Recommended Design Criteria for Rotating Biological Contactors...
General Considerations...
1. Applicability : The Rotating Biological Contactor (RBC) process may be used where the wastewater is amenable to biological treatment. The RBC process can be used in many modes to accomplish varied degrees of carbonaceous and/or nitrogenous oxygen demand reductions. The process is simpler to operate than activated sludge since recycling of effluent
or sludge is not required. Special consideration must be given to returning supernatant from the sludge digestion process
to the RBC's.
The advantages of RBC technology include a longer contact 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 the conventional activated sludge process. Whether used in small or large facilities,
the RBC process should be designed to remove at least 85 % 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.
2. Process Selection : Choice of the process mode most applicable will be influenced by the degree and consistency of treatment required, type of waste to be treated, site constraints, and capital and operating costs. The process design of
a RBC facility involves an accurate determination of influent, septage dumps, and sidestream loadings, proper media sizing, staging and equipment selection to meet effluent requirements, air requirements, and selection of an overall plant layout that shall provide for flexibility in operation and maintenance.
A comprehensive on-site pilot plant evaluation is recommended to incorporate the factors affecting RBC performance as an accurate source of information for a RBC design. Other approaches to determine the expected performance of RBC's may be
based upon results of similar full scale installations and/or through documented pilot testing with the particular wastewater. Small-diameter RBC pilot units are suitable for determining the treatability of a wastewater. If small-diameter units are operated to obtain design data, each stage must be loaded below the oxygen transfer capability of a full-scale
unit to minimize scale-up problems. Direct scale-up from small-diameter units to full-scale units is not possible because
of the effects of temperature, peripheral speed of media, and other process and equipment factors. In all RBC systems, the major factors controlling treatment performance are :
a. Organic and hydraulic loading rates;
b. Influent wastewater characteristics;
c. Wastewater temperature;
d. Biofilm control;
e. Dissolved oxygen levels;
f. Flexibility in operation.
Pretreatment...
Raw municipal wastewater shall not be applied to an RBC system. Primary settling tanks are required for effective removal
of grit, debris, and excessive oil or grease prior to the RBC process. In some cases, fine screens (0.03-0.06 inches) may
be considered. Screening and comminution are not suitable as the sole means of preliminary treatment ahead of RBC units.
Sulfide production must be considered in the system design. Separate facilities to accept and control feeding of septage waste or in-plant side streams should be considered where the potential for sulfide production or increased organic and ammonia nitrogen loadings will have a significant impact on the RBC system.
Basis of RBC Design...
1. Unit Sizing : Organic loading is the primary design parameter for the RBC process. This is generally expressed
as the organic loading per unit of media surface area per unit of time, or in units of pounds BOD5 per thousand square
feet per day. Wastewater temperatures above 55 degrees F have minimal affect on organic removal and nitrification rates; however, below 55 degrees F, manufacturers shall be contacted by the designer to obtain the various correction factors
that must be utilized to determine the needed additional media surface area. In determining design loading rates on RBC's, the following parameters should be utilized :
1. Design flow rates and primary wastewater constituents;
2. Total influent BOD5 concentration;
3. Soluble influent BOD5 concentration;
4. Percentage of total and soluble BOD5 to be removed;
5. Wastewater temperature;
6. Primary effluent dissolved oxygen;
7. Media arrangement, number of stages and surface area of media in each stage;
8. Rotational velocity of the media;
9. Retention time within the RBC tank(s);
10. Influent soluble BOD5 to the RBC system including SBOD from in-plant sidestreams, septage dumps, etc;
11. Influent hydrogen sulfide concentrations;
12. Peak loading, BOD5 max/BOD5 avg.; TKN max/TKN avg.
In addition to the above parameters, loading rates for nitrification will depend upon influent DO concentration, influent ammonia nitrogen concentration and total Kjeldahl nitrogen (TKN), diurnal load variations, pH and alkalinity, and the allowable effluent ammonia nitrogen concentration. Since soluble BOD5 loading is a critical parameter in the design of RBC units, it should be verified by influent sampling whenever possible.
2. Loading Rates : ( a ) When peak to average flow ratio is 2.5 to 1.0 or less, average conditions can be considered
for design purposes. For higher flow ratios, flow equalization should be considered. ( b ) The organic loading to the
first stage standard density media should be in the range of 3.5 to 6.0 pounds total BOD5 per thousand square feet per
day or 1.5 to 2.5 pounds soluble BOD5 per thousand square feet per day. First stage organic loadings above 6 pounds total BOD5 or 2.5 pounds soluble BOD5 per thousand square feet per day will increase the probability of developing problems
such as excessive biofilm thickness, depletion of dissolved oxygen, nuisance organisms and deterioration of process performance. The most critical problem in most instances is the structural overloading of the RBC shaft(s). ( c ) For
average conditions, the design loading should not exceed 2.5 pounds of soluble BOD5/1,000 square feet of standard media surface per day on the first stage shaft(s) of any treatment train. Periodic high organic loadings may require supplemental aeration in the first stage shafts. High density media should not be used for the first stage RBC's. ( d ) For peak conditions, the design loading shall not exceed 2.0 pounds of soluble BOD5/1,000 square feet for the first high density
media shaft(s) encountered after the first two shafts or rows of shafts in a treatment train. ( e ) For average conditions, the overall system loading shall not exceed 0.6 pounds of soluble BOD5/1,000 square feet of media. This soluble BOD5
loading to all shafts should be used to determined the total number of shafts required. The equation under section C.3.c could be used as an option to determine the number of stages required.
3. Staging Units : ( a ) Staging of RBC media is recommended to maximize removal of BOD and ammonia nitrogen
( NH3 - N ). In secondary treatment applications, rotating biological contactors shall be designed with a
minimum of three stages per flow path. For combined BOD5 and NH3 - N removal, a minimum of four stages is recommended per flow path. For small installations, multiple stages are acceptable on a single shaft if interstage
baffles are installed within the tank and introducing the flow parallel to the shaft. Whenever multiple process trains
are employed with three or more shafts in a row; the flow path should be introduced perpendicular to the shafts, and the wastewater should be distributed evenly across the face of the RBC's. ( b ) The organic loading must be accurately defined
by influent sampling whenever possible. For existing facilities that are to be expanded and/or rehabilitated it is unacceptable to only calculate the expected load to the shafts. Flow and load sampling must be done to demonstrate
the load which is generally accomplished by composite sampling after primary clarification. To predict effluent quality
for a range of loadings, the influent and effluent soluble-to-total BOD5 ratio can be assumed to be 0.5. ( c ) An
alternative method of estimating soluble organic removal in the interstages, devised by E.J. Opatken, utilizes a second
order reaction equation. The equation may be used for RBC design during the summer months; however, a temperature
correction factor should be used used for the cold winter months. Wastewater temperatures below 15OC decrease shaft rotational speeds and increase loping problems resulting with insufficient biomass sloughing. This equation is as follows :
C n = { - 1 + [ 1 + ( 4 ) ( k ) ( t ) ( Cn - 1 ) ] 1 / 2 } / [ ( 2 ) ( k ) ( t ) ]
Where :
C n = Is the concentration of soluble organics in the n th stage ( mg / L);
k = is the second-order reaction constant of 0.083 ( L / mg . hr );
t = is the average hydraulic residence time in the n th stage ( hour );
C n - 1 = is the concentration of soluble organics entering the n th stage ( mg / L)
The design engineer shall be aware that this equation shall be used only where appropriate, and that in the available RBC literature there may be a number of applicable equations.
4. Design Safety Factor : Effluent concentrations of ammonia nitrogen from the RBC process designed for nitrification
are affected by diurnal load variations. An evaluation of equalization vs. additional RBC media surface area is required
when consistently low ammonia nitrogen levels are necessary to meet effluent limitations. If flow equalization is not provided then it may be necessary to increase the design surface area proportional to the ammonia nitrogen diurnal peaking rates.
5. Secondary Clarification : The concentration of suspended solids leaving the last stage of an RBC system treating municipal wastewater is generally less than 200 mg/l when preceeded with primary clarification. To attain secondary effluent quality standards, secondary clarifiers must be used in conjunction with RBCs. The surface overflow rate, generally, should not exceed 800 gallons per day per square foot for secondary clarifiers. Consideration may be given to covering the clarifiers to improve efficiency.
Plant Design Considerations...
1. Enclosures : ( a ) Wastewater temperature affects rotating contactor performance. Year-round operation in cold climates requires that rotating contactors be covered to protect the biological growth from freezing temperatues and avoid excessive loss of heat from the wastewater. In order to prevent excessive heat gain during the summer, proper ventilation
of the insulated covers should be assured. ( b ) The enclosures should be individual removable covers or huts rather than
a building to house the RBC units. All enclosures shall be constructed of durable and corrosion-resistant materials. Snow loads should be considered in the design of individual covers. ( c ) In all RBC designs, convenient access to individual bearings, shafts, media, or mechanical equipment shall be provided for inspection, maintenance, and possible removal,
repair or replacement. The RBC design layout should consider the size, reach and accessibility of cranes for shaft removal. ( d ) If RBC's are installed in buildings; windows, louvered mechanisms and/or doors shall be installed to provide adequate ventilation. To minimize condensation on the walls and ceilings, the building shall be humidity controlled, heated and/or adequately insulated. The structure shall be designed such that it can be readily dismantled for removal of shafts and media at minimal cost and inconvenience of continued plant operation under expected environmental conditions. ( e ) Electrical equipment and controls shall comply with the National Electrical Code for Class I, Groups C and D, Division 1 locations.
The equipment, fixtures, lighting, and controls shall be located to provide convenient and safe access for operation and maintenance. A positive means of locking out each mechanical device shall be provided. ( f ) Pressurized water facilities shall be provided for spraying the media, washing tanks and other equipment. Potable water may be used but shall have backflow preventors. Non-potable wash water outlets shall be permanently posted indicating the water is not safe.
2. Flexibility and Flow Control : ( a ) Multiple treatment trains should be considered in the design for adequate flexibility in operation of the facility. For maximum flexibility, each train could have a separate basin for maintenance.
( b ) Positive and measurable flow control equipment must be provided for each unit or flow train for proper distribution
of the influent and effluent. Splitter boxes and/or weirs are preferred to long channels with slide gate controls.
( c ) Removable baffles shall be provided between all stages. ( d ) The capability to step feed stage(s) should be provided to reduce overloading. ( e ) Mechanical drive systems shall have provisions for variable speed rotation and supplemental
air capability in first and second stages of the RBC train. A rotational speed control of 1.6 rpm shall be provided with appropriate equipment supplied to reduce or increase speed as necessary to improve treatment efficiency and reduce energy costs. The mechanical drive systems should have provisions for reversing the direction of RBC rotation and supplemental air to promote biofilm stripping. ( f ) A means of adding chemicals, such as hydrogen peroxide or chlorine, should be considered for the media and/or wastewater to promote biofilm stripping and enhance oxygen transfer. ( g ) Air drive systems shall be provided with extra air capabilities. A rotational speed capability of 1.2 rpms must be provided. Air shall be supplied at
a rate to produce a range of shaft speeds throughout the year with one blower out of service plus additional air for
scouring the media. Generally, air flow requirements to provide the required seasonal shaft rotational speeds and extra
air for scouring, is approximately 400-600 cfm per shaft. ( h ) Air drive units or supplemental aeration units shall be provided with positive air flow control and metering to each shaft. ( i ) The capabilities of bypassing or recycling wastewater should be considered for adverse conditions. ( j ) Facilities for chemical addition following the biodiscs,
prior to the clarifiers, may be considered to allow polymer addition.
3. Monitoring : Dissolved oxygen (DO) monitoring shall be provided for in the first stage. DO monitoring for the other stages should be considered. The RBC system design shall provide for a positive DO level in all the stages. It is
recommended that the first two stages maintain at least 2 mg/l of dissolved oxygen.
4. Contactor Basins and Channels : ( a ) Drains shall be provided for each contactor basin. A sight tube may be included to montior sludge build-up. ( b ) The clearance between the basin floor and the bottom of the rotating media shall be from
4 to 9 inches. ( c ) The basins shall have a depth for submergence of at least 40% of the total media surface area at any
one time. ( d ) The basin volume-to-media surface area should be at 0.12 gallons per square foot. ( e ) Where deep channels are used to and from RBC basins, channel aeration or channel configurations shall be provided for scouring velocity. ( f )
Side wall slope of the biodisc basin is important to prevent sludge/solids accumulation.
Equipment Considerations...
1. Shafts : ( a ) RBC shafts are presently limited to approximately 27 feet in length. ( b ) Shafts shall be
fabricated from steel and be covered with a protective coating suitable for the humid and corrosive conditions. All fabrication during construction shall conform to American Welding Society (AWS) welding and quality control standards.
Media shafts shall be designed for unbalanced loads and cycle fatigue. ( c ) The design engineer should require the manufacturer to provide adequate assurance that the shaft(s), bearings, and media support structures are protected from structural failure for the design life of the facility. The manufacturer of the RBC units shall guarantee the shaft(s)
for 5 years. ( d ) Structural designs shall be based on appropriate AWS stress category curves modified as necessary to account for the anticipated corrosive conditions. ( e ) The design engineer shall specify a load bearing capacity for
each shaft considering the maximum anticipated biofilm growth and to include an adequate margin of safety.
2. Media : ( a ) Media materials shall be special manufactured material suitable and durable for the rotating biological contractor process. Media shall be resistant to disintegration, ultraviolet degradation, erosion, aging, all common acids, alkalies, organic compounds, fungus, and biological attack. ( b ) High density media shall not be used on the first two stages or rows of shafts of a treatment train for the purpose of BOD removal. ( c ) Media shall be fabricated with corrugations for stiffness and spacing. The media should not exceed 12 feet in diameter. Standard density media are considered as media with a surface area of 100,000 to 120,000 square feet and high density media are considered 150,000 square feet or more. ( d ) The manufacturer of the RBC media shall guarantee the media for 5 years. ( e ) All plastic
media shall be adequately supported on or attached to the shaft. ( f ) Air cups attached around the outer perimeter of
the media on an air driven unit shall be 6 inches in depth.
3. Drive Systems : ( a ) Mechanically driven RBC units shall have high efficiency motors and drive equipment which shall include variable speed capability. The electric motors used for mechanical drive RBCs shall be either 5 or 7.5 hp depending upon the actual energy requirements. The actual energy requirements for mechanically driven RBC units should be in the range from 1.05 kw/shaft to 3.76 kw/shaft. To evaluate the actual energy requirements for mechanically driven units, the design engineer must take into consideration the influences of drive train efficiency, biofilm thickness, media surface area, temperature, and rotational speed. ( b ) Air drive RBC units shall have high efficiency motors and blower systems which
shall include variable air flow requirements for rotational speed. The specific energy requirements for air driven units
can only be determined on a case-by-case basis and cannot be measured directly. For comparative purposes, an approximation can be made by dividing the blower KW by the number of driven shafts. The actual energy requirements should be within the range of 3.8 kw/shaft to 8.3 kw/shaft. To evaluate the actual energy requirements for air driven units, the design engineer must consider the desired rotational speeds, air flow, piping configurations, and blower efficiency. ( c ) Energy estimates used for planning and design should be based on anticipated operating conditions rather than general energy data supplied
by equipment manufacturers which may not be current or reflect actual field energy usage. If manufacturers energy guarantees or similar projected energy usage is considered, then testing procedures and calibration of equipment shall be included in the specifications. ( d ) The design engineer should consider providing power factor correction capacitors for all RBC mechanical and air drive systems. ( e ) All drive components shall be properly aligned.
4. Bearings : ( a ) Bearings shall be moisture resistant and self-aligning with oversize grease cups to increase lubrication intervals. ( b ) Bearings should be located outside the media covers and protected with cover plates on the
idle end of the shaft. ( c ) Center-carrier bearings with supports should be provided for shafts in excess of 20 feet in length.
5. Load Cells (Hydraulic or Electronic) : ( a ) Load cells shall be provided for all first and second row and/or stage shafts. Load cells are desirable for all shafts. ( b ) Hydraulic load cells shall be installed on the idle end of a mechanically driven shaft. A spare hydraulic load cell shall be provided as a back-up. ( c ) The electronic strain gage
cell with a companion converter unit is most desirable because it has continuous and direct read-out of total shaft weight.
Miscellaneous Considerations...
1. Stop motion detectors, rpm indicators, and clamp-on ammeters are desirable monitoring devices for individual RBC shafts.
2. Media, when stored on-site for installation, shall be properly protected from direct sunlight. Media can also be severely impacted by high wastewater temperatures above 95OF.
3. An O & M manual shall be provided specifying schedules for reading load cells, visual inspections of biofilm
growth, media integrity, and determining the status of mechanical and structural components. The manual shall outline remedial procedures to resolve identified operating problems. The manual should include provisions for daily and
analytical log recording.
4. All RBC units shall be equipped with appropriate safety features for protection of operators. Such features shall include drive mechanism enclosures, lighting, stairways, walkways, handrails, deck gratings, and slip-resistant surfaces.
5. Treatment plant operators should be supplied with sampling equipment, tools, and spare parts as may be necessary.
Deviations from Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.
