A COMPREHENSIVE NOTE ON MEMBRANE BIOREACTOR 1. INTRODUCTION TO MEMBRANE BIOREACTORS: The term ‘membrane bioreactor’ (MBR) applies to all water and wastewater treatment processes integrating a permselective membrane with a biological process. [1] Research into combining membranes with biological processes for wastewater treatment began over 30 years ago, and membrane bioreactors have been used commercially for the past 20 years. Today, over 500 membrane bioreactor processes have been commissioned to treat both industrial and municipal wastewaters, as well as for inbuilding treatment and reuse of greywater. In recent years the number of papers in journals, published case studies and conferences dedicated to these processes has risen exponentially. These meetings have brought together relevant biological and membrane fundamentals, the latest academic research findings, process developments and operational experiences from around the world. [1] 1.1 Early Development: Ultrafiltration as a replacement for sedimentation in the activated sludge process was first described by Smith et al., (1969). In another early report, Hardt et al., (1970) used a 10 l aerobic bioreactor treating a synthetic sewage with a dead end ultrafiltration membrane for biomass separation. The mixed liquor suspended solids concentration was high compared to conventional aerobic systems at 23 to 30,000 mg/l. The membrane flux was 7.5 l/ (m2 .h) and chemical oxygen demand (COD) removal was 98%. DorrOliver Inc developed the Membrane Sewage Treatment (MST) process in the 1960s (Bemberis et al., 1971). In the MST system, wastewater entered a suspended growth bioreactor where flow was continuously withdrawn via a rotating drum screen to an ultrafiltration membrane module. The membrane configuration was plate and frame and operated at inlet and outlet pressures of 345 kN/m 2 and 172 kN/m2 respectively, achieving a flux rate of 16.9 l/ (m2.h). [1] In the 1970s the technology first entered the Japanese market through a license agreement between Dorr-Oliver and Sanki Engineering Co. Ltd. By 1993, 39 of these external membrane bioreactor systems have been reported for use in sanitary and industrial applications. Today membrane bioreactor (MBR) systems are used widely in Japan with several companies offering processes for domestic wastewater treatment and reuse, and some industrial applications, mainly in the food and beverage industries where high COD wastes are common. [1] In 1982, Dorr-Oliver introduced the Membrane Anaerobic Reactor System (MARS) for the treatment of high strength food industry. The process used an external ultrafiltration membrane with an overall loading of 8 kgCOD/ (m3.d) achieving up to 99% removal of COD. [1] Assignment No. 3 1 1.2 The current status of membrane bioreactors for wastewater treatment: Full-scale commercial aerobic MBR processes first appeared in North America in the late 1970s and then in Japan in the early 1980s, with anaerobic processes entering the industrial wastewater market at around the same time in South Africa. The introduction of aerobic MBRs into Europe did not occur until the mid-1990s. [1] There are over 500 commercial MBRs in operation worldwide, with many more proposed or currently under construction. [1] Type of wastewater Approximate % of total MBRs Industrial 27 In-building 24 Domestic 27 Municipal 12 Landfill leachate 9 Table 1: Approximate global distribution of MBRs by wastewater type. Number of plants shown as a percentage of the total number of membrane processes. [1] 2. APPLICATIONS: The MBR process is very efficient in treating both municipal and industrial wastewater. It is particularly well adapted for: i. ii. iii. iv. Application in environmentally sensitive areas, Specific applications where conventional processes satisfactory water quality at reasonable cost, Water reuse applications, The upgrading of conventional treatment plants. [3] cannot produce 3. CLASSIFICATION OF MBRS: Membrane Bioreactors Recirculated or External Membrane Bioreactors Sawant A. Integrated or Submerged Membrane Bioreactors M.Tech Green Technology (2011-2013) Assignment No. 3 2 In an MBR, membrane filtration occurs either externally through recirculation (external loop) or within the bioreactor (submerged configuration) as shown in Figure 1 and 2, respectively. To perform well, the external loop configuration requires very high liquid velocity. This generates high operational costs compared to the submerged configuration, where aeration is the main operating cost component as it is required for both mixing and oxygen transfer. Figure 1: Principle of an external loop process. [3] Figure 2: Principle of a submerged process. [3] The configuration of the membrane, that is, its geometry and the way it is mounted and oriented in relation to the flow of water, is crucial in determining the overall process performance. [2] There are six principal configurations currently employed in membrane processes, which all have various practical benefits and limitations. The configurations are based on either a planar or cylindrical geometry and comprise: [2] Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 1. 2. 3. 4. 5. 6. 3 Plate-and-frame/flat sheet (FS) Hollow fiber (HF) (Multi) tubular (MT) Capillary tube (CT) Pleated filter cartridge (FC) Spiral-wound (SW) Of the above configurations, only the first three are suited to MBR technologies, principally for the reasons: the modules must permit turbulence promotion and regular effective cleaning. [2] 4. TYPES OF MEMBRANE BIOREACTOR: Combining membrane technology with biological reactors for the treatment of wastewaters has led to the development of three generic membrane bioreactors (MBRs): 1. For separation and retention of solids 2. For bubble-less aeration within the bioreactor 3. For extraction of priority organic pollutants from industrial wastewaters. Membranes when coupled to biological processes are most often used as a replacement for sedimentation i.e., for separation of biomass (Figure 3). However, membranes can also be coupled with bioprocesses for wastewater treatment in two other ways. Firstly, they can be used for the mass transfer of gases (such systems are not to be confused with so-called ‘membrane aerators’ which is a term used for some fine bubble diffusers), usually oxygen for aerobic processes (Figure 4). Secondly, membranes can be used for the controlled transfer of nutrients into a bioreactor or the extraction of pollutants from wastewaters which are untreatable by conventional biological processes (Figure 5). The target pollutants are then removed in a reactor with the correct environmental conditions for biological treatment. [1] Figure 3: Main features of the three different MBR processes: (a) solid-liquid separation MBR. [1] Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 4 Figure 4: Main features of the three different MBR processes: (b) oxygen mass transfer membrane bioreactor, in this case through a single hollow fiber with attached biofilm growth. [1] Figure 5: Main features of the three different MBR processes: (c) extractive membrane bioreactor (EMBR). [1] Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 5 5. ADVANTAGES AND DISADVANTAGES OF MBRS: Compared with conventional biologic treatment, the MBR process offers numerous advantages. The membrane is an absolute barrier to suspended matter and thus offers the possibility to operate the system at high mixed liquor suspended solids (MLSS) concentration (MLSS up to 15 g/l). The process can also be run at a long sludge ages (>20 days), which favors the development of slow-growing microorganisms leading to better removal of refractory organic matter. Long sludge ages are not possible with conventional activated sludge systems because they produce sludge that does not settle well. Finally, the use of the membranes makes the process very compact, with a significantly smaller aeration tank than conventional systems. [3] Advantages Disadvantages Membrane Separation Bioreactors Small footprint Complete solids removal from effluent Effluent disinfection Combined COD, solids and nutrient removal in a single unit High loading rate capability Low/zero sludge production Rapid start up Sludge bulking not a problem Modular/ retrofit Aeration limitations Membrane fouling Membrane costs Membrane Aeration Bioreactors High oxygen utilization Highly efficient energy utilization Small footprint Feed-forward control of O demand Modular/ retrofit Susceptible to membrane fouling High capital cost Unproven at full-scale Process complexity Extractive Membrane Bioreactors Treatment of toxic industrial effluents High capital cost Small effluents Unproven at full-scale Modular/ retrofit Process complexity Isolation of bacteria from wastewater Table 2: Advantages and Disadvantages of MBRs [1] Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 6 6. WHY ARE MBRs REQUIRED?: 6.1 Key driver for the growth of MBRs in India: Unlike many other regions of the world, legislation is not the key driver for the growth of MBRs in India since general discharged effluent standards prescribed by the CPCBs do not necessitate the use of MBR technology. [2] The most significant driver for MBRs in India is probably the shortage of clean water. Most new real estate projects are not necessarily supplied with adequate fresh water, and thus depend to a large extent on groundwater and rainwater. However, in most areas the groundwater is brackish and RO treatment costs are considered too high, while rainwater is not a guaranteed source. Water reuse has therefore become increasingly important and MBR technology more attractive, since it is the only system that can provide consistently good quality effluent for reuse. [2] Historically, India has been late in adopting the latest water treatment technologies. Though RO plants were marketed and sold from the late 1980s, it was only a decade after this that the technology was truly appreciated by the end user. Similarly, UF- and MF-based plants have been sold since the late 1990s, but it is only since the middle of the last decade that they have been adopted on a large scale in India. In keeping with this trend, whilst the water industry in India has been aware of MBRs for a number of years they have only been effectively marketed, promoted and sold since around 2007 and the end user is yet to fully recognize their benefits. The next five years are therefore likely to be critical to the growth of the MBR market in India. [2] 7. ECONOMICS: 7.1 Return on Investment: MBRs tend to be more costly and energy intensive than conventional processes, despite the significant decrease in membrane costs since the initial commercialization of the immersed configuration in 1990. Because of this and the perceived novelty of the technology, reflected in a paucity of extensive reference data needed to support investment decisions, there has in the past been some reluctance to invest in the process in some areas. However, the maturing of the technology and the much wider knowledge of the process, in particular the key aspects of energy optimization and process failure risk, have promoted greater confidence in the technology generally and subsequently greater willingness to invest in ever larger plant. [2] Membrane costs and, in particular, membrane life remain of key concern. Membrane purchase costs decreased almost exponentially over the course of the 1990s as a simple consequence of supply and demand, contributing to a decrease in the treated water cost of more than an order of magnitude. Given the generally lower production costs achievable in the highly industrialized Far Eastern countries of China and Korea, it seems likely that membrane costs will continue to decrease – though not as dramatically as during the 1990s. Membrane life, on the other hand, remains a challenging parameter to define. There is increasing evidence from some plants that membrane life can exceed a decade, and is more determined by the extent of manual intervention than any other Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 7 factor relating to routine operation. Provided a long membrane life can be assumed, then the costs of installing and running MBRs can be comparable with those of conventional treatment plants on a whole-life basis, with the added benefit of improved effluent quality. MBRs are also becoming more energy efficient, as new products materialize and means of operating existing plant at lower aeration demands are devised. [2] An additional consideration in some countries is the availability of state incentives. An example is the Enhanced Capital Allowance scheme introduced in the United Kingdom in 2001, whereby tax incentives are offered for water efficient technologies as part of the Green Technology Challenge. Other countries such as the USA, Australia, Canada, Finland, France, the Netherlands, Switzerland, Japan and Denmark, have all offered incentives in various forms to promote innovative water-efficient technologies and reduction in freshwater demand. The number of countries and governmental organizations offering such incentives is growing, essentially making more affordable advanced technologies such as MBRs and other membrane-based processes generally required to attain reusable water. Lastly, the small footprint generally incurred by MBRs compared with conventional processes provides a further financial incentive relating to the cost of land. [2] 7.2 Operating Conditions and Cost evaluation of submerged plate-and-frame, submerged hollow fiber, and side-stream configurations: Some general conclusions of a comparison between the three systems can be summarized as follows: Energy consumption and capital costs are lower with dead-end submerged systems. The energy consumption of crossflow design is about 10 times higher than that of dead-end systems. Therefore crossflow design should only be used when it is absolutely necessary. Norit X-flow have recently developed an MBR tubular side-stream system using an airlift system to scour the membrane, reducing the air energy consumption to 0.7 kWh/m3 in airlift mode. The costs of hollow fiber modules are lower than those of flat-sheet modules, but more equipment is required (backwash system, fine prescreen 1mm). A broader range of materials is available for flat-sheet and tubular membranes: they can have greater resistance to chemicals and temperature, and are sometimes required for difficult industrial applications. The membrane surface area needed for side-stream systems is smaller than that for submerged systems. Due to the higher MLSS concentrations, the side-stream systems are expected to be more compact, but with higher operating costs. Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 Unit Membrane/module type 8 Plate-andframe Flat-sheet polymer Hollow fiber Side-stream Bundles polymer Tubular ceramic Net flux 1/(m2h) 15-25 20-30 70-100 Recommended MLSS g MLSS/l 10-15 10-15 15-30 Fraction of aerobic volume % 30-100 10-40 External set-up Energy Consumption (membrane system only) kWh/m3 0.3-0.6 0.3-0.6 2-10 Cost ./m2 High Medium Very high pH-range - 1-12 2-11 1-13 T˚-resistance ˚C <60 <40 <100 Table 3: Description of membrane bioreactor design options: submerged versus sidestream systems. There are essentially three main operations of a membrane bioreactor (MBR) contributing most significantly to operating expenditure (OPEX). These are the following: a. Membrane permeability maintenance, b. Microbiology maintenance and c. Liquid and sludge transfer. [2] Of these, maintaining membrane permeability is the most significant, and impacts on OPEX through: i. ii. iii. Scouring and/or agitation by aeration (for immersed systems) and/or liquid crossflow (for sidestream systems); Cleaning, both physical (relaxation and/or backflushing) and chemical (maintenance and/or recovery); and Membrane replacement, should irreparable damage be sustained or otherwise recovery cleaning prove ineffective. [2] Since microbiology is also maintained by aeration – both for suspending the biomass and maintaining dissolved oxygen levels for sustaining microbiological activity – it follows that aeration energy is the most significant contributor to OPEX for immersed systems. Design of an MBR therefore demands knowledge of both of the feedwater quality, which principally determines the oxygen demand for biotreatment, and the aeration demand for fouling control. [2] Sawant A. M.Tech Green Technology (2011-2013) Assignment No. 3 9 REFERENCES: 1. Membrane bioreactors for wastewater treatment By T. Stephenson 2. The MBR Book: Principles and Applications of Membrane Bioreactors for Water and Wastewater Treatment By Simon Judd, Elsevier, 2011 3. Membranes for water treatment By Klaus-Viktor Peinemann, Suzana Pereira Nunes Sawant A. M.Tech Green Technology (2011-2013)