Hollow fiber membrane life in membrane bioreactors (MBR) - GE ...

Hollow fiber membrane life in membrane bioreactors (MBR)
Pierre Cote a, ⁎, Zamir Alam b , Jeff Penny b
a COTE Membrane Separation Ltd, 26 Tally Ho Road, Dundas, Ontario, Canada L9H 3M6
b GE Water & Process Technologies, Canada
article info
Article history:
Received 16 November 2011
Received in revised form 28 December 2011
Accepted 29 December 2011
Available online 28 January 2012
Keywords:
Membrane bioreactor
Membrane life
Ultrafiltration
Wastewater treatment
1. Introduction
abstract
The coupling of biological treatment with membrane separation in a
membranes bioreactor (MBR) is an established technology; the authors
estimate that the worldwide installed capacity was 5 million m 3 /d in
2010.
The invention of MBR is credited to researchers at Dorr–Oliver in the
late 1960s [1] . However, the commercial development in North America
was pioneered starting in the mid-1980s by Zenon Environmental Inc.
(now part of GE Water & Process Technologies) with a tubular pressurized
membrane [2]. Immersed membranes were developed starting
in the early-1990s by Zenon with the ZeeWeed hollow fiber, and by
Kubota Corporation with a flat sheet membrane [3]. Today,thereare
dozens of suppliers offering products using both immersed technology
platforms [4]. Flat sheet membranes are mostly used in small plants
(b5,000 m 3 /d); hollow fibers are used across the entire flow range and
dominate for larger plants (>10,000 m 3 /d). It is estimated by the authors
that about 75% of the total MBR installed capacity employs hollow fibers.
The rapid adoption of MBR has been primarily driven by superior
effluent quality that can meet ever more stringent discharge regulations
and by requirements for wastewater reuse [4]. In a recent paper,
Hospido et al. [5] presented a life cycle assessment of a number of
MBR configurations. MBR technology has also been widely adopted in
retrofit applications where the capacity of existing infrastructure requires
expansion and/or upgrade. Equally important is the fact that,
⁎ Corresponding author.
E-mail address: pierre@cotemembraneseparation.com (P. Cote).
0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2011.12.026
Desalination 288 (2012) 145–151
Contents lists available at SciVerse ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
The MBR technology has evolved rapidly over the past two decades with significant gains in performance and
reliability, and reductions of costs. While the MBR process is recognized for providing superior effluent quality
in a small footprint, it carries additional operating costs associated with the membrane system, including
the periodic replacement of the membranes which have a life shorter than the civil components of the plant.
The methodology used to determine membrane life was to analyze the entire data set of ZeeWeed MBR
membranes shipped to North American sites over the period of 15 years since their commercial introduction.
This analysis shows that membrane life for the current generation product should be greater than 10 years.
The analysis also considered the various mechanisms of aging and end-of-life triggers. It was determined
that a slow increase in operating pressure and the need for more frequent chemical cleaning should be the
dominant end-of-life trigger with the current product. The most effective method for membrane replacement
is a planned campaign where a fixed portion of the plant, typically dictated by the membrane train configuration,
is replaced on an annual basis in the period of time around the anticipated life of the membranes.
© 2012 Elsevier B.V. All rights reserved.
when compared to a conventional activated sludge (CAS) plant, a
MBR allows elimination of secondary clarifiers, construction of much
smaller aeration tanks and significant reduction of total plant footprint.
These savings essentially “pay” for the membrane system and make
MBR competitive with CAS from a capital cost point of view [6].
However, from an operation and maintenance point of view, a
MBR carries additional costs associated with the membrane system:
the energy cost for membrane aeration and for return activated
sludge (RAS) and permeate pumping, and the cost for membrane
replacement over the life of the plant. Significant progress has been
made over the past 20 years to minimize these costs. Membrane
replacement cost is a function of membrane price and life.
The literature contains little information on membrane life in the MBR
application. Two recently published books on MBR [3,4] do not provide
any data on membrane life. [7] analyzed 6 selected plants and concluded
that membrane life should exceed 6 years. DeWilde et al. [8] predicted a
membrane life of 13 years based on extrapolation after 3 years of operation
assuming that loss of permeability would lead to unsustainable operation.
The comprehensive analysis presented in this paper is the first time
that the entire database of membranes sold by a supplier is used to estimate
MBR membrane life. The purpose of this paper is to identify the factors
that have an impact on membrane life and to establish projected
membrane life based on actual experience with ZeeWeed MBR plants.
2. ZeeWeed product evolution
The presentation and interpretation of membrane life data are
complicated by the fact that the ZeeWeed product and the MBR

146 P. Cote et al. / Desalination 288 (2012) 145–151
Table 1
ZeeWeed major releases and key improvements.
Release
date
Key improvements
1995 ZW150
First commercial ZeeWeed module
Continuous aeration
1997 ZW500A
Improved manufacturing method
Increase of the surface area per cassette by a factor of 2.2 over ZW150
Design flux increase from membrane improvement
2000 ZW500C
Slimming down of module by elimination of bottom header
permeation
Increase of the surface area per cassette by a factor of 1.2 over
ZW500A
Introduction of 10 s/10 s cyclic aeration
2003 ZW500D
Return to permeation from both ends with slim module design
Increase of the surface area per cassette by a factor of 3.4 over
ZW500C
2006 ZW500D
Design flux increase from membrane improvement
Introduction of 10 s/30 s sequential aeration
2010 ZW500D
Increase of the surface area per cassette by a factor of 1.1 over
ZW500D
Design flux increase from membrane improvement
process have continuously evolved over the past two decades [9,10].
The purpose of this section is to present a brief history of the product
features and process conditions that are relevant to membrane life.
ZeeWeed was developed starting in the early 1990s and first commercialized
in 1995. The ZeeWeed membrane is a strong composite hollow
fiber made by coating a hollow braid with a polyvinylidene fluoride
(PVDF) membrane. The membrane was optimized over time but the
dimensions and materials have remained the same over the last two
decades.
The module and cassette have evolved through a number of redesigns
with major new releases introduced every 3–4 years as
described in Table 1. These releases corresponded to the introduction
of new modules and cassettes in 1997, 2000 and 2003 as described in
Table 2 and shown in Fig. 1, but also attention was given over
the years to improve the air scouring method in order to enhance performance
and reduce energy consumption.
The first ZeeWeed module built in 1993, called the ZW-145 (US
patent 5,248,424), was in the shape of an inverted “U” with an aerator
placed in between the two permeate headers. It was used in a number
of pilots and pre-commercial demonstration plants to validate the
concept of using immersed membranes in MBRs. This configuration
was eventually abandoned because it was complex to manufacture
and yielded a low packing density of about 60 m 2 /m 2 (m 2 membrane
surface area per m 2 of cassette footprint).
This was followed by a period of intense development where a number
of alternate configurations were investigated to increase packing
density without sacrificing air penetration in the membrane bundles.
It was first decided to simplify manufacturing by making a linear module
(shape of an “I”) with an elongated header at each end facing each
other. In an effort to promote air penetration in the membrane bundles,
Table 2
Key ZeeWeed cassette characteristics.
configurations where the hollow fibers were horizontal or at some
angle relative to rising air bubbles were evaluated. These approaches
were eventually abandoned in favor of side-by-side vertical modules
with aerators positioned in the gaps between modules.
The first commercial ZeeWeed module, the ZW150, was introduced
in 1995 (Fig. 1a). The cassette had a packing density of
168 m 2 /m 2 . While adoption of a linear configuration simplified
manufacturing, the design still had a number of drawbacks: i) the
hollow fibers were randomly located within the potting area, ii) the
permeate headers were removable and sealed with a flat gasket
held in place by bolts around the periphery, iii) the two headers
were held apart by U-shaped FRP beams, and iv) permeate collection
and air distribution pipes were attached to the outside of the cassette,
adding to the footprint. The ZW150 had a short but successful commercial
life. It served to demonstrate that an immersed hollow fiber
module in a vertical orientation was a viable MBR configuration. It
was the springboard for the ZW500A, which introduced a number
of manufacturing improvements to address product design limitations
and reduce manufacturing costs.
The ZW500A was introduced in 1997 (Fig. 1b). The module itself had
more than three times the surface area as compared to the ZW150, and
the cassette packing density increased by 65% to 278 m 2 /m 2 . It was the
first module designed for mass production. Key design improvements
over the ZW150 included: i) precisely positioned hollow fibers in the
header, ii) inclusion of a soft transition layer at the interface where
the hollow fibers enter the potting resin, iii) replacement of the Ushaped
FRP beams by FRP pipes that played the dual role of holding
the headers apart and serving as conduits for permeate and air, and
iv) integrating aerators into the bottom header. While the ZW500A
was a very successful product, operational issues were associated with
the effectiveness of aeration and the maintenance of clean hollow
fiber bundles. Coarse bubble aerator holes tended to plug as a result of
sludge drying around the holes; so an aerator flushing method using
permeate was developed to maintain uniform membrane aeration.
The ZW500B was a version of the ZW500A with increased surface
area for non-MBR applications. It was installed in a few MBR plants on
a trial basis, but the packing density was too high to allow sustainable
operation at similar design conditions.
The ZW500C released in 2000 (Fig. 1c) was developed to improve
operation over the ZW500A and included two major design changes.
The module was slimmed down to accommodate a thinner bundle
and permeate was extracted from the top header only. This allowed
opening up the bottom portion of the cassette for better air penetration.
This was the first design where the module headers were not held
apart individually, but attached to the cassette frame. The elimination
of the ZW500A FRP pipes allowed increasing the length of the header;
overall cassette packing density increased by 21% over the ZW500A to
336 m 2 /m 2 without increasing the local hollow fiber potting density.
Cyclic aeration was an independent innovation that was released
with the ZW500C. Cycling the air relatively frequently (i.e., 10 s on/
10 s off) resulted in a number of benefits: i) regular flooding of the
aerator pipes during the off period kept the aeration holes wet and
open without the need for permeate flushing, ii) unsteady-state aeration
enhanced air penetration in the membrane bundle, and iii) reduction
of the total airflow by 50% provided a corresponding reduction in
air scouring energy.
Characteristic Units ZW150 (1995) ZW500A (1997) ZW500C (2000) ZW500D (2003) ZW500D (2010) ZW500Ds (2010)
Modules per cassette 12 8 22 48 48 16
Surface area m 2
167 372 450 1516 1650 446
Permeate extraction Both ends Both ends Top only Both ends Both ends Both ends
Footprint m 2
1.0 1.3 1.3 3.7 3.7 1.3
Height m 1.8 1.8 2.1 2.5 2.5 2.1 (short)
Cassette packing density m 2 /m 2
168 278 336 411 448 346

c) ZW500C
The ZW500D introduced in 2003 (Fig. 1d) was a break in a chain of
backward retrofit-able cassettes. By that time, the market was exploding
and there was a need for significant scaling-up and cost reduction.
The size of the ZW500D cassette was set to the maximum
dimensions that can be transported in a standard ISO container.
Building a larger cassette (1516 m 2 , 3.4 times that of the ZW500C)
reduced the number of tie points and provided significant economies
of scale. Improvements made to that point were integrated in the
design, but the increase in module length dictated that permeate
had to be extracted from both ends to limit pressure losses in the hollow
fiber lumen; this was done without increasing the width of the
header or compromising on the improvement in scouring air penetration
realized with the ZW500C. Overall, cassette packing density
increased to 411 m 2 /m 2 , a 22% improvement over the ZW500C, due
mostly to the increase in module length.
P. Cote et al. / Desalination 288 (2012) 145–151
a) ZW150 b) ZW500A
d) ZW500D
Fig. 1. Pictures of ZeeWeed cassettes.
Two significant improvements were made to the ZW500D in 2006.
First, the membrane chemistry was optimized which resulted in a
doubling of permeability; after multiple pilot trials and observation
of full-scale cassette performance in operating plants, design fluxes
were conservatively increased. Second, a variation of cyclic aeration
called sequential aeration was implemented. Rather than turning
the air on and off under one cassette, this involved cycling the air between
four sets of aerators located under two different cassettes. This
was effectively equivalent to 10 s on and 30 s off but without leaving
the air off under a cassette for more than 10 s. 10/30 sequential aeration
cut scouring energy by half as compared to 10/10 cyclic aeration.
Two additional improvements were incorporated in the ZW500D
in 2010. Surface area per module was increased by 9% and design
fluxes were also increased taking further benefit from the permeability
advance in 2006.
147

148 P. Cote et al. / Desalination 288 (2012) 145–151
The ZW500Ds are a retrofit product for the ZW500A and ZW500C
installed in older plants and for smaller plants where the ZW500D
would be too large of a building block. It is based on either a standard
or a short ZW500D module, but with the modules positioned parallel
to the long side of the cassette in two rows.
The overall impact of these continuous improvement efforts over
a period of 15 years is illustrated in Fig. 2. Fig. 2a shows that the
filtration capacity per cassette increased from 68 to 1055 m 3 /d; this
significantly impacted capital cost as the cost of cassette holding
mechanisms, pipes and valves are very sensitive to the number of cassettes
installed in a plant. Fig. 2b shows how the improvement in cassette
packing density translates to membrane tank filtration loading
rate, increasing from 1.9 to 8.0 m 3 /h/m 2 (as a reference, secondary
clarifiers are normally designed for a loading rate of 1.0 m 3 /m 2 /h).
Fig. 2c shows that scouring aeration has been reduced by a factor of
10. Finally, Fig. 2d illustrates the decrease in the cost of membranes
by a factor of 6.2 expressed in $/m 3 /d (capturing the effects of flux
increase and reduction of cost per m 2 of membrane).
3. Membrane life
3.1. Methodology
Predicting membrane life is difficult given the fact it can vary
broadly with process and maintenance conditions. It is relatively
easy to focus on a few specific plant examples; this has often been
done but it can be misleading.
The approach employed in this study was to analyze the entire
data set of ZeeWeed membranes sold for the MBR application in
North America (all plants larger than 200 m 3 /d in the United States,
Canada and Mexico) over a period of 15 years (1996–2011). As a
membrane manufacturer, GE Water was in a unique position to access
and analyze these data. The data set included all the membranes
shipped to sites during that period (some 250 plants with a total
treatment capacity of approximately 850,000 m 3 /d or 225 MGD).
The membranes included all generations of modules listed in Table 2.
Fig. 2. ZeeWeed evolution.
The membranes shipped from the manufacturing plant were split
into three buckets: Installed, Warranty and Replacement.
• Installed membranes included initial and expansion membranes;
membranes from decommissioned plants were subtracted from
the cumulative amounts.
• Warranty membranes included mechanical module or cassette failures
that happened within the first year of service.
• Replacement membranes included all other membranes shipped to
existing plants.
It was sometimes difficult to assign a purpose to a given shipment;
the following guidelines were used. Addition of membranes to process
more flow was treated as either warranty (if the membrane
surface area installed initially was insufficient) or expansion (if the
flow rate at the plant was higher than original planned). Damaged
membranes caused by process failures were more difficult to categorize.
Damages resulting from failure of the air scouring system were
treated as warranty (i.e. before introduction of cyclic aeration). If
membranes were replaced for free they were put in the warranty
bucket; if the customer paid for them (such as in the case of the failure
of a third party component causing the damage), they were put in
the replacement bucket.
A review of the causes for warranty replacement revealed the
following:
i. No modules were ever replaced because of fiber breakage.
ii. There were only a very few instances of membrane delamination
and these were isolated to the earlier generations.
iii. There were some issues with failure of the fiber potting resin in
the very initial introduction of the technology.
iv. The vast majority of warranty issues were caused by breakage
of mechanical attachments of modules to cassettes and by failure
of the cassette frames themselves; these were solved by
design modifications but also by decreasing the stress level
applied to modules and cassettes as a result of the reduction
of specific aeration flow rates (i.e., Fig. 2c).

3.2. Results
The results of the analysis are presented in Fig. 3, where cumulative
surface area is plotted as a function of time. The Installed curve
includes initial membranes plus expansion membranes minus
decommissioned membranes, and totaled 1,765,000 m 2 at the end
of 2010 (15 year period); the curve also shows part of 2011 when
several large plants were commissioned, but these were not included
in the analysis below. The Replacement curve grows to 121,235 m 2 or
6.9% of the total Installed. The Warranty curve grows to 24,000 m 2 ,or
1.4% of the total Installed. The Installed curve can be used to calculate
an aggregated membrane life for the entire data set as represented by
the two solid lines labeled Hypothetical 5 year Life and Hypothetical
10 year Life. These curves were generated by assuming that Installed
membranes would have to be replaced exactly 5 or 10 years after
they were first put in service (accounting for 1, 2 or 3 replacements
as needed). The Replacement curve falls between the Hypothetical
5 year Life and Hypothetical 10 year Life curves, close to the Hypothetical
10 year Life curve. By trial and error, the best fit was found
to be 8 years (not shown). From this analysis, it can therefore be concluded
that the aggregated membrane life for all ZeeWeed membrane
products in the MBR application has been 8 years.
The analysis above is skewed by failures of first generation products
and the accelerating aging impact of early process issues. In
order to investigate this, the data set was split into two groups with
all the previous generations in the first group and the current generation
(ZW500D) in the second group. The results presented in Fig. 4
and Table 3 show only the first 8 years of each data set to facilitate
the comparison (since the ZW500D had only been in existence for
8 years at the time the analysis was done). For the previous generations,
the first 8 years corresponds to the period 1996–2003, while
for the ZW500D it corresponds to the period 2003–2010. This
reduced data set still included 87% of all Installed membranes in
Fig. 3. Installed capacity for the early generations (500A/B/C) grew
to 208,000 m 2 during the first 8 years; replacement membranes and
warranty represented 11.7% and 4.5% of that total, respectively. In
contrast, Installed capacity for the ZW500D grew to 1,320,000 m 2
during the same period (more than 6 times larger than the previous
generations), but replacement membranes and warranty represented
only 0.06% and 0.6%, respectively.
Membrane Surface Area (cumulative m 2 )
2,500,000
2,000,000
1,500,000
1,000,000
500,000
P. Cote et al. / Desalination 288 (2012) 145–151
Installed
Replacement
Warranty
Hypothetical 5y Life
Hypothetical 10y Life
It is obvious from this analysis that improvements to the product
and resolution of process issues have greatly reduced warranty
issues and all but eliminated early membrane replacement. The data
does not allow conclusive determination of a membrane life for the
ZW500D as was done above for the aggregated data set as not enough
time has passed, but it can be safely concluded that it should be
10 years or longer.
4. Analysis of end-of-life triggers
A number of different factors can trigger the decision to replace
membranes in a MBR plant. Ayala et al. [7] identified mechanical failure
of the membrane weld and loss of permeability as the dominant
factors to replace immersed flat sheet membranes. Corresponding
factors also exist for the ZeeWeed hollow fiber membrane (points 4
and 5 below), but the list was expanded as follows for the purpose
of the discussion in this paper:
4.1. Hollow fiber breakage
4.2. Mechanical module or cassette failure
4.3. Upgrade to a higher performance product
4.4. Weakening of the potting resin-membrane fiber bond
4.5. Increase in cleaning frequency to meet flow throughput
4.1. Hollow fiber breakage
Most MBR plants have limits on effluent quality as dictated by discharge
or reuse regulations. In a MBR, the removal of organic dissolved
species is achieved biologically while the solids are retained
by the membranes. In the following analysis we assume that the biology
is working well and examine the impact of membrane integrity
on effluent quality. Effluent turbidity can be monitored to provide
information on membrane damage or breaks. The data on membrane
warranty and replacement presented above indicates that integrity
failure of the ZeeWeed membrane has not been an issue. Breakage
of the ZeeWeed membrane has never been a problem thanks to the
fact that a strong reinforced hollow fiber structure has been used
from the very introduction of the first generation. There were a few
early occurrences of membrane delamination due to poor adhesion
of the membrane to the support, but none in the past ten years.
All Modules
-
1996 1998 2000 2002 2004 2006 2008 2010 2012
Year
Fig. 3. Cumulative ZeeWeed membranes in North America (all generations).
149

150 P. Cote et al. / Desalination 288 (2012) 145–151
Installed Surface Area (cumulative m 2 )
1,500,000
1,000,000
500,000
Damage to the membrane itself caused by sharp debris is self-healing
due to the presence of the reinforcing structure.
4.2. Mechanical module or cassette failure
Mechanical failures can be caused by factors internal or external to
the membrane product.
Internal factors include product design or manufacturing flaws;
these typically lead to failure within the first few months of operation
and are covered by manufacturer warranty. It is obvious from the data
presented above that the ZeeWeed module and cassette designs have
gone through a learning curve. However, the current ZW500D has
been stable for 8 years and mechanical failure modes of modules
and cassettes have been virtually eliminated.
External factors related to plant design and operation shortcomings
are more difficult to eliminate. The membrane manufacturer
can provide guidelines, but these must be implemented by others.
Overall however, the data presented above shows that process issues,
such as membrane aeration and pre-screening, have not led to membrane
replacement except for isolated cases.
4.3. Upgrade to a higher performance membrane
Previous Generations
ZW-500D
There are a number of plant-specific situations where membranes
that have not reached their end-of-life may be changed because it
is the most cost-effective solution to meet a planned expansion (i.e.,
it is less expensive to replace membranes within an existing system
than to add system components plus membranes to increase
throughput). This often happens during a period of intense innovation
when product performance significantly improves from generation
to generation. Given the fact that the ZeeWeed 500D has
0
1 2 3 4 5 6 7 8
Years since Product Introduction
Fig. 4. Cumulative ZeeWeed membranes in North America (previous generations and ZW500D).
Table 3
Warranty and Replacement statistics for early generations and ZW500D (through first
8 years of product introduction).
Installed Replacement Warranty
m 2
m 2
% m 2
%
Previous generations 208,476 24,450 11.7 9400 4.5
ZW500D 1,320,480 768 0.06 7808 0.6
become a mature product, this should not be a significant trigger for
membrane replacement in the future.
4.4. Weakening of the potting resin-membrane bond
Chlorine is the primary cleaner used both for maintenance and recovery
of permeability. Over time, chlorine exposure slowly weakens
the bond between the membrane fibers and the potting resin leading
to a potential failure mode where hollow fibers pull out of the potting
resin. The maximum annual chlorine exposure resulting the maintenance
and recovery cleaning regimes following manufacturer
recommendations is approximately 40,000 ppm-hours. Based on
the allowable lifetime exposure of 500,000 ppm-hours, this failure
mode is not a concern during the first 12 years of operation.
4.5. Increase in cleaning frequency to meet flow throughput
The analysis in this section leaves slow irreversible fouling as the
remaining membrane aging mechanism, and approaching maximum
trans-membrane pressure (TMP) as the trigger for membrane replacement.
Typically, as membranes age the frequency of cleaning
events increases and their hydraulic efficiency decreases. The increased
burden on operation eventually leads to the decision to replace
the membranes. The relationship between membrane life and
MBR design and operation conditions is complex and largely influenced
by plant-specific operating conditions, but it can be summarized
as follows. Operating at high fluxes will translate into a
steeper rate of TMP rise and more frequent chemical cleaning,
which will result in a reduced membrane life. Conversely, a conservative
flux can result in a significantly longer membrane life.
For the ZeeWeed product, if we accept that reaching maximum
TMP is the dominant end-of-life trigger, then membrane life is an economic
decision; decreasing flux will increase membrane life and vice
versa, shifting costs between capital and operation. It was shown in
this paper that current design practices with the ZW-500D correspond
to a membrane life of 10 years or more. While this may not
have been reached “by design”, the optimal trade-off between capital
and operation costs is best determined through a life cycle costs
analysis, which is increasingly the dominant procurement method.

5. A strategy for scheduled membrane replacement
It is clear from the data and from practical experience that membranes
do not reach their end-of-life and fail catastrophically and
unexpectedly. Instead, the operating burden increases over the
years through an increase in cleaning frequency and a decrease in
the recovery of permeability. By monitoring the operating pressure
after cleaning events, operators can ensure that the plant never
faces a situation where the incoming flow cannot be processed.
In general there is no single event that will trigger the decision to
replace membranes; membrane replacement should be a planned exercise.
For all but small plants, it is suggested that the entire inventory of
membranes be replaced over a 2–4 year period in sub-sets representing
10–25% of the total plant capacity. A sub-set should always be defined
as all the membranes connected to the same pumping or TMP control
system; new and old membranes should never be operated off the
same permeate pump as this will lead to uneven flow distribution and
rapid fouling of the new membranes. In this proposed strategy, membrane
cassettes that have been damaged (e.g., due to pre-screen failure)
can be consolidated into one train and replaced first. For example,
assuming a plant with 5 membrane tanks, replacements could be
done one tank at a time in years 8, 9, 10, 11 and 12. This will provide
an average membrane life of 10 years and, over time, result in evening
out cleaning requirements and plant hydraulic capacity.
6. Conclusions
The use of membrane bioreactors has grown exponentially over
the past two decades following the introduction of immersed membranes.
The technology has evolved rapidly with significant gains in
performance and reliability, and reductions of costs. While the MBR
process is recognized for providing superior effluent quality in a
small footprint plant, it carries additional costs associated with the
membrane system. One significant operating cost component of a
MBR is the periodic replacement of the membranes which have a
life shorter than the civil components of the plant. The focus of this
paper was to determine membrane life, analyze end-of-life triggers,
and use the information to propose a strategy for membrane
replacement.
The methodology to determine membrane life was to analyze the
entire data set of ZeeWeed membranes shipped to North American
sites over of period of 15 years since their commercial introduction.
These included initial membranes (new plants), expansion membranes,
warranty membranes and replacement membranes. By comparing
the growth of actual replacement membranes to predicted
replacement membranes, it was determined that the aggregate membrane
life for the ZW500 product in MBR applications since product
introduction has been 8 years.
P. Cote et al. / Desalination 288 (2012) 145–151
The data was then split into two groups including first all previous
generations, and second the current generation of ZW500 products in
order to measure the impact of product and process improvements.
This allowed determining that membrane life for the current generation
product (ZW500D) should be greater than 10 years.
The analysis also considered the various mechanisms of aging and
end-of-life triggers. While mechanical module or cassette failures or
process related failures may have been the cause of reduced membrane
life in the past, it was determined that a slow increase in operating
pressure and the need for more frequent chemical cleaning
should be the dominant end-of-life trigger with the current product.
To some extent, membrane life is an economic decision as it will
vary with design flux. Current design practices are based on minimization
of life cycle costs and correspond to a target membrane life of
10 years.
Based on the anticipated membrane life and dominant end-of-life
mechanism, the most effective method for membrane replacement is
a planned campaign where a fixed portion of the plant, typically
dictated by the membrane train configuration, is replaced on an
annual basis in the period of time around the anticipated life of the
membranes.
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