A COMPREHENSIVE NOTE ON MEMBRANE BIOREACTOR

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22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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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 in
-
building 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 mee
tings 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 sedim
entation 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/ (m
2
.h) and chemical oxygen demand (COD) removal was 98%. Dorr
-
Oliver 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bioreactor where flow was continuously withdrawn via a rotating drum screen to an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 configura
tion was plate and frame and
operated at inlet and outlet pressures of 345 kN/m
2

and 172 kN/m
2

respectively,
achieving a flux rate of 16.9 l/ (m
2
.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 off
ering 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/ (m
3
.d) achieving up to 99% removal of
COD.

[1]

Assignment No. 3

1


Sawant A.


M.Tech

Green Technology (2011
-
2013)

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 introductio
n
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.

Application in environmentally sensitive areas,

ii.

Specific applications where conventional processes cannot produce
satisfactory water quality at
reasonable cost,

iii.

Water reuse applications,

iv.

The upgrading of conventional treatment plants. [3]


3. CLASSIFICATION OF MBRS:






Membrane Bioreactors

Integrated or Submerged
Membrane Bioreactors

Recirculated or External Membrane
Bioreactors

Assignment No. 3

2


Sawant A.


M.Tech

Green Technology (2011
-
2013)



In an MBR, membrane filtration occurs either externally through recirculation (external
loop) or within the bioreactor (submerged conf
iguration) as shown in Figure 1

and 2,
respectively. To perform well, the external loop configuration requires very high l
iquid
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]


Assignment No. 3

3


Sawant A.


M.Tech

Green Technology (2011
-
2013)

1.

Plate
-
and
-
frame/flat sheet (FS)

2.

H
ollow fiber (HF)

3.

(Multi) tubular (MT)

4.

Capillary tube (CT)

5.

Pleated filter cartridge (FC)

6.

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 prom
otion 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

se
paration 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 aera
tors’ 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 a
re untreatable by conventiona
l 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]

Assignment No. 3

4


Sawant A.


M.Tech

Green Technology (2011
-
2013)



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]



Assignment No. 3

5


Sawant A.


M.Tech

Green Technology (2011
-
2013)

5. ADVANTAGES AND DISADVANTAGES OF MBRS:

Compared with conventional biologic treatment, the MBR process offers numerous
advantages. The membrane is an absolute barrier to suspe
nded 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 mic
roorganisms 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

Aeration limitations

Complete solids removal from effluent

Membrane fouling

Effluent
disinfection

Membrane costs

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



Membrane Aeration Bioreactors


High
oxygen utilization

Susceptible to membrane fouling

Highly efficient energy utilization

High capital cost

Small footprint

Unproven at full
-
scale

Feed
-
forward control of O demand

Process complexity

Modular/ retrofit



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]





Assignment No. 3

6


Sawant A.


M.Tech

Green Technology (2011
-
2013)

6
. WHY ARE MBR
s

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 M
BR 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 th
at 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 f
ive 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 b
een 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 a
nd 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 membra
ne life
can exceed a
decade, and is more determined by the

extent of manual intervention than any other
Assignment No. 3

7


Sawant A.


M.Tech

Green Technology (2011
-
2013)

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 ar
e 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 techn
ologies

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 tech
nologies 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
requ
ired 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 en
ergy 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 air
lift system to scour the membrane,
reducing the air energy consumption to 0.7 kWh/m
3

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 broa
der 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 syst
ems 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.






Assignment No. 3

8


Sawant A.


M.Tech

Green Technology (2011
-
2013)


Unit

Plate
-
and
-
frame

Hollow fiber

Side
-
stream

Membrane/module
type



Flat
-
sheet
polymer

Bundles
polymer

Tubular
ceramic

Net flux


1/(m
2
h)

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/m
3

0.3
-
0.6

0.3
-
0.6

2
-
10

Cost


./m
2

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 side
-
stream 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.

Scouring and/or agitation by aeration (for immersed systems) and/or liquid
crossflow (for sidestream systems);

ii.

Cleaning, both physical (relaxation and/or backflushing) and chemical
(maintenance and/or
recovery); and

iii.

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 fo
r 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]

Assignment No. 3

9


Sawant A.


M.Tech

Green Technology (2011
-
2013)

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

B
y Si
mon Judd
, Elsevier, 2011

3.

Membranes for water treatment By Klaus
-
Viktor Peinemann, Suzana Pereira
Nunes