;\1,
Unitsd. States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/033 June 1985
&ER& Project Summary
Review of Current RBC
Performance and Design
Procedures
U s Environmental Protection Agency
Region V, Library
23C South Dttrborn Strew
Chicago, Winds 606W ..;.^
This study investigates the perform-
ance and operating histories of rotating
biological contactors (RBC's) used for
secondary wastewater treatment. Site
visits and contacts with plant operators
were undertaken to provide data for
design engineers concerned with im-
proving these systems.
Plants surveyed complied better with
effluent requirements than with BOD
removal standards. Unacceptable per-
formance was traced to organic over-
loading or underloading. Equipment per-
formance is the most severe limitation
currently facing RBC systems, with
component failures prevalent in equip-
ment from all suppliers. Cumulative
RBC performance in terms of BOD
removal fell below the manufacturers'
design recommendations. Dissolved
oxygen limiting conditions were the
most significant RBC performance
problem. Costs of operating RBC units
were less than those for activated sludge
systems and more than those for trick-
ling filters.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati. OH, to announce
key findings of the research project that
is fully documented in a separate report
of the same title fsee Project Report
ordering information at back).
Introduction
Over the past two decades, rotating
biological contactor(RBC)technology has
become a viable alternative for secondary
wastewater treatment. As of 1979, ap-
proximately 263 municipal facilities in
the United States were using RBC's. The
rapid emergence of RBC systems as
viable secondary treatment processes
has increased the need to review their
performance history and provide back-
ground for the design engineer concerned
with improving the design, operation, and
reliability of these systems. This investi-
gation was designed to fulfill this need in
part by evaluating the performance and
operating history of RBC facilities that
treat municipal wastewater and approach
design flow conditions.
The investigation included site visits to
six plants for a review of process per-
formance and to 14 plants for power
measurements. In addition, process per-
formance data were collected from 16
plants, contacts were made with opera-
tors of eight additional plants with organic
overloading problems, and operators of
24 plants were contacted to review
maintenance requirements.
Process Performance
A survey of 16 RBC facilities with an
average influent BOD concentration of
124 mg/L revealed that these plants
have greater success in complying with
effluent concentration criteria (maximum
30 mg/L) than they have with BOD
removal standards (85 percent minimum).
Average monthly BOD removal for the
plants surveyed was 82 percent and
varied from 75 to 90 percent for one
standard deviation from the mean. The
average monthly effluent concentration
was 18 mg/L, with one standard deviation
from the mean producing a range of 12 to
28 mg/L. Unacceptable performance was
traced to either organic overload or
underload conditions.
Organically overloaded systems refer
to facilities in which the influent and
reactor organic concentrations result in
dissolved oxygen limitations in the initial
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stages of the RBC system and reduced
organic removals. In some facilities, the
first stage may be organically overloaded,
but subsequent stages are capable of
increasing their respective organic remov-
al rates in response to the higher organic
concentration of the influent. The result
is that final effluent concentration is still
acceptable. In instances where subse-
quent stages are not available, final
effluent quality does not meet secondary
treatment requirements.
Equipment Performance
A review of RBC units manufactured in
the United States revealed that they are
similar in size, shape, and materials of
construction and that component failures
were found in equipment from all sup-
pliers. From a survey of equipment fail-
ures at 17 operating RBC plants, it was
concluded that shaft failures have the
most serious impact on operations and
generally result from overloading. How-
ever, the survey revealed that radial arms
have the highest rate of equipment failure
(30 percent) of the five main RBC com-
ponents (shafts, bearings, radial arms,
media, and drive system). Equipment
performance was judged to be the most
severe limitation currently facing RBC
systems. As a result of this poor operating
history, it was concluded that design
engineers should require an RBC equip-
ment warranty sufficient to protect the
owner against equipment failure.
Design Procedures
Most municipal wastewater RBC sys-
tems are currently designed using
manufacturer-derived empirical design
relationships that are based on observed
municipal wastewater treatment perform-
ance. For the range of conditions observed
at the 16 RBC plants described above,
cumulative RBC performance in terms of
percent BOD removal fell below the
manufacturers' design recommendation.
Dissolved oxygen limiting conditions
were observed to be the most significant
problem associated with RBC perform-
ance. Only one manufacturer currently
recommends design modifications that
account for dissolved oxygen limitations.
This design criterion is defined in terms of
a limiting value for pounds of BOD per day
per 1,000 ft2. A review of 23 operating
RBC facilities revealed that nuisance
organism growths associated with dis-
solved oxygen limiting conditions were
reported for facilities experiencing a first-
stage organic loading greater than 6.4 Ib
of BOD per day per 1,000 ft2.
Organic overload should be avoided in
RBC design by limiting total BOD loading
per stage to 6.4 Ib per day per 1,000 ft2.
Organic loading should take into account
intermittent sidestream loadings, septage
loadings, and average hourly peak load-
ings as well as the average daily loadings.
RBC designers should make more use
of RBC kinetics, reactor design, and
system characteristics so that they may
incorporate alternative equipment layout,
flexibility, and quality control in the design
and selection of RBC equipment, particu-
larly when using empirical methods.
Additionally, greater flexibility to improve
RBC process control options is required.
Because no single design approach has
been completely acceptable for predicting
RBC performance, the RBC designer
should supplement existing methods with
field visits to similar facilities to define
operating problems and design deficien-
cies.
Operation and Maintenance
Requirements
Costs associated with operating RBC
units averaged less than those reported
for activated sludge systems but more
than those reported for trickling filters.
Labor costs to operate and maintain RBC
units (including inspection, lubrication,
and housekeeping) represent 60 percent
of the total RBC operation and mainte-
nance costs. Actual RBC maintenance
averaged 2.9 hr per week, or 2.7 percent
of the total plant requirements.
Power Requirements
A review of RBC manufacturers' power
studies together with the results of a
WESTON field power measurement pro-
gram resulted in the following conclu-
sions regarding mechanically driven
units:
1. The power consumed by a mechan-
ically driven RBC unit is directly
related to the surface area of the
unit (linear relationship).
2. The power consumed by a mechan-
ically driven RBC unit has been
reported to be proportional to the
rotational speed to the third power.
3. Initial RBC stages of mechanically
driven RBC units with thick biolog-
ical growths consume more power
than do later stages with thinner
growths.
4. Power consumption reported during
manufacturers' clean media tests is
significantly lower than power con-
sumption measured under field
conditions.
5. Power consumption by standard-
density media shafts (100,000 ft2)
and high-density media shafts
(150,000 ft2) driven by mechanical
drives at rotational speeds of ap-
proximately 1.6 rpm was observed
in the field to be approximately 2.3
and 3.4 kW per shaft, respectively.
6. Air-drive units with combined stan-
dard high-density media rotating at
1.2 rpm averaged approximately 3.6
kW per 100,000 ft2 of surface area
(for a combined standard and high-
density unit).
The full report was submitted in fulfill-
ment of Contract No. 68-03-2775 and No.
68-03-3019 by Roy F. Weston, Inc., under
the sponsorship of the U.S. Environmental
Protection Agency.
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This Project Summary was prepared by staff of Roy F. Weston, Inc., West Chester,
PA 19380.
Jon H. Bander is the EPA Project Officer (see below).
The complete report, entitled "Review of Current RBC Performance and Design
Procedures,' (OrderNo. PB85-180545/AS; Cost: $19.00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA contact person, Francis L Evans III, may be reached at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U.S. GOVERNMENT PRINTING OFFICE: 1985-559-016/27095
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
OCQC329
K AGENCY
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