Application of Rotating Biological Contactors (RBCs) in the Treatment of Industrial Waste – A Review

K. Stalin* Assistant Professor Dr. Pauls Engineering College, NH-66, Pulichpallam, Vanur Tk., Villupuram Dist. Tamilnadu Pin: 605 109 E-mail: nilats_mtech@yahoo.com

Dr. Y. R. M. Rao Principal, Dr. Pauls Engineering College, NH-66, Pulichpallam, Vanur Tk.,Villupuram Dist. Tamilnadu Pin: 605 109 E-mail: dryrmrao@rediffmail.com

Abstract

Rotating biological contactors (RBCs) constitute a very sole and superior alternative for biodegradable organic matter and nitrogen removal due to the reasons of their feasibility, simplicity of design and operation, short start-up, low land area requirement, low energy requirement, low operating and maintenance cost and treatment efficiency. The paper is mainly focused on the performance of RBCs like rotational speed, organic and hydraulic loading rates, retention time, bio film support media, staging, temperature, influent wastewater characteristics, bio film characteristics, dissolved oxygen levels, effluent and solids recirculation, step feeding and medium submergence. RBCs design considerations; operational problems and assessment with other wastewater treatment systems are also reported.

Key words: Biofilm, Grey water, Influent, Rotating Biological Contactor, Hydraulic Retention Time, organic loading, wastewater

1.0 Introduction

The upper crust of the earth surface is covered with water by 70% and land by 30 %. Almost 97 % volume of the total water is present in the sea, which is unusable due its high salt (mostly Nacl) concentration. Only 0.3 % of the total water available is usable for living organisms. With limited natural resources, demand for usable water is increasing day by day due to population explosion and industrial growth. As a result, India and other countries are going to meet water scarcity by 2030. Since the natural resources are limited, hence sustainable activities should be planned for future generation. Industrialists are generally of the opinion “spending money on waste treatment is waste” and hence treatment part is neglected. Hence, there is a need to treat the wastewater for recycling by recovering the useful ingredients. This will help in preserving the natural resources. (International water Management Company, May 2, 2011).

Earlier, the objectives of wastewater treatment were removal of suspended and floatable material, biodegradable organics and elimination of pathogens. The wastewater was normally treated with primary, secondary and advanced treatment methods. The primary treatment includes equalization, neutralization, screening, grit removal, primary sedimentation, etc and the secondary treatment processes include the biological and/or chemical treatment processes. The advanced treatment methods are namely denitrification, phosphorous removal, carbon adsorption, ion exchange, electrodialysis, reverse osmosis, polishing pond, etc. Activated sludge process, trickling filter, oxidation ditches, aerated lagoon, stabilization pond, rotating biological contactor, etc. are some of the biological treatment processes. (Metcalf and Eddy 2004).

The objective of secondary treatment is to remove the residual organics and suspended solids from the effluent of primary treatment. In the most cases, secondary treatment follows primary treatment and involves the removal of dissolved biodegradable and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter present in the wastewater, thereby producing more microorganisms and inorganic end-products like CO2, NH3, and H2O (Metcalf and Eddy, 2004).

2.0 ROTATING BIOLOGICAL CONTACTOR

A rotating biological contactor (RBC) is a type of an attached growth bioreactor that offers an alternative technology to the conventional activated sludge process. The first RBC was installed in West Germany in 1960, later it was introduced in the United States and Canada. In United States, rotating biological contactors are being used for industries which are producing wastewater with high BOD. The rotating packs of discs (known as the media) are installed in a tank or trough and rotate between 2-5 revolutions per minute. Polythene, PVC and expanded polystyrene are commonly used plastics for the media of the rotating discs. The shaft of the rotating disc is aligned with the flow of wastewater, so that discs rotate at the right angle to the flow with several packs. About 40% of the disc area is immersed in the wastewater. Biological growth is attached to the surface of the discs and forms a slime layer.

When the disc rotates, the wastewater comes in contact with the atmospheric air for oxidation. The rotation helps to slough off excess solids. The discs can be staged in series to obtain more detention time or degree of removal.

The discs consist of plastic sheets, ranging from 2 - 4 m in diameter and upto 10 mm in thickness. Several modules may be arranged in parallel and/or in series to increase the length of flow and to meet the treatment requirements. Most RBC systems design will include minimum 4 or 5 modules in series to obtain nitrification of wastewater (Rotating Biological contactor – Wikipedia).

Biofilms (the biological growth) attached to the discs, assimilate the organic materials of the wastewater. Aeration is provided by the rotating action, which helps to expose the media to the air after contacting with the wastewater, facilitating the degradation of the pollutants. The degree of wastewater treatment depends upon the surface area of the media, quality and volume of the inflow wastewater (A. P. Pajak and R. C. Loehr, 1976).
The inherent limitations of RBC, is the lack of operational flexibility. Once the plant is designed and installed, little can be done to alter the operation. On the other hand, the RBC’s lack of flexibility gives the process simplicity and stability. As long as the discs and media keep rotating and the hydraulic loading remains within the design capacity, the RBC normally will function properly. This process, like all other biological treatment processes, does have its limitations. Organic and hydraulic shock loading as well as toxic discharge will tend to decrease the process efficiency.

Excess slime, which has built up on the RBC has been sloughed off into the wastewater. Hence, after being treated in RBC, the wastewater is sent to the secondary settling tank (SST). This sloughed off slime and other sludge settle to the bottom of the SST and the effluent is disposed off. Sludge from the SST is pumped to the sludge digester for further treatment (A. P. Pajak and R. C. Loehr, 1976).

The use of simple mechanical components for the process, results in very low maintenance cost. Mainte-nance is only limited to greasing of bearings and inspecting the chains and sprockets. The low speed of disc is used in this process to achieve sufficient mixing and aeration with relatively low power consumption (Nahid P. et al 2001).

Since the metabolic rate of the bio-mass would be decreased with low temperature. RBCs are usually covered or enclosed. The rotating disc process is normally designed on the basis of hydraulic loading. At a specific hydraulic loading rate, specific percentage of BOD reduction is obtained. Thus, the percentage of BOD reduction is mainly a function of the hydraulic loading rate and hydraulic detention time (B. Guimaraesa et. al, 2005).

2.1 Advantages of RBC

● Consistent process results
● Stable operation without frequent supervision
● Lower expansion and retrofit costs due to modular construction and reduced excavation
● Short wastewater hydraulic retention time
● Minimal head loss through system
● Low energy consumption
● The only maintenance required is simple drive and bearing lubrication
● Reduced life cycle costs as compared to suspended growth systems
● Simple in operation since no sludge recycle
● Operator friendly
● Easy for up gradation and multiple application.

2.2 Factors Affecting the Performance

The performance of RBC depends upon several design parameters which include (i) rotational speed (ii) organic loading (iii) hydraulic loading rate (iv) hydraulic retention time (HRT) (v) RBC media (vi) temperature (vii) staging (viii) influent wastewater characteristics (ix) biofilm characteristics (x) step feeding (xi) disc submergence

2.2.1 Rotational speed

The rotational speed of the RBC discs is a very important parameter that affects nutrient and the oxygen mass transfer in the biofilm and consequently substrate removal. Usually, an increase on the speed of rotation increases the dissolved oxygen concen- tration available to the microorganism and as a result they are able to degrade the substrate at a higher rate (K.H Radwan and T.K.Ramanujam, 1995). However, increasing the rotational speed leads to higher power consumption, which may not be econo- mical for wastewater treatment applications. Besides, if the rotational speed is too high the microorganisms (bioflim) will be stripped off the media, deteriorating the effluent quality and lowering the biodegradation rate in the reactor. Packed supports will provide consi- derably more oxygenation than disc RBCs at the same rotational speed, but they require greater power consumption.

2.2.2 Organic loading

The variation of the organic loading rate is generally accomplished by changing the influent flow rate or the HRT, which also results in a change in the hydraulic loading (G. D. Najafpour and A. A. Zinatizadeh, 2005). As the applied organic loading rate increases the removal efficiency of RBC decreases. Reduction in efficiency may be an indica- tion of limitation in dissolved oxygen. Overloading problems can be avoided by removing baffles between the stages to reduce surface loading and increased oxygen transfer level. Oxygen transfer methods include (i) supplemental aeration (ii) step feeding (iii) recycling and (iv) introduction of an anaerobic system such as up flow anaerobic sludge blanket (UASB) reactor prior to the aerobic RBC systems.

2.2.3 Hydraulic loading

Increasing the flow rate through the bioreactor reduces the liquid retention time in the system and results in a reduction in removal efficiency. In defined conditions, increasing hydraulic loading also leads to an increase of attached biomass on RBC media surface (L. Alemzadeh and M. Vossoughi, 2001). Hydraulic loading rates vary widely depending on the design the substrate being removed and the effluent concentration desired (John N. Hochheimer and fred Wheaton, 1998). Typical hydraulic loading rate range recommended by RBC manufacturers (full-scale) is 1.292-6.833 dm3/h (G.Tchobanoglous and FL Burton, 1995). Due to the large amount of biological mass present (low operating feed / microorganisms), RBCs offer good stability under high or toxic hydraulic and organic loadings (S. Sirianutapiboon 2006).

2.2.4 Hydraulic retention time (HRT)

Longer contact times improve the diffusion of the substrate into the biofilm and its consequent removal of the influent (G.D Najafpour and A. A. Zinatizadeh, 2005). This trend is also verified with toxic and heavy metals substrates (S. C. Costley and F. M. Wallis, 2001). Too short HRT will result in low removal rates, whereas too long HRT will not be economically feasible. In order for a biological system to compete successfully with conventional physicochemical treatment methods, the shortest possible HRT associated with the most efficient removal rates is required (S. C. Costley and F. M. Wallis, 2001). Significant advantage offered by full scale RBCs is to require short hydraulic retention periods (generally less than 1h) (Zhongming zheng and Jeffrey Philip Obbard, (2002).

2.2.5 RBC Media

The media used for RBCs are actually produced from Styrofoam, Polycarbonate sheets or high density polyethylene (HDPE) and others. HDPE containing UV inhibitors such as carbon black is the material most commonly used and is provided in different configurations or corrugation patterns (G. Tchobano- glous and F. L. Burton, 1995). Corrugations enhance structural stability, improve mass transfer and increase the available surface area (Grady et. al, 1999). The type of biofilm supporting media are classified on the basis of surface area provided and are commonly termed as low or standard density, medium density and high density. Standard-density media are defined as having a surface area of about 115m2/m3 of reactor with larger spaces between media layers and are normally used in the lead stags of a RBC process. Medium and high density media have surface areas of about 135-200 m2/m3 of reactor and are used typically in the middle and final stages of a RBC system where thinner biological growth occurs (G. Tchobanoglous and F. L. Burton, 1995). Standard density media must be used in the first two stages that are highly loaded or where microbial growth possible.

Random packed media have been providing more surface area for attachment of the biofilm within the RBC reactor, contributing to higher mass transfer efficiency due to increased turbulence. Besides they have low energy consumption and the fabrication cost is nearly one third that of discs (A. J. Ware et al, 1990). Different types of packing such as pall rings, saddles and cylindrical plastic elements with distinctive sizes can be used in random packed RBC systems (P. Mathure and A. W. Patwardhan, 2005).

2.2.6 Temperature

Temperature is one of the most important factors that affect the rate of biological processes and consequently influences RBCs performance. At limited conditions, an increase in the influent temperature leads to an increase in the microbial activity and a higher substrate removal can be observed in all RBC stages (Banerjee, G. 1997b). Low influent temperatures can adversely affect biofilm establishment, particularly in its early stages. When waste water temperature is less than 13 °C organic and nitrogen removal rates may decrease. Generally, when the temperature drops from 13°C to 5oC nearly 2.5 times more media surface area is required for achieving the same performance (Rodgers M. and Zhan X-M (2003). In biofilms the nitrification process is less temperature – dependent than in activated sludge. The nitrification rate increases by about 4.5% per oC. Year round operation requires that rotating contactors be covered to protect the biological growth from freezing temperatures or excessive heat gain, which accelerates media deterioration. Covers also helpful to reduce heat are loss and minimize algae growth. Individual covers are preferable than entire installations being placed in buildings (G. Tchobanoglous and FL Burton, 1995).

2.2.7 Staging

Staging of RBC media is recommended to maximize removal of (NH in a 4 tank +-N). or Stages using are a BOD 5 and ammonia nitrogen accomplished by using baffles series of tanks. Typical RBC staging arrangements are illustrated in Fig.1 As the wastewater flows through the system; each subsequent stage receives an influent with an organic concentration lower than the previous stage. As the wastewater moves to the second and subsequent stages the RBC tends to first remove ammonia and then nitrite with the final product being nitrate, assuming that the RBC is sized and operated correctly (John.N.Hochheimer and fred Wheaton, 1998). When there is recycling of wastewater from the last stage to the first one, denitrification may be achieved in the first stage, where there is high organic loading and low dissolved oxygen content. Staging in the design of RBC systems is especially important at higher organic loadings and also if high effluent treatment quality is required. Moreover, staging of RBC decreases the detrimental effect of shock load on the performance of the system. The number of stages to be used depends on the organic content of the influent, flow rate and several other variables.

2.2.8 Influent wastewater characteristics

The substances in influent and their concentration levels may play a significant role in the operation of RBCs for example; the flux into the biofilm may be smaller for large and slowly biodegradable compounds. The presence of particulate organic matter can reduce the flux of soluble substrate since the particulate matter occupies space within the biofilm, which decrease the rate of biodegradation (Grady et. al, 1999). When sulphide is present, either in the influent wastewater or by its production deep within the biofilm, sulphide oxidizing bacteria such as beggiatoa will grow on the biofilm surface. The production of sulphide within the biofilm is due to oxygen depletion. Beggiatoa will compete with heterotrophic organisms for oxygen and in extreme cases will take over the first-stage of overloaded RBCs, shifting the load to the next stage and progressively taking over the system. RBC units properly designed and supplemented with essential nutrients consistently produce the best effluents and maintain biofilm on the media with better adhesion characteristics.

2.2.9 Biofilm characteristics

Biofilm present on disc media plays important role in moving the organic and inorganic substrate present in wastewater, which they used it for their metabolism and treating wastewater. Micro-organisms present in the wastewater adhere to the disc surfaces within 1 to 4 weeks and form a slime layer biofilm ranging from 1 to 4 mm in thickness. The biofilm after reaching a critical thickness, microorganism in deep are unable to receive nutrients and oxygen, they are no longer able to stick to disc and slough off. Large surface area allows a large, continuous and stable biomass population to develop on disc. The thickness of biofilm is not uniform since sloughing process occurs randomly. Sloughed biofilm and suspended solids are washed out of the contactor as the wastewater which is later removed in settling tank or at secondary clarification. The biomass contains different types of microorganisms. Initially brown color organisms is considered as healthy biomass where as white and grey biofilm are regarded as unhealthy ones. (N. E. Kinner and C. R. Curds, 1987).

2.2.10 Step-feeding

To increase the process capacity, to have a more robust performance and to reduce or prevent overloads, the capability to step-feed RBC stage(s) should provide. Working in step-feed mode will improve the removal rates and found higher dissolved oxygen values. The combined effect of step-feed and effluent recirculation has increased RBC performance, but for a simple soluble substrate. (Pradeep et. al, 2011).

2.2.11 Disc submergence

Disc submergence along with other factors affects the biological process. Generally, partially submerged RBCs are used for nitrification and fully submerged for denitrification. The experiment was carried out with three submergence levels from 23.7, 31.4 and 36%. As disc submergence was increased from 31.4 to 36% the removal efficiency of Total Chemical Oxygen Demand (TCOD) and Soluble Chemical Oxygen Demand (SCOD) improved and result obtained were 74.9 to 87.5% and 89.5% and 93.75% respectively (G.D Najafpour and A. A. L. Zinatizadeh, 2005). For aerobic RBC submergence more than 50% is not practically possible as the bearing holding the shaft will be immersed in wastewater and can get deteriorated affecting the working of shaft (Pradeep et. al, 2011).

3.0 PERFORMANCE EFFICIENCY OF RBC:

The performance of RBC is verified by various investigators under different operating conditions using various kinds of wastewater. The results obtained by them are consolidated and presented in Table1.

4.0 CONCLUSION

The management of the medium and small scale industries feel burden to treat wastewater if the cost involvement is high. Hence there is a broad scope for cheaper and compact unit processes or ideal solutions for such issues. Rotating biological contactor is most popular due to its simplicity, low energy and less land requirement. The rotating biological contactors are fixed film moving bed aerobic treatment processes and able to sustain shock loadings. Unlike activated sludge processes (ASP), trickling filter etc. rotating biological contactor does not require recirculation of secondary sludge and hydraulic retention time is also low.

Rotating biological contactor is very effectively used for treatment of wastewater to remove the very high organic loading. And now a days, the RBC is used for aerobic treatment process for removal of organic concentration, also anaerobic RBC is used for de-nitrification process. Scope for several structural modifications is there, such as changing rotational speed to increase dissolve oxygen level, variation of organic loading, disc submergence and step feeding during multiple staging to improve RBC’s performance.

ACKNOWLEDGMENT

The authors are thankful to Mr. N K Mandal and Mr. Kaviyarasan, Dr.Paul’s Engineering College, Villupuram district for extending support in preparing this paper.

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