United States Office of Water September 1984
Environmental Protection Program Operations (WH-546) 430/9-8*4-008 '
Agency Washington DC 20460
vvEPA Summary of
Design Information
on Rotating
Biological Contactors
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EPA-430/9-84-008
SUMMARY OF
DESIGN INFORMATION ON
ROTATING BIOLOGICAL CONTACTORS
by
James A. Heidman
Richard C. Brenner
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
and
Walter G. Gilbert
Municipal Construction Division
Office of Water Program Operations
Washington, D.C. 20460
Agency
OFFICE OF WATER PROGRAM OPERATIONS
OFFICE OF WATER
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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EPA REVIEW NOTICE
This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the Environmental Protection
Agency, nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
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FOREWORD
Rotating biological contactors (RBCs) are relatively new to secondary
wastewater treatment in the United States, with over 500 municipal facilities
being installed in the last decade. In addition to secondary treatment appli-
cations, RBCs have also been used successfully to upgrade marginal treatment
facilities and to provide nitrification. Because of a variety of problems
related to the design, construction, operation, and application of RBC facili-
ties throughout the country, the U.S. Environmental Protection Agency has
undertaken a number of research projects to investigate and identify the nature
and extent of these problems and to determine possible solutions. These
efforts have indicated that when properly designed, built, and operated, RBCs
can provide an acceptable alternative to conventional activated sludge systems.
The Wastewater Research Division of the Municipal Environmental Research
Laboratory, located in Cincinnati, Ohio, has conducted a comprehensive array
of research projects on various aspects of the RBC treatment process, including
the theoretical basis of the process, process design and operational considera-
tions, and equipment reliability and design. The results and findings of these
projects are detailed in a Municipal Environmental Research Laboratory publica-
tion entitled "Design Information on Rotating Biological Contactors" (EPA-600/2-
84-106). This publication is available from the National Technical Information
Service (NTIS PB84-199561).
This Summary document is extracted from the above publication and presents
in a concise form essential information on the design, construction, operation,
and application of the RBC process. It is intended to provide a basic under-
standing of the process and its application for the treatment of municipal waste-
waters, and will be of use to design engineers, governmental agency review
personnel, municipal officials, operators, and others with an interest in the
RBC process.
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CONTENTS
EPA Review Notice ............................ ii
Foreword .............................. ! *iii
1. Introduction .......................... i
2. Process Description .................... ] \ 2
3. Equipment Description ..................... 4
Introduction ........................ 4
Shafts ........................... 4
Media ........................... 4
Drive systems ....................... 8
Bearings ............ . ............. 3
Load cells ......................... 8
Covers ........................... 10
RBC arrangement ..................... [ iy
4. Equipment Reliability ..................... n
Introduction ........................ n
Shafts ......................... ! .* 11
Media ........................... 13
Drive systems ....................... 13
Bearings ....................... ! ! ! 13
Summary ........................ ! ! 14
5. Organic Removal ...................... ! ! 15
Wastewater carbonaceous characteristics ........ ! ! 15
Mass transfer of oxygen and organics ............ 15
First-stage loading limit ................. 19
Design approaches ................... ! ! 21
Secondary clarification ................ ! ! 25
6. Nitrification ...... . ......... ....!!!!! 27
Introduction ................. '.'.'.'.'.'.'. 27
Influence of organics ................... 27
Nitrification rates .................. ! ! 28
7. Energy Requirements .................... ! ! 34
Introduction ...................... ! ! 34
Mechanical drive systems .................. 34
Air drive systems ................... ! ! 36
8. Plant Design Considerations ............. ! ! ! ! ! 39
Introduction ..................... ! ! ! 39
Pretreatment ...................... ! ! 39
Equipment specifications, maintenance, and reliability . ! .' 39
Flow control ........................ 40
Biomass monitoring and control .............. * 41
Miscellaneous considerations ............... [42
References .............................. 44
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SECTION 1
INTRODUCTION
The relatively rapid introduction of rotating biological contactors
(RBCs) into the United States for municipal and industrial wastewater treat
ment has resulted in the widespread application of a technology with which
many design engineers are not intimately familiar. Of necessity, many RBC
designs initially were based solely on empirical design procedures generated
by various manufacturers. More recently, as interest in the process has
increased, alternative design approaches have begun to appear in the technical
literature.
This document highlights design information and supplements commonly
accepted RBC design methodology, such as manufacturers' design manuals and
empirical and deterministic models found in the literature, by providing
additional information and summaries of operating and performance data not
readily available to the design community. Most of the data used in evalua-
ting the RBC process were obtained from the technical literature, conference
proceedings, and the files of the manufacturers. Important design parameters
and relationships (or lack of them) are discussed to promote a more rational
RBC design approach.
The purpose of this document is to concisely summarize RBC design infor-
mation for municipal wastewater currently available in a number of EPA publi-
cations (1)(2)(3)(4). When a more in-depth discussion of a particular topic
is desired, the reader may wish to consult the original publications for
further details. Topics addressed include process and equipment descriptions,
equipment reliability, organic removal, nitrification, and energy requirements,
A major priority was given to emphasizing practical, usable design information
as well as important theoretical concepts. This summary is not intended to
serve as a "cook book" design reference or to replace other available design
guides from either manufacturers or the technical literature.
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SECTION 2
PROCESS DESCRIPTION
All RBC systems are cylindrical-type structures consisting of plastic
media attached to and/or supported by horizontal rotating shafts. The first
commercial RBC system was installed in West Germany in 1960. Units constructed
from this time to the early 1970s used flat 0.5-in. thick, 6.5- to 10-ft dia-
meter expanded polystyrene discs. All present systems use thin (0.04 to 0.06
in.), high density plastic media either formed as discs or sections of discs
and aligned perpendicular to the shaft, or spirally wound onto and aligned
parallel to the shaft. The media configuration, method of media attachment
to the shaft, available surface area per unit shaft length, method of rotating
the shaft, etc., vary with each manufacturer. An in-depth discussion of avail-
able equipment is presented in Section 3.
Approximately 70 percent of the RBC systems operating in the United
States and Canada are designed for organic carbon removal only, 25 percent
for combined organics removal and nitrification, and 5 percent for nitrifica-
tion of secondary effluent (5). When used for these applications, the RBC
shafts are positioned over the wastewater surface such that about 40 percent
of the media is submerged at any time. The shafts are rotated slowly (1 to 2
rpm) causing an alternating exposure of the media to the atmosphere and the
wastewater. Biological growth (biofilm) becomes attached to the surface of
the media and forms a slime layer over the entire surface of the discs. The
biological population that develops on each unit reflects the environmental
and loading conditions unique to each shaft, and simple visual observation
reveals a gradation in slime thickness and color in staged systems. The
first stage in a system operating within the proper organic loading range
exhibits a characteristic brownish-grey color, while terminal stages that are
nitrifying normally have a characteristic reddish-bronze color (6). The
rotation of the discs alternately contacts the biofilm with organic material
in the wastewater and then with the air. Shearing forces exerted on the bio-
film as it passes through the wastewater cause excess biological growth to be
sloughed off the media into the stage liquor where the turbulence created by
disc rotation maintains the sloughed biomass in suspension.
Microorganisms respond to the environment surrounding them, and, in the
RBC, that environment is continually changing. The requirement for movement
of organics and nutrients from the liquid phase into the biofilm and oxygen
from the atmosphere into the liquid film, biofilm, and bulk liquid makes it
necessary to consider physical mass transfer as well as microbial reaction
rates when rationally analyzing RBC performance. These considerations are
addressed in more detail in Section 5.
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A typical RBC application for secondary treatment is depicted in Figure 1.
Here, flow passes from the primary clarifier through the series arrangement
of RBC units and then to a final clarifier where the excess biofilm sheared
from the RBC unit is settled. In some installations, screens are used in
place of primary clarifiers with a resultant increased solids and biochemical
oxygen demand (BOD) load to the RBC system. If the RBC unit was receiving a
secondary effluent and was designed to provide nitrification, a clarifier
following nitrification would not necessarily be required.
When the dissolved oxygen (DO) level is at or near zero, many hetero-
trophic microorganisms are able to reduce nitrate nitrogen to nitrogen gas.
This phenomenon is used in wastewater treatment systems for nitrogen removal
(denitrification). Unoxidized nitrogen must first be converted to nitrate
nitrogen in an aerobic environment. Denitrification can then be accomplished
in separate-stage RBC systems where carbonaceous oxidation and nitrification
occur in the lead stage(s) of the RBC process train and a carbon source,
commonly methanol, is added to provide the energy for microbial denitrification
in the last (anoxic) stage. RBC systems can also be staged with the anoxic
unit as the first stage of the RBC with the organic carbon naturally present
in the incoming wastewater used for nitrate reduction. Nitrate nitrogen must
be introduced to this stage by recirculation of nitrified wastewater from the
downstream stages.
As no DO is desired in the denitrification reaction, RBC media should be
completely submerged in the wastewater for denitrification applications. The
submerged media are mechanically driven at a rotational velocity of about
1.6 rpm according to the current design procedure (7). Only one full-scale,
municipal RBC denitritrification facility was in operation in the United States
(8) as of August 1984.
RBC UNITS
PRIMARY TREATMENT
SECONDARY CLARIFIER
RAW
WASTE
WATER
SECONDARY
EFFLUENT
SOLIDS DISPOSAL
Figure 1. Typical RBC plant schematic for secondary treatment application
[from Autotrol design manual (7)].
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SECTION 3
EQUIPMENT DESCRIPTION
INTRODUCTION
The five major U.S. manufacturers marketing RBC equipment at the present
time are Clow, Crane-Cochrane, Envirex (formerly Autotrol), Lyco, and Walker
Process. Each manufacturer designs the RBC with an individuality that is
unique to its firm. Consequently, RBCs differ from one another in practically
every component used in their assembly, including shafts, plastic media con-
figurations, methods for separating the individual discs, methods of supporting
the plastic media, bearings, and drives. Competition among the manufacturers
encourages technical innovations that they expect to translate into marketing
advantages. Because of this competition, most of the manufacturers have pro-
gressed through several generations of design, production, and testing of the
individual components that make up their finished products.
SHAFTS
RBC shafts are used to support and rotate the plastic media. Maximum
shaft length is presently limited to approximately 27 ft, with 25 ft occupied
by media. Shorter shaft lengths are also available. The shafts are fabricated
from steel and are covered with a protective coating suitable for water and
high humidity service. Proper protective coating procedure requires sand
blasting of the steel prior to the coating application. A coal tar epoxy is
normally used as the protective coating with a minimum film thickness of 14
mils (0.014 in.).
Each manufacturer designs its own shape, size, and thickness of shaft.
The wall thickness of the shaft is governed by structural requirements, and
the shape is highly dependent on the method the manufacturer employs in sup-
porting the plastic media from the shaft. The five manufacturers each utilize
a shaft that differs from the others in either thickness, size, or shape, or
in some cases all three. Structurally, these differences are readily apparent
as shown in Figure 2 and identified in Table 1. Lyco currently manufactures
Series 300 circular shafts. The previous Lyco/Hormel Series 200 octagonal
shaft is also included in Table 1 because of the large number of installations
still using it.
MEDIA
The heart of the RBC process is the plastic media. In 1972, the high
density polyethylene (HOPE) disc was introduced as a cost reduction alterna-
tive to the previously used 0.5-in. thick polystyrene disc. The major
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in
CLOW
WALKER PROCESS
16-m. square
(outside
dimension)
Weld'
Locations
in
LYCO
ENVIREX/AUTOTROL
l-igure 2. Cross-sections of RBC shafts.
CRANE-COCHRANE
30 in.
outer face
to
outer face
ENVIREX
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TABLE 1. SHAFT CHARACTERISTICS
Manufacturer
Clow
Crane-Cochrane
Envirex
Envirex/Autotrol
Lyco
Lyco/Hormel
Walker Process
Shape
Round
Round
Octagonal
Square
Round
Octagonal
Round
Size
(in.)
30
30
30
16 x 16
28
24
30
Thickness
(in.)
0.625
0.75
0.625
1.00
0.75
0.75
0.75
Section Modulus
(in.3)
415
492
434
282
426
344
492
Ref.
9
10
11
7
12
12
13
advantage of polyethylene is its ability to be formed into various configura-
tions that require a thickness of only 40 to 60 mils (0.04 to 0.06 in.).
This innovation enabled 100,000 to 180,000 sq ft of surface area to be provided
on a 27-ft shaft with 12-ft diameter media. Today, all U.S. manufacturers of
RBCs utilize polyethylene as their plastic media.
Various media configurations or corrugation patterns have been selected
by the manufacturers, each with its own claimed advantages. Several examples
are shown in Figure 3. There are several reasons for using corrugations.
Corrugations add stiffness to the sheets and enable these sheets to be formed
with diameters as large as 12 ft. Corrugations increase the available surface
area by 15 to 20 percent. Corrugations cause the wastewater to follow a
tortuous path through the media, thus increasing wastewater exposure time to
the air for greater oxygen transfer in the atmospheric sector of the rotational
cycle. Finally, corrugations are used as spacers to keep the sheets separated.
Standard density media are normally used in the lead stages of an RBC
train. Standard density media are defined as media with a surface area of
100,000 sq ft supported on or from a 27-ft shaft in which the media diameter
is approximately 12 ft. By reducing the space required for the repeating
plastic corrugations by 33 percent, the available surface area can be effec-
tively increased by 50 percent; this results in shafts with 150,000 sq ft of
media, commonly called high density media. Some manufacturers are now also
offering media with densities of 120,000 and 180,000 sq ft per 27-ft long
shaft for increased design flexibility.
RBC manufacturers employ various methods for supporting plastic media
from their shafts. Clow, Crane-Cochrane, and Lyco rely on a coated steel or
stainless steel radial arm system to support the plastic media. Envirex
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FROM WALKER PROCESS BROCHURE (13)
COURTESY OF CRANE-COCHRANE
FROM AUTOTROL
DESIGN MANUAL (7)
COURTESY OF LYCO
COURTESY OF CLOW
Figure 3. RBC media configurations.
7
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employs plastic media hubs that fit onto its square and octagonal shafts with
the plastic media sheets thermally welded to the hubs. Walker Process attaches
the plastic media to the shaft with an epoxy bonding agent and stainless steel
strips with the media then spirally wound onto the shaft in 35-in. wide strips.
DRIVE SYSTEMS
Historically, RBCs have been driven mechanically and the drive assembly
has proven to be a very reliable equipment component. RBC manufacturers
specify factory-assembled drive packages for all mechanical drive equipment,
consisting of motors, speed reducers, and drive systems. Reduction of motor
output speed down to approximately 1.6 rpm can be accomplished through the
use of various combinations of multi-V-belts, gear boxes, and chain-and-sprocket
units. The electric motors presently used for mechanical drive RBCs are
normally high efficiency, 3-phase, 60-hertz units. The motors, designed with
protective coatings for high humidity environments, are capable of providing
long-term, reliable service. Energy requirements for mechanical drive systems
are discussed in Section 7.
If the designer desires, the manufacturers can modify their standard
drive packages to include variable speed capability. This provides the opera-
tor with additional flexibility for DU and biofilm thickness control. Methods
for achieving variable speed capability include positive infinitely variable
(P.I.V.) speed changers and variable frequency controllers, among others.
P.I.V. speed changers are hand-adjusted units and work on the principle of
changing mechanical gear ratios to obtain desired rotational velocity.
Variable frequency controllers enable a.c. motor speed to be changed directly
by varying the frequency of input current.
Envirex offers an air driven RBC unit. The air drive assembly consists
of 4- or 6-in. deep plastic cups welded around the outer perimeter of the
media and an air header placed below the media (Figure 4). Air is released
at a pressure of 3 to 4 psig into the attached cups, creating a buoyant force
that causes the shaft to turn. Approximately 20 to 30 percent of the air is
not captured by the cups and escapes into the radial passages where it flows
upward through the corrugated media. Air flow requirements are discussed in
Section 7.
BEARINGS
Some early RBC designs experienced deflections of longer shafts
causing unequal wearing of the shaft ends and bearings. The use of self-
aligning bearing units appears to have eliminated this condition. Pro-
tection from corrosion by the use of moisture resistant bearings and cover
plates on the idle end of the shaft has minimized another potential problem.
To permit easy access for lubrication and maintenance, the bearings should
be located outside the media covers.
LOAD CELLS
Hydraulic load cells are available and in use for periodically measuring
total shaft weight. The shaft weighing device consists of a load cell bearing
8
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RADIAL PASSAGES-
AIR CUPS
Figure 4. Air drive RBC schematic [from Autotrol design manual (7)].
installed on the idle end of a mechanically driven shaft. A hand-operated
hydraulic pump is attached to the load cell and used to lift the bearing off
its base while the shaft continues to rotate or is momentarily stopped. The
resulting hydraulic pressure is read from a gauge and the reading converted
to shaft weight, which in turn can be used to estimate biofilm thickness.
Such measurements are useful in determining conditions that may cause exces-
sive fatigue stress on the shaft and increased energy consumption.
The electronic strain gauge load cell is a recent development that
enables shaft load to be measured continuously without lifting the idle end
bearing off its base. A companion converter unit is available that when
plugged into this type of load cell provides a direct readout of total shaft
weight. Electronic strain gauge load cells are primarily applicable to new
installations as it is difficult and costly to modify existing RBC shaft end
walls to accept them.
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COVERS
RBC systems not housed in buildings are normally protected from the ele-
ments by manufacturer-supplied covers. These covers are made of fiberglass
or other reinforced resin plastics for durability and lightweight handling.
They all conform to the general shape of RBC media and permit sufficient
access to the units for observation and minor repairs. Most are designed in
sections that can be readily dismantled when major repairs or shaft removal
is required.
RBC ARRANGEMENT
BOD and
three or four
stages may be
For small
installing
Staging of RBC media is recommended to maximize removal of
ammonia nitrogen (NH3-N). In secondary treatment applications,
stages are generally provided for each flow stream. Additional
added for nitrification or for combined BOD and NH3-N removals.
installations, four stages can be provided on a single shaft by
three interstage baffles within the tank and introducing tne flow parallel to
the shaft. Installations requiring two RBC units may be placed in series
with a single baffle in each tank, thus providing four stages. Four or more
units can be placed in series, with each unit becoming a single stage. Various
schemes of staging RBC units are shown in Figure 5.
ONE UNIT,
FOUR STAGES
TWO UNITS IN SERIES,
TWO STAGES EACH
THREE UNITS IN PARALLEL,
FOUR STAGES EACH
LJ L J
MULTIPLE PARALLEL FLOW STREAMS,
FOUR OR MORE UNITS PER FLOW STREAM,
SINGLE-STAGE UNITS
Figure 5. Various schemes of staging RBC units.
10
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SECTION 4
EQUIPMENT RELIABILITY
INTRODUCTION
An EPA-sponsored study (3) of RBC facilities was conducted during the
period of September 1979 through September 1982. On-site visits and telephone
interviews were utilized to collect pertinent operation and maintenance (O&M)
and equipment-related data from 16 plants equipped with polyethylene media.
A partially updated summary of shaft and shaft-related failures from these 16
plants is presented in Table 2. For the 153 units included in this evaluation,
41 broken shafts, seven media attachment failures, and 16 bearing failures
were reported.
SHAFTS
The most serious equipment problem that can impact an RBC plant is a
shaft failure. A shaft failure involves a structural break in the horizontal
member itself, the loss of the unit, and damage to a portion of the media.
Repair requires that the damaged unit be removed and that a new shaft be
installed along with salvaged and/or new media. Depending on the site layout,
this may entail removal of protective media covers or relocation of the
entire shaft assembly outside a process building.
The vast majority of shaft failures can be attributed to known design
and manufacturing defects that have been addressed in the newer generations
of shafts currently being marketed. In particular, use of a discontinuous
backing strip during the welding of two 16-in. pieces of channel resulted
in a number of defective shafts being fabricated that subsequently failed.
Additional failures were also experienced from structural overloading of
shafts that were designed with inadequate section modulus.
The wide variation in shaft design procedures has aroused considerable
concern as to the adequacy of RBC shafts to provide 20 yr of reliable
service. Assuming satisfactory welding practice, the most likely cause
of shaft failure is fatigue. Extensive experimental testing of structural
members with applied cyclic loads has shown that the logarithm of fatigue
life is linearly related to the logarithm of the range of applied stress
between certain limits of the cyclic life. The stress range at which this
curve becomes flat is known as the fatigue limit and represents the theoreti-
cal value of the applied stress range for which no appreciable fatigue damage
occurs under atmospheric conditions. A member subjected to rotating/cyclical
loads with a maximum corresponding stress range less than the fatigue limit
will not fail (within a certain high level of confidence). The standards of
11
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TABLE 2. SUMMARY OF SHAFT AND SHAFT-RELATED FAILURES
No. of
Plant Shafts
Cheboygan, Mich.
Cleves, Ohio3
Edgewater, N.J.
Gladstone, Mich.
Gloucester, N.J.
Hamilton Township, N.J.
Hartford, Mich.
Ionia, Mich.
North Huntington, Pa.
Rhinelander , Wise.
Selden, N.Y.
Thermopolis, Wyo.
Voorhees Township, N.J.
Wappingers Falls, N.Y.
Washington Township, N.J.
Winchester, Ky.
8
6
4
6
4
48
2
12
4
10
12
2
6
2
3
24
RBC
System
Startup
Year
1978
1977
1973
1974
1974
1979
1978
1978
1975
1977
1974
1978
1976
1978
1974
19776
Broken
Shafts
3
0
0
1
1
6
0
12
3
6
0
0
3
0
0
6'
Bearing
Failures
0
0
1
0
2
0
0
0
2
0
8
0
0
1
0
2
Media
Support Corrective
Failures Action Taken
0
0
0
0
0
0
0
0
0
0
0
0
0
4
3
0
Replaced
Replaced
Welded in
Replaced
replaced
Replaced
Replaced
Replaced
replaced
Replaced
Replaced
Replaced
Replaced
Replaced
replaced
Replaced
Replaced
3 shafts
bearing
place
2 bearings ;
4 shafts
1 shaftb
12 shafts
all 4 shafts;
2 bearings
all 10 shaftsc
8 bearings
2 shaftsd
6 shafts
bearing;
radial arms
radial arms
2 bearings
a Air driven RBC units.
b One shaft replaced as of August 1984. Replacement of the 5 remaining broken shafts
is underway with completion projected by November 1984.
c All 10 shafts replaced due to anticipated failure of the remaining 4 units.
d Anticipated failure based on performance of similar units elsewhere.
e Plant upgraded from 4 to 24 shafts in 1980.
f Replacement of broken shafts under litigation.
12
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the American Welding Society (AWS) include stress curves for structural
members in several categories (14).
Geometry, detailing, and fabrication quality can each have a significant
effect on the fatigue resistance of a member. Transverse or longitudinal
welding, welded attachments, bolting, and weld toe treatment are factors that
influence fatigue behavior. The weldment fabrication procedures of the AWS
Structural Welding Code (14) should be adopted as the absolute minimum require-
ments. Fatigue resistance can also be affected by potential shaft corrosion
due to the expected operating environment.
The estimated fatigue lives for a number of shaft configurations, media
densities, and biofilm thicknesses were developed by Bowman and Gaunt (15).
Current manufacturers' shaft designs used in conjunction with standard density
media were projected to be resistant to failure from fatigue at biofilm thick-
nesses ranging from at least 75 mils (0.075 in.) to 150 mils (0.15 in.) or
greater with the range reflecting the different shaft configurations employed
by the various manufacturers. Further details are summarized elsewhere (1).
MEDIA
Six of the 16 plants visited in the aforementioned survey (3) reported
problems with the media component, including hub failures, shifting media,
media brittleness, and media breakage from unspecified causes. Failures can
occur due to degradation of the polyethylene media from exposure to heat and/or
concentrated organic solvents, or to UV degradation if anti-oxidants are not
added to the media formulations. Failures have also occurred because of poor
hub design, poor media welding procedures, and inadequately designed radial
arm systems.
The method of forming a media pack and attaching that pack to the shaft
determines whether an RBC assembly must be removed from its tank for field
replacement of damaged media. Present media pack attachment designs of Clow,
Crane-Cochrane, and Lyco do not require shaft removal to effect media replace-
ment. Envirex and Walker Process designs require shaft removal.
DRIVE SYSTEMS
Some of the more commonly used RBC drive systems have experienced opera-
tional problems. The most frequently occurring of these is misalignment of
drive components, which contributes to accelerated component wear and opera-
tional failures. Other concerns include the maintenance of proper tension on
belts in multi-V-belt systems to avoid accelerated wear of drive belts, and
decreased drive efficiency and wear pn chain-and-sprocket drives. Two of the
16 plants evaluated (3) reported broken drive chains.
BEARINGS
Six of the 16 plants evaluated (3) reported bearing failures. Some of
the bearings in earlier designs were tapered roller bearings that required
lubrication twice a week primarily to purge contaminants from the bearing
race. Inadequate lubrication contributed to the failures observed. Current
13
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practice is to use self-aligning bearings with oversized grease cups to
increase lubrication intervals.
SUMMARY
RBC systems have progressed through several generations of design. Sys-
tems presently being marketed have been improved compared to those surveyed.
Only time will tell whether present systems will prove reliable for the con-
templated project design life (normally 20 yr). Equipment reliability, con-
sequences of failure, equipment specifications and warranties, and operational
flexibility should be considered in all RBC designs. Design considerations are
discussed further in Section 8.
14
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SECTION 5
ORGANIC REMOVAL
WASTEWATER CARBONACEOUS CHARACTERISTICS
The biodegradable materials in municipal wastewaters are exceedingly
diverse, both with respect to number of components and range of particle
sizes. If wastewater is filtered through glass fiber filters, which normally
allow passage of particles up to about 0.3 to 2 j.im depending on the particu-
lar filter used, that material passing the filter is generally defined as
soluble. Primary effluent soluble BODs typically represents from 40 to 60
percent of the 8005 loading to a secondary RBC system. Where fine screens
are used in place of primary clarification, the soluble 8005 component may
comprise as little as 30 to 40 percent of the 8005 loading. Where an indus-
trial source comprises a significant portion of the load on a municipal
system, the ratio of soluble-to-total BOU5 may vary widely from these typical
values.
Soluble BOD loading is a key design parameter since smaller molecules
can exert a more rapid biological organic demand than larger particulate
materials (16). However, an RBC system must not only remove the soluble
wastewater components through a combination of biological oxidation and cell
synthesis, but must also agglomerate, bio-precipitate, biosorb, and/or metabo-
lize a substantial fraction of the incoming particulate material if a clari-
fied final effluent of acceptable quality is to be achieved.
MASS TRANSFER OF OXYGEN AND ORGANICS
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
gradients from the bulk liquid to biological reaction sites in the biofilm.
Oxygen transfer becomes limiting and controls the overall reaction rate in
heavily loaded systems. The importance of mass transfer can be visualized by
examining the changes depicted in Figure 6. Here the relative concentration
of oxygen and organic substrate are shown at different media locations for
one hypothetical loading condition and RBC speed. These relative values
will, of course, vary for any particular set of design conditions. When the
media are exposed to the atmosphere, the liquid film boundary at the air
interface immediately becomes saturated with DO as shown for Point A in
Figure 6. This saturation in turn results in an increase in the mass of
oxygen that diffuses into the biofilm. When the media are submerged, oxygen
transfer can occur either into or out of the biofilm depending on the bulk
liquid DO levels and the degree of mixing of the liquid film with the bulk
15
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LU
o:
LU
I
Q.
Q
D
O
D
CD
SUBMERGED
SECTOR
KS^-SUBSTRATE
RBC
MEDIA
POINT
D
LIQUID
FILM
Figure 6. Relative concentrations of oxygen and substrate for one
hypothetical loading condition and RBC rotational speed as a
function of media location.
16
-------
liquid. The substrate concentrations within the biofilm are also shown to
vary with position on the media, with the point of maximum substrate penetra-
tion into the biofilm occurring at the point of minimum DO level.
In addition to the oxygen transferred directly from the atmosphere to
the nonsubmerged biofilm, oxygen also enters the bulk liquid as the result of
turbulence generated by the rotation of the media and by the return to the
bulk liquid of wastewater lifted into the atmospheric sector that flows
freely back across the media. Where organic removals in the first stage of
heavily loaded systems have been limited by DO availability, it has been
shown that increasing the oxygen content in the atmospheric phase will in-
crease the organic removal rate (17)(18)(19). Gas transfer from the air
directly into the attached wastewater and biofilm represents the major oxygen
source for the organisms. The beneficial effect on DO of compressed air
injection in air drive RBCs is offset to some degree by their lower atmospheric
oxygenation rates (compared to mechanical drive units) resulting from lower
rotational velocities.
The total COD reduction across an RBC stage is a direct measure of
oxygen transfer capability provided that nitrogen and sulfur species are not
oxidized or reduced and influent and effluent DO levels are the same. Deter-
mination of total COD reduction requires that measurements be made on unsettled
samples entering and exiting a stage so that the reductions represent those
due to oxidative reactions only. COD balance studies conducted on a full-dia-
meter RBC unit (20) indicated a maximum oxygen transfer rate of approximately
1.5 Ib 02/day/1000 sq ft. This maximum transfer rate was achieved at a
rotational speed of about 1.6 rpm and is consistent with calculations of
oxygen transfer based on observed nitrification rates in full-scale units (see
Section 6).
The Autotrol design manual (7) indicates that zero-order removal of
soluble BOD5 at a maximum rate of 2.5 lb/day/1000 sq ft is possible for
mechanically driven units and 3 lb/day/1000 sq ft is possible for air driven
units. Soluble BOD removal represents both the fraction removed through oxi-
dative reactions and the fraction converted to new biomass. In the zero-order
removal range, BOD removal is controlled by oxygen diffusion (7). Consider
the data in Table 3 obtained on 11.8-ft diameter discs treating municipal
wastewater. A maximum soluble BOL>5 removal of about 2.6 lb/day/1000 sq ft
was achieved. It is clear, however, that soluble BODs removal was not constant,
nor was it a single-valued function of bulk liquid soluble BODs concentration.
The periods of lowest removal were partially related to periods of greatest
biofi1m thickness.
The above observations provide a background understanding of the key
factors affecting substrate removal and oxygen transfer in RBC systems. When
all of these factors are considered, it is apparent there is no reason to ex-
pect substrate removal from the RBC bulk liquid to follow any simple mathe-
matical model. The maximum oxygen transfer and substrate removal rates indi-
cated above are paramount in controlling RBC process performance and contribute
to the basis of the empirical design procedures discussed in the following
pages.
17
-------
TABLE 3. FIRST-STAGE RBC DATA REPORTED BY HYNEK AND CHOU (21)
00
Run No.
V-A
V-B
VI-A
VI-B
VI-C
VII
VIII
IX
X
Waste-
water
Temp.
53
59
73
64
66
65
63
57
54
Total B005
App 1 i ed
/ Ib/day \ /
yitlUU sq tt/ \
8.85
8.98
5.50
-
7.08
11.7
9.16
22.4
-
Mechanical
Applied
Ib/day \ /
Drive Soluble
Removed
Ib/day \ in
BOD5
Reactor
IOW sq tt/ \IOOO sq tt/ (mg/l)
4.25
2.86
2.52
1.29
2.50
4.20
3.54
6.26
5.54
2.41
1.5
1.44
0.76
1.47
0.94
0.76
2.14
1.78
41
40
31
16
32
45
40
44
53
Total BODs
Applied
/ Ib/day \
\ 1000 sq ft/
10.0
10.2
5.39
-
7.45
12.1
9.34
23.4
-
Air
Applied
/ Ib/day
Drive Soluble
Removed
\ I Ib/day
\ 1000 sq tt/ \IOOO sq tt
4.82
3.25
2.47
1.34
2.63
4.31
3.61
6.53
5.65
2.59
1.93
1.65
0.93
1.42
1.16
1.13
2.53
1.59
6005
)in Reactor
(mg/D
44
34
24
12
36
43
35
41
56
-------
FIRST-STAGE LOADING LIMIT
A major constraint in the design of any RBC system is limiting the
organic loading to the first stage(s) to values compatible with the oxygen
transfer capability of the system. With excessive organic loadings, the
biofilm thickness increases and a white/grey biomass is frequently observed.
Organic overloading is, therefore, characterized by the growth of an
excessively thick biological film on the media surface and can be accompanied
by or result in one or more of the following process performance and opera-
tional problems (22):
o DO deficit in the bulk liquid.
o Hindered oxygen transfer from the bulk liquid and air to interfilm
layers on the RBC discs. In severe cases, bridging of biological
growth between adjacent contactor discs can occur, effectively
clogging the media and virtually stopping both oxygen and substrate
transport.
« Anaerobic conditions within the biofilm leading to the dominance of
undesirable microorganisms. For example, sulfide formed deep within
the biofilm will diffuse outward and can lead to the proliferation of
nuisance organisms such as Beggiatoa, accompanied by potentially
reduced BOD removals.
« Development and predominance of microaerophiles throughout the bio-
film and reduced BOD removal due to their lower metabolic rates.
9 Creation of septic odors from both the bulk liquid and the biofilm.
• Elimination of the ability to control biofilm sloughing and thickness
via liquid shearing forces.
• Increased energy requirements.
« Unbalanced structural loading, structural overloading, and structural
failure of the media support framework and central shaft.
A survey of 23 RBC installations conducted for EPA related the presence
of sulfur oxidizing organisms to overloading caused by high hydraulic loading
and/or high influent BOD concentrations. The survey results are summarized
in Figure 7 (1). It can be seen that a first-stage loading limit in excess
of approximately 6.4 Ib BODs/day/lOOO sq ft was associated with the presence
of sulfur oxidizing organisms.
Based on these findings, first-stage loadings in the range of 6 to 8 Ib
total BOD5/day/1000 sq ft or 2.5 to 4 Ib soluble BOD5/day/1000 sq ft are
considered to be acceptable (2). Loadings in the higher end of these ranges
will increase the likelihood of developing problems such as heavier-than-normal
biofilm thickness and nuisance organism growth, depletion of DO, and deteriora-
tion of overall process performance. The structural capacity of the shaft,
19
-------
500
400
^
1
z
o
(-
(E 300
z
LU
O
z
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Lt.
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m
cc
100
0
l_l O£.
LEGEND:
O Plants reporting absence
of nuisance organism
growths
n Plants reporting
problems with nuisance
organism growths
^ Represents an organic
loading breakpoint of 6.4
I Ib BOD5/day/1000 sq ft
1 34 n
\ D33
\NOTE: Numbers correspond to
plant identifications
from survey.
\
•
\
\ 036
\ 35
10r\ D3°
6O . 31
\° o7
D 1
30 \ 11
140 • D D15 29°
^^. a 12
016 *>>^>^..
2004 '- ^
! | • i i 1 1 1 1 1
2 4 6 8 10 12 14 16 18 20
FIRST STAGE HYDRAULIC LOADING (gpd/sq ft
Figure 7. DO limiting conditions related to influent organic concentration
and hydraulic loading [from Brenner et al. (1)].
20
-------
provisions for stripping biofilm, consistently low influent levels of sulfur
compounds to the RBC units, the media surface area required in the remaining
stages, and the ability to vary the operating mode of the facility may justify
choosing a loading in the high end of the range, but close attention must be
given to process operations to minimize the possibility of structural over-
loading and/or operational problems.
DESIGN APPROACHES
Procedures for predicting organic removal relationships in the RBC
process continue to evolve as additional information becomes available.
Relationships that were recommended a few years ago have subsequently been
modified or replaced with alternative methodologies. Reported experimental
and pilot and field data exhibit sufficient variation such that support for
conflicting design approaches can usually be found.
Deterministic RBC Models
Mathematical models that take a completely deterministic approach by
attempting to incorporate all of the factors affecting RBC performance
provide considerable insight into those variables and ranges of variables
that impact the process. Employing this approach in design, however,
entails making certain assumptions about the wastewater film thickness in
the atmospheric portion of the cycle, mixing of this wastewater film with
the bulk liquid, the effect of RBC surface shape on mixing and surface
biofilm depth and uniformity, biofilm density, diffusion coefficient(s)
within the biofilm and possible variation with depth and/or the type of
organisms that predominate, and biochemical kinetic parameters. A mathe-
matical modeling approach has been applied in some design situations in
conjunction with pilot plant studies carried out on the wastewater in question.
Calibration of the model coefficients can be incorporated in the pilot plant
program and is a necessary step in the use of complex mathematical models for
design purposes.
Pilot Plant Studies
The best source of RBC design information is a comprehensive on-site
pilot plant evaluation. A common design practice in the past has been to
scale up small pilot plant results to full-scale applications by setting
the media perpheral speed at the same value for both size units, i.e.,
normally about 60 fpm for mechanically driven units. More recent studies
have shown that less-than-full-diameter units can exhibit greater removal
capacities per unit surface area (23)(24). The use of full-scale diameter
media is recommended in any pilot study to avoid scale-up problems. Insuffi-
cient information is presently available to accurately predict appropriate
scale-up factors. If small-diameter units must be operated to collect
design data, it is important that each stage be loaded below the oxygen
transfer capability of a full-diameter unit to minimize scale-up problems.
Manufacturers' Design Procedures
RBC manufacturers have adopted guidelines related to various design
21
-------
parameters based on field experience. For example, most manufacturers have
standardized tank volume-to-media surface area at 0.12 gal/sq ft based largely
on studies (6) that indicated higher ratios did not improve BOD removal. The
manufacturers have also found that loading variations do not adversely impact
process performance at peak-to-average flow ratios of 2.5 or less. Two design
manuals (7)(9) recommend utilizing peak flow and load conditions for design
at ratios of 2.5 or less and flow equalization for ratios above 2.5. All
manufacturers contend that wastewater temperature does not affect organic
removal above wastewater temperatures of 55°F. Below this temperature,
various correction factors are recommended as illustrated in Figure 8.
The number of stages utilized in an RBC design depends on several factors
including effluent requirements, first-stage and overall process loading cri-
teria, wastewater composition, and wastewater temperature, among others.
General staging recommendations developed by three manufacturers are summarized
below.
Envirex (7)
Target
Effluent
Soluble BOD5
(mg/D
>25
15-25
10-15
Recommended
Mi nimum
No. of
Stages
1
1 or 2
2 or 3
3 or 4
Clow (9)
At least
four stages
per flow path
recommended
Lyco (12)
Target
Total BOD5
Reduction
up to 40%
35 to 65%
60 to 85%
80 to 95%
Recommended
No. of
Stages
1
2
3
4
Minimum of four stages
recommended for combined
and NH-N removal
The depth of design information provided on various aspects of RBC
organic removal differs considerably among the various manufacturers. To
provide perspective on the range of predicted performance that results from
using these methods, predicted effluent quality was examined for a range of
loadings for the case where both influent and effluent soluble-to-total BODs
ratios were assumed to be 0.5. The results are presented in Figure 9. The
three Lyco predictions are based on total influent BODs concentrations of
100, 150, and 225 mg/1. Both Envirex and Clow predict identical relationships
for the loading ranges examined. Published design procedures for Crane-
Cochrane and Walker Process are not available. In lieu of establishing
published design procedures, both firms have indicated they prefer to evaluate
wastewater characteristics and process requirements of potential clients on an
individual basis before recommending specific loading criteria and projecting
anticipated performance.
22
-------
SURFACE AREA CORRECTION FACTOR
31
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10
ENVIREX
AND
CLOW
LYCO(150)
NOTE: Soluble BOD5/total BOD5 = 0.5
LYCO(100)
,LYCO(225)
NOTE: Numbers in ( ) are
influent total
BOD5 concentrations.
2345
ORGANIC LOADING RATE (Ib total BOD5/day/1000 sq ft)
Figure 9. Effluent BODs as a function of organic loading for selected RBC
manufacturers design techniques.
-------
Alternative Design Approach
In an attempt to devise an improved method of organic removal estimation,
Opatken (26) evaluated soluble organics interstage data by using the following
equation for a second order reaction (27):
4kt
_
n 2t
where Cn is the concentration of soluble organics in the nth stage (rng/1), k
is the second-order reaction rate constant (1/mg/hr), t is the average hydrau-
lic residence time in the nth stage (hr), and Cn_i is the concentration of
soluble organics entering the nth stage (mg/1). Interstage and final effluent
soluble BODs values predicted using this expression were compared against
measured values obtained at nine full-scale air drive plants. A k value of
0.083 was used for all installations. The predicted and measured values were
in good agreement for seven of the nine plants as shown by tne representative
results in Figure 10. The lack of close agreement at one plant could be
explained by inadequate oxygen transfer in the first stage to handle the nigh
influent organic load; the lack of close agreement at the other plant could
not be explained.
The second-order predicted values in general more closely approximated
measured soluble BODs concentrations than did values predicted by Envirex's
empirical organic removal design method for air driven RBCs. These results
indicate that second-order kinetics may offer an improved basis for predicting
interstage soluble organic removals in RBC systems tnat are not oxygen transfer
limited. As with the manufacturers' empirical techniques, the second-order kin-
etics approach only addresses the impact of soluble organics on effluent quality.
SECONDARY CLARIFICATION
The concentration of unsettled suspended solids (SS) leaving the last
stage of an RBC train treating municipal wastewater will normally be less than
300 mg/1 if primary clarification is not provided and less than 200 mg/1 where
primaries are used. A number of investigators have undertaken studies of sur-
face overflow rates of secondary clarifiers used in conjunction with RBCs to
achieve certain desired effluent qualities. The following is a summary of the
findings of some of those studies:
Surface Overflow Performance
Rate (gpd/sg ft) Objective Reference
550-650 <30 mg/1 SS Scheible and Novak (20)
740 >80% SS removal Srini vasaraghavan et al. (28)
<600 Maximize solids removal Murphy and Wilson (24)
800 30 mg/1 SS (t.f. upgrade) Smith et al. (29)
500 15 mg/1 SS (t.f. upgrade) Smith et al. 29
800 30/30 effluent Clow (9)
<500 10 mg/1 SS Clow (9)
1000-1200 Peak hydraulic rate DeCarlo (30)
25
-------
a>
§
in
O
o
CD
UJ
m
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DODGEVILLE, WISC.
2 4
TIME (hr)
80
60
D)
E.
in
O
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m 40
LLJ
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CD
D
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O
CO
20
WOODBURN, WASH.
LANCASTER, WISC.
2 4
TIME (hr)
100
WEST DUNDEE, ILL.
TIME (hr)
LEGEND:
O Measured values
A Predicted by 2nd-order
kinetics
TIME (hr)
Figure 10. Disappearance of soluble BOD5 with time at
four representative RBC plants.
26
-------
SECTION 6
NITRIFICATION
INTRODUCTION
The factors affecting nitrification in the RBC process include influent
organic concentration, influent nitrogen concentration and composition,
wastewater temperature, DO concentration, pH and alkalinity, and influent
flow and load variability, among others. 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. One manufacturer (31) recommends basing design on influent soluble
TKN (mg/1) minus 0.10 x soluble BOD5 removed (mg/1) to account for heterotro-
phic nitrogen metabolism. A less empirical approach is to base design on
influent total TKN (mg/1) minus 1.0 mg/1 refractory soluble organic nitrogen
minus 0.055 x soluble BODs removed, which assumes a yield factor of 0.6 and a
cell nitrogen content of 9.2 percent to estimate heterotrophic nitrogen metab-
olism (1).
The impact of flow and mass loading variations is usually more severe on
the nitrification efficiency of RBCs than on their organic removal performance.
Nitrifiers have long generation times compared to heterotrophs. Further,
external accumulation and/or internal storage of ammonia nitrogen for delayed
metabolism does not take place with nitrifiers. Consequently, influent
surges in flow or unoxidized nitrogen concentrations will be accompanied by
similar, delayed (roughly equal to reactor detention time) spikes of unoxidized
nitrogen in the effluent unless adequate RBC surface is provided to compensate
for expected variations. Where consistently low effluent ammonia nitrogen
residuals are required, flow equalization may be cost effective.
INFLUENCE OF ORGANICS
The normally staged configuration of an RBC system, which promotes
sequential or plug flow removal of substrate, is conducive to the development
of a nitrifying bacterial population. The degree of this development in any
stage depends primarily on the soluble organic concentration in the stage
bulk liquid. Population dynamics dictate that heterotrophic bacteria will
predominate in an RBC biofilm when the organic concentration is high. As the
organic concentration decreases to a level where the growtn rate of the
nitrifiers is greater than the rate of active biofilm sloughing, the percentage
of nitrifying bacteria in the biofilm will increase to a point where efficient
nitrification is possible. Selective predation of nitrifying bacteria by
higher life forms has been observed in tail-end RBC stages where high DO
27
-------
concentrations (6 to 8 mg/l) and low soluble 8005 levels (<5 mg/l) can coexist.
Sullivan (31) recommends limiting tail-end DO's to 3.5 mg/l through reduced
media rotational velocity, if possible, to discourage predation of nitrifiers.
Most empirical design procedures (7)(9)(12) are based on the assumption
that significant nitrification doesn't begin in an RBC system until bulk
liquid soluble BODs has been reduced to 15 mg/l. In combined carbon oxidation-
nitrification units, BODs values of this magnitude may first be encountered
in the second, third, or fourth stages, depending on influent strength, organic
loading rate, and wastewater temperature. In separate-stage nitrification
applications, the soluble BODs concentration of the wastewater entering the
RBC reactor is usually well below 15 mg/l and substantial nitrification is
evident in the first stage. Analysis of field data indicates that while
incipient nitrification is generally observed in RBC stages with soluble BODs
concentrations of about 15 mg/l, maximum nitrification rates are not attained
until bulk liquid soluble BODs drops to 10 mg/l or less (1).
NITRIFICATION RATES
Extensive testing and data evaluation were conducted by Autotrol to
model ammonia nitrogen oxidation in RBCs. The current Envirex procedure for
full-diameter RBC units operating at wastewater temperatures of 55°F or greater
is based on prior Autotrol testing and is summarized graphically in Figure 11.
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. It is interesting to note that a nitrification rate
of 0.3 Ib NH3-N/day/1000 sq ft represents an oxygen demand of about 1.4 Ib/
day/1000 sq ft, which corresponds closely with the estimates made by Scheible
and Novak (20) (see Section 5). Hence, the nitrification rate of 0.3 Ib
NH3-N/day/1000 sq ft in full-scale systems is apparently the result of oxygen
transfer limitations.
LU
(-
<
tr
if
tu >>
cc w
m~
z
0.4
0.3
0.2
0.1
0
WASTEWATER TEMPERATURE = 55°F
1.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0
STAGE NH3-N CONCENTRATION (mg/l)
13.5 15.0
Figure 11. Second-generation Envirex ammonia nitrogen removal rate curve
for full-scale RBCs [from Antonie (6)].
28
-------
Designs for nitrification using RBCs are often based on pilot-scale
testing. Although some pilot studies have reported nitrification rates as
high as 0.7 Ib NH3-N/day/1000 sq ft (32)(33)(34), it should be noted that
rates this high are not attainable with full-diameter units rotating at 1.6
rpm. As with organic removals, higher inherent rates of atmospheric surface
renewal and oxygen transfer associated with increased rotational velocity
explain the observed differences between small-diameter and full-diameter
units rotated at the same tip speeds.
Nitrification data obtained at three full-scale facilities when waste-
water temperatures were 55 +_ 2°F are shown in Figure 12. Each point repre-
sents data for 1 day for a given stage. For the combined carbon oxidation-
nitrification systems (Gladstone, Michigan, and Cleves, Ohio), ammonia nitrogen
removal data prior to the stage in the train where maximum nitrification rates
were observed were omitted on the assumption that organic removals were influ-
encing the nitrification rates. The curve shown in this figure essentially
duplicates the curve used in the current Envirex procedure. The zero-order
removal rate above bulk liquid ammonia nitrogen concentrations of 5 mg/1
corresponds to 0.3 Ib NH3-N/day/1000 sq ft.
If all other factors are not rate limiting, e.g., pH, DO, and/or oxygen
transfer, an increase in temperature above 55°F would be expected to increase
the NH3-N oxidation rate observed in RBC systems. Field data from five nitri-
fying plants operating at higher wastewater temperatures (65 + 5°F) are plotted
in Figure 13. While the data are more scattered than in the "5*5 _+ 2°F plot,
the average removal rate at concentrations above 5 mg/1 NH3-N is actually no
higher than the 0.3 Ib NH3-N/day/1000 sq ft rate estimated at 55°F. Compari-
son of Figures 12 and 13 supports the manufacturers' contention that wastewater
temperatures above 55°F do not result in higher nitrification rates on full-
scale units. The probable explanation for this phenomenon is that above 55°F
oxygen transfer capacity rather than biological growth rate controls the maxi-
imum nitrification rates attainable in full-scale systems.
Low wastewater temperature (47 + 2°F) nitrification data were available
for Gladstone (35). Evaluation of tFese data indicated that an estimate of
the amount of additional media surface area required to achieve an effluent
ammonia nitrogen level of 1.5 mg/1 at 47°F compared to 55°F was identical to
or within 10 percent of that predicted by the temperature correction curves of
Envirex, Clow, and Lyco illustrated in Figure 14. These data indicate that
as wastewater temperature drops increasingly below 55°F, biokinetic response
displaces oxygen transfer capacity as the dominant factor controlling full-
scale nitrification rates.
Nitrification rates can be significantly affected by pH, particularly
in unacclimated systems. One study (32) where pilot RBC units were allowed
to acclimate to each pH level tested reported a constant nitrification rate
between pH 7.1 and pH 8.6, 25 percent of this constant rate at pH 6.5, and
zero nitrification at pH 6.0. On another RBC pilot study (34), increasing
nitrification rates were observed as pH was increased from 6.5 to 8.5 Nitri-
fication enhancement with increasing pH values above the typical pH range for
raw wastewater of 7.0 to 7.5 has never been demonstrated, however, on full-
scale RBCs. As with wastewater temperature, oxygen transfer limitations may
29
-------
CT
if)
0.8 r
0.6
LEGEND:
O Gladstone, Mich. (35)
D Guelph, Ontario (36)
A Cleves, Ohio (37)
CJ
O
03
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LU
IJJ
DC
CO
I
0.4
0.2
O
o
o
o
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_L
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°u
-o
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O
D
NOTE: Temperature - 55±2°F
J_
4 6 8 10
STAGE NH3-N CONCENTRATION (mg/l)
12
Figure 12. Full-scale RBC nitrification rates at design wastewater
temperature (55° F).
-------
U.8
'day/1 000 sq
o
b>
-••.
£
UJ
$ 0.4
tr
0
£ 0.2
z
i5
0
LEGEND:
O Gladstone, Mich. (35)
D Lancaster, Wise. (37)
A Lower East Fork, Ohio (37)
© Columbus, Ind (37)
• Indianapolis, Ind. (38)
.
O «
**
- • o ° o o0
0 A Dn
<%
• 0 •
* • 0 •
A 0 o
0 ° o
0 o ° 0
°o o
0 0 0
0 NOTE: Temperature = 65±5°F
® .
10
12
14
16
18
STAGE NH3-N CONCENTRATION (mg/l)
Figure 13. Full-scale RBC nitrification rates at high wastewater temperatures.
-------
SURFACE AREA CORRECTION FACTOR
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obscure pH optimization of nitrification in 12-ft diameter field units rotated
at 1.6 rpm.
The process of nitrification consumes alkalinity. In poorly buffered
RBC nitrifying systems, alkaline chemicals such as lime, soda ash, or sodium
hydroxide may have to be added to the wastewater to maintain sufficient
alkalinity to prevent a precipitous drop in pH and nitrification rate.
33
-------
SECTION 7
ENERGY REQUIREMENTS
INTRODUCTION
Energy consumption in mechanically driven systems results from the power
used to overcome internal resistances and losses in the motor, 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 biofilm through the wastewater. The drag force, which constitutes
the major portion of the energy required for shaft rotation, is affected by
the amount of media surface area, the shape of that surface area, rotational
speed, wastewater viscosity (temperature), and the type and amount of biologi-
cal growth.
With air driven systems, compressed air is discharged beneath the RBC
media as previously shown in Figure 4. The rising air is captured by the air
cups, and the resulting buoyant forces provide the torque necessary for media
rotation. Energy is required in air driven systems for losses in the motor,
compressor, air headers, control valves, and diffusers, and also to overcome
the static head of wastewater in the RBC tank.
MECHANICAL DRIVE SYSTEMS
Manufacturers' estimates of energy requirements for mechanical drive
units rotating at approximately 1.6 rpm are as follows (39):
Media Type
Standard Density High Density
Manufacturer (kW/shaft) (kW/shaft)
Clow 2.1 2.4
Envirex 1.55(1.8)a 2.3(2.6)*
Lyco - . 1.7 - 2.1
Walker Process 2.0(2.6)b 2.3(3.0)&
a First number is mean of field measured values; second is 99.9
percent confidence level.
b First number is average value; second is maximum value.
Environmental Resources Management, Inc. (ERM), conducted an extensive
EPA sponsored field investigation to determine actual energy requirements on a
total of 105 mechanically driven units (4). A variety of shaft lengths, media
i
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surface areas, and manufacturers' equipment were included in the study A
summary of all field measured data is given below:
Motor Size kU/Shaft
(nP) AverageRange
5 2.02 1.05 - 3.76
7-5 2.05 1.32 - 2.99
Data from 80 of the units tested by ERM were selected as representative of
facilities with typical shaft lengths, media diameters, and rotational speeds
These data were combined with data from 12 units evaluated by EPA staff (40)
to ascertain the impact of media surface area on energy requirements. Of
these 92 units, a total of 55 were equipped with media having a surface area
in the range of 100,000 to 128,250 sq ft, a range 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
or high density media. The average measured energy consumption for the
standard density units was 2.09 kW/shaft with a standard deviation of 0 46
kW/shaft. For the high density units, the average power requirement recorded
was 2.40 kW/shaft with a standard deviation of 0.59 kW/shaft. These values
agree well with the manufacturers' estimates of mechanical drive energy re-
quirements.
Excessive energy consumption is generally indicated when field-measured
power levels exceed by one to two standard deviations the means indicated
above for standard and high density media. Higher-than-expected energy con-
sumption can be caused by equipment problems, heavier-than-normal biofilm
growth, or both. Potential equipment problem areas include improper belt align-
ment, inadequate lubrication, excessive rotational speed, excessive belt ten-
sion or belt slippage, and general wear and deterioration of the drive com-
ponents.
Power factor is an important parameter in determining the energy cost
associated with operating an induction motor. Many electric utilities have
demand charge schedules that penalize customers with power factors less than
u.9. The power factors for mechanically driven RBC units are typically auite
low as indicated by the following data from the 92 units summarized above-
Surface Area Range Motor Size Power Factor
(sq ft) (hp) Mean Range ~
100,000 - 128,250 5 0.51(28)3 0.26 - o.7l
100,000 - 128,250 7.5 0.38 27 a 0.18 - 0.82
138,000 - 180,000 5 0.49 20 a 0.26 - 0 68
138,000 - 180,000 7.5 0.38 17 a 0 16 - 0 63
a Number of shafts.
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The Upper Mill Creek (Cincinnati, Ohio) treatment plant utilizes mechani-
cally driven RBC units equipped with power factor correction capacitors.
Measurements made on seven units with power factor correction indicated that
the capacitors were increasing power factor from about 0.5 to 0.99 (1). The
resulting savings in apparent power are worth $17.30/month/shaft in lower
electrical costs at an assumed demand charge of $6.92/month/kVA. Based on
an approximate installed capacitor cost of $200, the payback period in this
case would be about 12 mo.
AIR DRIVE SYSTEMS
The energy consumption for an air drive RBC system can only be accurately
estimated once the motor and blower characteristics, line losses, and operating
range have been addressed. The sizing and design of the air distribution
system will directly affect the line losses and, therefore, the discharge
pressure at which the compressor must operate. The usual industrial practice
is to design for headlosses in the range of 0.1 to 0.6 psi/100 ft of line.
It is important that the design conditions stipulated for the centrifugal
compressors match actual operating conditions as closely as possible. Com-
pressor operation at conditions other than those for which the compressor was
designed can result in a substantial loss of efficiency. For those facilities
in which significant variations in air flow requirements are anticipated,
consideration should be given to using multiple compressors with different
design capacities, both for air flow and discharge pressure.
The air flow requirements to operate an air-driven RBC system are variable.
Envirex (31) recommends that 350 cfm of capacity be provided for each shaft.
The maximum normalized air flow at ambient conditions (68°F) with 4-in. air
cups for various rotational velocities is presently indicated by the manufac-
turer to be as follows (41):
Air Flow (acfma) at Indicated
Rotational Speed
Rotational Speed (rpm) 1.0 1.2 1.4
Standard Density Media 100 150 220
High Density Media 145 225 350
a acfm = actual cfm
The actual air flow requirements to operate a shaft at a given speed are also
reported to vary by 40 to 80 acfm, and the mean requirement over a range of
1.0 to 1.3 rpm will vary more than this (42).
Results of an EPA study (40) that evaluated the relationship between
rotational speed and air flow requirements for one process train at the Lower
East Fork (Batavia, Ohio) plant are presented in Figure 15. Also shown are
the manufacturer's design curves (41) for the range of conditions applicable
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500
400
E
t3
300
200
100
LEGEND:
— — — Autotrol curve for high
density media, heavy
growth, 6-in. cups, and
50° F
— —Autotrol curve for high
density media, normal
growth, 4-m. cups
53.6
"Letter designations from Table 4
1 i
0.5 0.75 1.0
ROTATIONAL SPEED (rpm)
1 ' ' _'
1.25
Figure 15.Summary of air flow versus rotational speed measurements made
at Lower East Fork-Little Miami River Regional Wastewater
Treatment Facility, Claremont County, Ohio.
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to the measurements made at this site. The operating conditions under which
the data represented by curves A to F were generated are summarized in Table
4. It can be seen that air flow requirements for a given rotational speed are
highly variable and unpredictable (compare curves B and C). The specific
energy requirements for an air driven RBC facility can only be determined on a
case-by-case basis.
TABLE 4. SELECTED OPERATING CONDITIONS OF LOWER EAST FORK PLANT
Curve Designation Air Cup Depth Date Measured Biofilm Thickness
in Figure (in.) Stage (1982) (In.)
A 6 1 2/11 0.050
B 6 1 3/15 0.044
C 6 1 3/17 0.042
D 43 3/15,3/17 0.038,0.037
E 45 2/23 0.045
F 49 2/23
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SECTION 8
PLANT DESIGN CONSIDERATIONS
INTRODUCTION
Proper design of an RBC facility entails, among other things, accurate
determination of influent and sidestream loadings, proper media sizing,
staging and equipment selection to meet effluent requirements, and selection
of an overall plant layout that provides sufficient flexibility to promote
good operation and maintenance practices. A number of design considerations
(in addition to those discussed in the preceeding sections) are germane to
the quality construction and successful operation of an RBC installation.
PRETREATMENT
Raw municipal wastewater should not be applied to an RBC system. Effec-
tive preliminary treatment is essential for good operation. Grit removal and
either primary clarifiers or screens are necessary to remove materials that
may settle in the RBC tankage or plug the RBC media. High influent grease
loads require the use of primary clarifiers or an alternate system for grease
removal prior to the RBC units. :
The impact of sidestreams from other unit processes must be considered.
Anaerobic digesters increase ammonia nitrogen loadings, and sludge conditioning
processes such as heat treatment contribute increased organic and ammonia
nitrogen loadings. Where septage addition is to be allowed, separate facili-
ties for its acceptance and controlled feeding to the RBC system should be
considered. Septic tank waste addition or a separate unit process located
ahead of the RBC system that has the potential for sulfide production, e.g.,
clarigesters, produces an incremental oxygen demand associated with the
sulfide that must be considered in system design.
EQUIPMENT SPECIFICATIONS, MAINTENANCE, AND RELIABILITY
The suggested ranges for first-stage organic loading rates as presented in
Section 5 are based on both process and structural considerations. First-stage
loadings in the upper end of the range may result in a variety of operational
and process problems, of which the most critical is the structural overloading
of the RBC shaft. Over the life of a facility, organic loadings will vary
considerably and excessive biofilm growths can frequently occur. Therefore,
the specifications for the load bearing capacity for each shaft snould consider
the maximum anticipated biofilm growth, the capacity to strip excess biofilm,
and an adequate margin of safety. In addition, structural designs of RBC
shafts should be based on appropriate AWS stress category curves (14)(see
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Section 4) modified as necessary to account for the expected corrosive environ-
ment.
Full building enclosures normally provide more convenient access to RBCs
for routine maintenance and visual observation than the use of individual
fiberglass covers. Housing RBC units within a building is undesirable, how-
ever, because of the corrosive atmospheric conditions associated with the
presence of hydrogen sulfide and high humidities. Condensation problems
have been encountered on interior building walls in cold climates, arid the
accompanying high humidities and ventilation requirements increase heating
costs. Where buildings are chosen, the building design must meet the addi-
tional requirements for ventilation, heating, and humidity control, and pro-
vide for removal of a shaft/media assembly should repair or replacement
prove necessary.
In all RBC designs, access to individual shafts for repair or possible
removal must be considered. Bearings should also be accessible for easy
removal and replacement. The weight of a 27-ft long shaft and media assembly
may be expected to range from 18,000 to 25,000 Ib for clean media depending
on whether standard or high density media is specified. A fully-loaded,
100,000-sq ft shaft with a 0.1-in. thick biofilm has a dead weight of about
70,000 Ib. Some manufacturers assemble their units in such a way that the
media can be removed from the shaft while the shaft remains in the RBC tank,
whereas others do not (see Section 3). Where all units in a large instal-
lation are physically located closely together, it has been necessary to
utilize large off-the-road cranes for shaft removal. Crane reach, crane
size, and the impact of being able to drain RBC tankage and dry a unit
prior to shaft removal should all be considered when designing the RBC
layout.
Equipment warranties can be negotiated with the manufacturers, and, in
some cases, extended equipment warranties have been obtained. This possi-
bility should be thoroughly considered in equipment specifications. Speci-
fications can set strict warranty requirements. Careful thought needs to
be given to the acceptability of field repairs versus new factory replace-
ment equipment, as well as other areas of potential conflict. Considering
the history of equipment failures, the specification of a rigorous performance
and replacement guarantee may offer the best protection available to the
treatment plant owner.
FLOW CONTROL
Whenever multiple process trains are employed, provision for positive
and measureable flow control to individual trains is essential. Use of
single, long influent cnannels 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. Provision of adequate flow control
equipment is especially important if individual trains are fed from a single
channel with rotation of some trains with and other trains against the
direction of plant flow.
40
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Feeding and discharge flexibility should be considered in RBC design.
Step feed capability can relieve overloaded first stage(s) and potentially
decrease or eliminate excessively tnick 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 treatment to meet effluent standards;
capability to bypass these terminal units would result in energy and O&M
savings. If sufficient flow flexibility is available, loss of an individual
unit need not result in shutting down an entire process train. Adjusting the
distribution of total flow and/or loading may be an operational necessity,
again emphasizing the need for positive and measurable flow control and/or
splitting.
The use of deep channels leading to and exiting from RBC tanks often
results in solids deposition and subsequent accumulation. Providing for
channel aeration (3.5 scfm/linear ft) or employing channel configurations
that promote adequate scouring velocity should overcome this problem.
Most RBC designs are based on the units operating at a submergence of
approximately 40 percent based on total media surface area. To avoid possible
shaft overstressing and inadequate media wetting, the RBC manufacturers
strongly recommend against dropping the liquid operating level below 35-percent
submergence. They also recommend a clearance of 4 to 9 in. between the tank
floor and the bottom of the rotating media to maintain sufficient bottom
velocities to prevent solids deposition in the tank.
BIOMASS MONITORING AND CONTROL
Organic loading limits and detrimental impacts resulting from organic
overloads were previously presented in Section 5. Since a higher percentage
of carbonaceous material is removed in the initial stages, the first and
second stages can be expected to experience the thickest biofilm growth and
heaviest shaft loads. Load cells (see Section 3) located at these and other
strategic locations can provide the operator with advance notice of a gradual
buildup in biofilm thickness.
DO monitoring is another operational tool that can be used advantageously
for process control. Falling stage liquor DO levels, particularly in the
lead stages, may forewarn of conditions conducive to the growth of undesirable
organisms and the development of excessive biofilm thicknesses. However,
increasing DO levels in the bulk liquid by itself will not necessarily overcome
process performance difficulties that may have developed and other actions to
strip excess biofilm may be required. Two of the plants surveyed by Roy F.
Weston, Inc., had nuisance organisms (Beggiatoa) present in the first stages
despite bulk liquid DO levels of 1.5 to 2 mg/1 (3). A similar observation
was also made at the RBC facility in Edgewater, New Jersey (20). A major
advantage of the air drive RBC system is claimed to be related to the increased
turbulence and stripping of excess biofilm induced as a portion of the air
bubbles rise through the media.
Sufficient operational flexibility must be provided to allow effective
response to organic overloads. Possible corrective action includes increasing
41
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rotational speed to enchance oxygen transfer and biofilm shearing force,
periodically reversing the direction of RBC rotation to promote biofilm
stripping, using supplemental aeration to increase bulk liquid DO concentra-
tion and induce biofilm stripping, increasing media surface area in the
affected stage by removing baffles to reduce overall loading, staging feed of
incoming flow, and temporarily adding chemicals such as hydrogen peroxide or
chlorine.
MISCELLANEOUS CONSIDERATIONS
As discussed in Section 5, influent soluble BOD is a critical parameter
in the design of RBC units. In the design of any facility, soluble BOD
levels should be verified by influent sampling whenever possible.
Depending on whether the media formulation includes carbon black, media
strength can be severely degraded by exposure to sunlight (ultraviolet degra-
dation). When RBC units are stored on-site for an extended period of time
prior to installation, provisions must be made to ensure that they remain
protected from direct sunlight. Media can also be severely impacted by high
wastewater temperatures (>95°F); this is a potential problem in some industrial
applications or in municipal installations that receive large industrial flows
or are located in desert-like environments.
High density media has been used advantageously in the middle and latter
stages of RBC trains where decreased availability of organic carbon results
in biofilm thicknesses of <50 mils (0.05 in.). A recommended general practice,
subject to site specific adjustment, is not to employ high density media
before the third stage of a sequentially-staged RBC module.
Air drive installations should be provided with positive air flow meter-
ing and control to each RBC unit. Operating an air drive facility without
these controls adds to the difficulty of appropriately responding to opera-
tional problems that may arise.
Stop motion detectors, rpm indicators, and clamp-on ammeters are poten-
tially useful monitoring devices for individual RBC shafts.
Energy requirements for RBC rotation increase exponentially with rota-
tional speed. Previous tests with clean media have shown that power consump-
tion increases as a cubic function of rotational velocity (3). Other energy-
consuming factors exist independent of speed, however. Accordingly, the
cubic relationship should not be used indiscriminately.
Nitrification is slow to develop in cold temperatures, and 8 to 10 wk may
be required before a nitrification system approaches equilibrium conditions.
Where seasonal standards are required for the final design, the transition
time and temperatures necessary to develop an adequate nitrifying population
must be considered.
O&M requirements associated with biofilm control, drive train and radial
support arm maintenance and repair, and media and shaft repairs and replace-
ments should be considered in laying out the RBC process configuration and
42
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selecting equipment. The O&M manual should specify a schedule for reading
load cells, visually inspecting biofilm growth and media integrity, and
determining the status of mechanical and structural components. The manual
should also outline procedures for dealing with identified problems.
43
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REFERENCES
1. Brenner, R. C., J. A. Heidman, E. J. Opatken, and A. C. Petrasek, Jr.
Design Information on Rotating Biological Contactors. EPA-600/2-84-106,
NTIS PB84-199561, June 1984.
2. Rotating Biological Contactors (RBCs) - Checklist for a Trouble-Free
Facility. USEPA, Office of Water Program Operations, Washington, D.C.,
May 1984 (rev.).
3. Review of RBC Design and Process Operation and Maintenance, Equipment,
and Power Performance. Final report prepared for USEPA, Municipal
Environmental Research Laboratory, Cincinnati, Ohio, under Contract
Nos. 68-03-2775 and 68-03-3019 by Roy F. Weston, Inc., July 1984,
(Publication pending).
4. MacGregor, A. and K. Raghavan. Evaluation of the Energy Requirements
for Rotating Biological Contactors (RBCs). Final report prepared for
USEPA, Office of Water Program Operations, Washington, D.C., under
Contract No. 68-01-6622 by Environmental Resources Management, Inc.,
May 1984, (Publication pending).
5. Hynek, R. J. and H. lemura. Nitrogen and Phosphorus Removal with
Rotating Contactors. In: Proceedings of the 1st National Symposium/
Workshop on Rotating Biological Contactor Technology, Vol. I, EPA-
600/9-80-046a, NTIS PB81-124539, June 1980. pp. 295-314.
6. Antonie, R. L. Fixed Biological Surfaces - Wastewater Treatment: The
Rotating Biological Contactor. CRC Press, Cleveland, Ohio, 1976.
7. Autotrol Wastewater Treatment Systems Design Manual. Autotrol Corpora-
tion, Bio-Systems Division, Milwaukee, Wisconsin, 1978.
8. Dallaire, G. U.S.'s Largest Rotating Biological Contactor Plant to Slash
Energy Use 30%. Civil Engineering, ASCE, 49(l):70-72, January 1979.
9. Clow Envirodisc Rotating Biological Contactor Systems Catalog. Clow
Corporation, Florence, Kentucky, 1980.
10. Personal communication from R. W. Hankes, Crane Company, Cochrane Environ-
mental Systems, King of Prussia, Pennsylvania, to R. C. Brenner, USEPA,
Municipal Environmental Research Laboratory, Cincinnati, Ohio, August 25,
1982.
44
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11. Personal communication from R. A. Sullivan, Envirex, Inc., Waukesha,
Wisconsin, to R. C. Brenner, USEPA, Municipal Environmental Research
Laboratory, Cincinnati, Ohio, May 1984.
12. Lyco Wastewater Products - RBC Systems Catalog. Lyco Division of
Remsco Associates, Marlboro, New Jersey, 1982.
13. Walker Process BioSpiral Rotating Biological Contactors Brochure. Walker
Process Corporation, Aurora, Illinois, 1979.
14. American Welding Society. Structural Welding Code-Steel. 5th Edition,
ANSI/AWS Dl.1-81, 1981.
15. Bowman, M. D. and J. T. Gaunt. Fatigue Behavior of Rotating Biological
Contactor Shafts. Internal report prepared for USEPA, Municipal Environ-
mental Research Laboratory, Cincinnati, Ohio, under Purchase Order No
C2159NAST, April 1982.
16. Balmet, J. L. Biochemical Oxidation of Various Particulate Fractions of
Sewage. Sewage Works Journal, 29:757, 1957.
17. Torpey, W., H. Heukelekian, A. J. Kaplovsky, and L. Epstein. Effects of
Exposing Slimes on Rotating Discs to Atmospheres Enriched with Oxygen.
Presented at the 6th International Conference on Water Pollution Research,
Jerusalem, June 18-23, 1972; Available in: Advances in Water Pollution
Research, Permagon Press, New York City, 1973. pp. 405-415.
18. Huang, J. C. Operational Experience of Oxygen Enriched Rotating Biologi-
cal Contactors. In: Proceedings of the 1st National Symposium/Workshop
on Rotating Biological Contactor Technology, Vol. I, EPA-600/9-80-046a,
NTIS PB81-124539, June 1980. pp. 637-659.
19. Binitanjia, H. H. J., J. J. Brunsmann, and C. Boelhouwer. The Use of
Oxygen in a Rotating Disc Process. Water Research, 10(6):561-565, 1976.
20. Scheible, 0. K. and J. J. Novak. Upgrading Primary Tanks with Rotating
Biological Contactors. In: Proceedings of the 1st National Symposium/
Workshop on Rotating Biological Contactor Technology, Vol. II, EPA-
600/9-80-046b, NTIS PB81-124547, June 1980. pp. 961-996.
21. Hynek, R. J. and C. C. Chou. Development and Performance of Air-Driven
Rotating Biological Contactors. In: Proceedings of the 34th Industrial
Waste Conference, Purdue University, West Lafayette, Indiana, May 8-10,
1979; Ann Arbor Science, Ann Arbor, Michigan, 1980. pp 805-815.
22. Evans, F. L., III. Consideration of First-Stage Organic Overloading in
Rotating Biological Contactor Design. Draft report prepared by USEPA,
Municipal Environmental Research Laboratory, Cincinnati, Ohio, (Publi-
cation pending).
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23. Reh, C. W., T. E. Wilson, and R. Srinivasaraghavan. An Approach to De-
sign of RBC's for Treatment of Municipal Wastewater. Presented at the
ASCE National Environmental Engineering Conference, Nashville, Tennessee,
July 1977.
24. Murphy, K. L. and R. W. Wilson. Pilot Plant Studies of Rotating Biologi-
cal Contactors Treating Municipal Wastewater. Report SCAT-2, Environment
Canada, Ottawa, Ontario, Prepared for Canada Mortgage and Housing Corpora-
tion, July 1980.
25. RBC DIS Subtask on RBC Design Approaches. Internal report prepared for
USEPA, Municipal Environmental Research Laboratory, Cincinnati, Ohio, under
Contract No. 68-03-3019 by Roy F. Weston, Inc., March 5, 1982.
26. Opatken, E. J. Rotating Biological Contactors - Second Order Kinetics.
In: Proceedings of the First International Conference on Fixed-Film
Biological Processes, Vol. I, EPA-600/9-82-023a, Kings Island, Ohio,
April 20-23, 1982. pp. 210-232.
27. Levenspiel, 0. Chemical Reaction Engineering. John Wiley and Sons, New
York City, 1972.
28. Srinivasaraghavan, R., C. W. Reh, and S. Liljegren. Performance Evalua-
tion of Air Driven RBC Process for Municipal Waste Treatment. In:
Proceedings of the 1st National Symposium/Workshop on Rotating Biological
Contactor Technology, Vol. I, EPA-600/9-80-046a, NTIS PB81-124539, June
1980. pp. 525-552.
29. Smith, D. A., C. P. Poon, and R. D. Miller. Upgrading DA Trickling-
Filter Sewage Treatment Plants. Technical Report No. N-102, USA Corps
of Engineers, Construction Engineering Research Laboratory, Champaign,
Illinois, May 1981.
30. Personal communication from D. A. DeCarlo, Burgess and Niple, Ltd.,
Columbus, Ohio, to J. A. Heidman, USEPA, Municipal Environmental Research
Laboratory, Cincinnati, Ohio, April 14, 1982.
31. Personal communications from R. A. Sullivan, Autotrol Corporation, Mil-
waukee, Wisconsin, to J. A. Heidman and R. C. Brenner, USEPA, Municipal
Environmental Research Laboratory, Cincinnati, Ohio, April 14, 1982.
32. Borchardt, J. A., S. J. Kang, and T. H. Chung. Nitrification of Secondary
Municipal Waste Effluents by Rotating Bio-Discs. EPA-600/2-78-061,
NTIS PB-285 240/8BE, June 1978.
33. Pano, A., E. J. Middlebrooks, and J. H. Reynolds. The Kinetics of Rota-
ting Biological Contactors Treating Domestic Wastewater. Water Quality
Series UWRL/Q-81/04, Utah State Unviersity, College of Engineering, Logan,
Utah, September 1981.
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34. Stratta, J. M. and D. A. Long. Nitrification Enhancement Through pH
Control with Rotating Biological Contactors. Final report prepared for
USA Medical Research and Development Command under Contract No. DAMD17-
79-C-9110 by Pennsylvania State University, September 1981.
35. Letter (data) communication from R. A. Sullivan, Autotrol Corporation,
Milwaukee, Wisconsin, to R. C. Brenner, USEPA, Municipal Environmental
Research Laboratory, Cincinnati, Ohio, March 12, 1982.
36. Crawford, P. M. Use of Rotating Biological Contactors for Nitrification
at the City of Guelph Water Pollution Control Plant, Guelph, Ontario,
Canada. In: Proceedings of the 1st National Symposium/Workshop on
Rotating Biological Contactor Technology, Vol. II, EPA-600/9-80-046b
NTIS PB81-124547, June 1980. pp 1247-1273.
37. Letter (data) communication from R. J. Hynek, Autotrol Corporation,
Milwaukee, Wisconsin, to E. J. Opatken, USEPA, Municipal Environmental
Research Laboratory, Cincinnati, Ohio, July 22, 1980.
38. Advanced Wastewater Pilot Plant Treatment Studies. Report prepared for
City of Indianapolis by Reid, Quebe, Allison, Wilcox, and Associates,
Inc., Indianapolis, Indiana, January 1975.
39. Gilbert, W. G., J. F. Wheeler, and A. MacGregor. Energy Usage and Other
Considerations in the Operation and Design of Rotating Biological Contac-
tor Facilities. Paper presented at 57th Annual Conference of the Water
Pollution Control Federation, New Orleans, Louisiana, October 3, 1984
(Publication pending).
40. Heidman, J. A., W. W. Schuk, and A. C. Petrasek, Jr. Field Measurements
of Power Consumption and Air Flow at RBC Installations. Internal report,
USEPA, Municipal Environmental Research Laboratory, Test and Evaluation
Facility, Cincinnati, Ohio, May 5, 1982.
41. Aero-Surf Energy Requirements. Autotrol Corporation internal report
Milwaukee, Wisconsin, April 1, 1981.
42. The Latest Word on RBC Reliability: Envirex. Envirex, Inc., internal
RBC status report, Waukesha, Wisconsin, undated.
47
U.S. Government Printing Office • 1984 -421-545/11830
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