5059
United States Revised
Environmental Protection May
Agency 1984
Washington DC 20460
Rotating
Biological
Contactors
(RBCs)
Checklist for
A Trouble-Free
Facility
CD
O
Ol
ro
Region V, Library
230 South Dearborn Street
Chicago, Illinois 60604
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Rotating Biological Contactors (I
Introduction
Rotating biological contactors (RBC's) are
relatively new to secondary wastewater treatment
in the United States. RBC technology consists of
plastic media, generally a series of vertical discs,
mounted on a horizontal shaft that slowly rotates,
turning the media into and out of a tank of
wastewater. RBC shafts are generally 25-27 feet in
length with a media diameter of 12 feet. About 40
percent of the media is submerged in the
wastewater at any one time. Media are available
in several different configurations for standard
density media (100,000 sq. ft. of surface area per
shaft) and high density media (150,000 sq. ft. of
surface area per shaft). Microorganisms on the
media oxidize organic wastewater constituents,
reducing these pollutants to more benign
components (biomass and gaseous by-products).
Benefits
The advantages of RBC technology include a
longer retention time (8 to 10 times longer than
trickling filters), a higher level of treatment than
conventional high-rate trickling filters, and less
susceptibility to upset from changes in hydraulic
or organic loading than conventional activated
sludge.
Whether used in a small facility or a large
municipal sewage treatment plant, the RBC
process can efficiently remove 85% or more of
the biochemical oxygen demand (BOD) from
domestic sewage. The process can also be
designed to remove ammonia nitrogen (NH3-N).
In addition, effluents and process wastewater
from dairies, bakeries, food processors, pulp and
paper mills, and other biodegradable industrial
discharges can be treated by the RBC process.
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IBCs)
History
During early pilot-scale operations, RBC's clearly
offered the potential of improved secondary
wastewater treatment. However, .the construction
and operation of full scale systems revealed
major difficulties. Equipment failure contributed
to some problems; design, construction, and
operational flaws led to others.
EPA initiated extensive study into the causes of
these problems. The breakdown of media, shafts,
and bearings was investigated as well as low
dissolved oxygen concentrations, nuisance
bacterial growths, solids accumulations in
undesirable locations, and periodic hydraulic
overloads.
RBC equipment manufacturers initiated their
own research and have modified their equipment
and design criteria. For example, more durable
shafts, bearings, and drive systems, and more
conservative organic loading design criteria are
now available to insure more efficient RBC
wastewater treatment.
In all RBC systems, major factors controlling
operation and performance are:
• Organic and Hydraulic Loading Rates
• Influent Wastewater Characteristics
• Wastewater Temperature
• Biofilm Control
• Dissolved Oxygen Levels
• Operational Flexibility
Why a Checklist?
The EPA research indicates that when properly
designed, built, and operated, RBC's can provide
an acceptable alternative to conventional
activated sludge systems. By heeding past
experience, designers, contractors, and operators
may avoid the difficulties encountered by some
of the first full scale systems.
Inside this folder is a checklist of RBC planning,
design, and construction considerations based on
the history of problems and the EPA research
findings. Utilization of this checklist as future
plants are planned and built will reduce the risk
of unforeseen treatment problems.
Other more detailed technical assistance will be
necessary when actually determining design,
construction, operation, and maintenance details.
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RBC Checklist: for Planning, Design, ai
The following checklist is based on treatment facility designs tl|
All design parameters should be compared with applicable
Organic loading to the first stage is a critical
factor in the design of an RBC system. Indications
from research and field observations are that
loadings in the range of 6.0-8.0 Ibs total
BOD5/1000 ft2/day or 2.5-4.0 Ibs soluble
BOD5/1000 ft2/day can be acceptable. Loadings in
the higher end of these ranges will increase the
likelihood of developing problems such as
heavier than normal biofilm thickness, depletion
of dissolved oxygen, nuisance organisms, and
deterioration of overall process performance. The
structural capacity of the shaft, provisions for
stripping biomass, consistently low influent levels
of sulphur compounds to the RBC units, the
media surface area required in the remaining
stages, and the ability to vary the operational
mode of the facility (=<=
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Construction of Rotating Biological Contaj
[at incorporate rotating biological contactors as the principal secon
|te design criteria to assure compliance with State agency requirer
structural failure for the design !ife of the facility.
Structural designs should be based on
appropriate American Welding Society (AWS)
stress category curves modified as necessary to
account for the expected corrosive environment.
All fabrication during construction should
conform to AWS welding and quality control
standards.
A means for removing excess biofilm growth
should be provided, such as air or water
stripping, chemical additives, rotational speed
control/reversal, etc.
Adequate flexibility in process operation should
be provided by considering one or more of the
following:
• Variable rotational speeds in first and second
stages.
• Multiple treatment trains.
• Removable baffles between all stages.
• Positive influent flow control to each unit or
flow train.
• Positively controlled alternate flow distribution
systems, such as step feed.
• Positive air flow metering and control to each
shaft when supplemental aeration or air drive
units are used.
• Recirculation of secondary clarifier effluent.
Effective treatment, through the use of primary
clarifiers or fine screens, must be provided ahead
of the RBC units.
Periodic high organic loadings may require
supplemental aeration in the first stage.
When peak to average flow ratio is 2.5 to 1.0 or
less, average conditions can be used for design.
For higher flow ratios, flow equalization should
be considered.
Available data indicate that organic removal and
nitrification rates diminish at wastewater
temperatures below 55°F. Below 55°F,
manufacturers utilize correction factors to
determine needed additional media surface area.
Nitrification with RBC units is sensitive to flow
and organic loading surges, requiring evaluation
of flow equalization vs. additional RBC media
surface when consistently low ammonia nitrogen
levels are required in the effluent.
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tor Treatment Facilities
pary unit process.
Small-diameter RBC pilot units are suitable for
determining the treatability of a wastewater.
However, direct scale-up from such units to full
scale is not possible because of the effects of
temperature, peripheral speed of media, and
other process and equipment factors.
Load cells should be provided for all first and
second stage shafts. Load cells for all other shafts
in an installation are desirable.
First stage dissolved oxygen (DO) monitoring
should be provided. The RBC unit should be
designed to maintain a positive DO level in all
stages.
Based on field measurements on 105 shafts at 22
installations in 1983, it was determined that
actual energy requirements for mechanically
driven RBC units ranged from 1.05 kW/shaft to
3.76 kW/shaft (average 2.03). Of the shafts
measured, 62% had 5 hp motors and the
remainder has 7.5 hp motors. The units measured
included both standard and high density media,
and the biofilm growths varied from very light to
medium. Current industry practice generally uses
5 hp motors, and manufacturer's estimates of
energy requirements fall within the range
identified above, with an average close to the
field measured units. In evaluating actual energy
requirements, the engineer should consider the
influences of drive train efficiency, temperature,
biofilm thickness, media surface area, and
rotational speed.
With air drive units, the energy requirements
(kW/shaft) can not be measured directly.
However, for comparative purposes an
approximation can be made by dividing the
blower kW by the number of driven shafts. Field
measured energy requirements for air driven RBC
units at 7 installations during 1983 ranged from
3.8 kW/shaft to 8.3 kW/shaft (average 5.2). Actual
energy requirements will depend on desired
rotational speed, air flow, piping configurations,
and blower efficiency.
Energy estimates used for planning and design
should be based on expected operating
conditions such as temperature, biofilm thickness,
rotational speed, and media surface area instead
of normalized energy data sometimes supplied by
equipment manufacturers. Care should be taken
to assure that manufacturer's data are current
and reflect actual field-validated energy usage.
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