20.1.1 General Description
A description of the RBC process is provided in Chapter 17 and immediately above. As indicated in Figure 20.1, a cover (typically fiberglass) is provided over each individual RBC unit for physical protection and process enhancement. Alternatively, entire installations can be placed in buildings, but this can result in a humid, corrosive atmosphere, leading to accelerated corrosion. Covers provide protection against inclement weather, freezing, and sunlight, which accelerates media deterioration. Covers also reduce heat loss, allow the offgas to be collected for odor control, and minimize algae growth.
Nearly all manufacturers produce individual RBC units to standard dimensions. A typical media bundle is 3.66 m in diameter and 7.62 m long, on a shaft that is 8.23 m long. Consequently, at a typical rotational velocity of 1.6 rpm the peripheral velocity of the disc is 18.3 m/min. The media is manufactured from high-density polyethylene containing UV inhibitors such as carbon black. The individual sheets are corrugated in much the same fashion as plastic sheet trickling filter media. Corrugations increase the stiffness of each disc, increase the available surface area, improve mass transfer, and serve to define the spacing between individual disks. Just as with plastic sheet trickling filter media, the size of the corrugations defines how closely together the individual sheets of media can be placed, thereby determining the media density. Standard density media has a specific surface area of about 115 mJ/m\ so each standard shaft provides 9,300 m: of media surface area. High-density media has a specific surface area of about 175 nv/m', providing 13,900 m'' of media surface area per shaft. Minor density differences occur from one manufacturer to another, so it is possible to purchase media with slightly larger or smaller surface areas per shaft. Like trickling filter installations, the media density used for a particular application is determined by the characteristics of the wastewater being treated and by the treatment objectives.
Individual RBC units (called shafts) generally are arranged in series to maximize capacity and treatment efficiency. Baffles are used to separate the RBC shafts into a series of completely mixed bioreactors, each referred to as a stage. The effects of staging on system performance are discussed in Sections 17.4 and 20.2.3. A single stage may contain more than one shaft, but because each stage is completely mixed, all shafts in a stage behave in the same manner. A volume of 45 m1 per shaft is typically used to size the bioreactor. As illustrated in Figure 20.2a, a typical system for removal of biodegradable organic matter, i.e., secondary treatment, might use
Standard Density Media
three stages in series, with the first stage containing two shafts. Such a series of stages is referred to as a treatment train. A mixture of standard and high-density RBC shafts can be used in a single train, although the initial shaft will generally contain standard density media. A larger number of RBC shafts in series will typically be used for advanced treatment applications, i.e., both carbon oxidation and nitrification, as illustrated in Figure 20.2b.
The axis of each individual RBC shaft is typically placed perpendicular to the direction of flow through the train. As indicated in Figure 20.1, the RBC shafts generally rotate in the direction that causes the top of the media to move opposite to the direction of flow. This minimizes short circuiting.
The baffles used to define the individual stages in a treatment train are typically not load bearing and are not capable of isolating an individual RBC shaft. Rather, they are often moveable to allow the number of stages and their sizes to be adjusted in response to long-term variations in process loadings. A typical inter-stage baffle is illustrated in Figure 20.1.
Staging can also be accomplished using a single RBC shaft, as illustrated in Figure 20.2c. The single shaft is placed in a bioreactor with a volume of 45 m \ with the shaft parallel to the long dimension of the bioreactor. Flow is parallel to the shaft, and inter-stage baffles are placed at various points along the shaft to provide the necessary staging. This arrangement is used in small wastewater treatment plants where only a small number of RBC units is needed.
Individual RBC trains are arranged in parallel with flow split equally to each train, as illustrated in Figure 20.3. Larger wastewater treatment plants will use several trains of parallel shafts. In smaller facilities, the "end flow" configuration illustrated in Figure 20.2c is used for each train. For systems removing organic matter, the effluent from the RBC trains will typically be combined and conveyed to secondary
Figure 20.3 Typical configuration of an RBC treatment facility.
clarifiers for the removal of sloughed biomass. Clarification of the RBC effluent may not be necessary for tertiary nitrification applications.
Treatment Objectives. RBCs are used to remove biodegradable organic matter and convert ammonia-N and organic-N to nitrate-N. As discussed in Section 20.2.1, operational problems caused by high unit organic loading rates restrict the use of RBCs for partial removal of organic matter, i.e., for "roughing" treatment. However they can be used quite effectively for substantial removal of organic matter. Process effluent, i.e., clarified, five day biochemical oxygen demand (BOD,) and total suspended solids (TSS) concentrations can easily be reduced to less than 30 mg/L each, and even lower concentrations can be obtained in some instances. This degree of treatment can be accomplished by applying proper organic and hydraulic loading rates, as discussed below.
Combined carbon oxidation and nitrification can also be accomplished in an RBC system. As discussed in Section 15.4, heterotrophic and autotrophic bacteria compete for space within the aerobic portion of a biofilm, causing heterotrophs to predominate when both organic substrate and ammonia-N concentrations are high. Consequently, the oxidation of organic matter will generally occur in the initial stages of the RBC train, just as it occurs in the top of a trickling filter. However, if the organic loading on the train is sufficiently low, the organic substrate concentration will be reduced sufficiently so that autotrophs will be able to compete in the latter stages. As with other aerobic fixed film processes, this occurs when the soluble substrate concentration is reduced to about 20 mg/L as chemical oxygen demand (COD) (15 mg/L as BOD,).2"27 " Thus, the primary distinction between a secondary treatment application (the removal of organic substrate alone) and a combined carbon oxidation and nitrification application (the removal of organic substrate and the oxidation of ammonia-N to nitrate-N) is the organic loading. A larger number of stages may be used for combined carbon oxidation and nitrification to increase the degree of staging and separate the heterotrophic and autotrophic bacteria.
Rotating biological contactors can also be used for separate stage nitrification; that is, to nitrify streams containing relatively high concentrations of ammonia-N and low concentrations of organic matter. Such applications may not require downstream clarification because of the low biomass production rates associated with nitrification. Separate stage nitrification applications are distinguished from combined carbon oxidation and nitrification applications by the characteristics of the wastewater being treated. If the concentration of organic substrate in the influent wastewater is low, relative to the concentration of ammonia-N, the biofilm that develops will be enriched in nitrifiers and the impact of the organic matter on process sizing will be negligible. Benchmarks for distinguishing a separate stage nitrification application for domestic wastewater treatment are an influent BOD,/TKN ratio less than about 1.0 and/or an influent soluble BOD, concentration less than about 15 mg/L.
Rotating biological contactors have also been used to accomplish denitrifica-tion. In these applications, the RBC unit is entirely submerged and covers are provided to exclude air. The influent is generally a nitrified secondary effluent, so an electron donor must be provided. These applications are quite limited and will not be discussed further.
Equipment Type. A motive force is necessary to rotate the RBC shaft. Two general approaches are used: mechanical drives and air drives. Mechanical drive systems consist of an electric motor, a speed reducer, and a belt or chain drive for each shaft. The electric motors are typically 3.7 or 5.6 kW, and the RBC rotational speed is typically 1.2 to 1.6 rpm. The capability to adjust speed and rotational direction can be provided by using speed reducers with multiple pulley or sprocket ratios or through a variable speed drive. These features can be used to control the buildup of excess biomass.
Air drive units increase the oxygen transfer capacity of an individual RBC unit and reduce the number of electric motors required. Cups are added to the periphery of the media and oriented to collect air injected under the RBC shaft. The cups are either 10 or 15 cm long, depending on the organic loading to the unit. The air flow per shaft ranges from 4.2 to 11.3 m'/min under standard conditions, which is typically sufficient to provide rotational speeds of 1.0 to 1.4 rpm. Air is generally provided to all shafts by a centralized blower system. The quantity of air required varies depending on the specific configuration and operating conditions.
Mechanically driven systems provide reliable, consistent rotation of the RBC shaft and media. However, they are susceptible to biomass buildup when they are organically overloaded or subjected to high sulfide loading, as discussed in Section 20.2.6. Air drive systems provide enhanced oxygen transfer, and the injected air can assist with the removal of excess biomass. Both of these effects can be beneficial in a heavily loaded unit. The primary disadvantage of air drive systems is that they are more susceptible to loping, which is uneven rotation caused by the development of nonuniform biomass growth around the circumference of the RBC media. Uneven rotational speed results as the heavier portion of the disc is lifted out of the liquid, rotated to the top, and allowed to descend by gravity back into the liquid.
A recent innovation is the submerged biological contactor, within which between 70 and 90% of the media is submerged. They are generally aerated. Claimed advantages include reduced structural loadings on the shaft and bearings, improved biomass control, the ability to use larger media bundles, and increased treatment capacity. To date these systems have received limited use.
Table 20.1 summarizes the primary benefits and drawbacks of the RBC process. It is mechanically simple, which simplifies normal equipment maintenance. It is also an uncomplicated process, thereby lessening the need for intensive daily process control actions. The energy requirements are relatively low, being only those required to rotate the media. Finally, it is modular in nature, which simplifies design and construction.
Its principal drawbacks are that process performance is sensitive to wastewater characteristics and loadings, and that it possesses little operational flexibility to adjust to varying loading and operating conditions. As discussed in Section 20.2.1, high organic loadings can result in excessive biomass growth, which structurally overloads the media and shaft. This problem is exacerbated by elevated levels of hydrogen sulfide in the influent wastewater. Although significant deterioration in treatment
Table 20.1 RBC Process Benefits and Drawbacks
Simple process, easy to operate Low energy requirements Modular configuration allows easy construction and expansion
Performance susceptible to wastewater characteristics Limited process flexibility
Adequate pretreatment required capacity and performance result from excessive biomass, the steps that can be taken to minimize its impact are relatively limited. Fortunately, the conditions leading to such cataclysmic declines in performance are now relatively well defined and can generally be avoided if the operating conditions for the facility are well defined.
An early claimed benefit of the RBC process was its ability to respond successfully to shock loads.: However, subsequent experience has demonstrated that its capability to respond to shock loads is much like that of the trickling filter process. Shock loads will not cause complete process failure, but the process does not generally possess sufficient reserve capacity to successfully treat shock loads.'"' Consequently, equalization should be provided upstream of an RBC process if the ratio of peak to average loading exceeds 2.5.27"
The size of an individual RBC shaft limits the maximum plant size for which the RBC process is practical. As discussed above, each shaft can contain a media surface area of 9,300 or 13,900 m\ Consequently, only for small to medium wastewater flow rates can sufficient media be provided by a reasonable number of RBC units. For example, a total media surface area of about 280,000 nr might be required to treat a typical municipal wastewater with a design flow of 15,000 mVday. This could be provided by 24 RBC shafts configured in six trains consisting of four shafts each. An equivalent media surface area would be provided by two trickling filters, each 16.4 m in diameter and 6.7 m deep. To treat a flow rate ten times as large, the number of RBC units required would increase ten-fold; in this case to 60 trains, each with 4 individual RBC shafts. In contrast, the equivalent trickling filter installation would still require only two trickling filters, although each would have to be 51.9 m in diameter and 6.7 m deep. Alternatively, four trickling filters, each 36.7 m in diameter and 6.7 m deep, could be used. From a cost, construction, and operational perspective, the smaller number of trickling filters would be more desirable. This factor tends to limit the use of RBCs to smaller wastewater treatment plants.
A final drawback of RBCs is the need for adequate preliminary treatment. Debris such as rags, plastics, and fibrous material can clog the RBC media if present in sufficient quantities, and grit will settle in the RBC bioreactor due to the relatively low level of turbulence provided. In general, the minimum degree of preliminary treatment required is fine screening (less than 1 mm opening) and excellent grit removal. Primary clarification is provided in many instances. The cost of the necessary preliminary treatment facilities must be included in any cost evaluation of the RBC process.
The biomass produced in the RBC process generally settles and thickens readily. Consequently, the waste solids stream leaving the final clarifier can either be recycled to the primary clarifier to be settled and thickened with the primary solids, or it can be thickened separately. Other solids thickening options can also be applied successfully.
During the early years of their application, RBC systems experienced a number of mechanical and structural problems, such as detachment of the media from the shaft and/or fatigue and failure of the shafts. The causes for these problems are now well understood by RBC manufacturers, and the expertise exists to manufacture equipment that is devoid of these defects. Thus, while numerous references can be found to these structural problems in the literature, they are no longer a major factor in the evaluation and selection of RBC systems.
Rotating biological contactors have typically been used to provide secondary treatment to municipal wastewater. They have also been used to nitrify municipal wastewaters, either in combined carbon oxidation and nitrification applications or in separate stage nitrification applications. Current estimates are that approximately 70% of the applications in the United States have been for removal of organic matter, 25% for combined carbon oxidation and nitrification, and 5% for separate stage nitrification.'"" Due to the poor economy of scale for this technology, it has been used most frequently for wastewater treatment plants with flows below about 40,000 m'/day. It has also been used successfully in a number of industrial applications, particularly those involving wastewaters of moderate- to low-strength and with low concentrations of hydrogen sulfide.
The reliability of RBCs has improved considerably in recent years. While many RBC installations have provided acceptable performance, many others have not met performance expectations. For example, a survey indicated that over 80%' of the RBCs designed before 1980 have experienced operational problems.1" Many of these problems have been solved by improved construction techniques and the use of appropriate organic and hydraulic loading rates. Consequently, RBC technology is now sufficiently well defined so that it is possible to clearly evaluate existing facilities for upgrades and new applications. In fact, new approaches are being investigated for utilizing existing RBC facilities to accomplish higher levels of treatment.1' Those interested in application of attached growth biochemical operations should monitor these developments for potential application elsewhere.
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