A SEMINAR REPORT ON

MEMBRANE BIOREACTOR FOR WASTEWATER TREATMENT

Submitted to the

Department of Chemical EngineeringBHARATI VIDYAPEETH UNIVERSITY

COLLEGE OF ENGINEERING

under the guidance of

Prof.S.J.ATTAR

Submitted By:-

PRATIK KUMAR ROLL NO-13 BE CHEMICAL

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BHARATI VIDYAPEETH UNIVERSITYCOLLEGE OF ENGINEERING

KATRAJ-DHANKAWADI, PUNE-411043

CERTIFICATE

This is to certify that the seminar report entitled “MEMBRANE BIOREACTOR FOR WASTEWATER TREATMENT.” carried out by PRATIK KUMAR of 4th year Chemical Engineering, during academic year 2009-10, is a bonafide work submitted to the Department of Chemical Engineering of B.V.U.C.O.E.

Prof. S.J.Attar

Project Guide Department of Chemical Engg.

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ACKNOWLEDGEMENT

When the compilation of the project comes to an end, the time comes to acknowledge all persons who have made it a success. It gives me immense pleasure to express my gratitude to each individual associated directly or indirectly with the successful completion of my seminar report. I would like to take this opportunity to especially thank my guide, Prof. S.J.Attar of Chemical Engineering Department, Bharati vidyapeeth University College of engineering, Pune for having trust in me and giving me such a challenging and demanding topic for my seminar. I would also like to thank him for all the materials he has provided me which proved to be of great importance in understanding the topic and also providing me the lab and internet facilities.

I would like to express my gratitude and appreciation to my friends and seniors for providing me with some valuable suggestions.

PRATIK KUMAR ROLL NO-13 BE CHEMICAL

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INDEX

S.NO. TOPIC PAGE NO.

1 INTRODUCTION Conventional waste water treatment plant. Membrane waste water treatment plant. Goals and application.

6-8

2 MEMBRANE SELECTION Operating range of membrane process. Types of membrane. Membrane geometry.

9-11

3 TYPES OF MBR Extractive Membrane Bioreactors. Bubble-less Aeration Membrane Bioreactors. Recycle Membrane Bioreactors. Membrane Separation Bioreactors

12-14

4 CHARECTERISTICS OF MBR Effluent Quality. Effectiveness. Sustainability. Economics.

15-19

5 DEVELOPMENTS IN MBR SYSTEM 20-22

6 ADVANTAGES AND DISADVANTAGES 23

7 CONCLUSION 24

8 REFERENCES 25

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ABSTRACT

The MBR process was introduced by the late 1960s, as soon as commercial scale ultra filtration (UF) and microfiltration (MF) membranes were available. Although the idea of replacing the settling tank of the conventional activated sludge process was attractive, it was difficult to justify the use of such a process because of the high cost of membranes, low economic value of the product (tertiary effluent) and the potential rapid loss of performance due to membrane fouling.

A membrane bioreactor (MBR) combines the biological degradation of waste compounds and the physical separation of the biomass and treated water by employing membranes. Of key importance is the MBR’s ability to produce high quality effluent while providing an integral pathogen barrier. . In addition to the operational benefits, economic benefits could also be realized in a small, remote community setting. The complete retention of biomass in the reactor allows smaller reactors to treat equivalent flows of wastewater.

One of the discussion points within the MBR industry is the ongoing argument concerning energy costs. There is no doubt that submerged MBR systems absorb less energy, but this only applies when membrane fouling is not a factor. 1 However, considerable progress is being made in the development of ‘low energy’ cross-flow MBR systems. One approach is the, so-called, “airlift” technique.

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1.INTRODUCTION

Membrane Bioreactor (MBR) systems essentially consists of combination of membrane and biological reactor systems. These systems are the emerging technologies, currently developed for a variety of advanced wastewater treatment processes. Bioreactors are reactors that convert or produce materials using functions naturally endowed to living creatures. Reactors using immobilized enzymes, microorganisms, animal, or plant cells and those applying new methodologies such as genetic manipulation or cell fusion are typical bioreactors. Bioreactors differ from conventional reactors as living organisms present in the reactors operate under milder conditions of temperature and pressure. The ranges of operating conditions within bioreactors are usually determined by the biocatalyst (organism) and are usually small. A membrane bioreactor (MBR) combines the biological degradation of waste compounds and the physical separation of the biomass and treated water by employing membranes. Of key importance is the MBR’s ability to produce high quality effluent while providing an integral pathogen barrier. Furthermore, MBRs exhibit good resistance to variations in hydraulic and organic loading. Replacing gravity settling used in conventional systems with membrane filtration in MBRs allows for extensive automation and presents possibilities for remote controlled monitoring and operation, thereby reducing the need for on-site expertise. In addition to the operational benefits, economic benefits could also be realized in a small, remote community setting. The complete retention of biomass in the reactor allows smaller reactors to treat equivalent flows of wastewater.

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1.1. Conventional wastewater treatment plant

The conventional activated sludge process (ASP) is the most common biological process in municipal wastewater treatment. Discovered in 1914 by Arden and Lockett, after that commercialized as a continuous process by John and Atwood in 1920, ASP is nowadays well understood and mathematically modeled. However, increasingly stringent effluent quality requirements in industrialized countries and a rising need for water reclamation call for further development of ASP. Current and impeding legislation on wastewater treatment effluent has led to the need of improved treatment processes capable of removing higher percentages of nutrients,Suspended solids, bacteria etc.A several of minimum standards for effluent concentrations exist. Requirements for effluents depend on the type of receiving water (e.g. lakes, lagoons, rivers, aquifers) and its quality category as well as on special demands locally adapted to the particular receiving water.

1.2. Membrane bioreactor Waste water treatment plant

The implementation of membrane bioreactors (MBR) to wastewater treatment offers the possibility of overcoming a lot of the current problems in activated sludge processes, which are mostly linked to the separation of biomass from the treated water. In MBR the settling process which is normally used for separation of biomass from treated water is replaced by micro- or ultra filtration. The filtration step can be realized in the form of external side-stream modules or directly immersed modules in the activated sludge tank. With a complete retention of bacteria and viruses this allows a very high effluent quality. Furthermore, it is possible to increase the biomass concentration considerably and thus decrease reactor volumes or sludge production rates. Technical SS concentrations in MBR vary between 8 and 15 g/L for municipal and up to 40 g/L for industrial wastewater treatment

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1.3. Goals and applications of MBR:

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2. MEMBRANE SELECTION

A membrane can be thought of as a material in which one type of substance can pass through more readily than others, thus presenting the basis of a separation process. It is therefore the property of the membrane to separate components of the water to be treated which is of key interest when selecting or designing membrane separation systems duties arising as such in the water industry. For many processes the membrane acts in a way to reject the pollutants, which may be suspended or dissolved and allow the “purified” water through it.

2.1. Operation range of membrane processes

The principal objective in membrane manufacture is to produce a material of reasonable mechanical strength and which can maintain a high throughput of a desired permeate with a high degree of selectivity. These last two parameters are mutually counteractive, since a high degree of selectivity is normally only achievable using a membrane having small pores and thus an inherently high hydraulic resistance (or low permeability). The permeability increases with an increase of density of the pores, implying that a high material porosity is desirable. The overall membrane resistance is directly proportional to its thickness. Finally, selectivity will be compromised by a broad pore size distribution. It stands to reason, therefore, that the optimal physical structure for any membrane material is based on a thin layer of material with a narrow range of pore size and a high surface porosity. The range of available membrane materials is very diverse. They vary widely both in chemical composition and physical structure, however the most fundamentally important property is the mechanism by which separation is actually achieved. On this basis, membranes may be categorized as either dense or porous. Separation by dense membranes relies to some extent on physico-chemical interactions between the permeatingComponents and the membrane material and relate to separation processes having the highest selectivity Reverse osmosis and nanofiltration processes are thus able to separate ions from water. Porous membranes, on the other hand, achieve separation mechanically (i.e. ostensibly by sieving) and are thus mechanistically closer to conventional filtration processes. Ultra filtration can remove colloidal and dissolved macromolecular species and as such their ability to reject material is defined by the molecular weight cut-off (MWCO) in Daltons (i.e. the relative molecular weight) of the rejected solute, rather than its physical size. Microfiltration, on the other hand, is capable of removing only suspended materials – generally down to around 0.05 μm in size. It is the porous membranes that are used in MBRs to retain the suspended solids material, mainly biomass, within the reactor while producing a clarified effluent.

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2.2. Types of membranes

2.2.1. Nanofiltration membrane

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2.2.2. Ultrafiltration membrane

2.3. Membrane geometry / configuration

The geometry of the membrane, i.e. the way it is shaped, is crucial in determining the overall process performance. Other practical considerations concern the way in which the individual membrane elements, that is the membranes themselves, are housed to produce modules. Some of these characteristics are mutually exclusive. For example, promoting turbulence results in an increase in the energy expenditure. Furthermore, direct mechanical cleaning of the membrane is only possible on a comparatively low area: volume units where the membrane is accessible. It is not possible to produce a high-membrane area to module bulk volume ratio without producing a unit that has narrow feed channels, which then adversely affect the cleaning regime and turbulence promotion.Characteristics of optimum geometry or configuration for an individual membrane element:

High membrane area to module bulk volume ratio. High degree of turbulence for mass transfer promotion on the feed side. Low energy expenditure per unit product water volume. Low cost per unit membrane area. A design that facilitates cleaning. A design that permits modularization.

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3.TYPES OF MBR:

In general, MBR applications for wastewater treatment can be classified into four groups, namely:

1. Extractive Membrane Reactors 2. Bubble-less Aeration Membrane Bioreactors 3. Recycle Membrane Reactors 4. Membrane Separation Reactors

3.1. Extractive Membrane Bioreactors

Extractive membrane bioreactors (EMBR) enhance the performance capabilities of biological treatment of wastewater by exploiting the membrane’s ability to achieve a high degree of separation while allowing transport of components from one phase to another. This separation aids in maintaining optimal conditions within the bioreactor for the biological degradation of wastewater pollutants. For example, degradable toxic organic pollutants from a wastewater could be transferred through a nonporous membrane, to a growth bio-medium for subsequent degradation. In this case the mass transfer of toxic compounds across the membrane takes place due to the presences of a concentration gradient, while the bio-medium functions as a sink. The extractive membrane bioreactor can be operated in two modes as illustrated in Figure:

• Mode 1-where the membrane is immersed in the bio-medium tank. Here the toxic wastewater is circulated through the membranes, and due to the concentration gradient, the toxic compounds are selectively transported to the surrounding bio-medium. Specialized microbial cultures could be cultivated in the reactor, which could be easily optimized for the degradation of the pollutants.

• Mode 2-where the membrane forms an external circuit with the bio-medium tank. The wastewater containing toxic organic pollutants is circulated on the shell-side of the membrane modules. While the bio-medium liquid is pumped through the membrane lumens. Due to the presence of a concentration gradient, the toxic pollutant is transferred to the bio-medium. Within the bio-medium the toxic pollutant is continuously degraded by specialized microorganisms operated in optimum conditions in pH, temperatures, dissolved oxygen, nutrient concentration etc. Whereas the organic pollutant extracted wastewater is removed on the other end of the membrane shell.

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3.2. Bubble-less Aeration Membrane Bioreactors

In a conventional aerobic wastewater treatment unit such as an activated sludge process, the process efficiency is controlled by the availability of air. Due to inefficient mode of air supply, 80-90% of the oxygen diffused as air in an activated sludge process is vented to the atmosphere. Oxygenation with pure oxygen as opposed to air as an aeration medium would lead to an increase in the overall mass transfer and biodegradation rate. However, since conventional aeration devices have high power requirements due to the high rate of mixing, these devices cannot be used with biofilm processes. Biofilm processes are advantageous as they enable retention of high concentrations of active bacteria. The membrane aeration bioreactor (MABR) process use gas permeable membranes to directly supply high purity oxygen without bubble formation to a biofilm. Here the bubble free aeration is achieved by placing a synthetic polymer membrane between a gas phase and a liquid phase. This membrane is used to transfer large quantity of air/oxygen into the wastewater. As the gas is practically diffuse through the membrane, very high air transfer rate is attained. The membranes are generally configured in either a plate-and-frame or hollow fibre module. However, current research has focused on the hollow fibre arrangement with gas on the lumen-side and wastewater on the shell-side. The hollow fibre modules are preferred since the membrane provides a high surface area for oxygen transfer while occupying a small volume within the reactor. Here the membrane also acts as a support medium for the biofilm formation, which reduces the potential for bubble formation and air transfer rate.

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3.3. Recycle Membrane Bioreactors

The membrane recycle bioreactor consists of a reaction vessel operated as a stirred tank reactor and an externally attached membrane module. The substrate (feed wastewater) and biocatalyst are added to the reaction vessel in pre-determined concentrations. Thereafter the mixture is continuously pumped through the external membrane circuit. The smaller molecular compounds, the end products of the biodegradation reaction, are permeated through the membrane. While the large molecular size biocatalyst are rejected and recycled back into the reaction tank.

The disadvantages of membrane recycle process are the loss of activity of between 10 to 90 % due to enzyme substrate orientation and diffusional resistances. However, research efforts of late have been directed toward the effective adsorption of biocatalysts onto the membrane surface thereby maximizing the degradation potential of the recycle membrane reactor. In industrial applications, the recycle membrane bioreactors are utilized essentially in two basic configurations, namely: tubular and beaker type. In the beaker type system, the feed wastewater together with the biocatalyst is placed in a beaker, which serves as the reaction vessel. U shaped bundle of fibres immersed into the beaker and product is continuously filtered through the membranes. Tubular configurations are preferred in large-scale industrial applications where the biocatalysts can be loaded or trapped either in the shell-side (annular space between the membrane fibres and the housing) or the tube side of a tubular membrane module this type of bioreactor has been tested on industrial scale for bioremediation activities, for the removal of aromatic pollutants and pesticides.

3.4. Membrane Separation Bioreactors

The activated sludge process is the most widely used aerobic wastewater treatment system to treat both municipal and industrial wastewater. Its operational reliability is one of the major reasons for the success for this technology. However, the quality of the final effluent from this treatment system is highly dependent on the hydrodynamic conditions in the sedimentation tank and the settling characteristics of the sludge. Consequently, large volume sedimentation tanks offering several hours of residence time are required to obtain adequate solid/liquid separation. At the same time, close control of the biological treatment unit is necessary to avoid conditions, which could lead to poor settle ability and/or bulking of sludge. Application of membrane separation (micro or ultra filtration) techniques for biosolid separation in a conventional activated sludge process can overcome the disadvantages of the sedimentation and biological treatment steps. The membrane offers a complete barrier to suspended solids and yields higher quality effluent. Although the concept of an activated sludge process coupled with ultra filtration was commercialized in the late 1960’s by Dorr-Oliver, the application has only recently started to attract serious attention with considerable development and application of membrane processes in combination with biological treatment over the last ten years.

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4. CHARACTERISTICS OF MBR:

4.1. Effluent Quality

The robustness of the MBR is exemplified in its ability to handle many different types and qualities of incoming waste yet maintain a high quality effluent. For example, a study performed on the biological degradation of landfill leachate showed that in a recent retrofit, the MBR system achieved acceptable effluent quality where biological oxidation alone failed. Prior to the retrofit, the BOD and COD removal efficiencies were 79% and 66% respectively. After the retrofit was implemented, the MBR BOD and COD removal efficiencies both increased to 97%.Membrane bioreactors have been used for agricultural wastewater treatment as well. A report by Cicek in 2003 highlighted the potential uses of the MBR for agricultural waste management and treatment. This report was corroborated by a 2005 study in Korea that showed rates of removal for BOD and TSS around 99.9%. Only slightly less effective, the study showed that COD, TN, and TP were removed by 92.0%, 98.3% and 82.7%, respectively.There have been several installations of MBRs to treat varying wastewater. The report highlighted several systems across North America that treated waste waters containing food ingredients, automotive factory waste, or pharmaceutical products. The broad range of wastes treated is a good indication of the flexibility of the MBR.

4.1.1. Comparison of effluent characteristics

Parameter (unit) SBR only SBR + Filters MBRBOD (mg/L) 16.1 + 11.7 30.3 +18.8 <6TSS (mg/L) 14.2 + 9.8 27.5 + 19.1 <5TKN (mg/L) 6.8 + 3.8 8.4 + 3.7 1.3 + 0.4TP (mg/L) 1.7 + 0.3 2.2 + 0.6 1.9 + 0.3NH3 (mg/L) 3.8 + 2.9 3.6 + 2.6 0.2 + 0.3NO3 (mg/L) 6.4 + 4.0 6.2 + 3.2 16.1 + 2.6

4.1.2. COD removal efficiency

Simply due to the high number of microorganisms in MBRs, the substrate uptake or reaction rate can be increased. This leads to better degradation in a given time span or to smaller required reactor volume. COD and BOD removal are found to increase with MLSS concentration. Arbitrary high MLSS concentrations are not employed; however, as oxygen transfer is limited due to higher and non-Newtonian viscosities. Kinetics may also differ due to easier substrate access. In ASP, flocs may reach several 100 μm in size. This means that the substrate can reach the active sites only by diffusion which causes an additional resistance and limits the overall reaction rate (diffusion controlled). Hydrodynamic stress in MBRs reduces floc size (to 3.5 μm in side stream MBRs and increases the apparent reaction rate.

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Parameters(mg/l) Primary Anaerobic Facultative MBR (%)

COD (mg/l) 1748 1104 690 199 89

BOD(mg/l) 350 21 94

Biological and MBR performance with average reduction of COD and BOD

4.1.3.BOD removal efficiency

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4.1.4. Total suspended solids

4.2. Effectiveness

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The effectiveness of membrane bioreactors has been well documented. The effectiveness of membranes actually increases with time. In the laboratory setting, the complete retention of the membrane separation unit allowed faster and more complete adaptation to influent wastewater. This may or may not be valid for real world application however, it was also stated that these rapid physiological adaptations occurred within the first few days of operation. This has implications for rapid and efficient system startup. It can also be used to explain system resilience to changing or adverse conditions. The ability of a microbial community to rapidly adapt to upset conditions would be extremely valuable in systems that have less supervisory support and operated with less technical knowledge. Along with biological degradation, the membrane itself physically removes some of the contaminants. This retention increases residence time and allows the biomass to have the opportunity to degrade these recalcitrant compounds. Unfortunately, it is these same compounds that are thought to lead to membrane fouling.

4.3. Sustainability

Sustainability implies that operation of an MBR wastewater treatment system could carry-on into perpetuity. Sustainable operation is measured by the external actions (i.e. to be sustainable the effluent must not damage the environment). Conversely, the MBR must be intrinsically adaptive to changing operational conditions. This includes a resistance to fluctuating influent wastewater quality as well as resilient to various mechanical failures.

4.4. Economics

The economics of any major project are the final arbiter of success. To be viable, the new technology must meet or reduce the overall costs of conventional treatment. While scientific studies may be in abundance, membrane technology can still be considered in its infancy with respect to wide-spread, full-scale operations. This is likely due to three primary factors. Firstly, membranes have the distinct reputation of increasing capital expense of any water and wastewater treatment project. This can likely be attributed to lack of competition in the marketplace. Secondly, the cost of operation of a membrane system is considered high. Reverse osmosis systems which are most widespread and understood, use large, high-power pumps to circulate the feed water. It is expected that the so-called rule of thumb that membranes require large quantities of power have been applied to all membrane systems, including MBRs. Thirdly, there is a fundamental reluctance to spend large sums of money on technology that does not have a long history. People in general are conservative in their approach to problem solving. Governments, the primary developer of water treatment infrastructure, are exponentially more conservative. Studies have confirmed that, in many instances, the use of membranes in wastewater treatment systems is more costly. An MBR system was compared to combined biological and chemical process (CBCP) in South Korea. As expected, it was found that the

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MBR process required less land and construction costs compared to the CBCP. However, the CAPEX of the membranes themselves outstripped these savings. The MBR’s operational costs were also much lower than the CBCP for chemicals, energy and sludge disposal. Municipal wastewater reuse has undergone limited economic comparison and not all studies have pointed to poor MBR economics. To be accurate, the economic study must look both at the cost of wastewater treatment and drinking water purification. The costs can vary depending on the ultimate end-use of the purified wastewater effluent.

5. DEVELOPMENTS IN MBR SYSTEM:-

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This emerging technology of biomass separation bioreactor is a combination of suspended growth reactor for biodegradation of wastes and membrane filtration. In this system, the solid- liquid membrane separation bioreactor employs filtration modules as effective barriers. The membrane unit can be configured either external to or immersed in the bioreactor. Figure 3 schematically represents the various stages of development of this MBR system, for biological wastewater treatment. The conventional approach to attain a reusable quality water from an activated sludge process is by applying tertiary treatment techniques such as multimedia filtration, carbon adsorption, etc., on biologically treated secondary effluents. As a first step, these tertiary treatment methods were replaced with membrane (ultra/micro) filtration, which ensures almost bacterial and viral free effluent in addition to colloids and solid removal without modifying existing treatment facilities. This type of coupling of membrane technology provides good quality effluent. Later, in view of utilizing membrane technology more effectively, the secondary sedimentation tanks were replaced with cross-flow membrane filtration. Here membranes are placed in an external circuit, where the biomass is circulated at a higher velocity on the membrane surface. The higher energy cost to maintain the cross flow velocity led to the development of submerging externally skinned membranes in the reactor itself and withdrawing the treated water through the membranes. As a further attempt at energy saving in membrane coupled bioreactors, the possibility of using jet aeration in the bioreactor was introduced. The main feature of this bioreactor is that the membrane module is incorporated into the liquid circulation line for the formation of the liquid jet, thereby accomplishing both operations of aeration and membrane separation using only one pump. The jet aeration works on the principle that, a liquid jet after passing through a gas layer plunges into a liquid bath entraining considerable amounts of air. Recently, the invention of the air back-washing technique for membrane de-clogging led to the novel approach of using the membrane itself as a clarifier as well as an air diffuser.

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5.1. Technological developments

One of the discussion points within the MBR industry is the ongoing argument concerning energy costs. There is no doubt that submerged MBR systems absorb less energy, but this only applies when membrane fouling is not a factor. For example, studies on the application of a submerged MBR treating landfill leachate have shown that as much as 2 kWh/m3 is being used solely to blow compressed air onto the underside of the membrane pack. This is in order to create sufficient bubbles to keep the membranes free of fouling deposits.1 However; considerable progress is being made in the development of ‘low energy’ cross-flow MBR systems. One approach is the, so-called, “airlift” technique, which is shown in the diagram below.The membrane modules are mounted in the vertical plane. Compressed air is injected into a specially designed air bubble distributor, located under the module tube plate. The bubbles rise inside the membrane tubes creating an upward flow of sludge. However, the main advantage is the scouring action created by the rising bubbles – these expand as they rise, wiping clean the membrane wall. This MBR ‘airlift’ system design has been in operation on a landfill site in the

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southern part of Germany for 18 months. The results, so far, have been excellent. The flux rate is reduced by 40%, due to the lower biomass flow velocity along the membrane tube, but the specific energy consumption is only 1.0 kWh/m3. This is a considerable saving in electrical power.

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6. THE ADVANTAGES AND DISADVANTAGES OF MBR:

Advantages Since suspended solid are totally eliminated through membrane separation, the settle

ability of the sludge, which is a problem in conventional activated sludge, has absolutely no effect on the quality of the treated effluent. Consequently, the system is easy to operate and maintain.

The overall activity level can be raised since it is possible to maintain high concentrations in bioreactors while keeping the microorganisms dispersed as long as desired and as a result, reactor volume will be reduced. In addition, the system requires neither sedimentation nor any post-treatment equipment to achieve reusable quality water, so the space saving is enormous.

Removal of bacteria and viruses can be expected, so the disinfection process is ecologically sound.

Compared to conventional activated sludge processes, maintaining low F/M (food/microorganisms) ratio will produce less excess sludge to be handled and treated.

The fluctuations on volumetric loading have no effect on the system hence a high sludge capacity can be maintained.

Disadvantages

High investment costs of membrane modules.

Membrane integrity (failure detection, lifetime). High operating costs (energy consumption).

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7. CONCLUSION

Membrane bioreactors• Several advantages compared to the conventional processes• A modern and highly effective process• Treatment costs are approximately competitive

Drawbacks• High energy consumption• High cleaning effort due to intensive fouling• reduced oxygen transfer rates for higher MLSS concentrations.

The review of the membrane bioreactors for the application of wastewater treatment has proven that this emerging technology has developed a niche in the wastewater treatment sector. The system was evaluated through a number of parameters include biochemical oxygen demand, chemical oxygen demand, nutrient loading and pathogen levels. In all categories, save pathogen reduction, the MBR met or exceeded environmental guidelines.While, research efforts of late have been directed towards application of membrane separation bioreactors to various wastewaters, the next step in its development would be to develop a membrane bioreactor process that is both robust and efficient for various wastewater applications.

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8.REFERENCES

BOOKS:

1. Charley, R.C.; Hooper, D.G.; McLee, A.G.: Nitrification kinetics in activated sludge at various temperature and dissolved oxygen concentrations.1387-1396.

2. Cicek, N.; Franco, Characterization and Comparison of a Membrane Bioreactor and a Conventional Activated Sludge System in the Treatment of Wastewater Containing High- Molecular-Weight Compounds.64-70.

3. Muller, E.B.: Bacterial Energetics in Aerobic Wastewater Treatment. Ph.D. Thesis, Vrije Universiteit, The Netherlands,1994.

4. Rosenberger, S.; Schreiner, A.; Wiesmann, U.; Kraume, M.: Impact of different sludge ages on the performance of membrane bioreactors.

5. Rosenberger, S.; Kubin, K.; Kraume, M.: Rheology of Activated Sludge in Membrane Bioreactors. Eng. Life.269-274.

6. Klatt, C. and T. LaPara .Aerobic biological treatment of synthetic municipalwastewater in membrane-coupled bioreactors. Biotech & Bioeng. 313-320.

WEB:

7. www.scribd.com.8. www.sciencedirect.com.

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