There s a lot to admire about membrane. Are MEMBRANE BIOREACTORS Ready for Widespread Application?

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1 Are MEMBRANE BIOREACTORS Ready for Widespread Application? Downloaded via on April 12, 2019 at 19:06:32 (UTC). See for options on how to legitimately share published articles. Developing countries have the most to gain from this technology. GLEN T. DAIGGER CH2M HILL BRUCE E. RITTMANN ARIZONA STATE UNIVERSITY SAMER ADHAM MONTGOMERY WATSON HARZA GIANNI ANDREOTTOL A UNIVERSITY OF TRENTO (ITALY) There s a lot to admire about membrane bioreactors (MBRs). This emerging wastewater-treatment technology combines a suspended growth biomass, similar to those used in the traditional activated sludge process, with a membrane system that replaces gravity sedimentation and that retains biomass and clarifies effluent (1 4). MBRs offer a host of technical advantages over activated sludge systems, such as small size, and seem to be well suited for applications such as water reuse. Figure 1 (on the next page) illustrates the components of an MBR and contrasts them with those of a traditional activated sludge process. In April 2003, the authors were part of a Rockefeller Foundation-sponsored team residency in Bellagio, Italy, that explored the potential of MBRs for sustainable, decentralized sanitation. A summary of the findings of the team 14 experts, who are listed in Supporting Information and our Bellagio Statement have been published in DiGiano et al. (5). Here, we describe our approach to assessing technical readiness. We also report on whether MBR technology is ready for more widespread application in developed and developing countries. Because developed nations are already adopting MBRs for certain applications, the key issue is their readiness for a wider range of uses. This sets the stage for adoption to be considered in developing countries, because developed and developing nations share many technical issues. However, the opportunities for MBR technology differ, in part because the technical infrastructure and social settings vary. ANTHONY FERNANDEZ 2005 American Chemical Society OCTOBER 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 399A

2 F I G U R E 1 Comparison of traditional and bioreactor methods (a) Traditional and (b) membrane bioreactor systems that use activated sludge have the same basic components, but the details differ when membranes replace the settling tank to separate out solids. RAS = return activated sludge; WAS = waste activated sludge. (a) RAS WAS (b) RAS Membrane separator WAS Why MBRs? MBR technology has been used for various specialty treatment applications for nearly 30 years (4). Replacing external membranes with immersed ones, which began in the early 1990s, reduced costs (capital and operating) and increased the range of applications for which MBRs can be cost-competitive (1). As discussed later, membrane costs have declined by an order of magnitude over the past decade, dramatically reducing MBR costs. For example, Adham and Trussell have demonstrated the cost-effectiveness of MBRs over conventional water reclamation systems for urban irrigation systems (6). Nevertheless, many practitioners are unfamiliar with MBR technology and, consequently, hesitate to use it. These practitioners still worry about cost, reliability, and operating problems. The current literature documents significant advantages for MBRs: Because the membrane separator retains most particulate matter, the effluent is very low in total suspended solids, turbidity, suspended biochemical oxygen demand (BOD), and most pathogens. The membrane also completely retains biomass, which makes retention of slowgrowing microorganisms with low yields more reliable. Reliable biomass retention enables MBRs to operate with high mixed-liquor suspended solids (MLSS) concentrations, which routinely are as high as g/l, whereas 3 g/l is typical for traditional activated sludge (7, 8). High MLSS concentrations allow moderately sized bioreactors to be used, despite relatively long solids retention times (SRTs). Moreover, MBRs have a small overall plant footprint because of the modestly sized bioreactors and the absence of external clarifiers or filters. With modern process-control equipment, such as programmable logic controllers, most of the operations can be automated. Finally, MBRs allow for exceptional versatility in the design of new plants or the retrofitting of existing wastewater-treatment facilities, because membranes can be added in modules. These advantages combine to provide a compact treatment system that can produce a very high quality effluent and can be operated remotely or with minimal attention in a decentralized setting. Their compact nature makes MBRs attractive for applications in crowded urban areas. The high quality of the effluent creates opportunities for reusing the treated effluent rather than discharging it to surface waters. Thus, MBRs offer a real solution for more sustainable approaches to urban water management in developed and developing countries. MBR installations now number >1000 in Asia, Europe, and North America. The widespread application demonstrates that MBRs are cost-effective for an increasing number and a growing variety of applications. But is MBR technology ready for widespread use in crowded urban areas, for water reuse, and with decentralized operation? Establishing readiness criteria Our investigation focused on assessing the technical readiness of MBR technology for more widespread application, rather than its societal readiness. In 400A ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2005

3 general, the team agreed that a technology is technically ready for application when the technical risks are sufficiently well characterized and manageable. Societal readiness is a broader topic that refers to the acceptability of the technology by the affected public and, therefore, is essentially a social-science issue. To conduct the technical analysis, the team established criteria that included fundamental engineering and science factors as well as commercial factors that reflect real-world constraints. Ultimately, the team used three criteria to assess whether MBR technology was ready for widespread application in urban water management systems: engineering, equipment, and verification. First, the engineering principles must be understood well enough to allow systems to be successfully implemented in a range of settings. Second, reliable equipment and technological support must be commercially available in sufficient quantities to meet existing and developing demand. Third, enough experience with the technology must exist to enable the verification of successful design and the identification of the factors required for successful design and operation. Another approach to assessing the readiness of a technology for widespread application is the technology adoption cycle, which is a well-characterized process that can be described by an S curve (9, 10). Adoption of a new technology is slow initially but picks up momentum as the new concept becomes more acceptable to a wider range of users. Eventually, growth in the use of the new technology slows and asymptotically approaches market saturation. The initial set of adopters, referred to as innovators, are motivated by interest in the technology and often play a crucial role in the technology development cycle by funding development. Innovators represent ~2 5% of the total user population. However, they do not motivate others to adopt the technology, because they often are not viewed as responsible users. Early adopters, on the other hand, do catalyze adoption by being opinion leaders, and they represent ~10% of the total users. These recognized leaders adopt technologies for competitive advantages and are watched closely by others within the particular user segment. Their adoption first catalyzes some others to adopt, and then, as reports of the technology s success spread, still more users follow suit. These early-majority and late-majority adopters represent about two-thirds of the total user base. Early- and late-majority adopters are characterized by their risk aversion and relative readiness to adopt new ideas. Finally come the laggards, who adopt new technology only when it becomes necessary. The technology no longer provides a competitive advantage, but it is an expectation or something necessary to compete with other members of the user community. Innovators and early adopters tend to address engineering principles and equipment/technical support in the early stages of the adoption process. Initial applications experience is accumulated during this phase as well. However, most of the applica- T A B L E 1 Analysis of applications in developed countries Criteria Engineering principles Equipment availability Applications experience Summary assessment Advanced removal of biological oxygen demand and nutrients for discharge demonstrated. Equipment widely available, along with associated technical support. Being used by innovators and early adopters. Progression to early majority either occurring or will begin soon. Transition from early adopters to early majority beginning. Adoption progressing in normal fashion. Pretreatment to produce highpurity water with reverse osmosis demonstrated. Equipment widely available, along with associated technical support. Pilot-scale demonstrations complete. Progression to early adopters likely to occur soon. Implementation of this application appears to be straightforward and likely to occur. Controlling micropollutants Mechanisms and effectiveness of membrane bioreactors (MBRs) to remove micropollutants evolving rapidly. Equipment widely available, along with associated technical support. Benefits becoming recognized by leading practitioners. Recycling gray water well developed and demonstrated. Equipment widely available, along with associated technical support. Coupled with source separation at the household level. Initial demonstrations of source separation just occurring. Implementation of this MBR technology for this op- application appears to be straightforward and likely to occur. tion well developed. Adoption depends on adoption of source separation at the household level. OCTOBER 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 401A

4 tions experience is gained during the early-majority phase. Thus, development of technical readiness is tied to the social process of technology adoption, and technology readiness is at least partially assessed by the stage of adoption. Said another way, assessing technical readiness using the three criteria used in this work implicitly addresses some aspects of societal readiness. The piggyback approach We concluded that MBR technology is ready for widespread application in developed countries. Thus far, it has been used mainly for advanced removal of BOD and nutrients from wastewater before discharge to surface waters, production of high-purity water as a pretreatment to reverse osmosis, advanced control of micropollutants, and recycling of gray water. Table 1 on the previous page summarizes the team s analysis of these four applications in terms of the three readiness criteria. Only the first application has moved past the early-adopters phase, but the good experience in that area suggests that technical roadblocks should not stop advancement for any of the applications. We evaluated each criterion largely on the basis of our experience with advanced wastewater treatment.. Given the similarities between traditional activated sludge and MBR processes, one might hypothesize that much of our extensive knowledge about the traditional process can be applied directly to the MBR process. Indeed, this is the case, as indicated by the following observations based on a series of recent studies (2 4, 11, 12). Although the precise number of MBR installations worldwide is not known, thousands exist. MBR MLSS aggregate, just as in the many variants of the activated sludge process. Both have a wide range of aggregate types that contain similar structures. And the stoichiometry and kinetics of the traditional and MBR processes appear to be consistent. Thus, established process-design procedures and models can be used successfully with the MBRs. Coincidentally, the microorganism growth rates in MBR systems are within the same ranges as those in traditional activated sludge setups. For example, the same SRT is required to achieve nitrification in traditional and MBR processes. MBR process configurations similar or identical to those used in traditional setups perform similarly. Moreover, the same equipment is used for oxygen transfer, mixing, and pumping. Equipment and support. The increased demand for MBR technology has piggybacked on the general expansion of the membrane industry. Membrane modules for MBRs are widely available from at least four manufacturers (Zenon, USFilter, Kubota, and Mitsubishi Rayon), with others also developing relevant products. Figure 2 illustrates the type of immersed-membrane equipment used in MBR facilities. Membranes require periodic cleaning to control biological and chemical fouling. Cleaning methods are well developed for membrane products typically used in MBRs. Membrane equipment and related facilities are essentially the same for MBRs and in drinking-water applications. MBRs require lower hydraulic application rates flow rate per membrane surface area, which is referred to as flux in the membrane industry than typical water-treatment applications. Several relatively large immersed-membrane installations demonstrate the capability of the industry to successfully implement large and small submerged-membrane installations (13). Although many immersed-membrane installations exist, the total installed capacity is relatively small at this time. However, production capacity of membranes does not constrain growing applications because immersed membranes are used in various water-treatment applications, including treatment of potable water. As a consequence, total production capacity far exceeds the current demand for membrane equipment for MBR applications. The costs for membrane equipment have been declining for more than a decade. Compared with the early 1990s, the cost of today s micro- and ultra-filtration membrane equipment has dropped by >90%. Several advances contributed to the reduced price, including development of better materials, more cost-effective configuration of membrane facilities, lower production costs as a result of greater economies of scale, more efficient production, and marketplace competition. Likewise, the costs for complete MBR facilities have also been declining. For example, in 2001, the total price of water for unrestricted urban irrigation produced by a 3800-m 3 /day (1 million gal/day) MBR was ~$0.80/m 3 ($3.05 per 1000 gal) (8). Three years later, the price for the same facility declined to ~$ /m 3 ($ per 1000 gal) (14). The declining costs, in turn, have encouraged membrane use in more applications, which has increased demand for equipment. The result is a virtuous cycle in which declining costs increase use, which funds innovation and increased production, both of which result in further cost reductions. MBR technology is a beneficiary of this virtuous cycle, as the corresponding improvements in membrane technology and reduced cost make MBRs more effective and cost-competitive relative to competing technologies. Applications. Although the precise number of MBR installations worldwide is not known, thousands exist. Most of these installations are small, but they apply to a wide range of wastewaters, including those from municipalities and industry. Capacity, however, is on the rise. A 40,000-m 3 /day MBR facility recently became operational in Brescia, Italy (15), a 32,000-m 3 /day facility recently started 402A ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2005

5 F I G U R E 2 Immersed-membrane modules (a) This individual immersed-membrane module has vertically oriented, hollow-fiber membrane strands arranged in horizontally oriented collectors. Individual modules are ganged together into cassettes to facilitate collection of effluent from individual membrane modules. In this design, the lower portion of the cassette also provides aeration to control fouling. (b) and (c) Membrane cassette manifolds can be assembled into a complete immersed-membrane system in a large process tank. Adjacent pumps and piping withdraw effluent. Blowers and chemical cleaning equipment are located nearby. (a) ZENON ZENON ZENON (b) (c) OCTOBER 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 403A

6 T A B L E 2 Analysis of unique applications in developing countries Criteria Engineering principles Equipment availability Applications experience Summary assessment Treating full sewage flows demonstrated, as discussed in connection with overall readiness assessment. Being used by innovators and early adopters in developing countries. technical support and further demonstrations application. Extracting useful resources from sewage understood for effluent. developing for extraction of other useful products (such as nutrients and energy). Being used for water extraction by innovators and early adopters in developing countries. Essentially no use to extract other products. technical support and further demonstrations application for irrigation water. Further technical developments needed for other applications. up in Traverse City, Mich. (16), and several installations of this capacity or larger are currently in various stages of construction. Furthermore, plans for facilities with capacities up to 150,000 m 3 /day are in the works. Evaluations completed for MBR facilities ranging in capacity from 300,000 to 800,000 m 3 / day find that implementation could be successful at this scale. As the technology is installed in larger facilities, the method of implementing MBRs is changing. In the past, MBR manufacturers sold complete treatment units, known as package plants. The small size of many existing units and the objective to minimize the need for operator attention often led to conservative designs that included very long SRTs of days and MLSS concentrations as high as 30 g/ L. Many assumed that these conservative operating parameters were necessary for successful MBRs. Subsequent research and experience have demonstrated, however, that designs based on specific process objectives but lower MLSS concentrations (generally <10 g/l) are more cost-effective for larger installations. Researchers evaluated MBR performance with MLSS concentrations ranging from 2 to Treating highly polluted environmental waters demonstrated, as discussed in connection with overall readiness assessment. No applications experience. technical support application. Technology demonstrations needed to develop local experience. 20 g/l and found no adverse impacts on effluent quality (6). At the same time, MBR design is now entering a new generation. The emphasis is on the economy of scale achieved when membrane manufacturers function solely as equipment suppliers, rather than as providers of complete systems. These concepts are being incorporated into the new, larger facilities being built. Another factor is that MBRs produce high-quality effluents that can be reused. In fact, some versions of the MBR process were specifically developed to allow small-scale reuse of nonpotable water. More recently, MBRs that produce effluents with sufficient quality for indirect potable reuse have been demonstrated in larger facilities. Future needs. Although MBRs are being implemented at an increasing rate, technical issues still demand advances. The recent use of lower MLSS concentrations has allowed higher water fluxes, and this factor has improved the cost-effectiveness of MBRs and helped accelerate demand. However, the lifetimes of the membranes operating with higher fluxes will not be established without several years of operating experience. Experience indicates that preliminary treatment can optimize membrane capacity and lifetime. An especially important pretreatment is removing fibrous material, such as hair. Screens with an opening of 2 mm are currently being used. The quantity and noxious nature of the materials removed by such fine screens pose problems for most operations. The proper balance between better screening to prolong membrane life versus the ongoing difficulties that the screenings create has not yet been established. Although MBRs are being implemented at an increasing rate, technical issues still demand advances. MBRs normally operate with a higher MLSS concentration than do conventional activated sludge processes, and this reduces the size and cost of the bioreactor. However, the trade-offs are increased membrane surface area and costs of oxygen transfer. These trade-offs are understood qualitatively, but more data are needed to quantify them. 404A ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2005

7 Other evolving capabilities of MBRs include removing micropollutants and/or extracting useful products, such as nutrients, from sewage. Research in these fundamental areas is ongoing. Much to gain Developing countries may have the most to gain from MBR technology because it can address their pressing needs for improved sanitation (5). In particular, the small footprints, flexible designs, and automation make MBRs ideal for rapidly growing urban areas, where large-scale public-works projects Equipment availability is an issue for all applications. are too expensive and completed too slowly. MBRs offer the potential for decentralized systems that make water management more sustainable, particularly in megacities in the developing countries. However, the introduction of MBRs in developing countries poses special challenges. Most stem from the lack of local resources: financial capital, manufacturing capability, technical support, and trained human resources. Taking these challenges into consideration, we identified three applications of MBRs that would be most beneficial for developing countries. The first is treatment of the full sewage flow in urban areas. In developing countries, urban areas do not have sewage-collection systems and rapid expansion in megacities makes it impossible for large-scale infrastructure to keep pace. Thus, full treatment requires localized treatment or decentralization (5). The second application is extraction of useful resources clean water, nutrients (nitrogen and phosphorus, mainly), and energy (in the BOD) from sewage. In developing nations, human wastes contain significant resource streams, but the resource value must be captured without polluting the environment or creating publichealth risks. The third application is treating polluted environmental waters. Today, waters in many developing countries are so highly polluted that they essentially need wastewater treatment before they are used for human activities or to prevent serious problems in the environment, such as odors, loss of fish habitat, and severe eutrophication. Table 2 shows an analysis of these applications for developing countries T A B L E 3 in terms of the three readiness criteria. MBRs also offer unique advantages and challenges relative to alternative technologies, and their readiness for application in developing countries is presented in Table 3. According to our analysis, the engineering principles exist for two of the three applications. The exception is extraction of nutrients and energy from sewage, because the principles are not yet in place. do support the unique MBR advantages of a small footprint and automated operation. Equipment availability is an issue for all applications. Import is possible, but cost may be a roadblock. Technical support must be provided, whether the equipment is or imported, and this may be the biggest constraint in developing countries. MBR installations that exist in developing countries can provide the experience necessary for expanded use. This is occurring in more traditional applications, such as conventional sewage treatment, and it is reasonable that this will expand to other nontraditional applications such as resource capture and treating environmental waters. Analysis of unique advantages of membrane bioreactors for developing countries Criteria Engineering principles Equipment availability Applications experience Summary assessment Small footprint Systematic benefits through smaller bioreactor, elimination of clarifiers and filters. Opportunity for further optimization through refinement of process parameters (mixed-liquor suspended solids, solids retention time, aeration). Being used by innovators and early adopters in developing countries. technical support application. Automated operation Development of training resources Hardware generally available in developing countries, but technical support not available. technical support application. Equipment well developed and widely availsionals generally Engineering profesable. Further work on present and being sensors needed, along educated in develop- with the development of process-control strategies. ing countries. However, technicians and other professionals not being recruited and educated. General lack of institutions for the recruitment and education of technical professionals. Methods for developing technical professionals in developing countries not demonstrated. When considered in connection with need for local technical support, may be critical barrier for widespread application of MBRs in developing countries. OCTOBER 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 405A

8 Widespread use of MBR technology in developing countries will demand attention to equipment availability and trained human resources. As with other new technologies, advances in workforce education could allow faster implementation of membrane technology. Glen T. Daigger is a senior vice president with CH2M HILL. Bruce E. Rittmann is a professor and director of the Center for Environmental Biotechnology at Arizona State University. Samer Adham is the manager of applied research and development with Montgomery Watson Harza. Gianni Andreottola is a professor at the University of Trento (Italy). Address correspondence regarding this article to Rittmann at rittmann@asu.edu. Acknowledgments The authors gratefully acknowledge the contributions of their colleagues to the work reported in this paper and express thanks to the Rockefeller Foundation for supporting the team residency. A complete list of residency participants is available as Supporting Information at acs.org/est. References (1) Adham, S.; et al. Feasibility of the Membrane Bioreactor Process for Water Reclamation. Water Sci. Technol. 2001, 43, (2) Gunder, B. The Membrane-Coupled Activated Sludge Process in Municipal Wastewater Treatment; Technomic Publishing Co.: Lancaster, PA, (3) van der Roest, H. F.; Lawrence, D. P.; van Bentem, A. G. N. Membrane Bioreactors for Municipal Wastewater Treatment; IWA Publishing: London, (4) Stephenson, T.; et al. Membrane Bioreactors for Wastewater Treatment; IWA Publishing: London, (5) DiGiano, F. A.; et al. Safe Water for Everyone: Experts Suggest That Membrane Bioreactors May be a Key to Global Water Sustainability. Water Environ. Technol. 2004, June, (6) Adham, S.; Trussell, R. S. Membrane Bioreactors: Feasibility and Use in Water Reclamation. San Diego, CA: Final Report; Water Environment Research Foundation: Alexandria, VA, (7) Rittmann, B. E.; McCarty, P. L. Environmental Biotechnology: Principles and Applications; McGraw-Hill: New York, (8) Grady, C. P. L., Jr.; Daigger, G. T.; Lim, H. C. Biological Wastewater Treatment, 2nd ed.; Marcel Dekker: New York, (9) Rogers, E. M. Diffusion of Innovations, 4th ed.; Free Press: New York, (10) Moore, G. A. Crossing the Chasm: Marketing and Selling High-Tech Products to Mainstream Customers, rev. ed.; Harper Business: New York, (11) Fleischer, E. J.; et al. Evaluation of Membrane Bioreactor Process Capabilities to Meet Stringent Effluent Nutrient Discharge Requirements. Water Environ. Res. 2005, 77, (12) Urbain, V.; et al. Integration of Performance, Molecular Biology and Modeling to Describe the Activated Sludge Process. Water. Sci. Technol. 1999, 37, (13) American Water Works Association Research Foundation. Water Treatment Membrane Processes; McGraw-Hill: New York, (14) DeCarolis, J.; et al. Cost Analysis of MBR Systems for Water Reclamation. In Proceedings of the Water Environment Federation 77th Annual Conference & Exposition, New Orleans, LA, Oct 2 6, 2004, CD-ROM, Water Environment Federation: Arlington, VA. (15) Cote, P.; et al. Brescia Large-Scale Membrane Bioreactor: Case-Study. In Membrane Academia Industry Network Conference, Cranfield, U.K., (16) Crawford, G.; Lewis, R. Exceeding Expectations. Civil Eng. 2004, 74, A ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2005