generally 200 ml, of final clarifier effluent were treated
first with activated carbon stock solution to either 150-mg/l or 200-mg/l
dosages. While being stirred at test speed, the coagulating agent(s) were
added, generally in the range of 2 to 5 mg/1. This range was broadened
to between 1 to 10 mg/1 if results seemed promising. Stirring time was
normally 10 min; however, this was shortened to 5 min if floe breakup was
observed during the longer period. Flash mixing at 400 rpm for less than
1 min after addition of the coagulating agent was evaluated and discon-
tinued as ineffective. Also discontinued as having no desirable effect
was a variation wherein the activated carbon was introduced at a timed
interval after introduction of the coagulating agent. Supernatant tur-
bidities were determined on most jars showing promise. A 5-min settling
period was allowed before sampling the supernatant for turbidity measure-
ment. Notes were made on the time required for the prominent floe
particles to settle the 3-1/2" liquid depth to the jar bottom.
Early results were discouraging. The majority of well known poly-
electrolytes tested showed no promise. Supernatant turbidities ranged
from 18-58 JTU except when bentonite-bearing compounds or polyelectrolytes
were used with bentonite. Rohm and Haas1 Primafloc C-7, used at Lebanon,
Ohio, was marginally effective; but at the required dosages of 5 to 10 mg/1
it was considered too costly. It performed better at lower concentrations,
2 to 5 mg/1, if the pH of the jar was adjusted to 8.5 to 9.0 with caustic
soda. Calgon's Cat-Floe and Dow's C-31 were tested at length with resultc
12
-------
slightly inferior to C-7. Supernatant turbidities remained in the
range of 8 to 14 mg/1.
Fresh stocks of Dow's C-31 and C-32 were obtained and additional
work indicated C-31 was as effective as C-7. The C-32 was found to pro-
duce a supernatant turbidity of 8-10 JTU at dosages ranging from 1-3 mg/1.
The optimum coagulation results recorded for the several polyelec-
trolytes showing promise was achieved using a solids recycle procedure.
The settled solids (floe) produced in one reaction were retained by de-
canting the supernatant to waste. A fresh aliquot of effluent was intro-
duced to these solids along with carbon and coagulant. The jar stirring
and settling were repeated and the solids again retained. This procedure
was continued through as many as seven cycles during which the floe char-
acteristics change and coagulant dosage changes may be evaluated. The
solids recycle procedure is particularly applicable to the type of
solids-contact units used in the pilot plant.
Typical data reporting (Table 2) for jar studies of this type indi-
cates the amounts and types of materials used, as well as notes on floe
formation, quiescent settling ability, and clarification results as
being poor, fair, or good. The resultant slurry volume existing at the
end of the settling period, as well as the number of previously formed
slurry volumes returned to each jar, are also recorded.
Even though the cationic polyelectrolyte, C-32, left a marginal
8 mg/1 supernatant turbidity, the low dosage level and potential cost
were acceptable and time did not permit further investigation in this
direction. Experience indicates that better coagulation can frequently
be achieved in a plant-scale continuous operation than in jars. So C-32
was selected for the pilot plant operation.
Little information was available concerning the effect of polyelec-
trolyte flocculation on the adsorptive capacity of powdered activated
carbon. It was desirable to learn if the adsorption system selected would
be adversely affected by the C-32.
Pairs of adsorption isotherms were developed using Darco S-51. One
isotherm of each pair was produced using the procedure described pre-
viously. The other incorporated a 3-mg/l C-32 dosage applied immediately
after the carbon addition to each of the five jars and at zero time to
the sixth or control jar. Optimization of the coagulant dosage was not
attempted.
Comparison plots of these isotherms (Figures 4 and 5) show displace-
ment of points for the polymer-treated samples downward and to the left
of those of the control. Note, also, that the differences are greater for
effluent samples having a higher original suspended solids content. A
speculative explanation for this shift is that coagulation of colloidal
13
-------
TABLE 2
Partial Coagulation Study Report
Evaluation of New Shipment of Purifloc C-31, and
Purifloc C-32 Polyelectrolytes with Cat-Floe
Effluent Sample 26151-T, pH 7.4
Temperature 26°C
200 rpm stir speed
Stir 10 min, Settle 5 min
Date: 9-12-68
TEST
A
1
2
3
4
5
6
S-51
mg/1
New
200
it
ti
ii
ii
ii
Poly
mg/1
Purifloc
3
3
3
3
2
2
Coag.
C-31
F
F-G
G
G
G
G
Sett.
P
P
F-P
F
F
F-G
Clar.
many fines
improved
ii
ii
JTU
15
10
12
12
%SV
<1
1
1
2
3
4
SVR
0
1
2
3
4
5
B
1
2
3
4
5
6
New Purifloc
200
3
3
3
3
2
2
C-32
F
F-G
G
G
G
G
P
P
F-P
F
F
F
many fines
improved
11
8
8
8
1
1
2
3
3
0
1
2
3
4
5
C-32 slightly better than C-31, also settling is
slightly improved
200
c
1
2
3
4
5
Cat-Floe
200
it
n
n
it
Cat-Floe
3
3
3
2
2
and
F-P
G
G
G
G
C-31 very
P
P
F
F
F
many
fines
improved
n
comparable.
C-32
12
9.5
12
better
<1
1
1
2
3
0
1
2
3
4
G-Good, F-Fair, P-Poor, %SV-percent of volume occupied by
slurry after 5 min settling, SVR-Slurry Volumes Returned,
to subject test from previous reactions.
14
-------
n—r
6C
4C
Effluent Sample 26204-T
Unfiltered COD = 125 mg/1
Suspended Solids = 65 mg/1
Filtered COD =25.4 mg/1
60 min contact
25°C
100 RPM
2$
WS
0
•H
a
a
O
•fi
•H
4J
a
'
© Darco S-51 with 3 mg/1 C-32
Darco S-51 without C-32
Control COD(C0):
With C-32, 20.8 mg/1
Without C-32, 25.4 mg/1
J i
J—L
8 10
15
20
30
40
Filtered COD Remaining, mg/1
FIGURE 4: Effect of polyelectrolyte on an adsorption isotherm
when the original sample contains a substantial
suspended solids concentration.
15
-------
T T
i—r
60
40
Effluent Sample 26197-T
Unfiltered COD =30.4 mg/1
Suspended Solids =6.0 mg/1
Filtered COD = 25.9 mg/1
60 min contact
25°C
100 RPM
20
00
c
•H
"O
CD
O
.-1
c
O
J3
M
03
CJ
0)
u
a)
>
^H
4J
U
10-
O Darco S-51 with 3 mg/1 C-32
Darco S-51 without C-32
Control COD (C0)
With C-32, 23.2 mg/1
Without C-32, 25.9 mg/1
JL
_L
J I
4 6 8 10
Filtered COD remaining, mg/1
15 20
30
40
FIGURE 5: Effect of polyelectrolyte on an adsorption
isotherm when the original sample contains
a low suspended solids concentration.
16
-------
organic solids not removed by filtration of the original clarifier
effluent in the polymer-dosed series enabled removal of some of this
material in the final filtration step of the test procedure. This would
not occur to the same extent in the control series. The resultant iso-
therm points plotted lower in both residual COD and carbon loading. If
adsorption had been adversely affected, these points would have plotted
at higher residual COD and lower carbon loading.
However, on the basis of this work, it was decided that control
isotherms should be developed using the same polymer dosage as in the
pilot plant to obtain the closest correlation between them and plant
performance.
Having now selected a suitable powdered activated carbon, Darco S-51,
and a coagulant, Dow C-32, operation of the pilot plant was initiated.
17
-------
PILOT PLANT PROGRAM
Apparatus and Procedure
A constant-rate pilot plant was constructed at the INFILCO Test
Facility adjacent to the Municipal Sewage Treatment Plant serving the
City of Tucson, Arizona.
The plant included two series-operated JBAS ACCELATORR clarifiers,
a type of recirculating-slurry, solids-contact unit which is the principal
component of a pre-engineered water treatment system used extensively in
the beverage industry. As depicted in Figure 6, the pilot plant schematic
flowsheet, the JBAS ACCELATOR clarifier contains an inlet-mixing chamber,
flocculators and a slurry recycle impeller on a common shaft, a slurry
concentrator, and a clarified water zone with effluent take off system.
Certain features of the standard JBAS unit were modified for this
application. A submerged-orifice peripheral launder was substituted for
the normal low-rate effluent collection system and the concentrator rim
was extended in the form of a sloped trough to enable extraction of solids
from the recirculating carbon slurry independent of the slurry-pool level.
Operation at throughput rates as high as 30,000 gpd was visualized,
resulting in an overflow rate of 1.6 gpm/ft2 of clarification area based
on the 12.9-ft2 cross-section of the annular clarification zone.
Each of the 730-gal JBAS clarifiers (4.5 ft diameter x 7.5 ft side-
sheet) included a recirculation zone volume of 225 gal. which provided
carbon contact times of 32, 16, and 11 min at operating rates of 10,000,
20,000, and 30,000 gpd, respectively.
In a solids-contact unit of this type, the throughput rate cannot
exceed the pumping capacity of the recirculation system if short-circuiting
to the solids separation zone is to be avoided. The estimated pumping
capacity of the slurry recirculation impeller in the pilot units was five
times the maximum operating rate. The recirculation flow also prevents
deposition of solids on the tank floor. Except for material isolated in
the slurry concentrator, all of the carbon inventory was maintained in
suspension during operation.
The pilot plant (Figure 7) was erected out-of-doors in an orientation
which differed only slightly from that shown in Figure 6. The feed
stream, pumped from the inner launder of one of the final clarifiers of
the adjacent activated sludge plant, was brought to the center front of
the layout through a rotameter after which it was split into two streams.
One of these operated a slurry-advance ejector and the other was throttled
as needed to set the combined flow to the inlet chamber of the first-stage
contactor-clarifier to the feed rate desired.
18
-------
Coagulant pumps
Rate set valves'
Flow indicator-
}
• -•
Carbon fee
/ —
-------
< I
o
FIGURE 7: Photograph of 30,000 gpd pilot plant,
-------
First-stage effluent flowed by gravity to the second-stage JBAS unit
and thence to a gravity sand filter for final treatment.
Carbon feed was countercurrent to the throughput flow. Fresh powdered
activated carbon was gravity fed by a volumetric slurry feeder to the
inlet chamber of the second-stage JBAS unit, along with a separate feed
of polymer flocculant. A second carbon feeder (the left-hand unit above
the ladder in Figure 7) was used during initial start-up to permit simul-
taneous development of slurry in both contactor-clarifiers and to provide
standby for the other feeder.
Flocculated carbon slurry recirculated within the second-stage JBAS
unit was permitted to accumulate to a manageable concentration which was
controlled by timer-actuated blowdown from the internal concentrator.
The Flexopulse repeating-cycle timer used afforded independent adjustment
of both the valve-open and valve-closed portions of the blowdown cycle
from one minute each up to a 2-hr total cycle time.
Used-carbon blowdown from this unit was collected in the right-hand
drum shown in pilot plant photograph and, together with a small flushing
stream of treated wastewater, transferred continuously to the inlet chamber
of the upstream contactor-clarifier by a Penberthy hydraulic ejector
operated with a portion of the feed stream. A secondary flocculant dosage
was also fed separately to this point.
Blowdown from the first-stage JBAS unit was to waste, except when
being measured. Both 55-gal. blowdown collection drums soon proved to be
too large and were replaced with 10-gal. polyethylene tanks.
Second-stage contactor-clarifier effluent could be divided so that
any portion or all of it could be wasted or flowed to a 2-ft square by
6-ft high sand filter. This gravity research unit, manufactured by
Filtration Equipment Corporation, was constructed of Transite and Lucite
with two opposing transparent sidewalls, allowing visual observation of
the contents. It included a FRE-FLO extruded asbestos-cement underdrain
system supporting 11 inches of graded gravel and 22 inches of filter
sand with a specified effective size between 0.45 and 0.55 mm and a maxi-
mum uniformity coefficient of 1.75.
A float-operated effluent control valve was added to this unit to
pace filtration with plant flows up to 12 gpm. During higher plant
operating rates, effluent from the second-stage JBAS unit was wasted to
limit the filtration rate to 12 gpm (3 gpm/ft^ of filter area). A trans-
parent plastic hose tapped into the underdrain and extended to the top
of the filter enabled loss-of-head measurement during filtration.
As a matter of convenience, locally available well water pumped
from a 2000-gal. storage tank was used for filter backwashing. The wash
rate was set manually while observing the extend of sand expansion and
computed from the duration of wash and drawdown in the storage tank.
21
-------
Sample taps were provided on each contactor-clarifier unit for slurry
withdrawal and determination of the slurry-clear water interface elevation.
A specially designed, multiple-dipper sampler to which streams of
plant feed, first- and second-stage contactor-clarifier effluents, and
filter effluent were conveyed continuously was used to obtain daily compo-
site samples collected at 1-min intervals over 24 hours. These samples
were accumulated in polyethylene containers refrigerated to a temperature
between 1° and 3°C.
The pilot plant program was conducted in six consecutive phases:
1. Brief operation of the two-stage system at 7 gpm and a carbon
dosage of 140 mg/1 for equipment shakedown and operator
training. This flow was selected as a practicable minimum
dictated by the water flow required to operate the slurry-
advance ejector plus a minimum manageable ejector bypass flow.
2. Limited operation at the same carbon dosage over a range of
flow rates to define the hydraulic capacity of the system.
3. Operation for ten event-free days at a 14-gpm flow selected
on the basis of data generated during Phase 2 and a carbon
dosage of 140 mg/1.
4. Operation over a similar period at the same flow and a 280-mg/l
carbon dosage.
5. Operation as in Phases 3 and 4 at a carbon dosage of 70 mg/1.
6. Operation of a single contactor-clarifier and the filter for
the same period at 14 gpm and a carbon dosage of 140 mg/1 to
evaluate performance of a single-stage system for comparison
with that of the two-stage system operated under closely
similar conditions.
During each of these runs, composite and grab samples were collected
for development of laboratory isotherms and analysis sufficient to define
process performance; i.e., pH, suspended solids, turbidity, COD, BODs,
and slurry and blowdown solids concentrations. Other data recorded com-
prised quantities and types of treatment chemicals; plant flow; rate and
volume of blowdown; slurry volume after five minutes of settling; slurry
level; filter head loss; and the filter backwash frequency, rate, and
duration. Records were also made of events which interfered with or
otherwise affected plant operation or performance.
Ten-day runs at each of the various operation conditions were
adopted to define mean system performance under the variable loading
imposed by the inconstant feed characteristics anticipated. And 24-hr
operation was undertaken to minimize the potential influences of biologi-
cal activity on BOD, COD, turbidity, and suspended solids.
22
-------
Experimental Results
Phase 1 - Initial operation of the pilot plant at 7 gpm for four
days provided an opportunity for equipment shakedown and operator train-
ing. After certain minor equipment repairs and revisions, this was
followed by five days of similar operation. Sampling and record keeping
procedures were initiated and analytical laboratory programming established
during this start-up period.
Performance of the carbon slurry feeders was excellent and no special
measures to wet the carbon were required.
It was established that powdered activated carbon slurry could be
produced in the contactor-clarifiers and controlled as desired. Operation
for 23 hours at a carbon feed of 140 mg/1 to the second-stage unit was
required for development of slurry to an extent such that blowdown and
carbon advance to the first-stage could be initiated. However, the single
4.7-gal. blowdown per hour produced at the minimum setting of the original
control system was deemed unsatisfactory and an adjustable 1-min resetting
timer triggered by the existing timer was added to the system. This per-
mitted limitation of blowdown duration to a fraction of a minute while
retaining the primary timer's flexibility for determining the interval
between blowdowns and reduced the unit discharge volume to 0.85 gal.
Slurry concentration and its control were two important additional
operational factors receiving attention during this program. When acti-
vated carbon is loaded with adsorbate to equilibrium, further removal is
impossible. Indeed, should the system equilibrium change, desorption may
occur. In a practical sense, the equilibrium loading is attained within
a relatively short time, but to blow down the carbon after this time
would result in an unmanageably low solids inventory. It was therefore
necessary to operate with slurry concentrations that were manageable
physically without regard to sludge age.
In this connection, brief comment on operation of ACCELATOR units
is pertinent. Recirculating slurry discharges as a moving stream into
the lower portion of the clarification region. The upper surface of the
slurry stream is the solids-liquid separation interface. Clarified liquid
rises to the effluent collection system, while solids carried in the
recirculating stream are drawn downward and re-enter the mixing and
flocculation zones. If the volume occupied by the slurry exceeds the
available volume, the interface rises into the clarification zone and a
so-called slurry pool forms above the streaming slurry. Treated liquid
escapes by filtering up through the slurry pool and slurry- or sludge-
blanket operation results. Solids in the blanket are non-uniform in
size and are not readily available for recovery and reuse by recirculation.
To establish the limiting slurry solids relationship which avoids
sludge-blanket operation at a particular installation, the unit is
23
-------
initially operated with little or no blowdown and the increase in solids
inventory is conveniently monitored by a simple 5-min slurry settling
test. When a limit is reached beyond which a slurry pool forms, the per
cent floe volume after five minutes of settling is noted and used as a
continuing operating datum.
Early operation of the pilot plant demonstrated superior settling
characteristics for the second-stage slurry and revealed that sludge-
blanket operation was not encountered in the first-stage unit until the
5-min settled slurry volume reached 30%. To provide margin for the in-
fluence of variable floe quality, to permit operation at greater flow
rates, and for operator convenience, an operating range of 10-15% was
selected for both contactors.
The relationship between the suspended solids content and 5-min
settled volume of draft-tube slurry samples collected at intervals through-
out the pilot plant operating period is shown in Figure 8. Except for a
few occasions when feed suspended solids (activated sludge carry-over)
were abnormally high, good correlation was noted. The second-stage unit
was affected only once when slurry with an appreciable content of activated
sludge was not wasted rapidly enough from the first-stage contactor and
carry-over resulted.
Filter sand was adequately cleaned by backwashing. However, inasmuch
as the pilot plant feed was unchlorinated sewage plant effluent, it was
not unexpected to note development of a rather heavy growth of algae and
slime in the filter underdrain, supporting gravel, and on the Lucite
sidewalls. Following the initial 4-day run, cover plates extending from
the sand-gravel interface to the filter floor were installed over the
Lucite to exclude light and thus inhibit algae growth. This was not com-
pletely effective, however, and growth of algae and slime below the sand
was to influence filter effluent quality throughout the study.
Phase 2 - Performance of the pilot plant at different flow rates
was studied over a 10-day period. Two 24-hr sampling runs at 7 gpm were
followed by four days at 10.5 gpm, two days at 14 gpm, and two days at
20 gpm. Carbon feed to the second-stage contactor was 136 mg/1 and 3 mg/1
of polyelectrolyte were fed to each unit, except for a brief period when
a feeder recharging error raised it to 4.3 mg/1.
The analytical data developed during this phase (Table 3) and the COD
results graphed in Figure 9 show that while over-all plant performance
was affected by variations in feed quality, any influence due to increased
flow rate is not evident-.
On the final day of the 20-gpm run, settling rates of 8.4 and 13.0
in./min were observed for draft-tube samples from the first- and second-
stage contactors, respectively. The equivalent overflow rates of 5.2 and
8.1 gpm/ft^ are far above conventional clarifier loadings and several
24
-------
20,
IS-
g3 161-
c
•H
a
i
M
3
i-H
C/5
0)
H
14
12
10
A Clarif ier //I, normal feed solids
0 Clarifier #2, " "
0 Single-clarifier run, normal feed solids
Solid Points:High feed solids (Activated Sludge)
1 1 1 1 I |_
_L
_L
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Suspended Solids, Weight %
FIGURE 8: Relationship between draft tube slurry 5 min
settled volume and suspended solids content.
25
-------
TABLE 3
Phase 2, 24-hour Composite Sample Analyses
DATE - 1968
Rate of Flow, gpm
Feed Stream
pH
Turb . , JTU
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #1 Eff.
pH
Turb . , JTU
Sus. Solids, mg/1
N COD filtered, mg/1
ot BOD, mg/1
Clarifier #2 Eff.
PH
Turb. , JTU
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Sand filter Eff.
PH
Turb . , JTU
Sus. Solids, mg/1
COD unfilt, mg/1
COD filtered, mg/1
BOD, mg/1
COD Red'n, %
BOD Red'n, %
Carbon loading %
10-6,7
7.0
7.7
11
10
25.0
10.2
7.8
16
17
13.4
4.7
7.9
4
6
5.4
4.0
7.9
1
4
6.3
5.8
—
78
61
14.0
10-7,8
7.0
7.5
17
12
26.7
—
7.7
14
11
12.7
-
7.7
4
4
6.7
—
7.7
2
2
7.6
6.2
-
77
-
14.6
10-8,9
10.5
7,5
12
13
25.8
8.0
7.7
12
18
14.3
4.7
7.8
3
7
8.5
2.4
7.8
2
2
9.0
8.5
2.8
67
66
12.3
10-10,11
10.5
7.8
22
20
27.2
19.0
7.9
13
16
16.6
7.9
7.9
2
3
8.8
4.0
7.9
2
2
9.6
8.8
2.3
68
88
13.1
10-11,12
10.5
7.6
14
11
24.5
10.1
7.7
9
10
13.6
3.6
7.8
2
3
5.2
3.4
7.8
2
2
6.6
5.2
2.2
79
78
13.8
10-12,13
14.0
8.0
12
8
28.2
8.4
8.0
5
6
16.2
5.2
8.0
2
2
7.9
2.6
8.0
3
2
8.8
8.3
2.6
72
69
14.5
10-13,14
14.0
8.0
4
5
24.6
9.1
8.0
2
4
14.7
5.5
8.0
1
2
7.5
2.6
7.9
2
1
7.9
7.5
3.5
70
72
12.2
10-14,15
20.0
8.0
25,
20
33.6
-
8.1
8
15
17.0
-
8.1
4
5
8.3
-
8.1
3
3
10.1
8.3
-
75
—
18.1
10-15,16
20.0
8.1
13
15
24.6
16.1
8.1
15
27
13.8
8.3
8.2
2
4
6.9
5.4
8.1
2
4
7.4
6.9
5.2
72
68
12.6
-------
DO
e
a
o
"O
0)
Clarifier #1 Effluent
Clarifier #2 Effluent
15r
10-
Sand Filter Effluent
Sand Filter Effluent Refiltered Through 0.45 pad
I I I I I I i I
2or
6
0.
oo
§
M-4
O
0)
4-1
15
10-
11 8-hr flow rate error, samples discarded
_L
_L
_L
Consecutive 24-hr Composite Sampling Periods
FIGURE 9: Filtered COD versus plant operating rate, Phase 2.
27
-------
times those of high-rate water treatment units. Understandably, the pilot
plant operating at 20 gpm (overflow rate 1.6 gpm/ft ) could contain the
carbon slurry even during periods when it was substantially degraded by
a large content of activated sludge solids.
The influence of solids carry-over from the sewage plant on opera-
tion during this phase is shown in Figure 10. Control of the second-stage
solids inventory was not difficult; but the first-stage contactor was more
sensitive, as expected.
Filtered COD removal through the pilot plant for the 10-day period
averaged 73 per cent. A COD loading of 19.4 mg on 136 mg of carbon was
attained, or 14.3 per cent by weight. Limited isotherm data generated
during this program indicated that achievement of this much COD reduction
by single-stage treatment would require about 750 mg/1 of carbon and load-
ing would drop to 3.5 per cent by weight.
Filter operating records (Figure 11) revealed progressively shorter
runs as the rate increased, as well as production of less finished water
per run as the pilot plant operating rate increased. Several events pro-
ducing significant head loss deviations are indicated as having been
operator induced. Other deviations of from 1-4 inches are attributed to
mechanical friction in the rate controller which did not provide precise
modulation.
A total of 172,000 gal. of treated wastewater was filtered during
five filter runs, the first of which started prior to initiation of this
operating phase. The rate and duration of backwash were determined by
observing sand expansion and cleaning effectiveness. An expansion of 8
inches or 36 per cent was achieved at wash rates of from 18 to 25 gpm/ft ,
depending on wash water temperature. A carbon penetration of several
inches below the sand surface was observed at times and the top inch of
sand retained a black cast throughout the study. Carbon floe was occasionally
observed migrating in the bed during backwash; but this appeared to break
up satisfactorily. Backwash duration evolved from an initial and inade-
quate five minutes to 10 minutes with manual surface agitation and freeboard
brushing. Wash water consumption was less than two per cent of the filtered
water volume.
The turbidity and suspended solids content of filter effluent are not
readily evaluated. The visual appearance of filter influent and effluent
was markedly dissimilar. The influent had a characteristic black cast,
with small carbon floe visible on occasion. Effluent was clear, but the
slight residue after Millipore filtration was brown or green rather than
black, evidently a result of biological activity in the lower portions of
the filter.
Phase 3 - This first constant-rate study followed the preceding program
without interruption. Without shutting down the pilot plant, chemical
28
-------
60
50
Clarifier #1, 10 day ave = 16.2 Vol %/5 min
§ 40
30
20
10
£ o
Act. Sludge in feed
CO
t)
50
Clarifier #2, 10 day ave = 13.4 Vol %/5 min
30
3
H
« 20
10
_L
_L
7 gpm
45
Elapsed Time,
Days
10.5 gpm
14 gpm —*- 20 gpm"
Flow Rate
FIGURE 10: Settled slurry volume versus pilot plant throughput
rate, Phase 2.
10
29
-------
00
0)
u
01
O
I
tn
0)
o
-o
c
CO
co
60
50-
40-
30-
20
10
Elapsed Time, Days
Performance Data
Run
Duration, hr
Filter rate, gpm
o
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
Clarifier overflow rate,
gpm/ft
10
1st 2nd 3rd 4th 5th
91* 71 25+28 20+18 28
7 10.5 10.5/14 14/12 12
1.75 2.63 2.63/3.50 3.50/3.0 3.0
38,000 44,700 39,300 29,800 20,200
7 10.5 10.5/14 14/20 20
0.55 0.74 0.74/1.1 1.1/1.6 1.6
*Run #1 was in progress at time zero.
FIGURE 11: Gravity Sand Filter Operating Data, Phase 2.
30
-------
feeders were reset and recharged and flow was set at 14 gpm, a rate
selected to avoid hydraulic gradient limitations encountered within the
pilot plant at the maximum operating rate of the previous study.
A target carbon dosage to the second-stage contactor of 140 mg/1,
chosen as a medium dosage level, was tested first in the constant-flow
series of programs because the plant was operating at this dosage and no
transition period would be required. Calculated from carbon consumption
over the 10-day run, the actual dosage was 146 mg/1.
Initially, 2.5 mg/1 of C-32 polyelectrolyte was fed to the inlet
chamber of each contactor and this concentration of coagulant was fed to
the second-stage unit throughout the run. Because of a noticeable in-
crease in effluent turbidity and suspended solids from the first-stage
contactor during the first day, C-32 feed to this unit was then increased
to 3.0 mg/1 and, although not particularly effective, maintained at this
level until the last day when it was again increased to 3.5 mg/1 as a re-
sult of continuing slurry deflocculation.
No unusual events were encountered during the 10-day study. Operators
were able to control blowdown from each contactor to maintain reasonable
draft-tube slurry conditions. There was some time lag in the blowdown
system which produced cyclic increases and decreases in results of the
5-min settled volume test, but averages for the run were close to the
target value (Figure 12).
A
Filter operation (Figure 13) at 3 gpm/ft* was good with runs in excess
of 60 hours when the filter was not backwashed prematurely. Occasional
dips in loss-of-head vs. time curves were not fully investigated. These
frequently occured within the first four hours of a sampling period.
Operator maintenance such as a 15-min flow interruption to clean the rate
of flow indicator or brushing algae from launders or the filter freeboard
may have contributed. Characteristically during this period, wastewater
temperature rose 4-6°F and a change in clarifier performance may have been
occuring. Whatever the causes, a collapse of and slight penetration of
material on the filter surface appeared to be occuring periodically.
Analytical data (Table 4) for this run show that uniform performance
was characteristic. Of interest is the decided increase in first-stage
contactor effluent turbidity and suspended solids over the feed levels.
This was counteracted in the second-stage contactor which delivered an
effluent of uniformly low turbidity and suspended solids. For the first
five days of the run, filter effluent solids were undesirably high. Follow-
ing a filter backwashing, suspended solids decreased for the balance of
the run. Filter effluent BOD shows a similar trend. Prior to the mid-
period backwash, final BOD exceeded filter influent BOD, whereas after
this backwash the reverse was true.
BOD reduction through the system averaged 69 per cent and mean COD
reduction was 72 per cent. The cumulative frequency distribution curves
31
-------
60
50j_
o
>
to
•o
o
en
Clarifier #1, 10 day ave =14.2 Vol %/5 min
Target value, 15 Vol %/5 min
4-
4-
Clarifier #2, 10 day ave = 15.2 Vol %/5 min
Target value, 15 Vol %/5 min
50-
3
rH
to
0)
H
j-1
IH
to
40-
3C -
20.
CM
I
u
bO
e
0
10
Elapsed Time, Days
FIGURE 12: Settled slurry volume, Phase 3.
32
-------
t/3
Elapsed Time, Days
Performance Data
Run
Duration, hr
Filter rate, gpm
2
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
ry
Clarifier overflow rate, gpm/ft
#1
49
12
3.0
36,300
14
1.1
#2
63
12
3.0
45,300
14
1.1
#3
60
12
3.0
43,200
14
1.1
#4
72
12
3.0
51,800
14
1.1
Total wash water used, 2160 gal = 1.2% at ave. rate of 20.7 gprn/ft"
FIGURE 13: Gravity Sand Filer Operating Data,
Phase 3.
33
-------
TABLE 4
Phase 3, 24-hour Composite Sample Analyses
CO
DATE - 1968
Feed stream
pH
Turb., JTU
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #1 Eff.
pH
r"
Turb . , JTU
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #2 Eff.
pH
Turb . , JTU
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Sand filter Eff.
pH
Turb., JTU
Sus. Solids, mg/1
COD unfilt, mg/1
COD filtered, mg/1
BOD, mg/1
COD Red'n, %*
BOD Red'n, %*
Carbon loading %
10-16
7.
8
13
24.
12.
7.
19
25
14.
8.
7.
3
4
7.
3.
7.
2
3
9.
7.
7 17
7
6
7
8
7
8
8
7
6
8
,1
,1
4.6
69
72
11.6
17.18
7.9
9
7
24.0
11.2
8.1
23
22
13.0
10.9
8.1
2
3
7.4
5.4
8.0
2
3
8.2
7.4
6.1
69
52
11.4
18.19
7.7
14
11
25.6
8.6
7.8
26
21
14.6
5.6
7.9
3
5
6.4
3.3
7.9
2
2
7.7
6.4
4.5
75
62
13.2
19.20
8.0
7
8
28.5
5.5
8.1
22
32
14.4
3.3
8.1
5
7
9.9
2.2
8.1
2
4
10.7
10.4
1.8
65
67
12.8
20.21
7.9
8
9
25.4
10.2
8.0
28
40
16.3
8.2
8.1
4
5
9.1
2.8
8.1
1
2
9.9
8.6
2.3
66
77
11.5
21.22
7.9
8
8
28.8
8.0
30
44
15.1
7.9
4
5
7.8
7.9
2
1
9.5
7.8
—
73
^
14.4
22.23
7.9
9
11
26.1
14.0
8.0
27
34
13.6
10.0
7.9
2
3
7.3
7.0
7.9
1
1
7.8
6.8
2.2
74
o /
84
•i *\ 1
13.3
23.24
7.7
9
6
25.6
12.5
7f\
.9
30
34
12.5
8.4
7f\
.9
2
2
5.7
6.5
7rt
. 9
2
1
7.3
5.7
2.6
78
"7 fl
79
ID ~J
13. 7
24.25
7.9
9
7
26.6
10.9
8rt
.0
28
31
13.6
10.2
8r\
.1)
3
3
7.3
4.5
70
. y
2
2
7.8
6.9
3.2
74
T 1 £
1J. 0
or,^ f -M f-
25.26
7.9
9
6
27.2
9.5
8f\
.0
27
27
13.1
7.1
8/1
. U
2
3
6.8
4.2
7Q
. y
2
1
7.2
6.3
2-*
. 7
77
70
/z
Uo
. J
-------
of Figure 14 show the COD of samples collected during the run from various
points throughout the treatment system.
These COD data are also portrayed in the adsorption isotherm analysis
of Figure 15, the construction of which requires explanation.
The 24-hr feed samples varied in COD (C0) and individual isotherms
varied significantly in slope (1/n) and carbon loading (X/M) achieved at
equilibrium. Taken separately, they are unsatisfactory for evaluation
of mean pilot plant performance.
Individual Freundlich isotherm data were accumulated throughout the
pilot plant operating period, after which all were transferred to a
composite data array (Figure 16). The points on this array were obtained
by selecting a specific COD remaining value (Ct), reading the carbon load-
ing (X/M) achieved at this Ct on each individual isotherm curve, calcula-
ting the mg/1 activated carbon dosage indicated by each, and then plotting
the dosage against the respective Co values. A curve approximating the
mean carbon dosage versus Co for that Ct was then drawn through the points.
This procedure was repeated for six Ct levels. The array of points dis-
closes the magnitude of variation in raw data encountered and emphasizes
the impracticality of attempting to evaluate a pilot plant run directly
with individual isotherms.
The Ct curves (Figure 16) determine six points for which loading (X/M)
versus COD remaining (Ct) can be derived and plotted as an isotherm for
any Co value. While this procedure is not absolute with respect to
accuracy it provides a basis for comparative run evaluation common to all
pilot plant studies. The performance of one study at a specific mean Co
can be evaluated against one or more other studies at their respective
mean Co conditions.
The mean-value single-stage isotherm for the Phase 3 study (Figure 15)
was derived in this fashion, as were others for the balance of the field
studies. From the slope and percentage of COD remaining at various points
on the single-stage isotherm, a two-stage countercurrent isotherm was
developed and added to the graph. Since published information for two-
stage countercurrent systems did not cover the entire range of adsorbate-
remaining values of interest , available data were extrapolated graphically.
The isotherm analysis graphs also include mean performance paths
which were calculated and plotted from the mean Co value and actual acti-
vated carbon feed utilized, and in the case of two clarifiers operated in
series, for the intermediate COD value (Ci) and the carbon feed. The
intermediate COD being the filtered COD in the effluent of the upstream
clarifier which is the feed to the downstream clarifier. For any given
feed COD and activated carbon feed, the carbon loading achieved when
plotted against COD remaining must fall on the performance path.
35
-------
40
30
20
10
9
So 8
? 7
6
§
0)
Feed Stream
Clarifier #1 Effluent
Clarifier #2 Effluent
•H
1*4
10
9
8
7
6
Sand Filter Effluent
I I
10
20 30 40 50 60 70 80
90
95
Percent of Observations Equal to or Less Than Stated Value
FIGURE 14: Composite-sample COD, Phase 3.
36
-------
-i—[—r
C
•H
T5
CO
O
C!
O
,£)
VI
rt
o
4-1
O
60
40 -
20
Total Pilot Plant
mean performance
Mean Performance Paths
for C
o
26.2 & M = 146
10
for
14.1 & M = 146
Clarifier #1, unit
mean performance
Clarifier #2, unit
mean performance
Daily Unit Performance
0 = 26.2 mg/1 COD
= 14.1 mg/1 COD
M = 146 mg/1 act. carbon
J I L
_L
_L
_l_
4 6 8 10 15
Filtered COD Remaining, mg/1
20
30
40
FIGURE 15: Adsorption Isotherm Analysis, Phase 3.
37
-------
3000
2000 -
1000 -
IP
0)
00
(0
0)
o
a
o
JO
n
(0
o
•o
4)
4J
O
0)
T3
500 -
I 1 1 1 1 I I I
100 -
21
22 23 24 25 26 27 28 29 30
Pilot Plant Feed Stream Filtered COD, mg/1
FIGURE 16: Single-stage adsorption isotherm data array.
38
-------
The performance paths intersect the isotherms and the intersections
can be considered predicted performance points for true s^gle-stage or
two-stage countercurrent processes.
A box labeled "applied load" appears on each analysis curve. The
ratio of applied COD to carbon feed for each day of operation was plotted.
The box encompasses these points and contributes some understanding of
the limitation on the magnitude of carbon loadings achieved.
Figure 15 includes the daily unit and total system performances
attained. These points deviate slightly from the mean performance paths
because they are calculated from daily Co and Ci values. Diamond-shaped
points designate mean unit and total performance for specific runs.
Observed mean and computed system performances were essentially
equal. COD loading on the carbon was 12.9 per cent by weight as compared
to 9.6 per cent predicted for single-stage adsorption.
First-stage contactor effluent, produced with carbon already partially
loaded with COD from the second-stage unit, was slightly inferior to that
predicted for a single-stage process using new carbon. Carbon loading
added in this unit was 8.3 per cent by weight as compared to 9.6 per cent
predicted for use of new carbon.
The second-stage unit, operating with a feed COD (Ci) of 14.1 mg/1,
loaded the new carbon to 4.5 per cent of its weight. A single-stage
loading was not predicted for this unit as insufficient isotherms were
generated to permit development of a mean-value curve. Performance in
this stage was sufficiently good to compensate for the first-stage defi-
ciency and bring the total system efficiency up to that predicted for
two-stage countercurrent adsorption.
Phase 4 - System performance at the same flow rate and a carbon
target dosage of 280 mg/1, double the previous concentration, was deter-
mined during this 11-day program. Calculated from carbon consumption over
this period, the actual dosage was 266 mg/1, excluding a period of six
hours near the end of the run when the feeder clogged.
Polyelectrolyte feed to both units was increased to flocculate the
higher carbon dosage. A dosage of 5.0 mg/1 to the first-stage contactor
was maintained throughout the run. The initial 4.0 mg/1 fed to the second-
stage unit was increased first to 4.5 mg/1 on the second day and then to
5.0 mg/1 on the fifth day to cope with carryover. During the last day of
the run, the point of application for each unit was changed from the inlet
chamber to the bottom of the inner draft tube to overcome a head-loss pro-
blem occasioned by clogging of the 2-in. transfer lines between the inlet
chambers and the draft tubes.
39
-------
Draft-tube slurry control was excellent for Contactor #2 but vari-
able for Unit #1 (Figure 17). The solids content of system blowdown of
19 per cent by weight made blowdown management difficult and this problem
persisted throughout the run. Blowdown volume was on the order of 0.1
per cent of the throughput.
In spite of the decreased suspended solids carryover to the filter
which characterized this run, filter runs shortened significantly
(Figure 18) and wash water consumption increased to 2.1 per cent.
Analytical results for the run (Table 5) are consistent with the
exception of two occurences. When polyelectrolyte feed to Unit #2 was
increased at the beginning of the fifth sampling period there was a slight
reduction in the turbidity and suspended solids content of final effluent
and the filter began discharging BOD at concentrations above those in its
influent.
The COD of samples collected during the run from various points in
the treatment system is presented in Figure 19. Mean COD reduction
through the plant was 84 per cent and BOD reduction, disregarding the
filter problem, was 78 per cent.
The isotherm analysis for this run (Figure 20) indicates that plant
performance exceeded two-stage countercurrent adsorption. The first-
stage unit very nearly equalled single-stage new carbon efficiency.by
picking up 6.6 per cent more COD loading on carbon already loaded to 2.1
per cent in Unit #2. COD equivalent to an additional 0.1 per cent load
was removed in the filter for a mean total loading of 8.8 per cent.
The predicted carbon requirement to achieve 84 per cent COD reduction
by two-stage countercurrent adsorption is 360 mg/1, or 34 per cent more
than was actually used.
Phase 5 - The final series study for 11 days at 14 gpm using two
JBAS clarifiers with countercurrent carbon advance was initiated without
plant shutdown. This investigation, using a low-level carbon dosage of
67 mg/1 (target dosage: 70 mg/1 or 25 per cent of the high-level concen-
tration) and an appropriate reduction in polyelectrolyte, was subject to
numerous difficulties.
Initial polyelectrolyte feed rates were inadequate and were increased
several times (Figure 21). High solids carryover and poor floe formation
are thought to be the result of the reduced ratio of activated carbon to
the normal organic solids input. The existing carbon slurry inventory in
the contactors which was brought forward to this study took 48 hours to
reach equilibrium at the new conditions. This transition was accelerated
on the third sampling day by reducing draft-tube slurry concentrations to
the target range of 10 vol Z/5 min (Figure 21). With the much lower rate
of slurry production, the concentration was dropped too far in Unit #1
40
-------
60
T
T
T
t>0
G
-------
10
11
Elapsed time, days
Performance Data
Run
Duration, hr
Filter rate, gpm
f\
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
f
Clarifier overflow rate, gpm/ft'
#1
42
12
3.0
#2
39
12
3.0
#3
40
12
3.0
#4
38
12
3.0
#6
33
12
3.0
#7
52
12
3.0
30,200 28,100 28,800 27,400 23,800 37,400
14 14 14 14 14 14
1.1
1.1
1.1
1.1
1.1
1.1
Total wash water used, 3690 gal or 2.1% at ave. rate of 22 gpm/ft
Note: Incomplete Run #5 ommitted from data table
FIGURE 18: Gravity Sand Filter Operating Data, Phase 4
42
-------
TABLE 5
Phase 4. 24-hour Composite Sample Analyses
DATE - 1968 10-26,27
Feed stream
pH
Turb., JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #1 Eff.
pH
Turb . , JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #2 Eff.
pH
Turb., JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Sand filter Eff.
pH
Turb . , JTU*
Sus. Solids, mg/1
COD unfilt. , mg/1
COD filtered, mg/1
BOD, mg/1
COD Red'n, %**
BOD Red'n, %**
Carbon loading %
7.9
10
10
28.5
10.6
8.0
15
12
10.9
9.9
8.0
5
3
4.2
6.9
7.9
2
3
5.7
4.2
3.3
85
69
9.1
27,28
7.8
9
6
23.1.
10.6
7.9
9
7
9.4
9.6
7.9
4
3
3.1
2.8
7.9
2
2
4.7
3.1
4.5
87
74
7.6
28,29
8.0
12
9
29.6
-
8.0
12
9
9.4
-
8.0
6
4
3.6
-
7.9
4
2
5.2
3.1
-
90
-
10.0
29.30
7.9
11
11
28.0
10.8
7.9
9
12
9.8
7.5
7.9
4
3
4.2
2.5
7.8
3
2
5.7
3.6
4.8
87
77
9.2
30.31*
7.9
10/3.2
7
27.5
13.3
7.9
9/1.2
5
9.8
7.6
7.8
3/0.5
2
4.7
1.4
7.8
2/0.4
2
5.7
4.2
5.4
85
90
8.7
31,11-1
8.0
11/3.1
7
25.4
12.3
8.0
7/1.1
11
9.4
8.1
8.0
4/0.7
4
4.7
1.6
7.9
3/0.35
2
5.2
4.7
6.3
82
87
7.8
1,2
8.0
10/2.4
11
31.2
11.4
8.1
8/0.66
7
10.4
6.6
8.0
4/0.62
4
5.2
1.8
8.0
2/0.10
1
5.7
4.7
7.2
85
84
10.0
2.3
7.8
8/2.3
5
29.3
6.4
7.9
10/1.2
7
11.4
4.3
7.9
4/0.67
3
5.7
1.5
7.8
2/0.58
2
6.1
5.7
4.0
81
77
8.8
3,4
7.8
8/2.3
7
26.4
9.1
7.9
7/1.4
8
9.8
5.4
7.9
4/0.68
4
5.7
1.7
7.8
1/0.65
1
6.6
5.2
5.4
80
81
8.0
4,5
7.8
9/2.6
7
29.6
7.9
13/1.1
13
11.3
"
7f\
.9
4/0.67
3
5.2
7O
.8
1/0.28
2
6.1
4.7
*~
84
~
9.4
j j"
5,6
7f\
. 9
8/2
12
26.4
11.7
8rt
. 0
9/1
15
10.8
6.1
7O
. 9
3/0.
4
5.2
1.6
7Q
. O
2/0.
3
5.7
4.7
6.1
82
O £L
86
8i~i
. 2
.2
.1
45
60
**Computed from lowest effluent value attained, disregards increases if any occuring in sand filter.
-------
50
20-
10-
9
8
7
6
Q
8
•o
-------
60
40
20
bO
C
i 10
o
G
O
O
TJ
0)
JJ
ta
•H
jj
o
"T 1 1 T
Total Pilot Plant
mean performance
Mean Performance Paths
For C0 = 27.
& M =266
For C-L =
& M =
Clarifier #2, unit
mean performance
Clarifier #1, unit
mean performance
© -f X Daily unit performances
27.8 mg/1 COD
10.2 mg/1 COD
266 mg/1 act. carbon
' i 1
J_
4 6 8 10 15
Filtered COD Remaining, mg/1
20
30 40
FIGURE 20: Adsorption Isotherm Analysis, Phase 4,
45
-------
60
50
30 -
20
10
Clarifier #1, 10 day ave = 11.7%
Target, 15%|Target, 10 Vol %/5 min
Ave, 14.8% lAve, 5.9 Vol %/5 min
I
I
o
oo
B
(M
CO
CJ
ro
u
o
Target
Ave, N
10%
.D.
Act. Sludge in Feed
**
eg
u
Q
50
40
30
20
10
Clarifier #2, 10 day ave = 11.5%
Target, 15%(Target, 10 Vol %/5 min
. M ^m t A n T TT_1 V / C _J_
Ave, 13.7% IAve, 8.7 Vol %/5 min
10
11
Elapsed Time, Days
FIGURE 21: Settled slurry volume, Phase 5.
46
-------
and took nearly two days to restore. The blowdown discharge pipes of
both units were equipped with 5/16" drilled orifices to further reduce
the volume per blowdown and permit more frequent cycling.
On the sixth sampling day activated sludge appeared in the feed
stream for six hours, shooting the draft tube slurry in Unit #1 up to
92 vol %/5 min in 4.5 hours elapsed time. The unit was filling _up with
this poor floe and if the flow had not been cut to 7 gpm to reduce the
solids input, slurry would have decanted to the second-stage unit. Ex-
tensive manually triggered blowdowns wasted much of this sludge volume
and two hours after the feed stream cleared, the flow rate was restored
to 14 gpm.
The seventh sampling period was uneventful with plant operation sub-
stantially restored to normal.
The eighth day started with a mechanical failure of the carbon feeder
supplying Unit #2 which shut down the pilot plant. Start-up on the ninth
day coincided with the reappearance of activated sludge in the feed
stream which continued for nine hours. Within 2.5 hours, the poorly con-
ditioned floe in Unit #1 had lost its characteristic black color due to
dilution by the organic solids and started decanting into Unit #2. Unit
#1 draft-tube slurry went up to 95 vol %/5 min and Unit #2 increased to
94 vol %/5 min.
Overflow of slurry continued until the flow rate was reduced to 7 gpm
(for 5 hours) just one hour before the feed stream cleared. The slurry
immediately settled down in Unit #2 but it required 1.5 hours of heavy
blowdown to lower the slurry in Unit #2 at this flow rate. No draft tube
slurry escaped from Unit #2 to the sand filter. Considering that all
activated sludge solids carried over to Unit #2 had to be removed from
the system by blowdown to Unit #1 (to maintain the carbon feed to Unit //I)
and then be wasted from here, it is understandable that recovery of the
system took 14 hours from the time the feed stream cleared. Recovery
was not seriously affected by another short slug of sludge received on
the tenth sample day.
The pilot plant was operated through one more 24-hr sampling period
to compensate for the day lost by mechanical problems.
Sand filter performance (Figure 22) during this period was poor. Runs
were short and wash water consumption jumped to 4.9 per cent.
Analytical data for the run (Table 6) reveal that the reduction in
polyelectrolyte feed was accompanied by deterioration of the effluents
of both clarifiers. The reduced coagulant dosage was inadequate to main-
tain the condition of floe brought forward from the preceding run, as
well as the 67 mg/1 of new carbon feed. Improvement was noted after the
dosage was increased to 4.0 mg/1 to Unit #1 on the sixth day. Unit #2
coagulant dosage was 3.0 mg/1 at this time and this rate was maintained
throughout the balance of the run.
47
-------
456
Elapsed time, days
Performance Data
Run
Duration, hr
Filter rate, gpm
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
Clarifier overflow rate, gpm/ft
Run
Duration, hr
Filter rate, gpm
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
Clarifier overflow rate, gpm/ft
#5
30
12
3.0
21,600
14
1.1
#7
35
12
3.0
25,200
14
1.1
#8
20
12/7
3.0/1.75
13,500
14/7
1.1/0.55
#9
24
12
3.0
17,300
14
1.1
#10
21
12/7
3.0/1.75
13,600
14/7
15
12/7
3.0/1.75
10,500
14/7
1.1/0.55 1.1/0.55
14
12
3.0
10,100
14
1.1
16
12
3.0
11,600
14
1.1
Total wash water used, 6070 gal or 4.9% at ave. rate of 20.5 gpm/ft'
Note: Incomplete Runs #1 and #6 omitted from data table.
FIGURE 22: Gravity Sand Filter Operating Data, Phase 5.
48
-------
TABLE 6
Phase 5, 24-hour Composite Sample Analyses
DATE - 1968
Feed stream
pH
Turb., JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #1 Eff.
pH
Turb., JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #2 Eff.
pH
Turb . , JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Sand filter Eff.
pH
Turb . , JTU*
Sus. Solids, mg/1
COD unfilt. , mg/1
COD filtered, mg/1
BOD, mg/1
COD Red'n, %**
BOD Red'n, %**
Carbon loading %
11-6,7
7.7
10/2.8
9
24.8
12.3
7.8
34/2.5
23
12.2
9.8
7.8
5/1.1
6
6.8
3.0
7.8
2/0.71
3
7.7
5.9
4.4
76
76
28.2
7,8
7.7
10/3.0
9
25.9
10.1
7.8
50/2.8
51
14.6
8.0
7.8
6/0.86
5
10.0
3.2
7.8
3/0.81
2
10.5
9.1
3.9
65
68
25.1
8,9
7.7
9/2.5
14
25.9
9.7
7.8
40/2.6
53
16.3
8.1
7.8
5/1.0
11
10.5
2.5
7.8
3/0.76
6
10.9
10.0
4.6
61
74
23.7
9,10
7.7
9/2.4
8
26.4
8.2
7.8
43/3.0
29
18.7
6.4
7.8
6/1.0
7
12.5
3.6
7.8
2/0.76
3
13.0
12.1
4.8
54
56
21.3
10,11
7.7
8/2.0
7
24.2
11.5
7.8
38/2.7
40
16.5
9.5
7.8
4/0.90
5
10.7
2.3
7.8
2/0.75
2
11.2
10.3
4.9
57
80
20.8
11,12
7.7
70/23
69
27.7
~~
7.8
13/3.1
13
17.9
~
7.8
5/1.0
7
11.2
_
7.8
2/0.74
4
12.1
10.7
—
61
25.4
12,13
7.7
8/2.5
6
25.6
13.3
7.8
17/2.8
24
19.0
10.3
7.8
3/0.90
5
12.0
2.1
7.8
2/0.82
4
12.4
11.0
6.5
57
84
21.8
14,15
7. 7
95/29
72
28.6
30
7.8
115/21
131
17.7
23
7.8
10/2.3
11
10.8
5.7
7.8
6/2.0
5
15.0
9.6
6.9
66
•~
28.4
15,16
7»*
.7
35/12
25
27.4
28.4
7.8
34/5.7
28
16.6
16.3
7O
.8
11/1.5
6
9.6
2.1
7.8
2/1.6
4
14.1
9.1
2.3
67
n n
93
ri /* f\
26.2
16,17
7*7
. 7
10/3.5
8
29.0
18.4
7rt
.8
11/2.2
12
21.6
14.5
70
. 0
31/5.4
26
13.6
3.1
7O
. 8
3/1.0
4
14.0
13.6
5.4
53
O *5
oJ
f\ n f\
22. 0
" JbJ.CLL.il liUUC J_ &.HJ\S A. UJ. LJ J-UJL.1UC U^-J- t- ^•C*V*.*.i.i£(i-» *-*^j^ ^.u. *. w* w *,».-— — — O t JT
**Computed from lowest effluent value attained, disregards increases if any occuring in sand filter,
-------
Results for the first two sample days are considered transitional
and have been discarded in evaluating performance. The COD of samples
collected from various points in the system is given in Figure 23. Mean
reduction through the plant was 60 per cent. Based on limited represen-
tative data, BOD removal was about 83 per cent.
The isotherm analysis of Figure 24 for this run shows a system per-
formance equivalent to that computed for two-stage countercurrent
adsorption..
First-stage performance equivalent to single-stage treatment with
fresh carbon was achieved, even though partially loaded carbon was used
here. The COD loading obtained on the carbon in this unit was 13.1 per
cent by weight in addition to the 10.0 per cent achieved in the second-
stage contactor. Including the small removal across the filter, total
carbon loading was 24.0 per cent by weight.
The solids content of blowdown from Units #1 and #2 was 21.5 and
16.7 per cent by weight, respectively, and system blowdown approximated
0.05 per cent of the throughput volume.
Phase 6 - In this investigation, a single contactor-clarifier and
the filter were operated for ten days at 14 gpm and a carbon dosage of
140 mg/1 to enable comparison of single- and two-stage treatment under
closely similar conditions.
Polymer dosage was 4.0 mg/1 initially, but was increased to 4.5 mg/1
during the first day. Continuing solids carry-over prompted an increase
to 5.0 mg/1 on the sixth day. On the last day, exploratory reduction of
the dosage to 3.0 mg/1 resulted in rapid deterioration of floe quality
and the feed was increased again to the end of the run.
Slurry control (Figure 25) was complicated by clogging problems which
were finally overcome by removal of the blowoff orifice. Sampled near
the end of the run, the solids content of system blowdown was 12.6 per
cent by weight and blowdown volume was about 0.1 per cent of the waste-
water treated.
Settling rates of 10-12 in./min for the fragile and readily defloc-
culated draft-tube slurry were measured on the sixth day, equivalent to
overflow rates of 6.2-7.5
Filter runs (Figure 26) averaged 28 hr. The relatively high wash
water consumption of 4.8 per cent is at least partially attributable to
modified procedure instituted during this run when the presence of mud
balls was noted. The latter most likely represented material accumulated
over the entire period of plant operation rather than being unique to
this run.
50
-------
00
e
(U
50
40
30
20-
10
9
8
7
6
20
10
9-
Q _
7-
6-
I I I I I
Feed stream
Clarifier #1 Effluent
Clarifier #2 Effluent
I I I I I
I I I I
Sand Filter Effluent
J L
I I \ L
5 10 20 30 40 50 60 70 80 90 95
Percent of Observations equal to or less than stated values
FIGURE 23: Composite-sample COD, Phase 5.
51
-------
60
40
20
S:
*
/-K
S3
00
c
Tl
"8 10
a
o
JO
"O
«) 5
4J J
CO
Mean performance paths
for C0 = 26.9 & M = 67
Total pilot plant
mean performance
For
18.1 & M = 67
Clarifier #l,unit -
mean performance
r #2, unit
:ormance
M
Daily unit performances
Transition performance
points discarded
=26.9 mg/1 COD
=18.1 mg/1 COD
= 67 mg/1 act. carbon
ii I
8 10
Filtered COD remaining, mg/1
15
20
30
40
FIGURE 24: Adsorption Isotherm Analysis, Phase 5
52
-------
c
o
•H
.U
cfl
^
4J
c
0
C/5 C
CO
(U
cd
CO
60
50
40
30-
20-
10
Single Clarifier, 10 day ave =10.7 Vol %/5 min
Target value, 10 Vol %/5 min
Activated Sludge in Feed
I I I I
0
Elapsed Time, Days
FIGURE 25: Settled Slurry Volume,
Phase 6
10
53
-------
S 60
x
o
e
•H
. 50
J,
O
I
0)
ffl
O
0)
4J
30
20
•a
I 10
#1
ii
#5
I
#2
#3
20
12
3.0
14,400
14
1.1
29
12
3.0
20,900
14
1.1
41
12
3.0
29,500
14
1.1
32
12
3.0
23,000
14
1.1
0 123456
Elapsed time, days
Performance _D_a_ta
Run #1
Duration, hr
Filter rate, gpm „
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
Clarifier overflow rate, gpm/ft
Run
Duration, hr
Filter rate, gpm
Surface loading, gpm/ft
Volume filtered, gal
Pilot plant flow rate, gpm
Clarifier overflow rate, gpm/ft
r
Total wash water used, 7,700 gal or 4.8% at ave. rate of 23.5 gprn/ft"
Note: Incomplete Run #9 omitted from data table.
FIGURE 26: Gravity sand filter operating data, Phase 6.
#7
10
#4
#8
16,
23
12
3.0
600
14
1.1
23
12
3.0
16,600
14
1.1
28
12
3.0
20,200
14
1.1
26
12
3.0
18,700
14
1.1
54
-------
Analytical data for the run (Table 7) disclosed that filter perfor-
mance was little changed from the previous study. Effluent suspended
solids were relatively high and BOD increased significantly across the
unit.
Clarifier effluent suspended solids averaged 14 mg/1 up to the
sixth sampling period when the polymer dosage was increased. The average
dropped to 8 mg/1 for the balance of the run which included a 2-hr influx
of activated sludge in moderate concentration which was handled without
difficulty.
COD at various points in the treatment system is given in Figure 27.
Mean overall reduction was 65 per cent and a BOD removal of 79 per cent
was achieved across the clarifier.
The isotherm analysis of Figure 28 places the mean plant performance
well above that predicted. The observed carbon loading was 13.1 per cent
by weight, 130 per cent of that predicted for single-stage treatment and
89 per cent of that calculated for a two-stage system. The 9.7 mg/1 of
residual COD represents a 30 per cent improvement over the effluent
quality predicted by the mean-value isotherm and approaches the theoreti-
cal effluent concentration of 7.5 mg/1 for two-stage countercurrent
treatment.
Supplemental Investigations
Additional studies conducted during the pilot plant program included
evaluation of the effect of observed temperature variation on system
performance and its analysis; consideration of adsorption theory; limited
review of the influence of the treatment process on nutrients, color, and
surfactants; and a comparison of the results of two methods of turbidity
measurement.
Temperature - Feed-stream temperature was measured hourly during
much of pilot plant program. Data presented in Figure 29 for two 60-hr
periods separated by four weeks show appreciable differences in range
and mean temperature. Although the adsorption process is temperature
sensitive, it was not expected that the observed temperature changes
over the period of plant operation would significantly affect the results.^
However, the validity of this assumption was assessed by a simple labora-
tory experiment.
Using procedure for determination of equilibrium COD described pre-
viously, feed-stream samples were contacted with carbon at temperatures
of 75, 80, and 85°F. Observed differences in equilibrium COD concentra-
tion in jars of identical makeup were within experimental error and no
influence of temperature over the range studies was evident. It was
concluded, therefore, that adjustment of the equilibrium adsorption data
of this study for temperature was not indicated.
55
-------
TABLE 7
Phase 6, 24-hour Composite Sample Analyses
DATE - 1968
Feed stream
pH
Turb., JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Clarifier #1 Eff.
pH
Turb., JTU*
Sus. Solids, mg/1
COD filtered, mg/1
BOD, mg/1
Sand filter Eff.
PH
Turb., JTU*
Sus. Solids, mg/1
COD unfilt. , mg/1
COD filtered, mg/1
BOD, mg/1
COD Red'n, %**
BOD Red'rv, %**
Carbon loading, %
11-17/18
7.7
10/3.3
9
27.0
14.7
7.8
14/2.6
11
10.6
2.7
7.8
3/1.1
3
11.4
10.2
4.5
62
82
12.0
18,19
7.7
9/3.0
10
27.7
—
7.8
10/1.6
17
9.9
—
7.8
3/1.5
5
12.0
9.9
—
64
-
17.8
19,20
7.8
12/3.7
10
29.0
11.6
7.9
15/1.9
16
9.9
3.7
7.8
3/1.0
4
10.4
9.1
5.7
69
68
14.2
20,21
7.7
9/3.6
11
26.9
15.2
7.8
12/2.6
18
9.5
4.2
7.8
2/1.4
4
9.9
8.7
6.4
68
72
12.9
21,22
7.7
11/3.8
11
27.4
14.2
7.8
10/2.0
10
10.4
2.7
7.8
3/1.4
3
11.2
9.5
6.0
65
81
12.8
22,23
7.7
10/2.9
10
28.2
13.2
7.8
7/1.6
8
10.4
2.8
7.8
3/1.2
3
10.8
10.4
6.3
63
79
12.7
23,24
7.7
17/6.7
16
28.2
17.9
7.8
10/2.0
8
9.4
2.9
7.8
3/1.3
2
9.9
8.4
7.2
70
84
14.1
24,25
7.7
10/2.6
7
27.7
12.1
7.8
9/1.5
6
9.9
1.6
7.8
3/1.1
3
10.4
9.4
7.7
66
87
13.1
25,26
7.8
12/3.7
9
31.2
—
7.9
11/2.0
9
6.0+
—
7.9
3/1.5
4
6.4+
5.5+
"~
82+
—
18.4
26,27
7.6
11/3.1
8
31.2
12.8
7.8
6/2.0
11
11.4
2.2
7.8
3/1.5
8
12.9
11.4
7.8
64
83
14.1
* Hach Model 2100 Turbidimeter readings appear at the right of spectrophotometer determinations.
**Computed from lowest effluent value attained, disregards increases if any occuring in sand filter.
+ Data discarded, analytical problem.
-------
Q
o
0)
)-i
0)
50
40
30
20
10
9
8
7
6
n i r
Feed stream
Clarifier #1 Effluent
1 I I I
20
10
Sand Filter Effluent
I
I
I
I
I
I
I
I
5 10 20 30 40 50 60 70 80 90 95
Percent of observations equal to or less than stated value
FIGURE 27: Composite sample COD, Phase 6.
57
-------
60
40
Total pilot plant
mean performance
20
Mean Performance Path
for Co = 28.1 & M = 140
ao
c
o
>-]
c
o
fi
0)
CJ
0)
•H
4J
u
10
o Daily pilot plant performance
C = 28.1 rag/I COD
M° = 140 mg/1 act. carbon
4 6 8 10 15 20
Filtered COD Remaining, mg/1
30 40
FIGURE 28: Adsorption Isotherm Analysis,
Phase 6.
58
-------
90-
80-
Vi
3
4J
CO
o.
75|- Late October, Weather: Clear, dry, hot
Nominal high daytime air temp. 85°F
70
85
80
75
Late November, Weather: Partly cloudy, moderate
Nominal high daytime air temp. 70°F
70
Noon
Mid-
Night
Noon
Mid-
Night
Noon
Representative diurnal feed stream temperature variations
90
Mid-
Night
October
-------
Adsorption Theory - With rare exception, single-stage laboratory
batch isotherms developed on the sewage plant effluent showed a COD/carbon
load limit of 15 per cent by weight. The data of Figure 2 are an excep-
tion and more typical isotherms are portrayed in Figures 3, 4, and 5.
Most of the individual isotherms leveled off on the low carbon dosage
end of the curve at or just under 15 per cent load. This limitation is
also evident in the mean single-stage isotherms used in evaluating pilot
plant performance.
Only the low-level dosage study (Phase 5) was characterized by a
carbon feed low enough to determine if loading in the pilot system could
surpass the limit implied by the single-stage isotherms. The results
(Figure 24) show conclusively that higher loading was possible. Mean
loading was 24 per cent by weight, with daily performance ranging between
21 and 28.5 per cent.
A laboratory generated isotherm featuring several points for low
carbon dosage (Figure 30) confirms the 15 per cent load limit.
The empirical Freundlich equation used in this work is not a pre-
ferred model for basic adsorption research because it is not based upon
theory and is unable to cope with limitations such as encountered here.
Its practical usefulness, however, is evidenced in the present study.
The Langmuir and the Brunauer-Emmett-Teller (B.E.T.) adsorption
models are used in basic studies. The Langmuir equation is valid only
for single-layer adsorption and assumes that maximum loading is reached
when a single complete layer of adsorbate molecules is formed on the sur-
face of the adsorbent. It is
„ XmbC
(1+bC)
in which X is the adsorbate loading per unit weight of carbon at the equili-
brium concentration C in solution. Xm is the ratio of adsorbate to carbon
when the former has formed a complete monolayer on the carbon surface and
b is a constant related to the energy of adsorption.
The B.E.T. equation assumes multilayer adsorption and further that
a given layer need not be completed prior to initiation of subsequent
layers. In one form it is written
cs
where X, Xm, and C are as defined above; A is an energy constant; and Cs
is the adsorbate saturation concentration. As this work deals with a mix
ture of unidentified adsorbate compounds of unknown saturation concentra-
tion, application of the B.E.T. model was not attempted.
60
-------
\ I
60
40-
Effluent grab sample (12-12)
60 min contact
25°C
100 RPM
Filtered C0 - 25.0 mg/1 COD
20
oo
10
3
c
o
cd
u
cfl
10U
140 mg/1,
200 mg/1
800 mg/1
50
30 mg/1
10 mg/1
mg/1
Treatment
Darco S-51, as indicated
Dow C-32, 3 mg/1
J.
_L
_L
4 6 8 10 15
Filtered COD Remaining, mg/1
20
30
40
FIGURE 30: Freundlich adsorption isotherm with emphasis on the
low carbon dosage end of the curve.
61
-------
The Langmuir equation may be linearized as follows:
C = C 1
X Xm
Figure 31, a graphical presentation of the eleven data points of Figure
30 in this form, indicates that only the low-dosage values are described
by this type of adsorption isotherm. The monolayer COD capacity, Xm,
calculated from the slope of the dashed line of Figure 31 is 14.5 per cent
by weight, a value confirmed by several other similar low-dosage isotherms
and implied closely by the Freundlich model.
Also plotted in Figure 31 are the mean pilot plant results. The
original COD concentration for the various studies varied slightly, invali-
dating direct comparison, but the relationships shown are informative.
Pilot plant performance at carbon dosages of 140, 146, and 266 mg/1 (in-
cluding the single-clarifier study) all approach the apparent monolayer
load capacity at equilibrium conditions unobtainable in batch laboratory
experiments. Of interest is the fact that plant performance at a carbon
dosage of 67 mg/1, a more favorable original COD-to-carbon ratio, falls in
the region of multilayer adsorption to the right of the dashed line.
Nutrients - Feed-stream phosphorus and ammonia nitrogen concentra-
tions were expected to be unaffected by the treatment process. These
were monitored for one week during the pilot plant program and the results
(Table 8) confirm this expectation as concerns phosphorus. Ammonia nitro-
gen, on the other hand, increased an average of 2.7 mg/1. A clear explana-
tion* for this is lacking and the point was not pursued. The sewage plant
can chlorinate at various points in the treatment process. It is possible
that the pilot plant feed contained unsuspected chloramines which affected
analysis by direct Nesslerization.
Chemical feeder dilution water from the test facility well contains
as much as 7 mg/1 of nitrate nitrogen. However, the dilution factor of
160 to 1 in the pilot plant reduces nitrogen contribution from this source
to an inconsequential level. Biological reduction of nitrate or nitrite
from whatever source is a possibility.
The polymer coagulant used is a polyamine, but tests indicated it was
not directly responsible for the observed results.
Color and Surfactants - During the pilot plant program, a limited
number of grab samples were analyzed for color. The apparent color of
the feed stream averaged 18 units, Clarifier //I effluent averaged 4 units,
and the average color of effluents from Clarifier #2 and the filter was
2 units.
The surfactant removal capabilities of activated carbon are well
known and this subject was not investigated quantitatively in this work.
62
-------
0.2C
0.15
0.10
M
U
0.05
800 mg/lO
400 mg/1
280 mg/B
Carbon doses
O 10 mg/1
20 mg/1
200 mg/1
140 mg/1
A, / Single-clarifier study, 140 mg/1 carbon
Two-Clarifier/Studies
A V/ Medium carbon dosage study, 146 mg/1 carbon
High carbon dosage study, 266 mg/1 carbon
Low carbon dosage study, 67 mg/1
NOTE: No adjustment made for slight differences in Co values.
JL
_L
5 10 15 20 25
Equilibrium filtered COD(C), mg/1
FIGURE 31: Langmuir single-stage adsorption isotherm.
63
30
-------
TABLE 8
Phosphorus and Ammonia Nitrogen
Content of 24-hr composite samples collected during
the Phase-6 (single-stage) pilot plant program.
DATE: Nov. 1968 20-21 21-22 22-23 23-24 24-25 25-26 26-27 Ave.
P. mg/1
Feed stream 10.1 11.3 12.1 10.8 10.8 12.3 12.4 11.4
Clarifier Eff. 9.5 11.1 11.7 11.7 10.7 12.4 10.6 11.1
Filter Eff. 9.1 11.1 11.7 11.7 10.7 11.7 10.2 10.9
NH3-N, mg/1
Feed stream 18.0 12.8 18.5 17.3 16.3 18.0 12.4 16.2
Clarifier Eff. 18.7 15.3 30.5* 18.8 17.0 19.3 18.8 18.0
Filter Eff. 20.0 15.8 19.3 18.8 16.3 21.6 20.2 18.9
^Questionable value, discarded.
64
-------
Foam to a depth of 2-3 inches existed much of the time in the inlet
chamber of the first-stage clarifier with the ejector and feed lines
jetting into it constantly. This trapped amounts of feed-stream solids
and recycled carbon floe but was easily dissipated by occasional brush-
ing. No other point in the treatment system foamed. Filter effluent
shaken vigorously in a stoppered bottle did not foam. It was concluded
from these indications of very low surfactant concentrations that quanti-
tative study of this point was of no practical value.
Turbidity Measurement - When the project was initiated it was decided
to use a standardized spectrophotometer for turbidity analysis. Part
way through the program it was requested that turbidity also be determined
with a Hach Model 2100 Laboratory Turbidimeter because this instrument
had been used during other FWQA treatment studies with powdered activated
carbon and it was recognized that results from the two methods of analysis
would undoubtedly differ.
The Hach instrument was used at the pilot plant site for determining
the turbidity of the four principal streams every two hours and that of
the composite samples each day before they were taken to the laboratory
where spectrophotometer turbidities were then measured. Data for the
composites are reported in Tables 5, 6, and 7. Inasmuch as they contribute
nothing to data comparison, the bihourly results are not reported here.
Feed-stream and filter effluent turbidities were produced by suspended
matter containing no activated carbon, whereas clarifier-effluent turbidi-
ties resulted primarily from the presence of black flocculated carbon.
Initially, it was felt that light absorption by the carbon floe would re-
sult in low light transmittance and turbidity readings as determined by
spectrophotometer. However, it developed that spectrophotometer turbidi-
ties were of the same order of magnitude as the suspended solids content
of all four sample streams. Both values were reported to the nearest
whole unit and frequently were in exact agreement.
The Hach instrument measures scattered light and is standardized
against a solidified Formazin suspension of known turbidity; hence its
readings may be quite different from those obtained by light transmittance
or absorption. Hach JTU readings ranged from 29 to 32 per cent of the
spectrophotometer turbidities for feed-stream samples and 23 to 45 per
cent for filter effluent. Carbon-bearing samples, however, produced
readings only 12 to 19 per cent of the spectrophotometer turbidities.
Thus, Hach readings were reduced by about 50 per cent when carbon floe
was present.
Comparison of the two sets of readings based on mean values for each
pilot plant run (Table 9) also shows a trend of increasingly higher Hach
readings as time elapsed which might have resulted from dust accumulating
in the instrument's optical system and increasing light scatter.
65
-------
TABLE 9
Ratio of Spectrophotometer to Hach Turbidities
Ist-stage 2nd-stage
Pilot Plant Feed Clarifier Clarifier Filter
Program Stream Effluent Effluent Effluent
Phase 4 3.5 8.5 6.1 4.4
Phase 5 3.3 8.2 5.4 2.7
Phase 6 3.1 5.3 - 2.2
66
-------
DISCUSSION
General Considerations
Inspection of the filter following completion of the pilot plant
program disclosed the presence of a few 1/4-inch carbon-sand mud balls
in the sand bed and larger aggregations of this material up to 1-1/2
inches in diameter were found on top of the supporting gravel. These
large balls, not visible through the transparent filter sidesheets, were
2-3 inches deep against the Transite sides. It is probable that this
material was the principal source of BOD and COD contributed by the
filter.
COD and BOD reductions through the pilot plant are summarized in
Table 10. In a practical sense, effluent COD for series adsorption was
equivalent to that expected from consideration of two-stage adsorption
theory. Mean residual COD for the high carbon dosage run (Phase 4) was
0.9 mg/1 lower than that predicted for two-stage countercurrent adsorption
and, while this may appear insignificant, it is noteworthy that a further
reduction of only 0.6 mg/1 represents the predicted achievement of a
three-stage system.
Residual COD for the single-clarifier study was 9.8 mg/1 versus pre-
dicted concentrations of 14 mg/1 and 7.5 mg/1 for single- and two-stage
treatment, respectively. The observed carbon loading of about 13 per cent
by weight for this run was close to that predicted for a two-stage system.
In general, however, performance was below that indicated by the earlier
single-stage model study during which carbon was fed directly in the
recirculating slurry.
Throughout the present field program, carbon was fed to the inlet
chamber of each solids-contact unit. At the 14-gpm plant operating rate,
each of these chambers provided a holding time of one minute during
which the principle of countercurrent adsorption was violated by concurrent
flow of the wastewater and carbon. As much as 50 per cent of the equili-
brium COD loading on the carbon may be obtained in this short time and
although the concurrent condition was not overlooked, its importance with
respect to the overall process was perhaps underestimated. The signifi-
cance of the brief concurrent contact merits further investigation.
Single- and two-stage countercurrent adsorption treatment has been
mentioned frequently. The greater the slope of the single-stage isotherm,
the greater the carbon economy in utilizing countercurrent systems. This
economy may extend to three- and even to four-stage processes, provided
extremes of treatment are required and cost and complexities can be justi-
fied. However, the saving in carbon dosage is not nearly as great between
two-stage and three-stage as it is between single-stage and two-stage
systems, etc.
67
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TABLE 10
Mean Pilot Plant Performance
Adsorption
System
Two-stage
Two-stage
Two-stage
Single-stage
Carbon
Dosage
mg/1
67
146
266
140
Filtered COD
Inf Eff Red'n
mg/1 mg/1 %
26.9 10.8 60
26.2 7.4 72
27.8 4.4 84
28.1 9.7 65
BOD*
Inf Eff Red'n
mg/1 mg/1 %
14.9
10.6
10.7
14.0
2.6
4.4
2.4
2.9
83
59
78
79
*Exclusive of sand filter
68
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For example, using data from the single-stage pilot plant study
(Figure 28) and assuming an objective of 80 per cent COD reduction, it
can be determined that a carbon dosage of 1,000 mg/1 is required for
single-stage treatment. The residual COD is 5.6 mg/1 and carbon loading
is only 2.2 per cent by weight. Two-stage treatment requires only 24
per cent of the single-stage carbon dosage, or 240 mg/1. And the dosage
for a three-stage system would approximate 14 per cent of the single-
stage dosage, or 140 mg/1.
The solids-contact clarifier offers certain simplifications and
economy for powdered activated carbon adsorption system design by com-
bining in one vessel the functions of mixing, flocculation, and solids
separation. Power use per stage is minimized and multiple tankage within
each stage is eliminated, thus reducing space requirements and piping
complexity.
An ACCELATOR clarifier produces a dense carbon slurry blowdown suita-
ble for direct transfer to a regeneration system. Should its internal
slurry concentrator prove troublesome due to polymer gelling of the slurry
as experienced at times in the pilot plant studies, external thickening
is practicable. Slurry at a concentration of about one per cent by weight
would be blown down to a thickener where it would concentrate to 10-20
per cent or more.
Process Economics
Based upon the design approach noted below, cost estimates have been
developed for 10-mgd and 100-mgd treatment systems of the type studied.
Plant Design - Each plant incorporates complete water recovery, elimina-
ting any requirement for separate disposal. All process water needs are
met with filter wash water which is pumped from storage to operate carbon-
slurry transfer ejectors, to fill slurry makeup tanks, and to dilute poly-
mer in day tanks. Excess backwash water is pumped to the second-stage
adsorption units for recovery.
Chemical feed systems are sized to provide maximum dosages of 400 mg/1
and 10 mg/1 of carbon and polyelectrolyte, respectively. Carbon recovery
via regeneration is visualized and 10 per cent loss per cycle is assumed.
The site requirement for the 10-mgd plant is an area 110 ft x 150 ft.
Two ACCELATOR mechanisms in basins 62 ft square x 17.5 ft deep handle the
adsorption duties of this plant and three 4-cell gravity sand filters
with GREENLEAF Filter Controls (a unique siphon valve system for controlling
the functions of multiple rapid-sand gravity filters) provide effluent
polishing at 3 gpm/ft2. These concrete structures, as well as a two-story
metal building housing the carbon regeneration system, polyelectrolyte
feed tanks and pumps, carbon makeup and polyelectrolyte storage, office,
and laboratory are constructed above grade.
69
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Agitated covered concrete tanks located below grade between the
ACCELATOR basins and the filters include three 6000-ft3 carbon-slurry day
tanks. During operation, one tank is in use, a second is being readied
with water plus carbon from regeneration and/or storage, and the third
is ready and on standby for use the following day. Two 3,400-ft3 mixed
tanks provide up to 12 hr of holding time for blowdown from the respec-
tive clarifiers, one supplying partially spent carbon to the first-stage
unit and the other furnishing fully spent carbon to regeneration.
The type of filter visualized requires no wash water storage. A
21,000-ft3 unmixed basin for backwash wastewater catchment completes the
below-grade tankage for this plant.
At a feed rate of 240 mg/1 (2 lb/1000 gal.), the daily carbon require-
ment is 20,000 Ib. The plant requires an in-service carbon inventory of
100,000 Ib and storage for two carloads of bagged carbon. A 36,000-lb
carload will meet maximum makeup requirements for 12 days. Drum delivery
of coagulant is assumed, since a 500-lb drum is sufficient for about a
week of operation.
A 100-mgd plant of similar general design occupies an area 450 ft by
450 ft. To provide appropriate flexibility and emergency operating capa-
bility, five 20-mgd ACCELATOR units operate in parallel as the first
contact stage, with five similar parallel units as the second stage.
These mechanisms are installed in common-wall concrete basins, each 88 ft
square x 21 ft deep. Fourteen 43-ft square, 4-cell filters with GREENLEAF
Filter Controls are required.
A two-story building covers two below-grade agitated blowdown and
three carbon slurry day tanks and houses the carbon regeneration system,
two polyelectrolyte day tanks, carbon feed systems, service pumps, offices,
and laboratory. Unit capacities of the blowdown and carbon slurry tanks
are 18,000 ft^ and 60,000 ft3, respectively.
In this plant, a conventional 67-ft diameter clarifier is used for
filter wash water recovery. The overflow furnishes all plant process
water and underflow solids are transferred by gravity to the first-stage
blowdown tank unroute to regeneration. As before, excess filter backwash
goes to the second contact stage for recovery.
The design also includes a 1-million gal. covered storage reservoir
to receive blowdown during periods of first-stage upset when the regular
holding tank could be totally inadequate. Carbon slurry so collected is
returned at a low rate to the first-stage contact-clarifiers to recover
both carbon and partially treated water. Carbon salvaged in this fashion
could amount to many thousands of pounds per occurence.
The in-service carbon inventory for this plant is about one million
pounds. At 240 mg/1, daily carbon use is 200,000 Ib and a make-up
70
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requirement of 20,000-30,000 Ib/day is indicated. Bulk storage is not
included. Rail delivery of bulk carbon is visualized with cars unloading
makeup quantities into the day tanks as required.
Polyelectrolyte coagulant is delivered by tank car to a 20,000-gal.
storage tank from where it is pumped as required to either of two day
tanks for dilution to feed strength. The diluted material is then pumped
to each contact-clarifier through variable-orifice rate control devices
installed along a distribution header serving all units.
Both plant designs assumed a mild climate location with weatherproof
equipment installed in the open. Carbon handling equipment is either of
corrosion resistant material or is protected with suitable coatings.
Essential instrumentation for flow metering and control and low-lift pro-
cess and service pumps are included. Treated water storage and high-
service pumps are not included.
Treatment Cost - The estimated construction cost of the 10-mgd plant,
including land but exclusive of the capital cost of a carbon regeneration
system, is as follows:
Equipment delivered ($268,200) and installed $333,000
Plant structures 215,200
Total plant cost $548,200
Engineering (10% of total plant cost) 54,800
Contractor's fee (10% of total plant cost) 54,800
Land aquisition (2% of total plant cost) 11,000
Contingencies and omissions (15% of total 82,200
plant cost)
Total capital cost $751,000
Annual operating cost breaks down as follows:
Capital ($751,000 for 20 years @6%) $ 65,500
Maintenance (3% of equipment + 1% of
structures) 12,150
Labor 66,600
Power (1C/KWH) 21,400
TOTAL $165,650
equivalent to 4.5
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supplied by regeneration and 10 per cent by fresh makeup, and modifying
the total carbon regeneration costs of Bloom, et al., *-® to reflect in-
creased interest rates, 90 rather than 95 per cent recovery, and 25 rather
than 50 per cent increased production predicted for processing 15 rather
than 10 per cent by weight slurry, chemical costs are:
C/1000 gal.
Regenerated carbon
(2 lb/1000 gal.) (2c/lb) (0.90) 3.6
Makeup carbon
(2 lb/1000 gal.) (13c/lb + 3c/lb freight) (0.10) 3.2
C-32 Polyelectrolyte
(0.067 lb/1000 gal.) (52.5c/lb + 3c/lb freight) 3.7
10.5
On this basis, the total cost of producing low-COD water on this scale is
15C/1000 gal.
Using the same approach, the estimated construction cost of the 100-
mgd installation is as follows:
Equipment delivered ($1,927,600) and installed $2,158,700
Plant structures 1,155,100
Total plant cost $3,313,800
Engineering (10% of total plant cost) 331,400
Contractor's fee (10% of total plant cost) 331,400
Land aquisition (2% of total paint cost) 66,300
Contingencies and omissions (15% of total plant
cost) 497.100
Total capital cost $4,540,000
The breakdown for annual operating cost exclusive of chemical cost
is;
Capital ($4,540,000 for 20 years (§6%) $ 395,800
Maintenance (3% of equipment + 1% of
structures) 76,300
Labor 199,800
Power (Ic/KWH) 122,500
TOTAL $ 794,400
72
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equivalent to 2.20/1000 gal. treated. The unit chemical costs noted above
remain unchanged, making the total cost of producing low-COD water on
this scale 12.70/1000 gal.
The significance of carbon dosage in establishing treatment cost is
most evident. For example, if 120 mg/1 (1 lb/1000 gal.) proves adequate,
unit costs for regenerated carbon, makeup carbon, and coagulant decrease
to 1.8, 1.6, and 1.80/1000 gal., respectively. Plant-scale total treatment
cost reduces to 9.70/1000 gal. for the 10-mgd installation and 7.40/1000
gal. for the 100-mgd facility.
Less costly activated carbons are available and if they perform
suitably their use would lower the cost of makeup carbon still further.
However, the 13o/lb carbon for makeup constitutes only 17 per cent of the
total cost for the 10-mgd plant at the 2-lb/1000 gal. feed rate and 13
per cent at the 1-lb/1000 gal. dosage. These figures for the 100-mgd
plant are 20 and 17 per cent, respectively. Use of 8o/lb carbon would
reduce total cost by 10/1000 gal. for 2-lb/1000 gal. treatment and by
0.50/1000 gal. for the 1-lb/1000 gal. dosage.
The projected total operating cost of a 10-mgd granular activated
carbon adsorption system using multiple downflow contact columns is
8.30/1000 gal.3 A plant of this type might require as much as 133,000 Ib
of in-service carbon inventory for each 1000 gpm of system capacity. The
powdered activated carbon system described is conservatively estimated to
require only 14,300 Ib of in-service carbon inventory for each 1000 gpm of
capacity. These inventories represent an initial carbon cost of $293,000
(31.5o/lb) for a 10-mgd granular carbon plant as compared to only $13,000
(13o/lb) for a powdered carbon plant of the same capacity, or $8,000
(8o/lb) if less costly carbon can be used. Both systems require facili-
ties to warehouse and handle 10-15 per cent makeup carbon.
Powdered carbon plant makeup requirements would commence within five
days of plant startup as compared to several months later for the granular
system. Powdered carbon utilization would range from 25-33 per cent of
the top utilization reported for granular systems, thus requiring regenera-
tion 3-4 times more frequently. Three or four times as much makeup carbon
is required, but this factor could be offset by the unit price differential
between powdered and granular carbons.
The cost of powdered carbon regeneration has been projected as 2o/lb
on a large scale. Considering that granular activated carbon has been re-
activated on a moderate scale for 1.50/lb, the recycle and regeneration
frequency for powdered carbon therefore results in a cost burden that must
be offset by lower capital, operating, and maintenance costs, reduction
in carbon loss, and eventually by lower regeneration cost and improved
carbon utilization.
73
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SUMMARY
Powdered activated carbon treatment of activated-sludge sewage plant
effluent in recirculating-slurry solids-contact clarifiers was evaluated
in this pilot-scale investigation. A single unit achieved 30 per cent
greater carbon efficiency than a conventional single-stage adsorption
system. Two such clarifiers operated in series with countercurrent carbon
transfer performed as well or better than conventional two-stage counter-
current adsorption systems requiring multiple agitated tanks, clarifiers
and filters. Series operation provided substantial process protection
during periods of gross deterioration of feed-stream quality.
Preliminary laboratory study of three powdered activated carbons
resulted in selection of Atlas Chemical Industries' Darco S-51 for the
pilot plant program and it was found that polyelectrolyte flocculation
was required to produce floe which settled well. For the system involved,
a study of 26 compounds disclosed that Dow Chemical Company's Purifloc
C-32 was the most effective and it was used throughout the pilot plant
work. A polyelectrolyte dosage of 6-7 mg/1 was required for effective
flocculation at carbon feed rates up to 140 mg/1. For a series study
using 266 mg/1 of carbon, 10 mg/1 of C-32 were required.
Slurry settling rates far exceeded requirements of the pilot plant
which was operated at hydraulic loads from 0.4 to 1.6 gpm/ft of clarifica-
tion area. In spite of this, it was necessary to reduce the pilot plant
throughput when influent suspended solids were high because of an inability
to remove solids as rapidly as they were accumulated within the system
during these periods.
The volume of system blowdown ranged from 0.05-0.1 per cent of the
throughput and its solids content of 13-22 per cent by weight would enable
economical recovery of carbon for reuse by reactivation without further
concentration.
A gravity sand filter operated at 3 gpm/ft2 on the downstream end
of the pilot plant was not completely successful. Shortly after initia-
tion of the pilot plant program, it was noted that effluent COD and BOD
occasionally exceeded the filter influent concentrations and this situa-
tion worsened with the passage of time. There was no evidence that carbon
fines were passed, but biological growth in the filter underdrain was
observed and at the conclusion of the program carbon-sand mudballs were
found at the gravel-sand interface. Wash water requirements ranged from
1.2 per cent of the filtered water volume to a high of 4.9 per cent when
extremes in backwash rate and duration were used in an effort to correct
the filter condition.
74
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Carbon was fed by a slurry feeder which provided continuous hori-
zontal agitation and wetting was never a problem.
Pilot plant influent filtered COD averaged 27.2 mg/1 and ranged from
23-34 mg/1 during the study program. Two-stage countercurrent treatment
with 67, 146, and 266 mg/1 of carbon achieved respective reductions of
60, 72, and 84 per cent. Residual COD concentrations observed were 10.8,
7.4, and 4.4 mg/1.
A COD reduction of 65 per cent was obtained across a one unit con-
tactor system with a carbon dosage of 140 mg/1. This compares with a
predicted single-stage removal of 50 per cent. Residual COD concentra-
tion observed was 9.7 mg/1.
Carbon loadings for the two-stage systems ranged from 9-24 mg of
COD/100 mg of carbon and a loading of 13.1 per cent by weight was obtained
during single unit treatment. There is reason to believe that modification
of the system as operated will improve carbon efficiency.
BOD removals for the respective systems, exclusive of the sand filter,
were 83, 59, 78, and 79 per cent. Considering the low average feed BOD
of 12 mg/1 and the substantial removals through the pilot plant, only
order-of-magnitude removals should be inferred here.
On a 10-mgd scale, assuming respective carbon and polyelectrolyte
dosages of 240 and 8 mg/1, the total estimated cost of treatment by the
pilot plant process is ISc/lOOO gal. If chemical feed requirements in
terms of effluent quality objectives could be reduced to 120 and 4 mg/1,
respectively, the total cost decreases to 9.7C/1000 gal.
A 100-mgd plant of similar design would produce treated water at a
cost of 12.7/1000 gal. at the higher chemical feed rates and 7.4<:/1000
gal. at the lower chemical dosages. Both plants feature 90 per cent
carbon recovery by regeneration and 10 per cent fresh carbon makeup.
Solid and liquid waste handling and disposal problems for either plant
are minimal.
75
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REFERENCES
1. Anon., "Taste and Odor Control in Water Purification." West Virginia
Pulp and Paper, Chemical Division, 1966.
2. Morris, J. C. and Weber, W. J., Jr., "Removal of Biochemically Resis-
tant Compounds by Adsorption." U. S. Department of Health, Education
and Welfare, Public Health Service Publication No. 999-WP-ll, AWTR-9.
3. "Summary Report, Advanced Waste Treatment Research Program, July 1964-
July 1967," U. S. Department of the Interior, Water Pollution Control
Research Series Publication No. WP-20-AWTR-19.
4. Beebe, R. L. and Stevens, J. I., "Activated Carbon System for Waste-
water Renovation." Water and Wastes Engineering, Jan. 1967, pp 43-45.
5. "Measuring Adsorptive Capacity of Activated Carbons for Liquid Purifi-
cation," Bulletin D-87, Atlas Chemical Industries, Inc., 1966.
6. "Standard Methods for the Examination of Water and Wastewater,"
12th Ed., American Public Health Assn., Inc., 1965.
7. Rinehart, T.M., Scheffler, G. H., Helbig, W. A., and Truemper, J. T.,
"A Symposium on Activated Carbon," Atlas Chemical Industries, Inc.,
1968.
8. Private communication, Cunningham, A. W., 1968 (Letter 7-16-68).
9. Weber, Walter, Jr., and Morris, J. Carrell, "Equilibria Capacities
for Adsorption on Carbon," Jour. ASCE San. Eng. Div., Vol. 90, No. SA3,
pp 79 (June 1964).
10. Bloom, R., Jr., Joseph, R. T., Friedman, L. D., and Hopkins, C. B.,
"New Technique Cuts Carbon Regeneration Costs," Environmental Science
and Technology, March 1969, pp 214-217.
76
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APPENDIX
Low-Level COD Determination
The procedure is essentially that given in the 12th Edition of
"Standard Methods for the Examination of Water and Waste Water," p. 513,
par. 4.6.
Attention is directed to the precaution concerning contaminated
glassware or contamination from atmospheric dust.
It has been found that ordinary laboratory grade distilled or de-
mineralized water used for pre-reflux dilution, blanks, rinsing, or final
dilution leads to highly erratic results. Therefore, this laboratory has
developed a special distillation procedure for producing suitable water.
A further source of error is in preparation of Ag2S04~H2S04 solution.
Every effort must be made to avoid contamination. It is essential that
all determinations and blanks in a single set be run with the same batch
of silver-acid. In this way any small contamination of the solution is
automatically corrected for by the blanks.
No significant error is introduced by the use of a polyethylene wash
bottle for rinsing down condenser tubes and tips.
The apparatus described is effective in preventing dust contamination
and in protecting the analyst from injury by bumping which may occasionally
occur during refluxing. It differs slightly from that described in
"Standard Methods."
1. Apparatus
1.1 500-ml short-necked round bottom boiling flasks. ST joint 24/40,
Corning #4320 or equal.
1.2 Allihn type condensers, 500-ml jacket, drop tip inner ST joint
24/40, top outer ST joint 24/40, Corning #2480 or equal.
1.3 Connecting tubes 75°, both ends inner ST joints 24/40, Corning
8920 or equal.
1.4 Tube stoppers, cap type, full length outer ST joint 24/40, 2
required per condenser.
1.5 Heaters, Precision Catalog #61560 or equal.
77
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1.6 Running water cooling bath large enough for required number
of flasks. The apparatus is assembled in such a way that the
complete glass assembly consisting of flask, condenser, and
upper connecting tube may be raised from or lowered to the
heater as required. The connecting tube is placed in the upper
end of the condenser pointed away from any work or chemical
storage area. The cap type stoppers are placed on condenser
tip and connecting tube tip when the apparatus is not in use,
to prevent entrance of dust particles. Both caps are, of
course, removed when the apparatus is in use.
2. Reagents
2.1 0.025N potassium dichromate, as per "Standard Methods" except
made up in special water (2.6).
2.2 0.01N ferrous ammonium sulfate. (For routine work this labora-
tory uses approximately 0.025N solution with reproducible results),
2.3 Ferroln indicator as per "Standard Methods."
2.4 Silver sulfate-sulfuric acid solution.
Prepare as follows; Place 31 grams reagent grade silver
sulfate in a scrupulously clean dry 2-liter beaker. Add
about 1 liter cone, reagent grade H2S04 from a newly
opened standard 9-lb bottle. With a flat tipped large
diameter glass stirring rod, carefully crush lumps of
Ag2S04 and stir thoroughly, repeating as often as neces-
sary. Complete dissolution may be obtained in 20 minutes
or less as compared with 1-2 days in "Standard Methods."
Return the solution to the 9-lb bottle. Cap securely and
mix thoroughly by inversion. Caution: Avoid exhaling
directly toward the beaker during dissolution step. Such
exhalation can obviously lead to particulate contamination.
2.5 Mercuric sulfate, reagent grade.
2.6 Blank water. Redistill a high quality distilled or demineralized
water from an all-glass ST joint apparatus. An activated carbon
purifier from which most of the chloride has been leached may
be used ahead of the redistillation. Proceed as follows;
Place in a 3-liter R.B. distilling flash containing several
boiling chips or beads, 500 ml water, 200 ml 0.25N (approx)
potassium dichromate, 200 ml Ag2S04-H2SC>4 solution, (2.4),
and about 1 gram HgSC^. Swirl to mix and dissolve the
HgSC«4. Mark level of this mixture on flask. Add about 1.5
78
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liters of water and mix. Bring to a boil on electric
heater. If a short-necked ST flask is used, a long glass
stirring rod may be kept in the flask while mixture is
heating. This helps to prevent bumping. Stir occasionally
until boiling starts. Allow to boil for 10 minutes wasting
steam to the atmosphere. Place connecting tube and conden-
ser, and waste steam through uncooled condenser for 2 or 3
minutes. Start cooling water and waste enough condensate
to rinse condenser.
Collect distillate in a glass bottle rinsed several times
with a little distillate. A 9-lb acid bottle, well cleaned
with chromic acid, makes a good container. Use an adapter
from condenser tip into bottle. The adapter tip should
enter the receiver bottle through a hole in a loose fitting
foil dust cap. Distill until level in flask drops to the
mark. A larger still should be used where the COD determina-
tion load is heavy. Keep mixture proportions the same.
Keep water storage bottles tightly capped when not in use.
The oxidizing mixture in the flask may be used repeatedly
until there is obvious discoloration.
3. Procedure
3.1 Two blanks should be run with each set of samples.
3.2 Place 50-ml blanks and samples in identified 500-ml boiling
flasks containing about 7 glass beads. Keep flasks capped with
small glass beakers at all times except when placing samples
and reagents.
3.3 Add 5 ml cone. H2S04 (not 2.4), swirling to avoid local heating.
3.4 Add approx. 0.5 gram HgSC>4 and swirl to mix until HgSO^ is
dissolved.
3.5 Add exactly 25 ml 0.025N potassium dichromate solution swirl-
ing to mix during addition.
3.6 Remove caps from reflux condensers and top connecting tubes.
Start cooling water through condenser jackets. Turn heaters
to "high."
3.7 To each flask, add without mixing 70 ml Ag2S04-H2S04 solution
(2.4).
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3.8 It is convenient to have a tray to hold flasks upright during
this and preceding steps. Carry tray to reflux bank and
taking each flask in turn swirl to mix contents quickly but
thoroughly and connect immediately to condenser. Clamp and
lower apparatus to heater. Boiling will start immediately
without bumping. When vigorous boiling is established, turn
heater down to a point to maintain boiling (about 75 on Pre-
cision heater scale). Reflux for 2 hours.
3.9 At end of reflux time, taking each unit in turn, in the same
order as in 3.8, turn off heater and raise apparatus several
inches above heater. Place an asbestos square over heater
top. Cool 15 minutes.
3.10 Now remove connecting tube and rinse down condenser with a
little blank water (2.6) Allow to drain for about 10-15 seconds
and holding flask by the neck lower it from condenser. Rinse
condenser tip into flask with a little blank water. Place
capped flask in cooling bath immediately. Replace condenser
and connecting tube caps.
3.11 While flasks are cooling, prepare a ferrous ammonium sulfate
standardization. Place 25 ml of 0.025N dichromate solution
and about 200 ml blank water in a 500-ml erlenmeyer flask. Add
with swirling, 30-35 ml cone. H2S04 (not 2.4). Cover and place
in water bath to cool. Note; If using 0.01N ferrous ammonium
sulfate, 10 ml of 0.025 dichromate solution is sufficient for
the standard.
3.12 When reflux flasks have cooled for about 15 minutes (cool to
the touch), add 200 ml blank water (2.6) carefully to each.
Mix and return to water bath.
3.13 When all flasks, including standard, have cooled, replace in
tray. Add 2-3 drops of ferroin indicator to each. Titrate
with ferrous ammonium sulfate directly in the boiling flask.
Do not transfer. This may require a little manipulative
practice, but is essential for consistent results.
3.14 Empty flasks without removing beads. Rinse flasks and beads
thoroughly 3 times with ordinary distilled water and 3 times
with small portions (about 10 ml) of blank water. Replace
caps immediately.
4. General Notes
4.1 Blanks should titrate within 0.2 ml of the standard if there
is no contamination. Different lots of Ag2S04-H2SO reagent
80
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will produce different blank titrations. With careful atten-
tion to cleanliness there will be less than 0.2 ml difference
in blanks. Within this limit use the highest blank, not the
average of two. For very precise work (non-routine) the two
blanks should agree.
4.2 All glassware must be kept clean and dust free. Use cleaning
acid followed by distilled water and finally blank water (2.6).
To illustrate the sensitivity of the method to contamination,
calculation indicates that in a 50 ml sample a speck of cellu-
lose dust weighing 0.05 milligrams would contribute about 1
mg/1 COD.
81
* U. S. GOVERNMENT PRINTING OFFICE : 1970 O - 408-097
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