United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/045 June 1986
Project Summary
Toxic Substance Removal in
Activated Sludge and PAC
Treatment Systems
Walter J. Weber, Jr. and Bruce E. Jones
The addition of powdered activated car-
bon (PAC) to activated sludge treatment
systems to enhance removal of specific
toxic organic compounds from waste-
water was evaluated. Little prior informa-
tion existed on the behavior of toxic organ-
ics during activated sludge treatment.
Thus a considerable effort was directed
toward identifying and quantifying the fate
of specific compounds in conventional ac-
tivated sludge systems as well as in those
to which carbon was added. Nine organic
compounds encompassing a range of sol-
ubility, volatility, biodegradability, and ad-
sorptlve properties were studied along
with selected commercial PAC's recom-
mended for use in integrated activated
sludge/carbon treatment systems. Com-
pletely-mixed-flow bioreactors equipped
with internal clarfflers and air-tight lids
were used to study the removal of each
compound. Completely mixed batch rate
and equMbrium studies were conducted to
quantify the removal mechanisms of vola-
tilization, biodegradation, biosorption, and
carbon adsorption.
Results from steady-state bioreactor
studies showed that the addition of less
than 100 mg/L PAC to the influent did not
enhance the removal of the biodegradable
compounds benzene, toluene, ethylben-
zene, o-xylene, chlorobenzene, and nitro-
benzene. Significantly improved removals
of the poorly degradable and nonbtode-
gradable compounds 1,2-dichlorobenzene,
1,2,4-trichlorobenzene, and lindane oc-
curred at influent powdered carbon con-
centrations in the 12.5- to 25-mg/L range.
Influent powdered carbon concentrations
of 100 mg/L effected overall removals of
greater than 90%. The addition of PAC not
only reduced effluent concentrations but
also reduced the amounts of the volatile
compounds stripped to the atmosphere.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
is fuHy documented In a separate report
of the same title (see Project Report order-
ing information at back).
Introduction
The presence of toxic organic com-
pounds in receiving waters and water sup-
plies has modified the emphasis of waste-
water treatment during the past several
decades. Wastewater treatment systems
historically have been designed to remove
BOD, suspended solids, pathogenic organ-
isms, and nitrogen and phosphorus, but
the removal of these substances alone is
no longer sufficient to protect the quality
of receiving waters. Current discharge re-
gulations impose limitations on specific
toxic substances such as the 114 organic
compounds designated as priority pollut-
ants by the U.S. Enviromental Protection
Agency (EPA).
The presence of toxic organic com-
pounds in municipal and industrial waste-
waters necessitates an understanding of
the capabilities of conventional waste-
water treatment systems for removing
such substances. Some organic com-
pounds are adequately removed, but the
specific removal mechanisms must be
identified and quantified so that treatment
systems can be operated to achieve max-
imum effectiveness. Other compounds are
not removed to levels considered accep-
table for discharge. Treatment of these
substances will require modifications or
alternatives to existing treatment systems.
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Various modifications and alternatives
to conventional biological treatment pro-
cesses have been suggested and evalu-
ated in bench-, pilot-, and full-scale treat-
ment systems during the past several de-
cades. The addition of PAC to the aeration
basins of activated sludge facilities (a pro-
cess known as PACT*) has emerged as an
attractive, integrated application of both
biological degradation and adsorption
technology to wastewater treatment for
removal of organic compounds. Though
this integrated process was originally in-
troduced as a method for upgrading the
performance of activated sludge systems
with respect to conventional parameters,
its potential ability to remove toxic and
carcinogenic organic compounds may be
at least as significant. However, little infor-
mation exists on either the fate of specific
organic compounds during such treatment
or on the factors that control the effec-
tiveness of their removal.
This project was designed to study the
fate and behavior of specific organic com-
pounds in bench-scale activated sludge
and integrated activated sludge and car-
bon treatment systems. The effectiveness
of adding PAC was evaluated by compar-
ing the removals of specific compounds in
integrated systems with removals in ac-
tivated sludge systems.
Experimental Approach
Compounds Studied
Nine specific organic compounds were
included in the study (Table 1). They en-
compass a broad range of volatility, bio-
degradability, and adsorptive properties.
One feature of the study was the use of
pollutants known to be present in munic-
ipal and industrial wastewaters and at
concentrations representative of those
actually measured in the field.
Adsorbents
Background adsorption studies were
conducted with a variety of adsorbents to
select one PAC as the primary adsorbent
for use in the continuous-flow bioreactor
studies. Based on adsorptive performance
in equilibrium isotherm studies and com-
mercial availability for wastewater treat-
ment, Hydrodarco C (ICI Americas) was
chosen as the primary adsorbent for the
bioreactor studies.
"Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
Bioreactors
Completely-mixed-flow (CMF) bioreac-
tors containing 10-L aeration sections and
1-L internal clarifiers were used to study
the fate of the organic compounds during
activated sludge and integrated activated
sludge and carbon treatment. Each bio-
reactor was fitted with an air-tight lid that
enabled collection and sampling of bio-
reactor off-gases for each organic com-
pound. The air-tight lids, which were a
unique feature of the experimental sys-
tem, facilitated formulation of a mass
balance for each compound and determin-
ation of its fate during treatment. A
schematic diagram of the experimental
system appears in Figure 1.
Removal Mechanisms
Batch equilibrium and rate studies were
conducted to evaluate independently the
mechanisms that affect the fate of organic
compounds during activated sludge and
integrated activated sludge and carbon
treatment. Four removal mechanisms were
identified: volatilization, biosorption, bio-
degradation, and PAC adsorption. A sum-
mary of each of these studies is given
below. The full report details the exper-
imental approach and complete results
from each study.
Volatilization
Batch air-stripping studies were con-
ducted over a range of aeration rates in
10-L bioreactors without activated sludge
present to determine the rates at which
individual compounds, were volatilized
from the experimental system. First-order
volatilization rate coefficients were
determined from the slope of semi-loga-
rithmic plots of concentration versus time.
A linear relationship between the volatiliza-
tion rate coefficient and the aeration rate
was observed for each compound. Batch-
generated volatilization rate coefficients
were used to predict the fate of the seven
volatile compounds in continuous-flow
bioreactors without activated sludge. The
agreement between the predicted and
measured effluent and off-gas values
(Table 2) demonstrates that volatilization
was well characterized in the experimental
system. The results indicated that volati-
lization effected at least 90% reduction of
the volatile compounds in the aqueous
concentrations.
Biosorption
Batch and rate equilibrium sorption
studies were conducted with the three
compounds expected to sorb most signif-
icantly onto activated sludge based on
octanol/water partition coefficients—
namely, 1,2-dichlorobenzene, 1,2,4-trichlo- M
robenzene, and lindana Results from sorp- "
tion isotherm studies indicated a reason-
ably linear partitioning, and the rate
studies demonstrated that equilibrium
conditions occurred within approximately
15 min. A summary of biomass partition
coefficients for the three compounds ap-
pears in Table 3.
Biodegradation
Completely mixed batch biodegradation
rate studies were conducted with acti-
vated sludges from acclimated 10-L biore-
actors to serve two purposes: 1) to con-
firm the biodegradation observed in 10-L
CMF activated sludge units; and, 2) to
measure pseudo first-order biodegradation
rate coefficients for selected compounds.
These studies were conducted under aero-
bic conditions without aeration to evaluate
independently the removal mechanism of
biodegradation. The full report details ex-
perimental procedures and results.
PAC Adsorption
Equilibrium adsorption isotherms were
conducted with Hydrodarco C and each of
the nine organic compounds in back-
ground solutions of bioreactor effluent.
Results from these studies adhered to the
Freundlich isotherm model, as shown by
the data plotted in logarithmic form in t
Figure 2 for five of the compounds. The
compounds could generally be ranked in
the following decreasing order, based on
the amount adsorbed for a given concen-
tration of an equilibrium solution: 1,2,4-tri-
chlorobenzene > lindane > 1,2-dichloro-
benzene = nitrobenzene > chlorobenzene
= ethylbenzene > o-xylene > toluene =
benzene. The full report contains results
from additional isotherm studies.
Activated Sludge Bioreactor
Studies
Activated sludge studies were conduct-
ed to fulfill three primary objectives:
1) To provide a reference point to which
the results from bioreactor studies
with carbon could be compared to
evaluate the effect of carbon addi-
tion on the removal of the organic
compounds;
2) To determine which of the organic
compounds could be biodegraded by
activated sludge under conditions
representative of municipal activated
sludge treatment; and,
3) To evaluate the relative importance
of volatilization, biodegradation, and
biosorption as removal mechanisms.
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Table 1. Physical Properties of Toxic Organic Compounds
Compound
Purgeable:
1. Benzene IBZ)
2. Toluene (TL>
3. Ethylbenzene (EB)
4. o-Xylene (XL)
5. Chlorobenzene
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Table 3. Biomass Partition Coefficients
Compound
LogKt
•B
1,2-Dichlorobenzene
Undone
1,2,4-Trichlorobenzene
2.40
2.75
3.01
3.38
3.72
4.26
*Biomass partition coefficient.
+ Octanol/water partition coefficient.
10
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700
o
I
8
o
a Benzene
o Chlorobenzene
o 1,2-Dichlorobenzene
A 1,2,4-Trichlorobenzene
0 7 14 21 28 35 42 49 56 63 70 77
Figure 3. Effect of increased chlorination on the biodegradation (overall removal) of volatile
organic compounds during single-solute activated sludge studies.
the removal of toxic organic compounds
in activated sludge systems receiving PAC.
Conclusions
This project provides information on the
potential for conventional activated sludge
systems to effect significant biodegrada-
tion of trace concentrations of organic
pollutants. The study also demonstrates
that the addition of activated carbon to ac-
tivated sludge systems can be an effective
technique for providing enhanced removal
of nonbiodegradable and poorly biode-
gradable compounds. Major conclusions
that can be drawn from the study are as
follows:
• Benzene, toluene, ethylbenzene, o-xy-
lene, Chlorobenzene, and nitrobenzene
are effectively biodegraded in activated
sludge systems;
• Carbon dosages of 100 mg/L had no
significant effect on the steady-state
effluent and off-gas concentrations of
the biodegradable compounds;
• 1,2-Dichlorobenzene, 1,2,4-trichloroben-
zene, and lindane are not effectively
removed during activated sludge
treatment;
• Carbon dosages of 25 to 50 mg/L
effected 80% to 95% removal of
1,2-dichlorobenzene, 1,2,4-trichloroben-
zene, and lindane;
• Addition of carbon resulted in approxi-
mately equal percent reductions in both
effluent and off-gas concentrations of
volatile compounds; and,
• The overall removal of nonbiodegrad-
able compounds under steady-state
operating conditions was a function of
the influent carbon dosage and not of
the mixed liquor concentration of
carbon.
The full report was submitted in fulfill-
ment of Grant No. CR 806030 by the
University of Michigan under the sponsor-
ship of the U.S. Environmental Protection
Agency.
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200
£ 700
I
I I
O Measured Data Points •
— Predicted if no Biodegradation
i i i I I I
\ \
.
ki
6.0
4.0
2.0
T r r \i
\ \
I
CO
IB
(3
1200
soo
400
I I
i r
I
1.0
0.5
0.0
0 7 14 21 28 35 42 49 56 63 70 77
Time (days)
Figure 4. Benzene influent, effluent, and off-gas concentrations and fractional recoveries,
N/No, measured during a 6-day SRT activated sludge study. (Dashed lines give
values predicted by air-stripping, assuming no biodegradation.)
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Table S. Overall Percent Removals Measured at Various Influent Carbon Dosages*
Compound
Poorly Biodegradable:
1,2-Dichlorobenzene
Nonbiodegradable:
1,2,4-Trichlorobenzene
Lindane
Carbon Dosage
Control
12.5 mg/L
25mg/L
50 mg/L
1OO mg/L
200 mg/L
Biodegradable:
Benzene
Toluene
Ethylbenzene
o-Xylene
Chlorobenzene
Nitrobenzene
83
84
81
78
82
97
ND+
NO
NO
ND
ND
ND
79
ND
79
ND
82
ND
84
ND
85
82
85
97
86
ND
87
ND
90
99
86
90
87
ND
90
ND
36
0
0
ND
ND
68
61
69
84
71
77
92
93
94
ND
94
94
ND
"Overall Removal = (1-N/Ng) x 100
N0 = influent flux
N = sum of effluent and off-gas fluxes
+ND = Not Determined
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200
5 700
I
1
D 100 mg/L PAC Bioreactor
O Control Activated Sludge
I i t t \
I t \ \
4.0
2.0
0.0
CO
CO
600
400
O 200
\ I
\\
\ \ \ \ \ \ \ \ \
1.0
0.5
0.0
\ \ \ \
0 7 14 21 28 35 42 49 56 63 70 77
Time (days)
Figure 5. Effect of a 100-mg/L influent PAC dose of Hydrodarco C on benzene effluent and
off-gas concentrations and fractional recoveries, N/N&
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200
c '00
I
D 100 mg/L PA C Bioreactor
O Control A ctivated Sludge
I
I
\
18.0
12.0
It 6.0
ki
7200
800
400
1.0
0.5
o.o
14 21
28 35
Time (Days)
42 49
56
Figure 6. Effect of a 100-mg/L influent PAC dose ofHydrodarco C on 1,2,4-trichlorobenzene
effluent and off-gas concentrations and fractional recoveries, N/N*
-------
Walter J. Weber, Jr.. and Bruce E. Jones are with the University of Michigan. A nn
Arbor. Ml 48109.
Sidney A. Hannah is the EPA Project Officer (see below).
The complete report, entitled "Toxic Substance Removal in Activated Sludge and
PA C Treatment Systems, "(Order No. PB 86- J 82 425/AS; Cost: $40.95, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
10
#U.S.GOV«NMINT PRINTING OfflO: 1986/646-116/20853
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