Rotating Biological Contactor RBC

Rotating Biological Contactors (RBCs) are used in the treatment of wastewater as a secondary treatment process. The RBC process involves allowing wastewater to come in contact with a biological medium in order to remove contaminants in sewage before discharge of the treated wastewater to the environment, usually a river.

The construction of an RBC consists of a series of plastic discs, the media, mounted on a driven shaft that is contained in a tank or trough. Commonly used plastics for the media are polythene, PVC, and expanded polystyrene. The shaft is aligned with the flow of sewage so that the discs rotate at right angles to the flow, with several rotors usually combined to make up a treatment train. About 40% of the disc area is immersed in the sewage.

The biological growth that becomes attached to the discs assimilates the organic materials in the wastewater. Aeration is provided by the rotating action, which exposes the media to the air after contacting them with the wastewater, facilitating the digestion of the organic compounds that need to be removed. The degree of wastewater treatment is related to the amount of media surface area and the quality and volume of the inflowing wastewater.

The RBC process may be used where the wastewater is suitable for 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 because recycling of effluent or sludge is not required. Special consideration must be given to returning supernatant from the sludge digestion process to the RBCs. 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 suscepti-

Table 4.1. Trickling filter types in biological wastewater treatment (adapted from Metcalf and Eddy, Inc., 1991).

Hydraulic

bod5

bod5

Recir

Loading

Loading

Removal

Depth

culation

Film

Filter Type

Filter Medium

(gal/ft2-min)

(lb/ft3-day)

(%)

(ft)

Ratio

Sloughing

Nitrification

Low rate

Rock & slag

0.0-0.06

0.005-0.025

80-90

6-8

0

Intermittent

Well

Intermediate

Rock & slag

0.06-0.16

0.015-0.03

50-70

6-8

0-1

Intermittent

Partial

rate High rate

Rock

0.16-0.64

0.03-0.06

65-85

3-6

1-2

Continuous

Little

Super high rate

Plastic

0.2-1.2

0.03-0.1

65-80

10^10

1-2

Continuous

Little

Roughing

Plastic &

0.8-3.2

0.1-0.5

40-65

15^10

1^1

Continuous

None

redwood

Two-stage

Rock & plastic

0.16-0.64

0.06-0.12

85-95

6-8

0.5-2

Continuous

Well

bility 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 BOD from domestic sewage. The process can also be designed to remove ammonia nitrogen (NH3-N). In addition, the RBC process can treat effluents and process wastewater from dairies, bakeries, food processors, pulp and paper mills, and other biodegradable industrial discharges.

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 an RBC facility involves an accurate determination of influent, septic dumps, and side-stream 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 RBCs may be based upon results of similar full-scale installations and/or thorough documented pilot testing with the particular wastewater. Small-diameter RBC pilot units are suitable for determining the treatability of 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

° Organic and hydraulic loading rates ° Influent wastewater characteristics ° Wastewater temperature ° Biofilm control ° Dissolved oxygen levels ° 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.

Design criteria

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°F have a minimal effect on organic removal and nitrification rates; however, below 55°F, manufacturers shall be contacted to obtain the various correction factors that must be utilized to determine the needed additional media surface area. In determining design-loading rates on RBCs, the following parameters should be utilized:

• Design flow rates and primary wastewater constituents

• Total influent BOD5 concentration

• Soluble influent BOD5 concentration

• Percentage of total and soluble BOD5 to be removed

• Wastewater temperature

• Primary effluent dissolved oxygen

• Media arrangement, number of stages, and surface area of media in each stage

• Rotational velocity of the media

• Retention time within the RBC tank(s)

• Influent soluble BOD5 to the RBC system, including soluble BOD5 from in-plant side-streams, septage dumps, etc.

• Influent hydrogen sulfide concentrations

• Peak loading, BOD5 max/BOD5 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.

Because soluble BOD5 loading is a critical parameter in the design of RBC units, it should be verified by influent sampling whenever possible.

Loading rates

When peak:average flow ratio is 2.5:1.0 or less, average conditions can be considered for design purposes. For higher flow ratios, flow equalization should be considered.

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).

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 RBCs.

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.

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 determine the total number of shafts required. The equation in the later section of could be used as an option to determine the number of stages required.

Staging units

Staging of RBC media is recommended to maximize removal of BOD and ammonia nitrogen (NH3-N). In secondary treatment applications, RBCs shall be designed with a minimum of three stages per flow path. For com bined 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 RBCs.

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 calculate only the expected load to the shafts. Flow and load sampling must be done to demonstrate the load that is generally accomplished by composite sampling after primary clarification. To predict effluent quality for a range of loadings, the influent and effluent soluble:total BOD5 ratio can be assumed to be 0.5.

An alternative method of estimating soluble organic removal in the interstages, devised by E. J. Opatken (1986), 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 for the cold winter months. Wastewater temperatures below 15°C decrease shaft rotational speeds and increase loping problems resulting in insufficient biomass sloughing. This equation is as follows (Equation 4.23):

where: Cn is the concentration of soluble organics in the nth 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 nth stage (hour), and Cn— 1 is the concentration of soluble organic matters entering the nth stage (mg/l).

The design engineer shall be aware that this equation may be used only where appropriate, and that in the available RBC literature there may be a number of applicable equations.

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 versus additional RBC media surface area is required when consistently low ammonia nitrogen levels are necessary to meet ef

2kt fluent limitations. If flow equalization is not provided, it may be necessary to increase the design surface area proportional to the ammonia nitrogen diurnal peaking rates.

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 preceded 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.

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