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  1. 1. Prashant A. Kadu, Dr. Y. R. M. Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.2149-2153 A Review of Rotating Biological Contactors System Prashant A. Kadu*, Dr. Y. R. M. Rao***(Associate Professor, Department of civil Engineering, Prof. Ram Meghe Institute of Technology & Research, Badnera, Dist:Amravati 444701 ( M.S.) INDIA,) **(Principal, Dr. Paul’s Engineering College, Paul’s Nagar, Vannur, Villupuram Dist.,Tamil Nadu State, INDIA)ABSTRACT The rotating biological contactor process middle of 20th century. It increased after newoffers the specific advantages of a biofilm system biofilm media material and reactor configurationsin treatment of wastewater for removal of soluble were developed [2]. The attached growth biofilmorganic substances and nitrogen compounds. It is systems rendered several advantages over thea unique adaptation of the moving-medium suspended growth biomass systems. The specificbiofilm system which facilitates easy and effective advantages vary with the type of biofilm system andoxygen transfer. Media in the form of several reactor configuration. In general, a biofilm systemlarge flat or corrugated discs with biofilm offers the following advantages [3]:attached to the surface is mounted on a commonshaft partially submerged in the wastewater and High biomass packing density and reactorrotated through contoured tanks in which compactness due to a large specific surface area.wastewater flows on a continuous basis. The Short contact periods and cohabitation of -compactness of the system and its economical aerobic and anoxic micro-organisms within theoperation makes it a viable option specially same ecosystem.suited for decentralized wastewater treatment Reduced sludge bulking and better sludgetechnologies. The process optimisation and thickening qualities.adaptability under different environmental Lower sensitivity and better recovery fromconditions and influent characteristics remain shock loadings.challenging tasks for the efficient use of this Low energy requirements and more economy intechnology. operation and maintenance. Low sludge production and superior proess cKey words: Biofilm, denitrification, nitrification, substrate, RBC. Simple in operation and maintenance.1. INTRODUCTION The rotating biological contactor (RBC) is The implementation of suitable methods a unique adaptation of the moving-medium attachedfor the disposal of wastewater dates back to the growth biofilm system which offers an alternativetimes of Roman civilization. However, it was only technology to the conventional ASP treatmentin the later part of the 19th century that a spurt of process. Media in the form of several large flat oractivity in the realm of wastewater treatment took corrugated discs with biofilm attached to the surfaceplace. The growth of the human population, are mounted on a common shaft partially submergedurbanization and industrialization necessitated the in the wastewater and rotated through contouredtreatment of wastewater. It became evident that the tanks in which the wastewater flows on a continuousuntreated wastewater which was discharged directly basis. RBC systems offer a typical specific surfaceinto water bodies caused pollution and posed health area of the order of 150-250 m2/m3 of liquid. Thehazards. A lot of research followed in the late 19th principal advantage of the RBC system stems fromcentury and led to the development of the biological its high oxygen transfer efficiency which providestreatment process using aerated suspended biomass, greater economy in the long run compared to otherknown as activated sludge process (ASP). This was processes employing surface aerators or diffusers. Itadapted for large-scale treatment applications and is operationally very economical and efficient at lowinvolved separate aeration and recirculation power consumption values. Though RBC systemsmechanisms. In 1923, Los Angeles became one of are inclined to be sensitive to temperature, andthe first big cities to use an activated sludge process involve capital costs initially, they have proved to bein its wastewater treatment plant. very efficient systems with excellent sludge quality However, the advent of fixed biofilm and low sludge volume index values in thesystems as a secondary wastewater treatment secondary clarifier [4]. Properly designed RBCsprocess seems to precede the use of ASP process. provide other specific advantages such as highThe instance came with first full-scale operation of capacity to withstand fluctuations arising in thetrickling filters in early 1880s in Wales [1]. But the wastewater characteristics and substrateapplication of biofilm systems was limited till the concentrations and to dampen shock loadings [3]. 2149 | P a g e
  2. 2. Prashant A. Kadu, Dr. Y. R. M. Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.2149-2153Estimations reveal that RBCs require about 40-50% novelty processes like fluidised bed reactors,of the energy requirements of an activated sludge membrane reactors and moving bed biofilm carriers.system [5] and 70-80% of a Trickling filter system However, many of these state-of-the-art biofilm[2]. treatment processes come with extra costs which The first instance of the use of RBC as a need to be compensated with treatment efficiency.biofilm remediation technology is documented in The criterion for selection of a suitable system is1928 [6]. The availability of polystyrene marked the often a question of economy, quality of raw influent,beginning of commercial application of RBCs with desired treatment efficiency and availability ofthe first full-scale RBC being installed in Germany space. In some processes such as membrane filters,in 1958. There are several different designs the capital costs may be very high, but theavailable today world-wide depending upon specific operational costs are minimal and it is oftenrequirement criteria. More than 16% of all economical in the longer run. Each process has itswastewater treatment plants in Switzerland and own specific advantage and disadvantage and itnearly 31% of the small treatment units with a remains for the designer and the users to choose acapacity of the equivalent of a population of 5000 suitable treatment based on specific requirementsare RBCs [7]. Today, the increasing complexity and and economic factors.inadequate efficiency in operation and maintenanceof large and sometimes mammoth sized wastewater 3. FEATURES OF A RBC SYSTEMtreatment plants based on ASP process has paved RBCs offer a low cost solution with highthe way towards the concept of small wastewater efficiency and inherent advantages. The most stand-treatment plants (decentralized wastewater alone feature is facilitating easy transfer of oxygentechnologies). The increasing Introduction 25 from air while enabling biofilm affected treatmentdemand for such small and medium sized plants for simultaneously. Due to remarkably low energyurban or sub-urban habitations created the need for consumption, efficient aeration of water due toalternative processes which are required to be rotation while simultaneously making efficientequally effective and more economical. The RBC contact between biofilm and water, they offer anconcept fits in ideally in such cases. Trickling filters ideal choice as small wastewater treatment unitshave also proven to be an effective biofilm under situations where space is limited [12]. Oxygentreatment system but the disadvantage is their remains as one of the most limiting substrates incomplexity in operation and requirement of proper biofilm treatment, and the deeper and faster therecirculation [8]. Apart from the easy and effective oxygen diffusion is inside the biofilm, the better theoxygen transfer in RBCs, the compact design with aerobic treatment. The advantages of this system areseparate compartments provides the advantages of a extremely low cost for mixing and high oxygenplug-flow system to sustain load surges and provide provision [2]. The system is also less susceptible tohigh efficiency without the requirement of flow fluctuating hydraulic loadings as compared to therecirculation. trickling filters. In its simplest form, an RBC unit consists of a series of closely placed discs that are2. BIOFILM REACTORS IN WASTE mounted on a horizontal shaft and are partiallyWATER TREATMENT submerged in wastewater. In other words, a part of Different reactors have been developed to the discs is immersed in the bulk liquid while thetake advantage of the biofilm processes in other part remains exposed in air. In some cases, thewastewater treatment. The oldest is the traditional disc is completely submerged in bulk liquid tobiofilter, introduced before the 20th century. It was initiate anoxic conditions of treatment. The shaft isinitially used as screening device but later on it driven mechanically using an electric motor orbecame clear that the mechanism of purification was pneumatically with compressed air drive so that thenot purely physical screening, but biological as well discs rotate perpendicular to the flow of wastewater.[9].This led to the trickling filters where the biofilm The rotation helps to maintain oxygen diffusiongrows over stone or plastic carrier material which through the air liquid interface and enables aerobicacts as a media bed and the wastewater trickles biofilm growth and resultant nutrient removal. Indown through this media bed. Few years later, at the practice, RBCs are preceded by a primary clarifierturn of the century, RBCs were introduced using and succeeded by a secondary clarifier. As stated,wood as solid media for the attachment of the the wastewater previously undergoes primarybiofilm [10]. The original patent of RBC was filed treatment in settling tanks before entering into theby A.T. Maltby in 1928 [11], The essential RBC unit. The effluent from the RBC also requiresdifference between the two aerobic systems is that a secondary clarifier to separate out the sludge fromin trickling filters, the media remain fixed in the the treated wastewater. The repeated rotation of thereactor bed and the water flows over the biofilm disc media by the shaft not only supplies oxygen toexposed in air, while in RBCs, the media rotates the micro-organisms in the biofilm but also to thethrough a nearly stagnant bulk water and air. Further suspended biomass in the bulk liquid. The discs aredevelopment in biofilm technology led to usage of usually made of plastic (polythylene, polyvinyl 2150 | P a g e
  3. 3. Prashant A. Kadu, Dr. Y. R. M. Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.2149-2153chloride or polystyrene) and are contained in a Recirculation ratetrough so that about 40% of their area is typically Drive mechanismsimmersed in wastewater. They are arranged in Shaft arrangement (common shaft with single rpmgroups or packs with baffles in between them to or separate shaft for each stage)reduce surging or short-circuiting. Normally RBCs oxygen transfer rateare designed and operated in a series of stages, ambient and wastewater temperatureranging from three to four stages, separated by media densitybaffles. The purpose of staging is to make the RBCbehave like a plug-flow system and eliminate short The most important physical factorscircuiting. Typically, the first stage always receives affecting the overall removal efficiency of thethe highest organic loading and provides maximum system are oxygen mass transfer rate andorganic removal efficiency. The latter stages are temperature. Oxygen transfer rate is againused for nitrification as well as residual organic dependent on operating temperature and physicalcarbon removal. After the fourth stage, set-up of the system. The thickness of the biofilm isimprovement of organic removal is insignificant controlled by the availability of nutrients and[2].When there is recycling of wastewater from the surface turbulence due to rotational speed.last tank to the first one, denitrification may beachieved in the first tank, where there is high 3.1 PROCESS DESIGN AND NUTRIENTorganic loading (due to the influent) and low REMOVAL PERFORMANEdissolved oxygen content. In specific set-ups, the 3.1.1. ORGANIC SUBSTRATE REMOVALrotational speed may be varied in each stage The critical hydraulic retention time fordepending upon the oxygen requirement at that removal of carbonaceous substrate in RBCs is aboutstage so as to optimise removal efficiency under 3-4 hours and studies have revealed that furthergiven loading conditions and to enable variation of increase in retention has little effect on improvementhydraulic shear forces to initiate detachment of in performances. For a given system, as the appliedbiomass. This can help to control biofilm thickness. loading rate increases, the removal efficiencyNormally the rotational speed cannot be made very decreases. Under normal operating conditions,high and there is disc size restriction. This checks carbonaceous substrate is mainly removed in initialhigh peripheral velocities, which may cause stages of the RBC.shearing off of the liquid film and consequentlysubstrate limitations in the biofilm. To protect the 3.1.2 NITRIFICATIONbiofilm from exposure to temperature extremities The oxidation of ammonia is an importantand heat loss and to prevent the growth of algae, feature in assessing the performance of a biologicalRBC units are almost always covered. It is also reactor. Heterotrophic bacterias offer strongimportant to protect the plastic discs from direct competition to nitrifiers in the initial stages withexposure to UV rays and weather. Discs may also be high BOD concentrations. So the maximumimmersed up to 80-90% submergence, which nitrification rate occurs when the soluble BOD loadprovides less loading on the shafts due to buoyancy reduces sufficiently. Studies suggest that fulland larger media contact volume. This design is nitrification can only be achieved when the organicused especially suited for denitrification effects [13]. loading rate is less than 5gBOD/( [15]. TheBut for aerobic treatment of wastewater, deeper recommended initial BOD5 loading rate as persubmergence of discs does not allow adequate ATV-DVWK standard is 8-10g/(m2.d). Therefore,aeration and dissolved oxygen levels in the liquid nitrification always occurs prominently in the laterlower down. Consequently, additional aeration units stages of RBC set-up. The highest nitrification rateneed to be used to provide sufficient aeration and depends upon oxygen concentration in the boundarysuspension of biomass in the trough. Fully layer and dissolved oxygen concentration in the bulksubmerged RBCs can be used for anaerobic mode of liquid, which should not be oxygen limited. Alsotreatment [14]. Overall performance of RBC concentration of ammoniacal Nitrogen should notsystems for nutrient removal from wastewater go below 3-5 mg/l in the bulk liquid for best resultsdepends upon several factors: bulk liquid for best results.Influent wastewater characteristics Hydraulic loading rate 3.1.3 DENITRIFICATION Organic loading rate The usage of RBC systems for Ammonium loading rate denitrification is not very widespread. Laboratory pH scale studies indicate that for an influent NO3-NSystem configuration concentration of 50mg/l, the maximum Rotational speed denitrification rate is 15.2gNO3-N/ at a Specific surface area of discs rotational speed of 2rpm [13].In case of liquid Disc submergence temperature going below 13°C, temperature Number of stages correction factors need to be taken into account. 2151 | P a g e
  4. 4. Prashant A. Kadu, Dr. Y. R. M. Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 5, September- October 2012, pp.2149-2153These can be obtained from pilot studies and 3.3 OPERATIONAL PROBLEMSliterature. In general, when the temperature drops Mechanical failures often occur in RBCfrom 13 to 5°C, nearly 2.5 times more media surface units. The most common are shaft failures, bearingarea is required for achieving the same performance failures and media support structure failures. This[2]. may arise due to overloading conditions from high hydraulic loading rate, excess biofilm growth,3.2 RBC BIOFILM microbially influenced corrosion, low frequency A layer of biological growth, slimy in corrosion fatigue, improper greasing and inadequatenature and about 1-3 mm in thickness is established locking of nuts and bolts [20]. If the RBC unit is noton the surface of the disc. This biological slime that housed properly, the discs may get exposed to UVbecomes attached to the disc surface assimilates the radiations and bad weather conditions in tropicaldissolved and suspended organic materials from the climates, which can damage the disc material.wastewater. Excess biomass is sheared off into the Another problem associated with this is excesstank, while the rotating action of the discs maintains growth of algae which may clog the shaft and discthe solids in suspension. Eventually, the effluent movement. So the RBC units need to be adequatelycarries these solids out of the system and into a covered.secondary clarifier, where they are separated.Structurally the composition of the biofilm is highly 4. CONCLUSIONS:heterogeneous. It is made up of a matrix of RBCs enable high DO concentrations inmicrobial cell clusters and intermediate voids with the bulk liquid due to diffusive transfer of oxygenspatial distribution of autotrophs, heterotrophs, other from air into the exposed liquid film surface.bacterial cells and protozoa. Microscopic studies Therefore, requirement of external aeration in thereveal that the outer biofilm layer is more reactor compartment can be avoided. The adoptionheterogeneous and complex. It is composed of of the new boundary layer concept reveals thatfilamentous bacteria, protozoa, eukaryotic algae and average DO levels in the liquid film usually remainssmall metazoans. The inner layers are relatively higher than in the bulk liquid.more uniform and compact [16]. During the initial Under variations of nutrient and hydraulicstages of RBC set-up for BOD removal, loading rates in influent, RBCs can sustain suchheterotrophs compete with autotrophs (nitrifying fluctuations within a tolerable range and performbacteria) in the outermost biofilm layers for oxygen efficiently. Although this is valid for most biofilmand space. The bacterial population gets diminished systems, RBCs may provide an economicalin the inner biofilm layers, because it contains a advantage.major fraction of non viable bacteria compared to Temperature increase shows an improvement in thethe outer layers [2].The active metabolic cell overall removal efficiency.fraction reduced from 35 in the outermost biofilm Denitrification is only partial in RBC systems andlayer to 15 in the innermost biofilm layer [17]. occurs predominantly in the initial stages of theFilamentous mirco-organisms present in the biofilm RBC where high heterotrophic population andsuch as Beggiatoa ssp and Sphaerotilus natans often anoxic ambience is readily available.cause flotation problems and the sheared sludge Flow recirculation reveals little improvement in therefuses to settle [18]. Excess growth of Beggiatoa overall removal efficiency of the RBC system.serves as a caution signal to the performance of Submergence level of 40-42% is optimum for allRBC units because the blooming of these sulphur stages and shows best results.oxidising bacterias prevents the sloughing of thickbiofilm from the discs, which may result in REFERENCESoverloading on the media supports [19]. Studies [1] Lazarova, V. and Manem, J. (2000).show that there are several means to control the Innovative biofilm treatment technologiesbiofilm thickness. These are increasing rotational for water and wastewater treatment. In:speed, reversing the rotation-direction periodically Bryers, J.D.(ed.), Biofilms II: Processto develop shear stresses in opposite direction, Analysis and Applications, Wiley-Liss,supplementary aeration for pre-oxidation of New York, pp. 159-206.reduced-sulphur so that Beggiatoa growth is [2] Rodgers, M. and Zhan, X.-M. 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