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bioreactor where flow was continuously withdrawn via a rotating drum screen to an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]
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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
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Membrane Bioreactors
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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]
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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]
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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]
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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]
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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
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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.
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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]
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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
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