Metcalf & Eddy,
9.3
ROTATING BIOLOGICAL CONTACTORS (RBCs)
Rotating Biological Contactors (RBC’s) were first installed in West Germany in 1960 and later introduced in the United States. Hundreds of RBC installations were installed in the 1970s and the process has been reviewed in a number of reports (U.S. EPA. 1984, 1985, and 1993; WEF, 1998 and 2000). An RBC consists of series of closely spaced circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated through it (see figure 9-11). The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approximately 3.5 m (12 ft) in diameter and 7.5 m (25 ft) in length. The surface area of the disks for a standard unit is about 9300 m² (100,000 ft²), and a unit with a higher density of disks is also available with approximately 13,900 m² (150,000 ft²) of surface area. The RBC unit is partially submerged (typically
Figure 9-11
40 percent) in a tank containing the wastewater, and the disks rotate slowly at about 1.0 to 1.6 revolutions per minute (see Fig.9-11a). Mechanical drives are normally used to rotated the units, but air-driven units have also been installed. In the air-driven units, an array of cups (see Fig.9-11c) is fixed to the periphery of the disks and diffused aeration is used to direct air to the cups to cause rotation. As the RBC disks rotate out of the wastewater, aeration is accomplished by exposure to the atmosphere. Wastewater flows down through the disks, and solids’ sloughing occurs. Similar to a trickling filter, RBC system require pretreatment of primary clarification or fine screens and or secondary clarification for liquid/solids separation.
A submerged RBC design was also introduced in the early 1980s but has seen limited applications. The submergence is to 70 to 90 percent and air-drive units are used to provide oxygen and rotation. The advantages claimed for the submerged unit are reduced loadings on the shaft and bearings, improved biomass control by air agitation. However, because of the comparatively low levels of dissolved oxygen-limited. To prevent algae growth, protect the plastic disks from the effects of ultraviolet exposure, and to prevent excessive heat loss in cold weather. RBC units are covered (see Fig. 9-11b).
The history of RBC installations has been troublesome due to inadequate mechanical design and lack of full understanding of the biological process. Structural failure of shafts, disks, and disk support system has occurred. Development of excessive biofilm growth and sloughing problems has also led to mechanical shaft, bearing, and disk failures. Many of these problems were related to lack of conservatism in design and scale-up issues from pilot-plant to full-scale units. Many of the problems associated with earlier installations have been solved and numerous RBC installations are operating successfully.
Process Design Considerations
There are many similarities between RBC design considerations and those described for trickling filters. Both systems develop a large biofilm surface area and rely on mass transfer oxygen and substrates from the bulk liquid to the biofilm. The complexity in the physical and hydrodynamic characteristics requires that the design of the RBC process be based on fundamental information from pilot-plant and field installations. As for trickling filters, the organic loading affects BOD removal efficiency and the nitrogen loading after a minimal BOD concentration is reached affects the nitrification efficiency. In contrast to the trickling filter where the wastewater flow approaches a plug flow hydraulic regime, the RBC units are rotated in a basin containing the wastewater, so that separate baffled basins are needed to develop the benefits of a staged biological reactor design. The design of an RBC system must include the following considerations: (1) staging of the RBC units, (2) loading criteria, (3) effluent characteristics, and (4) secondary clarifier design. Typical design information for RBC’s is presented in Table 9-8.
Staging of RBC Units
Staging is the compartmentalization of the RBC disks to form a series of independent cells. Based on mass transfer and biological kinetic fundamentals, higher specific substrate removal rates will occur in RBC biofilms at higher bulk liquid substrate concentrations. Because a low effluent substrate concentration and high specific substrate removal rates are generally the ultimate treatment goal, reduced disk area requirements can be realized only by using stage-RBC units.
The RBC process application typically consists of a number of units operated in series. The number of stages depends on the treatment goals, with two to four staged for BOD removal and six or more stages for nitrification. Stages can be accomplished by using baffles in a single tank or by use of separate tanks in series. Staging promotes a variety of conditions where different organisms can flourish in varying degrees from stage to stage. The degree of development in any stage depends -
Table 9-8
Typical design information for rotating biological contactors
Parameter
Unit
Treatment Level®
BOD Removal
BOD Removal and Nitrification
Separate Nitrification
Hydraulic Loading
m³/m².d
00.8-0.16
0.03-0.08
0.04-0.10
Organic Loading
g sBOD/ m².d
4-10
2.5-8
0.5-1.0
g sBOD/ m².d
8-20
5-16
1-2
Max. 1st-stage organic loading
g sBOD/ m².d
12-15
12-15
g sBOD/ m².d
24-30
24-30
NH3 Loading
g N/ m².d
0.75-1.5
Hydraulic Retention Time
H
0.7-1.5
1.5-4
1.2-3
Effluent BOD
mg/l
15-30
7-15
7-15
Effluent NH4-N
mg/l
<2
1-2
® Wastewater temperature above 13°c (55°F)
Note: g/ m².d x 0.204 = lb/10³ ft².d.
m³/ m².d x 24.5424 = gal/ ft².d.
primarily on the soluble organic concentration in the stage bulk liquid. As the wastewater flows through the system, each subsequent stage receives an influent with a lower organic concentration than the previous stage. Typical RBC staging arrangements are illustrated on Fig. 9-12.
For small plants, RBC drive shafts are oriented parallel to the direction of flow with disk clusters separated by baffles (see Fig. 9-12a). In larger installations, shafts are mounted perpendicular to flow with several stages in series to form a process train (see Fig. 9012b). To handle the loading on the initial units, step feed (see Fig. 9-12d) or a tapered system (see Fig. 9-12e) may be used. Two or more parallel flow trains should be installed so the units can be isolated for turndown or repairs. Tank construction may be reinforced concrete or steel, with steel preferred at smaller plants. Treatment systems employing RBCs have been used for BOD removal, pretreatment of industrial wastewater, combined BOD removal and nitrification, tertiary nitrification, and denitrification. The principal advantages of the RBC process are simplicity of operation and relatively low energy costs.
The History of RBC Loading Criteria
Based on experience, the performance of an RBC system is related to the specific surface loading rate of total and soluble BOD for BOD removal an NH4-N for nitrification. For successful treatment, the loading rates must be within the oxygen transfer capability of the system. Poor performance, odors, and biofilm sloughing problem have occurred when the oxygen demand due to the BOD loading has exceeded the oxygen transfer capability. A characteristic of this problem is the development of Beggiatoa a reduced-sulfur oxidizing bacteria, on the outer portion of the biofilm, which prevents sloughing (S.S. EPA, 1984). A thick biofilm can develop to create enough weight to stress the structural strength of the plastic disks and shaft.
Under overloaded conditions, anaerobic conditions develop deep in the attached film. Sulfate is reduced to H2S, which diffuses to the outer layer of the biofilm, where oxygen is available. Beggiatoa, a filamentous bacteria, which is able to oxidize the H2S and other reduced sulfur compounds, form a tenacious whitish biofilm that does not slough under the normal RBC rotational -
Figure 9-12
sheer conditions. In designing RBC units, it is important to select a low enough BOD loading for the initial units in the staged design to prevent overloading. Odor problems are most frequently caused by excessive organic loadings, particularly in the first stage.
Because the soluble BOD is used more rapidly in the first stage of an RBC system, most manufacturers of RBC equipment specify a specific soluble BOD loading in the range of 12 to 20 g sBOD/ m².d. (2.5 to 4.1 lb sBOD/10³ ft².d) for the first stage. Assuming a 50 percent soluble BOD fraction, the total BOD loading ranges from 24 to 30 g BOD/m².d. For some designs that involve higher-strength wastewaters, the loading criteria are met by splitting the flow to multiple RBC units in the first stage or using a step feeding approach as shown on Fig. 9-12d.
For nitrification, the design approach for RBC systems can be very similar to that shown for tertiary nitrification trickling filters after the sBOD concentration is depleted in RBC units preceding nitrification. An sBOD concentration of less than 15 mg/l must be met before a significant nitrifying population can be developed on the RBC disks (Pano and Middlebrooks, 1983). The maximum nitrogen surface removal rate has been observed to be about 1.5 g N/m².d (U.S. EPA, 1985), which is quite similar to values observed for trickling filters.
Effluent Characteristics
Treatment systems with RBCs can be designed to provide secondary or advanced levels of treatment. Effluent BOD characteristics for secondary treatment are comparable to well-operated activated-sludge processes. Where a nitrified effluent is required, RBCs can be used to provide combined treatment for BOD and ammonia nitrogen, or to provide separate nitrification of secondary effluent. Typical ranges of effluent characteristics are indicated in Table 9-8. An RBC process modification in which the disk support shaft is totally submerged has been used for denitrification of wastewater (see Sec. 9-7).
Physical Facilities for RBC Process
The principal elements of an RBC unit and their importance in the process are described in this section. The suppliers of RBC equipment differ in their disk designs, shafts, and packing support, and configuration designs. The principal elements of an RBC system design are the shaft, disk materials and configuration, drive system, enclosures, and settling tanks.
Shaft
The RBC shafts are used to support and rotate the plastic disks. Maximum shaft length is presently limited to 8.23 m (27 ft) with 7.62 m (25 ft) occupied by disks. Shorter shaft lengths ranging from 1.52 to 7.62 m (5 to 25 ft) are also available. Shaft shapes include square, round, and octagonal, depending on the manufacturer. Steel shafts are coated to protect against corrosion and thickness range from 13 to 30 mm (0.5 to 1.25 in) (WEF, 1998). Structural details and the life expectancy of the disk shaft are important design considerations.
Disk Materials
High-density polyethylene is the material used most commonly for the manufacture of RBC disks, which are available in different configurations or corrugation patterns. Corrugations increase the available surface area and enhance structural stability. The types of RBC disks, classified based on the total area of disks on the shaft, are commonly termed low- (or standard) density, medium-density, and high-density. Standard-density disks, defined as disks with a surface area of 9300 m² (100,000 ft²) per 8.23 m (27 ft) shaft, have larger spaces between disks and are normally used in the lead stages of an RBC process flow diagram. Medium-and high-density disk assemblies have surface area of 11.000 to 16,700 m² (120,000 to 180,000 ft²) per 8.23-m (27-ft) shaft, and are used typically in the middle and final stages of an RBC system where thinner biological growth occur.
Drive Systems
Most RBC units are rotated by direct mechanical drive units attached directly to the central shaft. Motors are typically rated at 3.7 or 5.6 kW (5 or 7.5 hp) per shaft. Air-drive units are also available. The air-drive assembly consists of deep plastic cups attached to the perimeter of the disks, an air header located beneath the disks, and air compressor. Airflows necessary to achieve design rotational speeds are about 5.3 m³/min (190 scfm) for a standard-density shaft and 7.6 m³/min (270 scfm) for a high-density shaft. The release of air into the cups creates a buoyant force that causes the shaft to turn. Both systems have proved to be mechanically reliable. Variable-speed features can be provided to regulate the speed of rotation of the shaft.
Tankage
Tankage for RBC systems has been optimized at 0.0049 m³ (12.000 gal) for a shaft with a disk area of 9300 m². Based on this volume, a detention time of 1.44 h is provided for a hydraulic loading of 0.08 m³/m².d (2 gal/ft².d). A typical sidewater depth is 1.5 (5 ft) to accommodate a 40 percents submergence of the disks.
Enclosures
Segmented fiberglass reinforced plastic covers are usually provided over each shaft. In some cases, units have been housed in a building for protection against cold weather, to improve access, or for aesthetic reasons. RBCs are enclosed to (1) protect the plastic disks from deterioration due to ultraviolet light, (2) protect the process from low temperatures, (3) protect the disks and equipment from damage, and (4) control the buildup of algae in the process.
Settling Tanks
Settling tanks for RBCs are similar to trickling filter settling tanks in that all of the sludge from the settling tanks is removed to the sludge processing facilities. Typical design overflow rates for settling tanks used with RBCs are similar to that described for trickling filters with plastic packing in Sec. 9-2.
RBC Process Design
Empirical design approaches have been developed for RBC systems based on pilot-plant and full-scale plant data and that consider such fundamental factors as the disk surface area and specific loadings in term of g/m² disk area.d. Approaches for designing staged RBC systems for BOD removal and nitrification are presented in this section.
BOD Removal
Design models for BOD removal in RBC systems are reviewed in WEF (2000). In a design comparison, the models generally resulted in lower recommended BOD loadings than that determined from manufacturer’s literature and were, in some cases, similar for BOD removals below 90 percent. Of these, a second-order model by Opatken (U.S. EPA 1985) is selected to estimate RBC surface area requirements, as the models was developed with data from nine full-scale plants and includes staged reactor designs.
The second-order model was converted to SI units by Grady et al. (1999), and terms were converted to account for disk surface are. The model can be used to estimate the soluble BOD concentration in each stage.
(Eq. 9-27)
Where
Sn = sBOD concentration in stage n, mg/L
As = disk surface area on stage n, m²
Q = flow rate. m³/d
Because Eq. (9-27) applies only to sBOD concentrations, a secondary clarifier effluent sBOD/BOD ratio of 0.50 is assumed to design for an effluent BOD concentration. Similarly, without sBOD concentration data for the primary effluent fed to the RBC system, an sBOD/BOD ration of 0.50 to 0.75 can be assumed. Because the design is based on sBOD, the first-stage RBC soluble unit organic loading rate should be equal to or less than 12 to 15 g sBOD/m².d to determine the first-stage disk area and effluent sBOD concentration from Eq. (9-27). The computational procedure used to size an RBC system for BOD removal is summarized in Table 9-9 and illustrated in Example 9-7.
Table 9-9
Computation procedure for the design of a rotating biological contactor (RBC) process
Item
Description
1
Determine influent and effluent sBOD concentration and wastewater flowrate
2
Determine the RBC disk area for the first stage based on a maximum sBOD of 12 to 15 g sBOD/m³.d
3
Determine the number of RBC shafts using a standard disk density of 9300 m²/shaft
4
Select the number of trains for the design, flow per train, number of stages, and disk area/shaft in each stage. For the lower loaded stages a higher disk density may be used
5
Based on the design assumptions made in Step 4, calculate the sBOD concentration in each stage. Determine in the effluent sBOD concentration will be achieved. If not, modify the number of stages, number of shafts per stage, and/or disk area per stage. If the effluent sBOD concentration is met, evaluate alternatives to further optimize the design. Note that the procedure lends itself to spreadsheet calculations
6
Develop the secondary clarifier design
Note: g/m³.d x 0.00624 = lb/10³ ft³.d
Example 9-7
Stage RBC Design for BOD Removal
Given the following design conditions, develop a process design for a stage RBC system.
Parameter
Unit
Primary Effluent
Target Effluent
Flow rate
m³/d
4000
BOD
g/ m³
140
20
sBOD
g/ m³
90
10
TSS
g/ m³
70
20
Note: g/ m³ - m/L
Solution
1. Determine number RBC shafts for the first stage
Assume 1st stage sBOD = 15 g/m².d
sBOD loading = (90 g/ m³) 4000 m³/d = 360.000 g/d
Disk Area Required = = 24.000 m²
Use 9300 m²/shaft
Number of shafts = = =2.6
Use 3 shaft for first stage at 9300 m²/shaft
2. Select number of trains and number of stages.
Assume: 3 trains with 3 stages/train
Flowrate/train = = 1333.3 m³/d
3. Calculate sBOD concentration in each stage using the shaft area and flow to each train. Use Eq. (9-27)
a. Stage 1
S1 =
S2 = 90 g/m³
As/Q = 9300 m²/(1333.3 m³/d) = 6.97 d/m
S1 = = 29.8 g/m³
b. Repeat calculation similar to (a) above. Solving for S2 and S3 yields
S2 = 14.8/m³
S3 = 9.1 g/m³
Because the goal was 10 g/m³ for S3, the proposed design is satisfactory
4. Determine the organic and hydraulic loadings
a. First stage organic loading
Lorg = = 12.9 g sBOD/m².d
b. Overall organic loading
Lorg = = 6.7 g BOD/m².d
c. Hydraulic loading
HLR = = 0.05 m³/m².d
5. Summary
Parameter
Unit
Value
No. of trains
Number
3
Flowrate/train
m³/d
1333.3
No. of stages
Number
3
Total disk area/stage
m²
9300
First-stage sBOD loading
G BOD/m².d
12.9
Total number of shafts
Number
3
Overall organic loading
G BOD/m².d
6.7
Hydraulic loading shaft
m³/m².d
0.05
Comment: At the lower concentrations in stages 2 and 3, some nitrification is likely.
Nitrification
Treatment systems employing RBC units can be used to develop nitrifying biofilms for nitrification of secondary effluents or at low sBOD loadings where nitrification can occur in BOD removal systems. For tertiary nitrification the same procedure used for the design of trickling filters (Sec. 9-2) can be followed. A value of 1.5 g N/m².d is recommended based on field test results (U.S. EPA,1984). For combined BOD removal and nitrification will be prevented or inhibited by the addition of sBOD to the RBC unit. The nitrifying bacteria can complete for space on the RBC disk once the sBOD concentration is reduced to 10 to 15 mg/L. The sBOD concentration remaining in an RBC tank will be related to the sBOD loading. Pano and Middlebrooks (1983) provide a relationship to show the effect of the sBOD loading on the nitrification rates.
F rn = 1.00 – 0.1 sBOD (Eq. 9-28)
Where
F rn = fraction of nitrification rate possible without sBOD effect
sBOD = soluble BOD loading, g/m².d
At an sBOD loading rate of 10g sBOD/m².d, the nitrification rate is predicted to be zero.
ROTATING BIOLOGICAL CONTACTORS (RBCs)
Rotating Biological Contactors (RBC’s) were first installed in West Germany in 1960 and later introduced in the United States. Hundreds of RBC installations were installed in the 1970s and the process has been reviewed in a number of reports (U.S. EPA. 1984, 1985, and 1993; WEF, 1998 and 2000). An RBC consists of series of closely spaced circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated through it (see figure 9-11). The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approximately 3.5 m (12 ft) in diameter and 7.5 m (25 ft) in length. The surface area of the disks for a standard unit is about 9300 m² (100,000 ft²), and a unit with a higher density of disks is also available with approximately 13,900 m² (150,000 ft²) of surface area. The RBC unit is partially submerged (typically
Figure 9-11
40 percent) in a tank containing the wastewater, and the disks rotate slowly at about 1.0 to 1.6 revolutions per minute (see Fig.9-11a). Mechanical drives are normally used to rotated the units, but air-driven units have also been installed. In the air-driven units, an array of cups (see Fig.9-11c) is fixed to the periphery of the disks and diffused aeration is used to direct air to the cups to cause rotation. As the RBC disks rotate out of the wastewater, aeration is accomplished by exposure to the atmosphere. Wastewater flows down through the disks, and solids’ sloughing occurs. Similar to a trickling filter, RBC system require pretreatment of primary clarification or fine screens and or secondary clarification for liquid/solids separation.
A submerged RBC design was also introduced in the early 1980s but has seen limited applications. The submergence is to 70 to 90 percent and air-drive units are used to provide oxygen and rotation. The advantages claimed for the submerged unit are reduced loadings on the shaft and bearings, improved biomass control by air agitation. However, because of the comparatively low levels of dissolved oxygen-limited. To prevent algae growth, protect the plastic disks from the effects of ultraviolet exposure, and to prevent excessive heat loss in cold weather. RBC units are covered (see Fig. 9-11b).
The history of RBC installations has been troublesome due to inadequate mechanical design and lack of full understanding of the biological process. Structural failure of shafts, disks, and disk support system has occurred. Development of excessive biofilm growth and sloughing problems has also led to mechanical shaft, bearing, and disk failures. Many of these problems were related to lack of conservatism in design and scale-up issues from pilot-plant to full-scale units. Many of the problems associated with earlier installations have been solved and numerous RBC installations are operating successfully.
Process Design Considerations
There are many similarities between RBC design considerations and those described for trickling filters. Both systems develop a large biofilm surface area and rely on mass transfer oxygen and substrates from the bulk liquid to the biofilm. The complexity in the physical and hydrodynamic characteristics requires that the design of the RBC process be based on fundamental information from pilot-plant and field installations. As for trickling filters, the organic loading affects BOD removal efficiency and the nitrogen loading after a minimal BOD concentration is reached affects the nitrification efficiency. In contrast to the trickling filter where the wastewater flow approaches a plug flow hydraulic regime, the RBC units are rotated in a basin containing the wastewater, so that separate baffled basins are needed to develop the benefits of a staged biological reactor design. The design of an RBC system must include the following considerations: (1) staging of the RBC units, (2) loading criteria, (3) effluent characteristics, and (4) secondary clarifier design. Typical design information for RBC’s is presented in Table 9-8.
Staging of RBC Units
Staging is the compartmentalization of the RBC disks to form a series of independent cells. Based on mass transfer and biological kinetic fundamentals, higher specific substrate removal rates will occur in RBC biofilms at higher bulk liquid substrate concentrations. Because a low effluent substrate concentration and high specific substrate removal rates are generally the ultimate treatment goal, reduced disk area requirements can be realized only by using stage-RBC units.
The RBC process application typically consists of a number of units operated in series. The number of stages depends on the treatment goals, with two to four staged for BOD removal and six or more stages for nitrification. Stages can be accomplished by using baffles in a single tank or by use of separate tanks in series. Staging promotes a variety of conditions where different organisms can flourish in varying degrees from stage to stage. The degree of development in any stage depends -
Table 9-8
Typical design information for rotating biological contactors
Parameter
Unit
Treatment Level®
BOD Removal
BOD Removal and Nitrification
Separate Nitrification
Hydraulic Loading
m³/m².d
00.8-0.16
0.03-0.08
0.04-0.10
Organic Loading
g sBOD/ m².d
4-10
2.5-8
0.5-1.0
g sBOD/ m².d
8-20
5-16
1-2
Max. 1st-stage organic loading
g sBOD/ m².d
12-15
12-15
g sBOD/ m².d
24-30
24-30
NH3 Loading
g N/ m².d
0.75-1.5
Hydraulic Retention Time
H
0.7-1.5
1.5-4
1.2-3
Effluent BOD
mg/l
15-30
7-15
7-15
Effluent NH4-N
mg/l
<2
1-2
® Wastewater temperature above 13°c (55°F)
Note: g/ m².d x 0.204 = lb/10³ ft².d.
m³/ m².d x 24.5424 = gal/ ft².d.
primarily on the soluble organic concentration in the stage bulk liquid. As the wastewater flows through the system, each subsequent stage receives an influent with a lower organic concentration than the previous stage. Typical RBC staging arrangements are illustrated on Fig. 9-12.
For small plants, RBC drive shafts are oriented parallel to the direction of flow with disk clusters separated by baffles (see Fig. 9-12a). In larger installations, shafts are mounted perpendicular to flow with several stages in series to form a process train (see Fig. 9012b). To handle the loading on the initial units, step feed (see Fig. 9-12d) or a tapered system (see Fig. 9-12e) may be used. Two or more parallel flow trains should be installed so the units can be isolated for turndown or repairs. Tank construction may be reinforced concrete or steel, with steel preferred at smaller plants. Treatment systems employing RBCs have been used for BOD removal, pretreatment of industrial wastewater, combined BOD removal and nitrification, tertiary nitrification, and denitrification. The principal advantages of the RBC process are simplicity of operation and relatively low energy costs.
The History of RBC Loading Criteria
Based on experience, the performance of an RBC system is related to the specific surface loading rate of total and soluble BOD for BOD removal an NH4-N for nitrification. For successful treatment, the loading rates must be within the oxygen transfer capability of the system. Poor performance, odors, and biofilm sloughing problem have occurred when the oxygen demand due to the BOD loading has exceeded the oxygen transfer capability. A characteristic of this problem is the development of Beggiatoa a reduced-sulfur oxidizing bacteria, on the outer portion of the biofilm, which prevents sloughing (S.S. EPA, 1984). A thick biofilm can develop to create enough weight to stress the structural strength of the plastic disks and shaft.
Under overloaded conditions, anaerobic conditions develop deep in the attached film. Sulfate is reduced to H2S, which diffuses to the outer layer of the biofilm, where oxygen is available. Beggiatoa, a filamentous bacteria, which is able to oxidize the H2S and other reduced sulfur compounds, form a tenacious whitish biofilm that does not slough under the normal RBC rotational -
Figure 9-12
sheer conditions. In designing RBC units, it is important to select a low enough BOD loading for the initial units in the staged design to prevent overloading. Odor problems are most frequently caused by excessive organic loadings, particularly in the first stage.
Because the soluble BOD is used more rapidly in the first stage of an RBC system, most manufacturers of RBC equipment specify a specific soluble BOD loading in the range of 12 to 20 g sBOD/ m².d. (2.5 to 4.1 lb sBOD/10³ ft².d) for the first stage. Assuming a 50 percent soluble BOD fraction, the total BOD loading ranges from 24 to 30 g BOD/m².d. For some designs that involve higher-strength wastewaters, the loading criteria are met by splitting the flow to multiple RBC units in the first stage or using a step feeding approach as shown on Fig. 9-12d.
For nitrification, the design approach for RBC systems can be very similar to that shown for tertiary nitrification trickling filters after the sBOD concentration is depleted in RBC units preceding nitrification. An sBOD concentration of less than 15 mg/l must be met before a significant nitrifying population can be developed on the RBC disks (Pano and Middlebrooks, 1983). The maximum nitrogen surface removal rate has been observed to be about 1.5 g N/m².d (U.S. EPA, 1985), which is quite similar to values observed for trickling filters.
Effluent Characteristics
Treatment systems with RBCs can be designed to provide secondary or advanced levels of treatment. Effluent BOD characteristics for secondary treatment are comparable to well-operated activated-sludge processes. Where a nitrified effluent is required, RBCs can be used to provide combined treatment for BOD and ammonia nitrogen, or to provide separate nitrification of secondary effluent. Typical ranges of effluent characteristics are indicated in Table 9-8. An RBC process modification in which the disk support shaft is totally submerged has been used for denitrification of wastewater (see Sec. 9-7).
Physical Facilities for RBC Process
The principal elements of an RBC unit and their importance in the process are described in this section. The suppliers of RBC equipment differ in their disk designs, shafts, and packing support, and configuration designs. The principal elements of an RBC system design are the shaft, disk materials and configuration, drive system, enclosures, and settling tanks.
Shaft
The RBC shafts are used to support and rotate the plastic disks. Maximum shaft length is presently limited to 8.23 m (27 ft) with 7.62 m (25 ft) occupied by disks. Shorter shaft lengths ranging from 1.52 to 7.62 m (5 to 25 ft) are also available. Shaft shapes include square, round, and octagonal, depending on the manufacturer. Steel shafts are coated to protect against corrosion and thickness range from 13 to 30 mm (0.5 to 1.25 in) (WEF, 1998). Structural details and the life expectancy of the disk shaft are important design considerations.
Disk Materials
High-density polyethylene is the material used most commonly for the manufacture of RBC disks, which are available in different configurations or corrugation patterns. Corrugations increase the available surface area and enhance structural stability. The types of RBC disks, classified based on the total area of disks on the shaft, are commonly termed low- (or standard) density, medium-density, and high-density. Standard-density disks, defined as disks with a surface area of 9300 m² (100,000 ft²) per 8.23 m (27 ft) shaft, have larger spaces between disks and are normally used in the lead stages of an RBC process flow diagram. Medium-and high-density disk assemblies have surface area of 11.000 to 16,700 m² (120,000 to 180,000 ft²) per 8.23-m (27-ft) shaft, and are used typically in the middle and final stages of an RBC system where thinner biological growth occur.
Drive Systems
Most RBC units are rotated by direct mechanical drive units attached directly to the central shaft. Motors are typically rated at 3.7 or 5.6 kW (5 or 7.5 hp) per shaft. Air-drive units are also available. The air-drive assembly consists of deep plastic cups attached to the perimeter of the disks, an air header located beneath the disks, and air compressor. Airflows necessary to achieve design rotational speeds are about 5.3 m³/min (190 scfm) for a standard-density shaft and 7.6 m³/min (270 scfm) for a high-density shaft. The release of air into the cups creates a buoyant force that causes the shaft to turn. Both systems have proved to be mechanically reliable. Variable-speed features can be provided to regulate the speed of rotation of the shaft.
Tankage
Tankage for RBC systems has been optimized at 0.0049 m³ (12.000 gal) for a shaft with a disk area of 9300 m². Based on this volume, a detention time of 1.44 h is provided for a hydraulic loading of 0.08 m³/m².d (2 gal/ft².d). A typical sidewater depth is 1.5 (5 ft) to accommodate a 40 percents submergence of the disks.
Enclosures
Segmented fiberglass reinforced plastic covers are usually provided over each shaft. In some cases, units have been housed in a building for protection against cold weather, to improve access, or for aesthetic reasons. RBCs are enclosed to (1) protect the plastic disks from deterioration due to ultraviolet light, (2) protect the process from low temperatures, (3) protect the disks and equipment from damage, and (4) control the buildup of algae in the process.
Settling Tanks
Settling tanks for RBCs are similar to trickling filter settling tanks in that all of the sludge from the settling tanks is removed to the sludge processing facilities. Typical design overflow rates for settling tanks used with RBCs are similar to that described for trickling filters with plastic packing in Sec. 9-2.
RBC Process Design
Empirical design approaches have been developed for RBC systems based on pilot-plant and full-scale plant data and that consider such fundamental factors as the disk surface area and specific loadings in term of g/m² disk area.d. Approaches for designing staged RBC systems for BOD removal and nitrification are presented in this section.
BOD Removal
Design models for BOD removal in RBC systems are reviewed in WEF (2000). In a design comparison, the models generally resulted in lower recommended BOD loadings than that determined from manufacturer’s literature and were, in some cases, similar for BOD removals below 90 percent. Of these, a second-order model by Opatken (U.S. EPA 1985) is selected to estimate RBC surface area requirements, as the models was developed with data from nine full-scale plants and includes staged reactor designs.
The second-order model was converted to SI units by Grady et al. (1999), and terms were converted to account for disk surface are. The model can be used to estimate the soluble BOD concentration in each stage.
(Eq. 9-27)
Where
Sn = sBOD concentration in stage n, mg/L
As = disk surface area on stage n, m²
Q = flow rate. m³/d
Because Eq. (9-27) applies only to sBOD concentrations, a secondary clarifier effluent sBOD/BOD ratio of 0.50 is assumed to design for an effluent BOD concentration. Similarly, without sBOD concentration data for the primary effluent fed to the RBC system, an sBOD/BOD ration of 0.50 to 0.75 can be assumed. Because the design is based on sBOD, the first-stage RBC soluble unit organic loading rate should be equal to or less than 12 to 15 g sBOD/m².d to determine the first-stage disk area and effluent sBOD concentration from Eq. (9-27). The computational procedure used to size an RBC system for BOD removal is summarized in Table 9-9 and illustrated in Example 9-7.
Table 9-9
Computation procedure for the design of a rotating biological contactor (RBC) process
Item
Description
1
Determine influent and effluent sBOD concentration and wastewater flowrate
2
Determine the RBC disk area for the first stage based on a maximum sBOD of 12 to 15 g sBOD/m³.d
3
Determine the number of RBC shafts using a standard disk density of 9300 m²/shaft
4
Select the number of trains for the design, flow per train, number of stages, and disk area/shaft in each stage. For the lower loaded stages a higher disk density may be used
5
Based on the design assumptions made in Step 4, calculate the sBOD concentration in each stage. Determine in the effluent sBOD concentration will be achieved. If not, modify the number of stages, number of shafts per stage, and/or disk area per stage. If the effluent sBOD concentration is met, evaluate alternatives to further optimize the design. Note that the procedure lends itself to spreadsheet calculations
6
Develop the secondary clarifier design
Note: g/m³.d x 0.00624 = lb/10³ ft³.d
Example 9-7
Stage RBC Design for BOD Removal
Given the following design conditions, develop a process design for a stage RBC system.
Parameter
Unit
Primary Effluent
Target Effluent
Flow rate
m³/d
4000
BOD
g/ m³
140
20
sBOD
g/ m³
90
10
TSS
g/ m³
70
20
Note: g/ m³ - m/L
Solution
1. Determine number RBC shafts for the first stage
Assume 1st stage sBOD = 15 g/m².d
sBOD loading = (90 g/ m³) 4000 m³/d = 360.000 g/d
Disk Area Required = = 24.000 m²
Use 9300 m²/shaft
Number of shafts = = =2.6
Use 3 shaft for first stage at 9300 m²/shaft
2. Select number of trains and number of stages.
Assume: 3 trains with 3 stages/train
Flowrate/train = = 1333.3 m³/d
3. Calculate sBOD concentration in each stage using the shaft area and flow to each train. Use Eq. (9-27)
a. Stage 1
S1 =
S2 = 90 g/m³
As/Q = 9300 m²/(1333.3 m³/d) = 6.97 d/m
S1 = = 29.8 g/m³
b. Repeat calculation similar to (a) above. Solving for S2 and S3 yields
S2 = 14.8/m³
S3 = 9.1 g/m³
Because the goal was 10 g/m³ for S3, the proposed design is satisfactory
4. Determine the organic and hydraulic loadings
a. First stage organic loading
Lorg = = 12.9 g sBOD/m².d
b. Overall organic loading
Lorg = = 6.7 g BOD/m².d
c. Hydraulic loading
HLR = = 0.05 m³/m².d
5. Summary
Parameter
Unit
Value
No. of trains
Number
3
Flowrate/train
m³/d
1333.3
No. of stages
Number
3
Total disk area/stage
m²
9300
First-stage sBOD loading
G BOD/m².d
12.9
Total number of shafts
Number
3
Overall organic loading
G BOD/m².d
6.7
Hydraulic loading shaft
m³/m².d
0.05
Comment: At the lower concentrations in stages 2 and 3, some nitrification is likely.
Nitrification
Treatment systems employing RBC units can be used to develop nitrifying biofilms for nitrification of secondary effluents or at low sBOD loadings where nitrification can occur in BOD removal systems. For tertiary nitrification the same procedure used for the design of trickling filters (Sec. 9-2) can be followed. A value of 1.5 g N/m².d is recommended based on field test results (U.S. EPA,1984). For combined BOD removal and nitrification will be prevented or inhibited by the addition of sBOD to the RBC unit. The nitrifying bacteria can complete for space on the RBC disk once the sBOD concentration is reduced to 10 to 15 mg/L. The sBOD concentration remaining in an RBC tank will be related to the sBOD loading. Pano and Middlebrooks (1983) provide a relationship to show the effect of the sBOD loading on the nitrification rates.
F rn = 1.00 – 0.1 sBOD (Eq. 9-28)
Where
F rn = fraction of nitrification rate possible without sBOD effect
sBOD = soluble BOD loading, g/m².d
At an sBOD loading rate of 10g sBOD/m².d, the nitrification rate is predicted to be zero.