Are MEMBRANE BIOREACTORS Ready for Widespread Application? Developing countries have the most to gain from this technology.
T
here’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
© 2005 American Chemical Society
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 FERNANDE Z
GLEN T. DAIGGER CH2M HILL BRUCE E. RITTM A NN A RIZONA STATE UNIV ERSIT Y SA MER A DH A M MONTGOMERY WATSON H AR Z A GI A NNI A NDREOTTOL A UNIV ERSIT Y OF TRENTO (ITALY )
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FIGURE 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 10–20 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). 400A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2005
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
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-
TA B L E 1
Analysis of applications in developed countries
Criteria
Advanced removal of biological oxygen demand and nutrients for discharge
Pretreatment to produce highpurity water with Controlling reverse osmosis micropollutants
Recycling gray water
Engineering Engineering principles Engineering principles Mechanisms and ef- Engineering principles well principles well developed and well developed and fectiveness of mem- developed and demondemonstrated. demonstrated. brane bioreactors strated. (MBRs) to remove micropollutants evolving rapidly. Equipment Equipment widely Equipment widely Equipment widely Equipment widely available, availability available, along with available, along with available, along with along with associated techassociated technical associated technical associated technical nical support. support. support. support. Applications Being used by innova- Pilot-scale demonBenefits becoming Coupled with source sepaexperience tors and early adopt- strations complete. recognized by leading ration at the household levers. Progression to Progression to early practitioners. el. Initial demonstrations of early majority either adopters likely to ocsource separation just ococcurring or will be- cur soon. curring. gin soon. Summary Transition from early Implementation of this Implementation of this MBR technology for this opassessment adopters to early ma- application appears application appears tion well developed. Adopjority beginning. Adop- to be straightforward to be straightforward tion depends on adoption tion progressing in and likely to occur. and likely to occur. of source separation at the normal fashion. household level.
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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. Engineering principles. 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. Mem402A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2005
brane 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-m3/day (1 million gal/day) MBR was ~$0.80/m3 ($3.05 per 1000 gal) (8). Three years later, the price for the same facility declined to ~$0.48–0.58/m3 ($1.80−2.20 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-m3/day MBR facility recently became operational in Brescia, Italy (15), a 32,000-m3/day facility recently started
FIGURE 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.
ZENON
(b) ZENON
(a)
ZENON
(c)
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20 g/L and found no adverse impacts on effluent quality (6). At the same time, MBR design is now entering a Analysis of unique applications in developing countries new generation. The emphasis is on the Treating economy of scale achieved when memExtracting useful highly polluted brane manufacturers function solely as Treating full resources from environmental equipment suppliers, rather than as Criteria sewage flows sewage waters providers of complete systems. These Engineering Engineering principles Engineering principles Engineering principles concepts are being incorporated into principles well developed and well developed and well developed and the new, larger facilities being built. demonstrated, as dis- understood for efflu- demonstrated, as disAnother factor is that MBRs procussed in connection ent. Engineering prin- cussed in connection duce high-quality effluents that can with overall readiness ciples developing for with overall readiness be reused. In fact, some versions of assessment. extraction of other assessment. the MBR process were specifically useful products (such developed to allow small-scale reuse as nutrients and enof nonpotable water. More recently, ergy). MBRs that produce effluents with sufEquipment Equipment can be Equipment can be Equipment can be ficient quality for indirect potable reavailability manufactured local- manufactured local- manufactured localuse have been demonstrated in larger ly or imported. Suffily or imported. Suffi- ly or imported. Suffacilities. cient local support ficient local support cient local support Future needs. Although MBRs are not available. not available. not available. being implemented at an increasing Applications Being used by innova- Being used for water No applications experate, technical issues still demand adexperience tors and early adopt- extraction by innova- rience. vances. The recent use of lower MLSS ers in developing tors and early adoptconcentrations has allowed higher countries. ers in developing water fluxes, and this factor has imcountries. Essentially proved the cost-effectiveness of MBRs no use to extract othand helped accelerate demand. Hower products. ever, the lifetimes of the membranes Summary Development of local Development of local Development of looperating with higher fluxes will not assessment technical support and technical support and cal technical support be established without several years further demonstraneeded for widefurther demonstraof operating experience. tions needed for wide- tions needed for wide- spread application. Experience indicates that prelimispread application. spread application for Technology demonnary treatment can optimize membrane irrigation water. Fur- strations needed to capacity and lifetime. An especially ther technical devel- develop local experiimportant pretreatment is removing opments needed for ence. fibrous material, such as hair. Screens other applications. with an opening of ≤2 mm are currently being used. The quantity and noxious nature of the materials removed up in Traverse City, Mich. (16), and several installaby such fine screens pose problems for most operations of this capacity or larger are currently in varitions. The proper balance between better screening ous stages of construction. Furthermore, plans for to prolong membrane life versus the ongoing diffifacilities with capacities up to 150,000 m3/day are culties that the screenings create has not yet been in the works. Evaluations completed for MBR faciliestablished. ties ranging in capacity from 300,000 to 800,000 m3/ day find that implementation could be successful Although MBRs are at this scale. As the technology is installed in larger facilities, being implemented the method of implementing MBRs is changing. In the past, MBR manufacturers sold complete treatat an increasing rate, ment units, known as “package plants”. The small size of many existing units and the objective to mintechnical issues still imize the need for operator attention often led to conservative designs that included very long SRTs of demand advances. 30−50 days and MLSS concentrations as high as 30 g/ L. Many assumed that these conservative operating MBRs normally operate with a higher MLSS conparameters were necessary for successful MBRs. Subsequent research and experience have demcentration than do conventional activated sludge onstrated, however, that designs based on specific processes, and this reduces the size and cost of the process objectives but lower MLSS concentrations bioreactor. However, the trade-offs are increased (generally
