United States
Environmental Protection
Agency
Research and Development
Water Engineering Research
Laboratory
Cincinnati OH 45268
EPA/600/S2-86/018 Apr. 1986
4*EPA Project Summary
Rotating Biological Contactors
Hydraulic Versus Organic
Loading
Wayne F. Echelberger, Jr., John S. Zogorski, Tim W. McDaniel,
Daniel J. Scheidt, Chad P. Gubala, David R. Walker, Larry D. Good,
Michael J. Meyer, Marvin Lambert, and Garry S. Pugh
A study was undertaken to provide
plant-scale data to develop design and
operating criteria for rotating biological
contactor (FIBC) wastewater treatment
facilities. The study determined RBC
effectiveness over varying flowrates
and organic loads fortwo different flow
schemes—one based on hydraulic load-
ing rates with stages of equal media
surface area, and the second based on
organic loading rates with each stage
receiving approximately equal mass
loadings of organic matter (carbona-
ceous biochemical oxygen demand, or
cBOD). The flowrate was varied from
50% to 200% (430 to 1720 gpm) of
design flow. The hydraulic and organic
bays operated in parallel at and below
125% (1075 gpm) of design flow.
After treatment by 400,000 ft2 of
surface area, the hydraulic bay had
soluble cBOD concentrations that were
2 to 5 mg/L lower than those in the
organic bay in the parallel flow experi-
ments. Above 400,000 ft2, no statis-
tically significant difference occurred in
soluble cBOD concentrations. At flows
up to 125% of design flow, ammonia
nitrogen concentrations in the hydraulic
bay were from 0 difference to 4 mg/L
lower than those in the organic bay after
400,000 ft2 of treatment, and up to 1
mg/L lower in the hydraulic bay after
600,000 ft2 of medium treatment. After
this point, there was no statistical
difference between ammonia nitrogen
concentrations in the two bays. At flows
greater than 125% of design, no signifi-
cant nitrification occurred in either
train.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The wastewater treatment plant at
Columbus, Indiana, is a 12.4-million-
gallon-per-day (mgd) facility using rotat-
ing biological contactors (RBC) for
secondary treatment and nitrification.
Expansion and improvement to the plant
occurred in 1976 with the selection of the
RBC process to provide removal of carbona-
ceous biochemical oxygen demand (cBOD)
and conversion of ammonia-nitrogen to
other nitrogen forms. The RBC portion of
the plant consists of 10 treatment bays
with 8 shafts each. Because the size of
the Columbus installation allowed the
plant to operate at near-design conditions
while using only a portion of the facility, a
number of RBC units were available for
experimentation. Wastewater that has
undergone conventional primary treat-
ment enters the RBC train for secondary
treatment. The treated wastewater is
then passed on to a clarifier and a
chiorination tank before being discharged
to the East Fork of the White River.
The overall objective of this study was
to provide plant-scale experimental data
to develop appropriate design and opera-
tional criteria for RBC wastewater treat-
ment facilities. The specific goal of this
study was to determine RBC effectiveness
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over varying flow rates and organic loads
for two different flow schemes operating
side by side. The first flow scheme was
operated on the basis of hydraulic loading
rates with stages of equal media surface
area. The second flow scheme was
operated on the basis of organic loading
rates with each stage receiving approxi-
mately equal mass loadings of organic
matter (cBOD). Hydraulic and organic
loading rates varied from 50% to 200%
(430 to 1720 gpm) of design flow to
determine treatment effectiveness and
limitations for the two operational
schemes.
Figure 1 depicts the flow schemes for
the hydraulic and organic bays. A portion
of the primary treated wastewater was
diverted to the two experimental bays.
The hydraulic bay consisted of eight
shafts in conventional series configura-
tion. In this bay, stages and shafts were
synonymous. The organic bay consisted
of eight shafts in a tapered configuration.
The primary treated wastewater was
evenly divided among four parallel shafts
in stage one, followed by two shafts in
stage two, and one shaft each in stages
three and four. A side stream of the
effluent from each bay was diverted to a
pilot clarifier to characterize settled ef-
fluent quality.
Experimental Methods
The Columbus RBC Research Project
consisted of two full-scale trains of eight
shafts, each of which contained 100,000
ft* of media. One train was arranged for
hydraulic loading and one for organic
loading. Monitoring requirements includ-
ed flow (continuous), dissolved oxygen
(continuous and spot check), pH (con-
tinuous and spot check), temperature
(continuous and spot check), biochemical
oxygen demand (nitrification inhibited),
chemical oxygen demand, total filterable
residue, ammonia-nitrogen, nitrite-
nitrogen, nitrate-nitrogen biomass, and
power (see Figure 1). All analytical tests
were performed on daily composite waste-
water samples in accordance with U.S.
Environmental Protection Agency (EPA)
approved procedures. A laboratory quality
control/quality assurance program was
developed, reviewed, and accepted by
EPA and was rigorously adhered to
throughout the study.
A typical experiment lasted 14 to 21
days, with all parameters being monitored
throughout the experiment. Following
each process adjustment (flow increase
or decrease), a 10- to 14-day period of
stabilization was included in the experi-
mental protocol.
Selected Results
The concentration of residual soluble
cBOD depended on the cumulative waste-
water retentioh time in either the hy-
draulic or organic bays. In Figure 2, the
soluble cBOD versus retention time data
are illustrated for both bays. These data
tend to overlap one another, except for
retention times of less than 1 hr, when
the organic bay seems to have a slightly
higher remaining soluble cBOD concen-
tration. The reason for this somewhat
reduced level of treatment in the organic
bay is not specifically known, but it may
Influent from Primary Treatment
Hydraulic
Bay
Effluent
Monitored Parameters:
Auto Sampler
Dissolved Oxygen
pH Value
Temperature
(T) Load Cell
(Cn Pilot Clarifier
Figure 1. Sta'ge configuration of experimental bays.
Effluent
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have resulted from the increased staging
at the 1 -hr retention time in the hydraulic
bay compared with that in the organic
bay. These observations generally reveal
that RBC treatment is a high-rate process
that achieves low effluent cBOD concen-
trations in a short wastewater retention
time.
The total cBOD removal by both the
hydraulic and organic bays was quite
impressive throughout the study (Table
1). The increasing flowrate did not signifi-
cantly influence the treatment ability of
the two flow schemes, as demonstrated
by the relatively stable removal efficien-
cies for the 50% to 175% (430 to 1510
gpm) design flow experiments. When the
flow was increased to 200% (1720 gpm)
of the design flow, some deterioration
occurred in effluent quality, probably
because of the reduced residence time in
the treatment bays. The higher flowrate
could also have caused increased shear-
ing of biological slime from the media.
Table 1. Comparison of Total cBOD Removals in the Hydraulic and Organic Bay Experiments
Percent Removal of Total cBOD
Percent or uesign
Flowrate
50*
75*
100*
125
150
175
200
Hydraulic Bay
83+t
S3+'t
S3t
83^
73
83
67
Organic Bay
79*
85*
85 •
85*
82
80
72
*Side-by-side evaluation.
* Average of two replicate experiments.
•\Based on average of 700,000 and 800,000 ft2 of treatment.
thereby increasing the suspended solids
in the effluent and reducing total cBOD
removal efficiencies.
One of the major advantages of the
organic bay flow scheme is its ability to
lower the organic loading rate applied
during early stages of treatment. At the
Columbus RBC research facility, this
50
40
30 -
•8
Cb
•§
20 ~
10
Key
Hydraulic Bay
(_) Organic Bay
Figure 2.
7 2 3.4 5
Cumulative Fluid Retention Time, hours
Comparison of mean soluble cBOD concentration versus cumulative wastewater
retention time for the hydraulic and organic bay experiments.
lower rate was achieved by having
400,000 ft2 and 200,000 ft2 of media
surface area in the first and second stages
of treatment, respectively. The ability of
the organic bay flow scheme to distribute
the organic loading equally among stages
is apparent in Figure 3, which depicts the
organic loading rate versus the stage of
treatment for both the hydraulic and
organic treatment bays. Data from Ex-
periment 4 (100% design flow, 860 gpm)
and Experiments 8 and 11 (175% design
flow, 1510 gpm) are shown in Figure 3 to
illustrate how the organic loading varied
under average and high flowrate condi-
tions. Note especially the ability of the
organic bay flow configuration to maintain
a relatively constant organic loading rate
at each stage of treatment. In addition,
the loading to each stage was less than
2.5 to 4.0 Ib/day per 1000 ft2—the range
of maximum first-stage loadings recom-
mended by EPA to avoid the problems
associated with organic overloading. In
contrast, the organic loading in the hy-
draulic bay exceeded the EPA guideline in
many of the earlier RBC stages. These
results indicate that the organic flow
scheme allows equalization of the organic
load among stages, which may help to
avoid many of the problems that have
occurred when RBC units are organically
overloaded. Such conditions include ex-
cessive biomass growth with possible
structural failures, low oxygen or anoxic
conditions, undesirable odors, nuisance
organisms, and poor cBOD removal.
Conclusions
Specific conclusionsfrom the Columbus
RBC research project are as follows:
1. The mean total cBOD concentration
in the effluent for the various ex-
periments ranged from 63 to 95
mg/L and from 72 to 95 mg/L for
the hydraulic and organic bays,
respectively. Total cBOD removal
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efficiency in the hydraulic bay was
stable at 83% for all the design flow
experiments, with exceptions noted
at 150% (1290 gpm) and 200%
(1720 gpm} of design flow. In the
organic bay, the removal efficiency
measured 79% to 85% as the flow
changed from 50% to 75% (430 to
650 gpm) of design flow, and it
remained steady at the 85% level
through the 125% (1080 gpm) de-
sign flow studies. When the flow
was further increased, total cBOD
removal efficiency fell to 72% at
200% (1720 gpm) of design flow in
the organic bay and to 67% as the
flow changed from 175% to 200% of
design flow in the hydraulic bay.
The 4-2-1-1 shaft-staging configu-
ration of the organic bay allowed
soluble cBOD loadings to the first
stage of this treatment train to be
less than the range of maximum
loading recommended by EPA (2.5
to 4.0 Ib cBOD/day per 1000 ft2).
First-stage soluble cBOD loadings
ranged from a low of 0.44 Ib cBOD/
day per 1000 ft2 in the 50% (430
gpm) design f lowrate experiment to
a high of 2.2 Ib cBOD/day per 1000
ft2 in the 200% (1720 gpm) design
f lowrate experiment. In contrast to
the organic bay, first-stage soluble
cBOD loadings in the hydraulic bay
ranged from a low of 1.8 Ib cBOD/
day par 1000 ft2 in the 50% (430
gpm) design f lowrate experiment to
a high of 7.3 Ib cBOD/day per 1000
ft2 in the 150% (1290 gpm) design
f lowrate experiment. I n addition, the
first-stage cBOD loadings exceeded
even the upper limit of EPA's guide-
line in all experiments where the
flowrate equalled or exceeded 100%
(860 gpm) of the design flowrate.
The ability to achieve low effluent
cBOD concentrations and comply
with the EPA organic loading guide-
line is an advantage of the organic
bay's staging configuration com-
pared with the more commonly used
flow scheme in the hydraulic bay.
The experimental rneansfor soluble
cBOD concentration in the influent
to the experimental RBC treatment
trains ranged from 29 to 47 mg/L
and from 26 to 46 mg/L for the
hydraulic and organic bays, respec-
tively. Both treatment trains consis-
tently reduced the influent soluble
cBOD to less than 10 mg/L under
flowrate variations between 50%
and 200% (430 and 1720 gpm) of
Hydraulic Bay
t§
.c
ll
1!
345
Stage of Treatment
\ Stage of Treatment
i
Figure 3. Comparison of mean soluble cBOD loading rates for selected experiments in the
hydraulic and organic bays.
the design flowrate. Mean effluent
soluble cBOD levels as low as 2 to 4
mg/L were achieved in both treat-
ment configurations at rates lower
than 100% [(860 gpm) of the design
flow. Effluent levels of soluble cBOD
were slightjly higher when the ex-
perimental flow exceeded 75% (650
gpm) of the design flow. The highest
mean effluent soluble cBOD level
for the organic bay was 8.3 mg/L at
200% (1720 gpm) of design flowrate
(Experiment 12). Similarly, the
highest effluent soluble cBOD con-
centration in the hydraulic bay was
9.2 mg/L, again at 200% (1720
gpm) of the design condition (Ex-
periment 9). Removals of mean
soluble cBOD exceeded 80% for all
experiments except those in the
hydraulic bay at 200% of the design
flow where a 73% removal was
1
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recorded. Soluble cBOD removals
between 87% and 94% were evident
for both the hydraulic and organic
bays at flowrates at or below 100%
(860 gpm) of the design flowrate.
4. The removal of soluble chemical
oxygen demand (COD) followed a
pattern similar to that of soluble
cBOD. Mean soluble COD concen-
trations in the effluent from the
hydraulic and organic bays were
always below 60 mg/L. Effluent
COD levels of 30 to 40 mg/L were
achieved in both treatment trains at
flowrates equal to or less than 100%
(860 gpm) of the design flowrate—
except for hydra ul ic bay Experiment
3 (75% of design flow, 650 gpm), in
which an uncommonly high mean
effluent COD value of 52 mg/L was
recorded. Cumulative removals of
COD ranged from 49% to 74% and
from 59% to 72% in the hydraulic
and organic bays, respectively. As
expected, COD removals increased
when the experimental flowrate
was decreased.
5. The hydraulic treatment bay
achieved slightly lower soluble
cBOD concentrations after the
400,000 ft2 surface area treatment
than did the organic bay. This
observation was based on the com-
parison of daily soluble cBOD con-
centrations for those experiments
during which the hydraulic and
organic bays were compared on a
true side-by-side basis. The greater
soluble cBOD removal in the hy-
draulic bay at 400,000 ft2 of treat-
ment was confirmed from statistical
analysis with a 90% confidence
level. Again, when comparing data
from parallel flow experiments, the
concentrations of soluble cBOD in
the hydraulic and organic bays were
not statistically different after treat-
ment by 600,000 ft2 (or more) of
media surface area.
6. In both the organic and hydraulic
treatment bays, the concentration
of soluble cBOD and COD at each
stage of treatment appeared to be
related to the cumulative fluid re-
tention time. A majority of the
observed removals occurred within
the first 2 hr of treatment, after
which the removal rates of both
cBOD and COD became extremely
slow. For practical purposes, the
ultimate removal achieved was
nearly complete after 2 hr of reten-
tion time. This observation confirms
that RBC treatment, regardless of
the flow configuration, is a high-
rate process in which low effluent
cBOD and COD concentrations may
be obtained with a short wastewater
retention time. For example, the
relationship of soluble cBOD re-
maining versus wastewater reten-
tion time (which was developed
from the hydraulic bay experiments)
would predict an effluent soluble
cBOD concentration of 10 mg/L
after 1 hr of treatment and a con-
centration of 5 mg/L after 2 hr of
treatment.
7. The organic bay flow configuration
offers several operational advan-
tages over the commonly used
hydraulic bay configuration. The
equalization of organic loading
among stages that is accomplished
in the organic bay flow configuration
allows this treatment system to (a)
meet EPA's organic loading guide-
line; (b) better withstand organic
shock loadings; (c) control the
growth of nuisance organisms,
odors, and similar problems associ-
ated with organic overloading; (d)
maintain acceptable dissolved oxy-
gen values in all stages of treatment,
thereby maintaining an aerobic
treatment process and eliminating
oxygen transfer as a rate-limiting
step in the removal of organic
matter; and (e) control the occur-
rence of excess biomass growth
that can cause structural failure of
RBC shafts. These operational ad-
vantages are attained in the organic
bay flow configuration with no
adverse impact on treatment per-
formance. As noted previously (see
Concusion 5), a slightly lower cBOD
was obtained in the organic bay
with 400,000 ft2 of media area than
in the hydraulic bay. The difference
was small, however, and was not
recorded after treatment by 600,000
ft2 of surface area.
8. The level of ammonia-nitrogen
present in the effluent from the
hydraulic and organic bays varied
depending on the flowrate and
organic loadings. In experiments
below 100% (860 gpm) of the design
flowrate, both treatment bays were
able to reduce the influent ammonia-
nitrogen level to less than 1.0 mg/L,
indicating that effective nitrification
was occurring. For the 100% to
150% (860 to 1290 gpm) design
flowrate experiments, some reduc-
tion in the concentration of
ammonia-nitrogen was evident, al-
though only partial nitrification of
the effluent was accomplished. Very
little to no change in the ammonia-
nitrogen concentration was evident
between stages in either the organic
or hydraulic treatment bay at the
175% and 200% (1510 and 1720
gpm) design flowrate experiments.
This result shows that nitrification
was not significant at these high-
flow conditions.
9. In the parallel flow experiments in
which the hydraulic and organic
bays were compared directly, the
ammonia-nitrogen concentrations
after 400,000 and 600,000 ft2 of
treatment were lower in the hydrau-
lic bay. Statistical analysis supports
this conclusion with a 95% confi-
dence level. The superior perform-
ance of the hydraulic bay is thought
to have resulted from the increased
level of staging in this bay's flow
scheme. Increased treatment per-
formance is expected with increased
staging for processes in which the
kinetic order is something other
than zero.
10. Nitrite- and nitrate-nitrogen produc-
tion was closely associated with the
removal of ammonia-nitrogen. As
the flowrate was increased, the
concentration of nitrite- and nitrate-
nitrogen decreased and the produc-
tion occurred in the later stages of
treatment. At flowrates exceeding
150% (1290 gpm) of design, little to
no nitrite- and nitrate-nitrogen pro-
duction was evident in either the
organic or the hydraulic bay. This
result indicates that nitrification
was not occurring at either the
175% or the 200% (1510 or 1720
gpm) flow conditions.
11. The organic bay showed an ability to
resume nitrification shortly after a
reduction from 200% to 75% (1720
to 650 gpm) of the design flowrate.
This result suggests that nitrifying
organisms were still present in the
200% (1720 gpm) flowrate experi-
ment but that overriding factors
rendered them ineffective in lower-
ing the ammonia concentration.
Conversely, increasing the flowrate
from 50% to 200% (430 to 1720
gpm) of design resulted in an im-
mediate and significant decrease in
the removal of ammonia.
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The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
807463 by the City of Columbus, Indiana,
under the sponsorship of the U.S. Envi-
ronmental Protection Agency.
W. F. Echelberger, Jr., J. S. Zogorski, T. W. McDaniel, D. J. Scheldt, C. P. Gubala,
andD. R. \l\ialkerare with Indiana University, Bloomington, IN47405; L D. Good
andM. J. Meyer are withSIECO, Inc., Columbus, IN 47201; M. Lambert, andG.
S. Pugh are with City Utilities, Columbus, IN 472O1.
Edward J. Opatken was the EPA Project Officer (see below for present contact).
The complete,report, entitled "Rotating Biological Contactors—Hydraulic Versus
Organic Loading." (Order No. PB 86-160 322/AS; Cost: $16.95, subject to
change) wifl be available only from:
National Technical Information Service
5285, Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For further information, Francis L. Evans, III, can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
•&U. S. GOVERNMENT PRINTING OFFICE: 1986/646-116/20801
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United States
Environmental Protection
Agency
Center for Environmental JResearch
Information I
Cincinnati OH 45268
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POSTAGE & FEES PAID
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Official Business
Penalty for Private Use $300
EPA/600/S2-86/018
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