\ I /
United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-106 July 1984
Project  Summary
Design  Information  on  Rotating
Biological  Contactors
Richard C. Brenner, James A. Heidman, Edward J. Opatken, and Albert C.
Petrasek, Jr.
  A document summarizing design
information for  rotating biological
contactors  (RBC) was developed to
provide  in-depth  data on the critical
features of this process  and the key
factors  affecting its operation and
performance. The information provided
is intended  to supplement and qualify
that available from RBC manufacturers
and the published  literature. Topics
addressed include process and design
considerations for  carbonaceous re-
moval, nitrification, and damnification;
equipment reliability and service life;
power requirements for air driven and
mechanically driven RBC units; and
general system design considerations
involving structural, hydraulic, and
operational flexibility. Practical, usable
design information was  emphasized
along  with important theoretical con-
cepts.
  This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory. Cincinnati. OH.
to announce key findings of the design
information document that is available
in its entirety in a separate report  of the
same  title (see Project Report ordering
information at back).

Introduction
  RBC's for municipal wastewater treat-
ment  have been  introduced relatively
rapidly to the United States. The  result
has been widespread application of a
technology that is not familiar to  many
design engineers. Out of necessity, many
RBC designs were initially based  solely
on proprietary-generated empirical design
procedures. The lack of a comprehensive
appraisal of modified media configurations
left design  engineers and equipment
purchasers with an inadequate basis for
comparing performance of alternative
equipment designs and effectively relating
anticipated performance to loading. As
interest in the  process has increased,
more  complex,  deterministic design
approaches have begun to appear in the
technical literature.
  The  purpose  of this  project was to
prepare a design information document
that would  supplement commonly ac-
cepted RBC design methodology (such as
manufacturers' design manuals and
deterministic models) by providing appro-
priate qualifiers and  summaries of
operating and  performance data not
readily available to the design community.
Important design parameters and  rela-
tionships (or lack of them) are discussed
to promote a more rational RBC design
approach. Many aspects of the informa-
tion contained in the document are
equally applicable to industrial wastewa-
ter treatment. The document is not
intended to serve as a cookbook-style
design reference or to replace any of the
design guides mentioned above.
  The  full report considers the equipment
and design practices of the five current
U.S. RBC manufacturers — Autotrol (now
Envirex), Clow, Crane-Cochrane, Lyco,
and Walker Process.* Significant varia-
tions in shaft, media configuration, and
media attachment designs are evident
among the five  and must be taken into
account in designing an RBC system.
•  Information was originally presented
on the equipment and design practices of
a sixth U.S. RBC manufacturer.  This
manufacturer  has  recently stopped
•Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.

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marketing RBC units and is referred to
hereinafter as Manufacturer X. Descrip-
tions of Manufacturer X's equipment
have subsequently been removed from
the full report and this Project Summary,
but published  references to and discus-
sions of its previous design methods have
been retained  where appropriate.


Equipment  Description and
Performance
  The most serious equipment problem
that can  affect an  RBC system  is the
structural breaking (failure) of a shaft. A
shaft failure usually involves temporary
loss of the affected RBC unit and perhaps
its entire train, damage to a portion of the
media, and replacement of the  broken
shaft with a  new  shaft along with
salvaged and/or new media. Numerous
failures of first-generation shafts  in this
country  have been  attributed to poor
fabrication and welding practices. Im-
proved second-generation designs have
corrected most of the early manufacturing
deficiencies.  Shaft failures have also
occurred because of fatigue resulting
from overstressing of the member's load-
carrying capacity.
  A matrix of estimated shaft  fatigue
limits and shaft fatigue  lives was devel-
oped for the shaft cross-sections shown
in  Figure 1  as a function of shaft
attachment detail, shaft wall thickness.
         and  combined media and biofilm  load.
         Appropriate fatigue provisions of the
         American Welding Society (AWS) Struc-
         tural Welding Code for Design of Tubular
         Structures (Section  10) and Design of
         New Bridges (Section  9) were used to
         determine the fatigue limit of each shaft
         design and the estimated shaft life
         when  imposed loads exceed fatigue
         limits.  A  shaft  member subjected to a
         stress range below its fatigue or endurance
         limit will  not fail (with a high degree of
         confidence) as  a result  of structural
         fatigue.
           Fatigue loading limits for RBC shafts
         can  be expressed as a function of the
         thickness of the biofilm attached to the
         rotating  media.  Estimates of allowable
         biofilm thickness that keep the  imposed
         load below the respective fatigue limits of
         the six shafts varied depending on  shaft
         structural dimensions  and characteris-
         tics. The  ranges  calculated  were 75 to
         >150  mils (0.075  to >0.15 in.) for
         standard  density media and  50 to  125
         mils (0.05 to 0.125 in.) for high density
         media.
           These estimates of shaft fatigue limits
         were made assuming  that  no welding
         deficiencies or shaft deterioration existed
         as a result of corrosion. The weldment
         provisions of the AWS Structural Welding
         Code should be used as  an  absolute
         minimum to ensure satisfactory weld-
         ment fabrication. Epoxy  coatings are
         Clow
     -28-in. O.D.
                                 Walker Process
16-in. square
  (outer
  dimension)
                                       ion)
                               Weld
                               Locations-~-
           — 1 in.
                                 Autotrol
         Lyco
     Crane- Cochrane

     outer face
-24 in.   to   —
     outer face
                            Weld
                           \Locations__|_
                                                        Lyco/Hormel
 Figure 1.    Cross-sections of RBC shafts.
routinely used by RBC manufacturers to
protect shafts from the corrosive effects
of wastewater. Available information
from coating manufacturers indicates
that epoxy coating materials have an
expected life of 5 to 10 years or more, but
long-term data are limited.
  Shaft fatigue protection  may also  be
provided  by  increasing shaft section
modulus (additional shaft wall thickness)
to decrease the applicable cyclic stress
range and compensate for possible metal
loss resulting from corrosion. In selecting
and specifying RBC shafts, the designer
should require evidence from the shaft
manufacturer that satisfactory fatigue
stress protection (including protection
from possible corrosive effects) has been
provided to cover the design life of the
RBC  system  under  expected loading
conditions, either by providing  reliable
protective coating material or by modifying
the shaft design to increase the  section
modulus.
  Design techniques  can be used to
minimize shaft overstressing from exces-
sive biofilm growth. These techniques
include (1) incoming load manipulation to
bypass temporarily the organically over-
loaded  stages (usually the  first  two)
and/or to distribute that load to  greater
media  surface  through step feeding or
removal of baffles and (2) variable speed
mechanical drives to reduce biofilm
thickness by means of increased shear.
For some mechanically driven systems,
strategic placement of supplemental  air
headers to increase biofilm  shearing
rates may also represent a cost-effective
method. A secondary  benefit of variable
speed  drives and/or supplemental  air
would be the ability to increase tempora-
rily the concentration of bulk liquid dis-
solved oxygen (DO) in specific stages.
  The plant operator  must be provided
with sufficient flexibility to respond to
changing load and process conditions. In
addition to mechanisms for manipulating
load and increasing  DO  concentration
and biofilm shearing potential,  recom-
mended operational  tools include (1)
shaft  load cells for estimating  biofilm
weight (particularly in the critical lead
stages), (2) DO monitoring equipment to
aid in process  control, (3) flow  control
devices to equalize hydraulic and organic
loads to  parallel trains, and (4) air flow
measuring equipment to adjust air flow
rates  properly  in  air  drive RBC installa-
tions.'
  High  density polyethylene (HOPE) is
used by all manufacturers  in the fabrica-
tion of RBC media. The method of forming
a media pack and attaching it to the shaft
determines whether an RBC assembly

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must be removed from its tank for field
replacement of damaged or deteriorating
media. In designing an RBC system, the
engineer should consider the operation and
maintenance ramifications of field media
replacement.  If  a  particular  design
requires removal  of the RBC assembly
from  its tank  to perform  this  task,
permanently installed shaft removal
equipment  may be  cost  effective. The
alternative is to bring in an over- or off-
the-road crane as needed. Present media
pack attachment designs of Clow, Crane-
Cochrane,  and  Lyco  do not require
removal from the tank for media replace-
ment,  but  current Autotrol, Tait/Bio-
Shafts, and Walker Process designs do.
  A standard RBC assembly consists of
media  approximately 12  ft in diameter
supported on or from a 27-ft-long shaft
(25-ft media length).  Standard  density
media are  defined  as those with a
nominal  surface  area of 100,000 sq
ft/25  ft of media length. High  density
media are formed by reducing the space
between  repeating plastic corrugations
by 33  percent, thus achieving a media
configuration with 50 percent  more
available surface  area (i.e.,  a nominal
surface area of 150,000  sq ft/25 ft of
media length).
  Standard density media are normally
used in the front or lead stages of an RBC
train where organic strength is highest
and  biofilm growth  is thickest.  High
density media have been  used primarily
in the middle and final  stages where
lower  organic concentrations will not
promote and sustain thick biofilm growth.
Excepting for second-step nitrification
applications where the organic load  is
known to be low, use of high  density
media  in  lead  RBC  stages  should be
avoided because of potentially excessive
biofilm development per unit volume of
media  (resulting in shaft  overstressing)
and the increased possibility of inducing
biofilm bridging between adjacent media
sheets. Recommended rules of thumb are
to limit media  biofilm  thickness, if pos-
sible,  to 75 mils (0.075 in.) for standard
density media and  50 mils (0.05 in.)
for high  density  media to provide an
acceptable margin of safety against shaft
overload and possible fatigue failure
These  recommendations do not apply to
Walker Process and Lyco because of the
much higher fatigue limits of their shafts.
   Early media designs were subject to
degradation from  the ultraviolet compo-
nent of sunlight. The use of anti-oxidants
in HOPE formulations and protective
covers have essentially eliminated this
problem. Prolonged  exposure to high
temperature environments and concen-
trated organic solvents can also lead to
media degradation and/or brittleness.
Cases of media rupture resulting from the
poorly understood phenomenon of stress
cracking have also been reported. Media
can be broken while in service as a result
of lateral  shifting in assemblies using
radial arm support systems if inadequate
bracing and tightening capability are not
provided. On the average, however,
media failures have not been encountered
as often as shaft failures.
  The drive assembly has proven to be the
most reliable  equipment component in a
mechanically  driven RBC system. Occa-
sional broken drive chains are  the only
problem of note.  Operating  experience
with air driven systems is too limited to
date to make definitive projections of
equipment performance. The critical
aspects of air  driven system performance
currently appear to relate more  to
achieving desired air distribution among
multiple shafts and maintaining uniform
rotational speeds of individual shafts
than they  do to reliability  of the  air
delivery equipment itself.
  In  a  few severe cases where biofilm
growth on RBC media  has  become
grossly  uneven or unbalanced, lack of
positive rotational control with air driven
systems has  brought shaft rotation to a
complete  halt. Providing the operator
with load manipulation capability and/or
equipment to achieve rapid biofilm
stripping  (e.g.,  high pressure water
injection) are the most effective techniques
for responding to these  situations.
  The full report summarizes the struc-
tural and mechanical performance of 17
RBC plants surveyed using questionnaires
and onsite visits. Data are presented for
the three major RBC system component
units (shaft, media, and drive).

Organic Removal
  Under most circumstances,  mass
transfer is the dominant factor affecting
organic removal in an  RBC  system.
Mass transfer resistances associated
with both the liquid phase and the biofilm
result in significant concentration gradi-
ents from the bulk liquid to biological
reaction sites on the media, and frequently
these resistances control system perform-
ance. Oxygen  transfer becomes rate
limiting and controls the overall reaction
rate m heavily loaded systems.  After all
of the factors affecting substrate removal
and oxygen transfer are considered, no
reason exists to expect substrate removal
from the -RBC bulk  liquid to  follow any
simple mathematical model.
  Oxygen loading to any stage of an RBC
system should not exceed values compa-
tible with the oxygen transfer'capability of
the system. COD balance studies conduc-
ted on  a full-diameter RBC unit  have
indicated  a maximum oxygen transfer
rate of approximately 1.5 Ib Oz/day/1000
sq  ft. Most oxygen transfer in an RBC
takes place in the atmospheric portion of
the rotational cycle.
  The presence  of Beggiatoa organisms
on  RBC media is associated with biofilm
DO depletion and  is considered  an
indication of organic overloading. In the
absence of DO, sulfide  essential for
Beggiatoa growth is generated by means
of  sulfate  reduction and/or anaerobic
decomposition deep  within the biofilm.
When designing a mechanically driven
RBC (System, research and field observa-
tion^ indicate that safe, conservative
firstfstage  loading figures to avoid
orgahic  overloading  are  2.5 Ib soluble
BODs/day/sq ft  or 6 Ib total  BODs/day/
100d sq ft. These figures may be increased,
but the designer  must recognize that this
increase  may lead to heavier-than-
normal  biofilm growth, bulk liquid DO
depletion, development of sulfide oxidizing
nuisance organisms,  deterioriation of
overall  process and/or mechanical
performance, etc.
  A first-stage  loading exceeding the
above figures may be justified depending
on  the degree of operational and main-
tenance attention the plant will receive,
the structural capacity of the selected
shaft, the ability to strip excess biomass
from  the media, the  levels of sulfur
compounds  in the RBC system influent,
the media surface area required in the
remaining stages, and the ability  to vary
the operational  mode of  the plant. The
loading, however, should  generally not
exceed 4 Ib soluble BOD5/day/1000 sq ft
or 8 Ib total  BOD5/day/1000 sq ft.
  Mass organic removal rates in  an
overloaded RBC stage may exceed those
of a stage operating in a proper loading
range if sulfide levels are low enough to
avoid extensive Beggiatoa  growth. In
many instances,  however, mass removal
rates  in overloaded stages may actually
decrease as Beggiatoa organisms begin to
compete seriously with desirable hetero-
trophs for oxygen and space on  RBC
media. In extreme cases, the takeover of
the first stage  of an  overloaded  RBC
system by Beggiatoa can shift the load to
the next stage,-leading to a  progressive
Beggiatoa takeover of the entire system
and significant deterioration of effluent
quality.
  Present procedures for scaling up pilot
test data collected  on less-than-full-
diameter RBC  units to full-scale  design
are inadequate. Mass removal  data

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generated on less-than-full-diameter
units operated at equivalent f ieldscale tip
speeds (60 fpm ±) tend to be optimistic
because  of  the higher  rates of oxygen
transfer and atmospheric surface renewal
inherently achieved. The distortion  is
magnified as the scale decreases.
  Deterministic mathematical  models
that attempt to incorporate all of the
factors affecting the RBC process have
been used in some design situations  in
conjunction with the results of smaller-
scale RBC pilot studies carried out on the
wastewater in  question. Though these
models provide considerable insight into
the  variables and ranges of variables
affecting RBC performance,  their suc-
cessful  application to design depends
greatly on accurate calibration of model
coefficients.
  Very small-scale RBC pilot units (1.5-ft
diameter or less) are  more useful  in
determining the basic treatability of a
wastewater than in establishing full-
scale design parameters. If small-diam-
eter units must be used to collect design
data, however, it is important that each
stage be loaded  below the  oxygen
transfer capability of a full-scale unit to
minimize scale-up  considerations.  Full-
diameter (12 ft ±) or nearly full-diameter
RBC pilot studies are recommended where
feasible, since they offer the  highest
probability  of obtaining  reliable  RBC
design data.
  Additional factors affecting  organic
removal in RBC's include, among others,
the ratio of tank volume to media surface
area (detention time), influent wastewater
strength,  influent flow and load variability,
wastewater temperature, and  staging
configuration. Most RBC manufacturers
have standarized ratios of tank volume-
to-media  surface area at 0.12 gal/sq ft,
since higher ratios did not improve BOD
removal at equal hydraulic loading rates.
For municipal wastewaters,  percentage
BOD removal  normally  increases with
increasing wastewater strength; the con-
verse is generally true for high-strength
industrial wastewaters.
  Loading variations are  not assumed by
the manufacturers to affect process per-
formance adversely at peak-to-average
flow ratios of 2.5 or less. The Autotrol and
Clow design manuals recommend using
either peak flow and load conditions for
design or flow equalization for ratios
above 2.5.
  The manufacturers universally contend
that about 55°F wastewater temperature
does not  affect organic removal design.
Below 55°F, varying degrees of decreased
biological activity are  predicted  by the
manufacturers.
   The optimal number and size of stages
 and the overall staging configuration for a
 given  design  are frequently difficult to
 predict. Flexible  hydraulic designs that
 enable the plant  operator to change the
 size and  number of stages (e.g.,  step
 feeding and stage  bypass provisions,
 removable baffles, etc.) will enhance the
 potential for maximum performance from
 any RBC facility.
   No single best  design procedure or set
 of relationships has been found that can
 universally predict RBC organic removal.
 Empirical design approaches used by four
 manufacturers (Autotrol, Clow, Lyco, and
 Manufacturer X) exhibit considerable
 variation  in  organic loading  versus
 predicted  effluent quality  (Figure 2).
 Except for Lyco,  these empirical design
 techniques are based on predictions of
 effluent soluble  BOD5. Effluent total
 BOD5 projections are then made assum-
 ing a set ratio (e.g., 0.5) of soluble-to-total
 BODs  in the effluent. This  assumption
 ignores (1) the impact of solids settling
 characteristics on final clarifier perform-
 ance   and (2) the wide variations  in
 effluent suspended solids and paniculate
 BOD5  that are possible with any,given
 concentration of effluent soluble BODs. In
 the absence of full-diameter pilot plant
 data for the design in question, any RBC
 design guidelines should be used with
 discretion.
  To devise  an  improved method  for
 estimating organic removal, soluble
 organics interstage data were evaluated
 using  second  order kinetics. Interstage
 and final  effluent soluble BODs values
                                         predicted using this approach are com-
                                         pared in the full report with measured
                                         values obtained  at  nine  full-scale,  air
                                         driven RBC plants.  The  predicted and
                                         measured values are in good agreement
                                         for seven of the nine plants. The lack of
                                         close agreement at one of the other two
                                         plants could be explained  by inadequate
                                         oxygen transfer in the first stage to handle
                                         the high  influent organic loading. The
                                         lack of close agreement at the ninth plant
                                         could not be explained.
                                           The second-order predicted values
                                         more closely  approximated measured
                                         soluble  BODs concentrations than  did
                                         values predicted by Autotrol's empirical
                                         organic removal design method for air
                                         driven RBC's. These results indicate that
                                         second-order kinetics may offer  an
                                         improved basis for predicting interstage
                                         soluble organic removals in RBC systems
                                         that are not oxygen transfer limited; they
                                         also suggest that further evaluations be
                                         conducted for both mechanically and air
                                         driven options as additional interstage
                                         data become available. As with manufac-
                                         turers' empirical techniques, the second-
                                         order kinetic approach only addresses the
                                         impact of soluble  organics  on  effluent
                                         quality.

                                         Nitrogen Control

                                         Nitrification
                                           The factors affecting nitrification in the
                                         RBC process are representative of those
                                         that affect any attached growth biological
                                         process.  They  include  influent  organic
                                         concentration, influent  nitrogen concen-
   40

a  20
,o
   10
                                     AUTOTRQL
                                        AND
                                       CLOW
                                                     Lyco ffSOJ
        Note: Soluble BODs/totalBODs =

                          Lyco (JOO}
                                                                Lyco 1225)

                                                                 MFR X  1301
                                                      <(20)
                                                   Note: Numbers in f ) are
                                                         influent soluble BODi
                                                         concentrations for
                                                         Manufacturer X and total
                                                         BODs concentrations
                                                         for Lyco.
Figure 2.
               1         2         3          4         5          6        7
                    Organic Loading Rate lib total BODs/day/ WOO sq ft)

           Effluent BODs as a function of organic hading for selected RBC manufacturers
           design techniques.

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tration  and composition, wastewater
temperature, DO concentration, pH and
alkalinity, and influent flow and load
variability, among each others.
  Most  empirical design procedures are
based on the assumption that significant
nitrification does  not begin  in an RBC
system until bulk liquid soluble BOD5 has
been reduced to 15 mg/L, or total BOD5 to
30 mg/L. In combined carbon oxidation/
nitrification units,  BOD5  values  of this
magnitude  may first be encountered in
the second,  third,  or fourth stages,
depending on  influent strength, organic
loading  rate, and wastewater tempera-
ture. In  separate-stage nitrification RBC
applications, the soluble  BOD5 concen-
tration of the  wastewater entering the
RBC reactor  is usually well below 15
mg/L and substantial nitrification is
typically evident in the first  stage.
Analysis of field data indicates that while
incipient nitrification is generally observed
in RBC stages with soluble BOD* concen-
trations of 15 mg/L ± maximum nitrifica-
tion rates are not attained until bulk liquid
soluble  BODs drops to 10 mg/L or less.
  Numerous investigators have found
that RBC  nitrifier growth kinetics  and
ammonia nitrogen oxidation follow the
classic Monod expression.  When substrate
(ammonia nitrogen) concentration is low
compared with the half-velocity constant,
ammonia nitrogen oxidation approaches
first-order removal. Conversely, when
substrate concentration is high in relation
to  the  half-velocity  constant, oxidation
approaches a constant zero-order removal
rate.
  The  zero-order  ammonia  nitrogen
removal rate observed for RBC's and the
ammonia nitrogen concentration transi-
tion point at which it begins depend on
scale and wastewater temperature. Zero-
order removal  rates as high as 0.8 Ib NH3-
N /day/1000 sq ft have been measured
on small pilot units.  For full-diameter
RBC's,  the maximum attainable zero-
order removal rate is generally acknow-
ledged to be about0.3 lbNH3-N/day/1000
sq  ft. As with organic removal, higher
 inherent rates of atmospheric surface
 renewal and  oxygen transfer are the
 probable reasons for the higher mass nitri-
fication  rates  attainable  with pilot RBC
 units.
   Because of the  varying  degrees of
 hydrolysis that can occur in sewer lines,
 RBC  nitrification  designs should not be
 based  on  influent  ammonia nitrogen
 concentration alone. To do so risks serious
 undersizing if substantial amounts of
 organic nitrogen are present.
   Nitrification rates drop  sharply at
 wastewater temperatures below 55°F.
The manufacturers have found, however,
that wastewater temperatures above 55°F
do not affect RBC nitrification. Nitrification
temperature correction factors recom-
mended by four  manufacturers are
given in the full report.
  Minimum DO levels of 3 to 4 mg/L are
recommended in RBC nitrifying stages to
ensure that nitrification is not limited by
oxygen transfer info the biofilm. Lower
DO levels are more likely to occur in the
transition stage from organic removal to
incipient nitrification and in the next
succeeding stage than in the final stages.
The beneficial effect on DO of compressed
air injection  in  air  driven  RBC's is
approximately offset by their lower
atmospheric oxygeriation (compared with
mechanically driven units), which results
from reduced rotational velocities (typi-
cally 65  to 75 percent of the standard
mechanically driven velocity of 1.6 rpm).
  The literature recommends optimal pH
values for nitrification ranging from 7.0to
9.0.  If pH drops much below 7.0 in
unacclimated systems, the nitrification
rate will be seriously retarded, decreasing
to zero somewhere between pH 6.0 and
5.0. Since nitrification reactions consume
alkalinity,  alkaline addition may be
required  for RBC  nitrification systems
treating wastewaters with low or highly
variable alkalinity.
  Nitrification rates have been enhanced
through upward pH  adjustment  in pilot-
scale RBC units, with the optimum rate
occurring at pH 8.5. Similar response has
not been demonstrated on full-diameter
RBC's.
  The impact of flow and mass loading
variations is usually more severe on the
nitrification efficiency of  RBC's than on
organic removal performance. Nitrifiers
have  long  generation  times  compared
with heterotrophs. Furthermore, external
accumulation and/or internal  storage of
substrate for delayed metabolism  does
not take place with  nitrifiers as  with
heterotrophs. Consequently, influent
surges in flow or unoxidized nitrogen
concentration (either of which increases
nitrogen  mass loading) will be accompa-
nied  by similar delayed (roughly equal to
reactor detention time) spikes of unoxi-
dized nitrogen  in  the effluent  unless
adequate RBC  surface is  provided to
compensate for expected variations. In
design situations that specify consistently
low effluent ammonia nitrogen residuals
(1  to 2  mg/L),  the amount of media
surface required  should be estimated
with and without prior flow equalization
to determine which option is more cost
effective.
   Since  the mid-1970's, Autotrol has
conducted extensive testing and data
evaluation to  model ammonia nitrogen
oxidation in RBC's. Their current empirical
procedure  for full-diameter RBC  units
operating at wastewater temperatures of
55 °F or greater is based on a Monod-type
curve (Figure 3). This curve projects first-
order removal (oxidation) of ammonia
nitrogen  at concentrations in the stage
liquid below about 5 mg/L. Above 5 mg/L
NH3-N, removal is claimed to proceed at a
zero-order  rate of  approximately 0.3 Ib
NH3-N/day/1000  sq ft. This rate is
consistent with the maximum potential
nitrification rate (0.33 Ib NH3-N/day/1000
sq ft) that can  be calculated based on an
assumed peak oxygen transfer capability
for 12-ft diameter  media of  1.5 Ib
O2/day/1000 sq ft (see the earlier
section on Organic Removal).
  Applied to a staged RBC reactor, the
above design  basis predicts that when
soluble BODs drops below 15 mg/L,
ammonia nitrogen will be oxidized at a
constant (zero-order) rate  as it  passes
through  succeeding stages down to a
bulk liquid concentration of about  5
mg/L, and thereafter at an approximate
first-order rate. Nitrification performance
data from three full-scale RBC facilities
were evaluated for wastewater tempera-
tures of 55 ±  2°F.  The resulting best fit
curve of ammonia nitrogen removal rate
versus ammonia nitrogen concentration
closely approximates the Autotrol design
curve (Figure 3).

Denitrification
  Denitrification can be accomplished
with RBC's using two different process
configurations. In the first, representing
the traditional biological denitrification
approach, organic removal and nitrifica-
tion occur upstream of the denitrification
reactor in either a lead-stage(s) RBC unit
or some other type of biological system. A
supplemental carbon source (commonly
methanol) must be added to the waste-
water to  provide the required energy for
microbial denitrification  in the denitrifi-
cation (anoxic) stage. A short-term, final,
aerated  RBC  stage  or  other polishing
biological unit  may also have to be added
to oxidize any residual methanol not used
for denitrification.
  A possible alternative  RBC denitrifica-
tion configuration would be to place the
anoxic stage before the organic removal
and nitrification reactor(s). This technique
is well  known  for activated sludge
systems, but thus far it has not been used
in any full-scale RBC installations. The
organic carbon naturally present in the
incoming wastewater provides the neces-
sary energy for reducing nitrate nitrogen

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      0.4
      0.3
  •S  0.2
       0.1
                                     Wastewater Temperature = 55°F
              1.5    3.0    4.5    6.0    7.5    9.0    10.5

                            Stage NH3-N Concentration (mg/L)
                                                 12.0   13.5   15.0
Figure 3.
Second-generation Autotrol ammonia nitrogen removal rate curve for full-scale
RBC's (from: Antonie, R.L.. "Nitrogeii Control with Rotating Contactors," Autotrol
Corporation, Undated; Reprinted with*permission).
recirculated to the anoxic stage from the
downstream reactor(s). Though  the
maximum potential  denitrification effi-
ciency at recirculation-to-plant influent
flow ratios of 1 to 3 is reportedly only
about 75 percent, this method has the
advantage of not requiring a supplemental
carbon source.
  To  keep  the denitrification  stage
anoxic, the RBC media must be complete-
ly submerged inthewastewater. Rotation
is achieved with mechanical drives at an
angular velocity of approximately 1.6
rpm. At this  time, Autotrol (now Envirex)
is the only manufacturer marketing
RBC's for denitrification.
  Autotrol's  design procedure is based on
pilot  studies indicating  that  in  the
presence of  an  adequate carbon source
the  denitrification rate  in  RBC's is
independent of bulk liquid nitrate nitrogen
concentration down to  1  mg/L, with a
zero-order  removal rate  at 55°F of
approximately 0.9 Ib NOa-N/day/IOOO
sq ft. Other  pilot plant results, however,
have exhibited a first-order relationship
between nitrate removal  rate and bulk
liquid nitrate nitrogen concentration in
the 0- to 6-mg/L range.

Power Consumption
  In mechanically driven RBC systems,
power is used to overcome internal
resistances  and losses  in the motors,
friction losses in gear reduction and drive
belts  or chains, friction losses in shaft
support bearings,  and the  drag forces
resulting from rotation of the media and
attached biof ilm through the wastewater.
The  drag forces,  which represent  the
largest power drain,  are affected by the
amount and  shape of the media .surface,
rotational speed, wastewater  viscosity,
and the type and amount of  biological
growth.
                               In air driven systems, compressed air
                             discharged beneath the RBC media rises
                             and is captured by air cups mounted to
                             the periphery of the media. The resulting
                             buoyant forces provide the torque neces-
                             sary for media rotation. Power is required
                             to overcome (1) losses in the compressor,
                             compressor motor, air headers, control
                             valves, and diffusers and (2)  the static
                             head of wastewater in the RBC tank.
                               Power factor is an important parameter
                             in determining the energy cost associated
                             with operating an induction motor. Power
                             factor is defined as actual power divided
                             by apprarent  power. Polyphase  watt
                             meters  measure actual  power  (kW)
                             drawn, but demand charges are based on
                             apparent power (kVA). Most electric
                             utilities have demand charge schedules
                             that  penalize customers with power
                             factors of less than  0.9. The use of
                             capacitors to increase the power factors
                             of  induction  motors can  significantly
                             reduce  power costs in RBC  plants,
                             particularly plants that have large
                             numbers of mechanically driven units.
                               The Upper Mill Creek treatment plant in
                             Butler County, Ohio, uses mechanically
                             driven units equipped with power factor
                             correction capacitors. Measurements
                             made on seven units indicated that the
                             capacitors were increasing power factor
                             from an average of about 0.5 to 0.99. The
                             resulting  2.5-kVA savings  in apparent
                             power is worth $17.30/month/shaft in
                             lower electricity costs at  an assumed
                             demand charge of $6.92/month  kVA.
                             Based on  an  approximate  installed
                             capacitor cost of $200, the payout period
                             in this case would be about 12 months.
                               Field  measurements  of the power
                             required  for  mechanically  driven RBC
                             units  are reported for  16 facilities (92
                             shafts) in the full design information
                             dopument. The media surface area for the
shafts monitored varied from 100,000 to  ™
180,000 sq ft. The measured mean power
requirement was 2.98 hp/shaft, with a
standard deviation of 0.71  hp/shaft and
recorded high and low readings of 5.10
and 1.62 hp/shaft, respectively.
  Of the 92 mechanical drive units
monitored, a total of 55 were equipped
with media  having a surface  area of
100,000 to 128,250 sq ft. Media having a
surface area in this range are generally
regarded in the industry as representing
standard density media. The  media
surface area of the other 37 units varied
from 138,000 to 180,000  sq ft,  a range
characteristic of high density media. The
average  measured power consumption
for the standard density units was 2.80
hp/shaft with  a standard deviation of
0.62 hp/shaft. For the high density units,
the average power requirement recorded
was 3.22 hp/shaft  with a standard
deviation of 0.79 hp/shaft.
  An earlier EPA survey  indicated that
manufacturers' estimated power require-
ments for mechanically  driven RBC's
range  from 2.7 to 3.4  hp/shaft  for
standard density media and from 3.5 to
4.2 hp/shaft for high density media. The
above field-measured values agree well
with the  manufacturers'  esitmates  for   ^
standard density media and are slightly   m
lower than the estimates for high density   ~
media.
  When field-measured, mechanical
drive power levels  exceed the  means
indicated  above for standard and high
density media  by one to  two standard
deviations ot more,  the operator should
investigate whether the   higher power
consumption is being caused by equip-
ment problems, heavier-than-normal
biofilm growth, or both. Potential equip-
ment problem  areas  include improper
alignment, inadequate lubrication, ex-
cessive rotational speed,  excessive belt
tension or belt slippage, and general wear
and deterioration of the drive components.
  Power consumption in air driven RBC
systems is affected by motor and blower
characteristics and line losses as well as
rotational speed and biofilm growth. The
type of blower and its proximity to the
RBC trains can significantly affect overall
power requirements. Information on
rotational speed versus air flow relation-
ships is thus of greater value to the design
engineer than are indiscriminate, overall
power  measurements.
  Because the  type of information dis-
cussed above is essentially unavailable to
the design community at  large, studies
were conducted at two air driven installa-   _
tions (Lower East Fork and Indian Creek)  fl
in the Cincinnati, Ohio, area. In general,  ^

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Autotrol's design curves underestimated
the actual air requirements measured at
the two plants. Load  cell readings  at
Lower East Fork did not indicate that the
biofilm being carried on those units was
excessive. Though data from two plants
are not sufficient to describe the rotational
responses that may  prevail at other RBC
air  driven installations, they do indicate
that the recommended design relation-
ships are not always applicable.

General Plant Design
Considerations
  In general, housing RBC units within a
building (as opposed to using individual
covers for the RBC units) is undesirable
because of the  high humidities and the
corrosive atmospheric  conditions  asso-
ciated with  H2S release. Condensation
problems have been encountered on
interior building walls  in cold climates,
and the associated high humidities and
ventilation requirements increase heating
costs. If buildings are chosen to house an
RBC  unit, the building design  must
provide for  removal of a shaft/media
assembly should repair or replacement
prove necessary. In contrast to individual
fiber  glass covers,  full building cover
normally provides more convenient
access to RBC's for routine maintenance
and visual observation.
  In all RBC designs, access to individual
shafts for repair or possible removal mus t
be considered. Bearings should also be
accessible for easy removal and replace-
ment  and should also be accessible for
easy  removal  and replacement  and
should be equipped with oversized grease
cups to minimize manpower requirements
for lubrication.
  Whenever multiple-process trains are
employed, provision for  positive  and
measurable flow control  to individual
trains is essential.  Use of single, long
influent channels with slide gate control
for individual trains makes it difficult for
the operator to locate flow maldistributions
and implement appropriate  corrective
procedures. Splitter boxes and/or weirs
are low-cost solutions to this problem.
Adequate flow control equipment is
especially important if  individual  trains
are fed from a single channel, with some
trains  rotating  with and  other trains
against the direction of plant flow.
  Feed and discharge flexibility should be
considered in RBC  design.  Step feed
capability can relieve overloaded first
stage(s) and potentially decrease or
eliminate excessively thick biofilm growth.
Removable baffles may also be effective
in this regard. In underloaded  plants, the
final stages frequently must be operated
to keep suspended solids in suspension
rather  than  to provide additional  treat-
ment to meet effluent standards.  Thus,
the capability for temporarily  bypassing
these terminal units would yield savings
in energy and operation and maintenance.
  Load cells  can provide useful operating
and shaft load data, especially  in the first
stages. Where  parallel  trains are  in
operation, they can  pinpoint overloaded
or underloaded  trains.  Stop  motion
detectors,  rpm indicators, and clamp-on
ammeters are also potentially useful
monitoring instruments.
  The  use of deep  channels  leading to
and exiting from  RBC tanks has resulted
in solids deposition  and subsequent ac-
cumulation at a number of installations.
Providing for channel  aeration (3.5
scfm/linear ft) or using channel configura-
tions that promote adequate scouring
velocity should overcome  this  problem.
  Equipment warranties can be negoti-
ated with the manufacturers, and in some
cases,  extended  equipment warranties
have been  obtained. This possibility
should be thoroughly considered  in
equipment specifications.  Although RBC
manufacturers continue to make improve-
ments  in their equipment, major equip-
ment problems and failures have occurred
at some installations.
  How sidestreams from  other unit
proceses affect  RBC performance  must
be considered.  Anaerobic  digesters
increase ammonia nitrogen loadings, and
sludge  conditioning processes such as
heat treatment contribute increased
organic and ammonia nitrogen loadings.
Whenever septic tank discharges are part
of the influent wastewater or when any
unit processes are  employed that may
produce sulfide ahead of the RBC units,
the additional oxygen demand associated
with sulfide  must  be considered  in
system design. Prechlorination or pre-
aeration may represent potentially cost
effective means of eliminating the oxygen
demand posed by incoming sulfide. High
influent grease loads require the use of
primary clarifiers instead of screens.
  Nitrification is slow to develop in cold
temperatures, and 8 to 10 weeks may be
required before an RBC nitrification
system approaches equilibrium condi-
tions. Where seasonal standards for
nitrification are required, the final design
must consider  the  transition time and
temperatures needed to develop an ade-
quate nitrifying population.
  Most existing air driven installations do
not have provisions for measuring and
controlling air flow to individual RBC
units. Furthermore,  some plants cannot
easily verify whether some of the air
driven diffusers have plugged. Operating
an  air  driven  facility  under such blind
conditions makes it  more difficult  to
respond  appropriately to operational
problems that may arise. The question of
how much plant blower capacity should
be provided is open  to debate. A critical
need exists for more information on the
normal range of air flows that should be
expected with air driven units.
  The EPA authors Richard C. Brenner. James A. Heidman, Edward J. Opatken.
    and Albert C. Petrasek. Jr.. are with the Municipal Environmental Research
    Laboratory, Cincinnati. OH 45268.
  The  complete report, entitled  "Design Information on Rotating Biological
    Contactors," (Order No. PB 84-199 561; Cost: $ 17.50, 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 authors can be contacted at:
          Municipal Environmental Research Laboratory
          U.S. Environmental Protection Agency
          Cincinnati, OH 45268
                                                                                                *USGPO:  1984-759-102-10622

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United States                        Center for Environmental Research                                            BULK RATE
Environmental Protection               Information                                                          POSTAGE & FEES PAID
Agency                             Cincinnati OH 45268
                                                                                                         PERMIT No. G-35
Official Business
Penalty for Private Use $300
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