vvEPA
     United States
     Environmental Protection
     Agency
Wastewater Management Fact Sheet
Membrane Bioreactors
INTRODUCTION
The technologies most commonly used for per-
forming  secondary  treatment  of  municipal
wastewater rely on microorganisms suspended in
the wastewater to treat it. Although these tech-
nologies work well in many situations, they have
several drawbacks, including  the difficulty  of
growing the right types of microorganisms and
the physical requirement of a large site. The use
of   microfiltration   membrane   bioreactors
(MBRs), a technology that has become increas-
ingly used in the past 10 years, overcomes many
of the limitations of conventional systems. These
systems have the  advantage of combining a sus-
pended growth biological reactor with  solids
removal via filtration. The  membranes  can be
designed for and operated in small  spaces and
with high  removal efficiency of contaminants
such  as  nitrogen,  phosphorus,  bacteria,  bio-
chemical  oxygen demand, and total suspended
solids. The membrane filtration system in effect
can replace the secondary clarifier and sand fil-
ters  in  a typical  activated sludge treatment
system.  Membrane  filtration  allows  a higher
biomass concentration to be maintained, thereby
allowing smaller bioreactors to be used.

APPLICABILITY
For new installations, the  use of MBR systems
allows for higher wastewater flow or improved
treatment performance in a smaller space than a
conventional design, i.e., a facility using secon-
dary  clarifiers and  sand  filters. Historically,
membranes have been used for smaller-flow sys-
tems  due  to  the  high  capital  cost  of  the
equipment and high operation and maintenance
(O&M) costs. Today however, they are receiving
increased use in  larger systems. MBR systems
are also well  suited for some industrial and
commercial applications. The high-quality efflu-
ent produced by MBRs makes them  particularly
applicable  to reuse  applications and for  surface
                  water discharge applications requiring extensive
                  nutrient (nitrogen and phosphorus) removal.

                  ADVANTAGES AND DISADVANTAGES
                  The advantages of MBR systems  over conven-
                  tional biological systems include better effluent
                  quality, smaller space requirements, and ease of
                  automation.  Specifically,  MBRs  operate at
                  higher volumetric loading rates which result in
                  lower hydraulic retention times. The low reten-
                  tion  times  mean that  less space is  required
                  compared to a conventional system. MBRs have
                  often been operated with longer solids residence
                  times (SRTs), which results in lower sludge pro-
                  duction;  but this is not a requirement, and more
                  conventional SRTs have been used (Crawford et
                  al. 2000). The effluent from MBRs contains low
                  concentrations of bacteria, total suspended solids
                  (TSS), biochemical  oxygen demand (BOD), and
                  phosphorus. This facilitates high-level disinfec-
                  tion.  Effluents are readily discharged to surface
                  streams or can be sold for reuse, such as irrig-
                  tion.

                  The primary disadvantage  of MBR systems is
                  the typically higher capital and operating costs
                  than conventional systems for the same through-
                  put. O&M costs include membrane cleaning and
                  fouling  control,  and  eventual membrane  re-
                  placement. Energy costs are also higher because
                  of the need for air scouring to control bacterial
                  growth on the membranes. In addition, the waste
                  sludge from such a system might have  a low
                  settling rate, resulting in the need for chemicals
                  to produce biosolids  acceptable  for  disposal
                  (Hermanowicz et al. 2006). Fleischer et al. 2005
                  have demonstrated that  waste sludges  from
                  MBRs can be processed  using standard  tech-
                  nologies used for activated sludge processes.

-------
MEMBRANE FILTRATION
Membrane filtration involves the flow of water-
containing pollutants across a membrane. Water
permeates through the membrane into a separate

channel for recovery (Figure 1). Because of the
cross-flow movement of water  and  the waste
constituents, materials left behind do not accu-
mulate at the membrane surface  but are carried
out of the system for later recovery or disposal.
The water passing through the membrane is
called the permeate, while the water with the
more-concentrated  materials is  called the  con-
centrate orretentate.
Figure 1.   Membrane filtration process
(Image from Siemens/U.S. Filter)

Membranes are constructed of cellulose or other
polymer material, with a maximum pore size set
during the manufacturing process. The require-
ment is that the membranes prevent passage of
particles the size of microorganisms, or about 1
micron (0.001 millimeters), so that they remain
in the system. This means that MBR systems are
good  for removing solid  material, but the re-
moval of dissolved wastewater components must
be facilitated by using additional treatment  steps.

Membranes can be configured in a number of
ways. For MBR applications, the two configura-
tions most often used are hollow fibers grouped
in bundles, as  shown  in  Figure  2, or as  flat
plates. The hollow fiber bundles are connected by
manifolds in  units  that are designed  for easy
changing and servicing.
Figure 2.   Hollow-fiber membranes (Image
from GE/Zenon)


DESIGN CONSIDERATIONS
Designers of MBR systems require only basic
information about the wastewater characteristics,
(e.g.,  influent characteristics,  effluent  require-
ments, flow data) to  design  an MBR system.
Depending on effluent  requirements,  certain
supplementary options can be included with the
MBR system. For example, chemical addition (at
various places in the treatment chain, including:
before the primary settling tank; before the sec-
ondary settling tank [clarifier]; and before the
MBR or final filters) for phosphorus removal can
be included in an MBR  system if needed to
achieve low  phosphorus concentrations in the
effluent.

MBR systems historically have  been used for
small-scale treatment applications when portions
of the treatment system were shut down and the

-------
wastewater routed around (or bypassed)  during
maintenance periods.

However, MBR systems are now often used in
full-treatment applications. In these instances, it
is recommended that the installation include one
additional membrane tank/unit beyond what the
design would nominally call for. This "N plus 1"
concept is a blend between  conventional acti-
vated sludge and membrane process design. It is
especially important to consider both operations
and  maintenance requirements when selecting
the number of units for MBRs. The inclusion of
an extra unit gives  operators  flexibility and  en-
sures that sufficient operating capacity will be
available (Wallis-Lage et al. 2006). For example,
bioreactor  sizing is often limited by  oxygen
transfer,  rather  than the volume  required  to
achieve the required SRT—a  factor that signifi-
cantly affects  bioreactor  numbers and  sizing
(Crawford et al. 2000).

Although  MBR  systems  provide operational
flexibility with  respect to flow rates, as well as
the ability to readily add or subtract units as con-
ditions  dictate,  that   flexibility   has   limits.
Membranes typically require that the water sur-
face  be maintained  above a minimum elevation
so that the membranes remain wet during opera-
tion.  Throughput limitations are dictated  by  the
physical  properties  of the membrane,  and  the
result is  that peak  design  flows  should  be no
             more than 1.5 to 2 times the average design flow.
             If peak flows exceed that limit, either additional
             membranes  are needed simply to process the
             peak flow, or equalization should be included in
             the  overall design. The equalization is done by
             including a separate basin (external equalization)
             or by  maintaining  water in the aeration and
             membrane tanks at depths higher than those re-
             quired   and  then  removing  that  water  to
             accommodate higher flows when necessary (in-
             ternal equalization).

             DESIGN FEATURES
             Pretreatment
             To  reduce  the  chances of membrane  damage,
             wastewater should undergo a high level of debris
             removal prior to the MBR. Primary treatment is
             often provided  in larger  installations, although
             not  in most  small to medium sized installations,
             and is not a requirement.  In  addition, all MBR
             systems require 1- to 3-mm-cutoff fine  screens
             immediately before the membranes,  depending
             on the MBR manufacturer. These screens require
             frequent cleaning. Alternatives for reducing the
             amount of material reaching the screens include
             using two stages  of  screening and locating the
             screens after primary settling.

             Membrane Location
             MBR systems  are configured with  the mem-
                        Mixed
                        Anoxic
Aerobic + ZeeWeed
                                 Sludge
                                 Recycle
                                                   Blowers
                                     Sludge Wasted
                                      [1-1.2 wt% TS
     Figure 3.   Immersed membrane system configuration (Image from GE/Zenon)

-------
                                Membrane Operating System
      Figure 4.  External membrane system configuration (Image from Siemens/U.S. Filter)
branes actually immersed in the biological reac-
tor  or, as an alternative, in a separate vessel
through which mixed liquor from the biological
reactor is circulated. The former configuration is
shown in Figure 3; the latter, in Figure 4.

Membrane Configuration
MBR manufacturers employ membranes in two
basic  configurations: hollow fiber bundles and
plate  membranes. Siemens/U.S.Filter's  Memjet
and Memcor systems, GE/Zenon's ZeeWeed and
ZenoGem  systems,  and  GE/Ionics' system use
hollow-fiber,  tubular membranes  configured  in
bundles. A number of bundles are connected by
manifolds  into units that can be readily changed
for maintenance or replacement. The other con-
figuration,   such   as   those  provided   by
Kubota/Enviroquip, employ membranes in a flat-
plate  configuration,  again with manifolds to al-
low a number of membranes to be connected in
readily changed  units.  Screening  requirements
for both systems differ: hollow-fiber membranes
typically  require 1- to  2-mm  screening, while
plate membranes require 2- to 3-mm screening
(Wallis-Lage et al. 2006).

System Operation
All MBR systems require some degree of pump-
ing to force  the  water  flowing through  the
membrane. While other membrane systems use a
pressurized system to push the water through the
membranes,  the  major systems used in MBRs
draw a vacuum through the membranes so that
the water outside is at ambient pressure. The
advantage of the vacuum is that it is gentler to
the membranes; the advantage of the pressure is
that throughput can be controlled. All systems
also include techniques for continually cleaning
the system to maintain membrane life and keep
the system operational for as long  as  possible.
All the principal membrane  systems used  in
MBRs  use an air  scour technique to reduce
buildup of material  on the membranes. This is
done by blowing air around the membranes out
of the manifolds. The GE/Zenon systems use air
scour,  as well as a back-pulsing technique,  in
which  permeate  is  occasionally pumped  back

-------
into  the membranes to  keep the pores  cleared
out.  Back-pulsing is typically done on a timer,
with the time  of pulsing accounting for  1 to 5
percent of the total operating time.

Downstream Treatment
The  permeate  from an MBR has low levels of
suspended solids, meaning the levels of bacteria,
BOD,  nitrogen,  and phosphorus  are also low.
Disinfection is easy and might not be required,
depending on permit requirements..

The  solids  retained by the membrane are recy-
cled  to the biological reactor and build up in the
system. As in conventional biological systems,
periodic  sludge  wasting  eliminates   sludge
buildup and controls the SRT within the MBR
system. The  waste  sludge from  MBRs goes
through standard  solids-handling  technologies
for thickening,  dewatering, and ultimate dis-
posal.  Hermanowicz  et al. (2006)  reported  a
decreased ability to  settle in waste MBR  sludges
due to increased amounts of colloidal-size parti-
cles  and filamentous bacteria. Chemical addition
increased the ability of the sludges to settle. As
more MBR facilities  are built and operated, a
more definitive understanding of the characteris-
tics of the  resulting biosolids will be achieved.
However, experience to date indicates that con-
ventional biosolids processing  unit operations
are also  applicable to the waste  sludge from
MBRs.

Membrane Care
The  key  to the  cost-effectiveness of an MBR
system is  membrane  life.  If membrane  life is
curtailed  such that  frequent replacement is re-
quired,   costs   will    significantly  increase.
Membrane life can be increased in the following
ways:

   - Good screening of larger solids before the
   membranes to protect  the membranes from
   physical damage.
   - Throughput rates that are not excessive, i.e.,
   that do  not push the system to the limits of
   the design. Such rates  reduce the amount of
   material that is forced into the membrane and
   thereby reduce the amount that has to be re-
   moved by cleaners or that will cause eventual
   membrane deterioration.

   - Regular use of mild cleaners. Cleaning so-
   lutions most often used with MBRs include
   regular bleach (sodium) and citric acid. The
   cleaning should be in accord with manufac-
   turer-recommended maintenance protocols.

Membrane Guarantees
The  length  of  the  guarantee provided by the
membrane system provider is also important in
determining the cost-effectiveness of the system.
For  municipal   wastewater treatment, longer
guarantees might be more readily available com-
pared to those available for industrial systems.
Zenon offers a  10-year guarantee;  others range
from 3 to 5 years. Some guarantees include  cost
prorating if replacement is needed after a certain
service time. Guarantees are typically negotiated
during the purchasing process.  Some manufac-
turers' guarantees are tied directly to screen size:
longer membrane warranties are granted when
smaller screens  are used  (Wallis-Lage  et al.
2006). Appropriate membrane  life  guarantees
can be secured using appropriate membrane  pro-
curement strategies (Crawford et al.  2002).

SYSTEM PERFORMANCE
Siemens/U.S. Filter Systems
Siemens/U.S.Filter offers  MBR systems under
the Memcor and Memjet brands. Data provided
by U.S. Filter for  its Calls Creek (Georgia) facil-
ity are summarized below. The system, as Calls
Creek retrofitted it, is shown in Figure 5. In es-
sence, the membrane filters were used to replace
secondary clarifiers  downstream  of  an  Orbal
oxidation  ditch.  The system  includes a  fine
screen (2-mm cutoff) for inert solids removal just
before the membranes.

The facility  has an average flow of 0.35 million
gallons per day  (mgd) and a design flow of 0.67
mgd. The system has 2 modules, each containing
400 units, and  each unit consists of  a cassette
with manifold-connected membranes. As shown
in Table 1, removal of BOD, TSS, and ammonia-
nitrogen is excellent; BOD and TSS in the efflu-
ent are around the detection limit. Phosphorus is
also removed well in the system, and the effluent

-------
   Upgraded
   3 Channel
Orbal® Process
   (add aeration
   capacity and
  rework transfer
ports for hydraulics)
                                                        ..xxxxxxxxxx,
                                                        'Fine Screen and)
                                                        f                 S,
                                                        - Inerts Removal '
                                                         L
                                                         (2) Membrane
                                                             Tanks
 (Future i
3rd Tank)i
                                                                        UV-fr»
        Figure 5.  Calls Creek flow diagram (courtesy of Siemens/U.S. Filter)
                                           Table 1.
                                   Calls Creek results 2005
Parameter
Flow (mgd)
BOD (mg/L)
TSS (mg/L)
Ammonia-N (mg/L)
P (mg/L)
Fecal conforms (#/100 mL)
Turbidity (NTU)
Influent
Average
0.35
145
248
14.8
0.88
-
-
Average
-
1
1
0.21
0.28
14.2
0.30
Effluent
Max Month
0.44
1
1
0.72
0.55
1 20
1.31
Min Month
0.26
1
1
0.10
0.12
0
0.01








has very low turbidity. The effluent has consis-
tently met discharge limits.

Zenon Systems
General Electric/Zenon provides systems under
the ZenoGem  and ZeeWeed brands.  The Zee-
Weed  brand refers  to  the membrane,  while
ZenoGem is the process that uses ZeeWeed.

Performance data for two installed systems are
shown below.
                 Cauley Creek, Georgia. The Cauley Creek fa-
                 cility  in Fulton County, Georgia, is  a 5-mgd
                 wastewater  reclamation  plant.  The   system
                 includes biological phosphorus  removal, mixed
                 liquor surface  wasting, and  sludge thickening
                 using  a  ZeeWeed system to minimize  the re-
                 quired volume of the aerobic digester, according
                 to information provided by GE. Ultraviolet disin-
                 fection is  employed to meet regulatory limits.
                 Table  2  shows that the removal for all  parame-

-------
                                            Table 2.
                          Cauley Creek, Georgia, system performance
Parameter
Flow(mgd)
BOD (mg/L)
COD (mg/L)
TSS (mg/L)
TKN (mg/L)
Ammonia-N (mg/L)
TP (mg/L)
Fecal conforms (#/100 mL)
NO3-N (mg/L)
Influent
Average
4.27
182
398
174
33.0
24.8
5.0
-
—
Average
-
2.0
12
3.2
1.9
0.21
0.1
2
2.8
Effluent
Max Month
4.66
2.0
22
5
2.9
0.29
0.13
2

Min Month
3.72
2.0
5
3
1.4
0.10
0.06
2

ters is over 90 percent. The effluent meets all
permit limits,  and is reused for  irrigation and
lawn watering.

Traverse City,  Michigan.  The  Traverse City
Wastewater Treatment  Plant  (WWTP)  went
through an upgrade  to  increase plant capacity
and produce a higher-quality effluent, all within
the facility's  existing plant footprint (Crawford
et al. 2005). With the ZeeWeed system, the facil-
ity was able to achieve those goals. As of 2006,
the plant is the largest-capacity MBR facility in
North America. It has a design average annual
flow of 7.1 mgd, maximum monthly flow of 8.5
mgd,  and peak  hourly  flow of  17  mgd.  The
membrane  system consists of a 450,000-gallon
tank with eight compartments of equal size. Sec-
ondary  sludge  is  distributed  evenly  to  the
compartments. Blowers for air scouring, as well
as permeate and back-pulse pumps, are housed in
a nearby building.

Table 3 presents  a summary of plant results over
a 12-month period. The facility provides excel-
lent removal  of BOD, TSS, ammonia-nitrogen,
and phosphorus.  Figure 6 shows the influent,
effluent, and flow data for the year.

Operating data for the Traverse City WWTP
were  obtained for the same period. The mixed
liquor suspended  solids over the period January
to August averaged 6,400 mg/L, while the mixed
liquor volatile suspended solids averaged 4,400
mg/L. The energy use for the air-scouring blow-
ers averaged 1,800 kW-hr/million gallons (MG)
treated.

COSTS
Capital Costs
Capital costs for MBR systems historically have
tended to be higher than those for conventional
systems with comparable throughput because of
the initial costs of the  membranes. In certain
situations, however, including  retrofits, MBR
systems can have lower or competitive capital
costs compared with alternatives because MBRs
have lower land requirements and  use  smaller
tanks, which can reduce the costs for concrete.
U.S. Filter/Siemen's  Memcor  package  plants
have installed costs of $7-$20/gallon treated.

Fleischer et al. (2005) reported on a cost com-
parison of technologies for a 12-MGD design in
Loudoun County, Virginia. Because of a chemi-
cal oxygen  demand limit,  activated  carbon
adsorption was included with the  MBR  system.
It was found that the capital cost  for MBR plus
granular activated carbon at $12/gallon treated
was on the same order of magnitude as  alterna-
tive processes,  including multiple-point  alum
addition,  high  lime   treatment,  and   post-
secondary membrane filtration.

Operating Costs
Operating costs for MBR systems are typically
higher  than those for comparable conventional
systems.  This  is because of the  higher energy

-------
                                           Table 3.
                  Summary of Traverse City, Michigan, Performance Results
Parameter
Flow(mgd)
BOD (mg/L)
TSS (mg/L)
Ammonia-N (mg/L)
TP (mg/L)
Temperature (deg C)
Influent
Average
4.3
280
248
27.9
6.9
17.2
Average
-
<2
< 1
<0.08
0.7
-
Effluent
Max Month
5.1
<2
< 1
<0.23
0.95
23.5
Min Month
3.6
<2
< 1
<0.03
0.41
11.5
            350
            300
             0
             Aug-05
                      Sep-05
                               Nov-05
                                        Jan-06
                                                 Feb-06
                                                          Apr-06
                                                                   Jun-06
                                                                            Jul-06
            -Inf. BOD •
                    -Inf. TSS •
                            -Inf. NH3-N •
                                     -Inf. PO4-P •
                                              -Eff. BOD •
                                                      -Eff. NH3-N •
                                                               -Eff. TSS •
                                                                       -Eff. PO4-P •
                                                                                -Flow(MGD)
        Figure 6.  Performance of the Traverse City plant
costs if air scouring is used to reduce membrane
fouling. The amount of air needed for the scour-
ing has been reported to be twice that needed to
maintain aeration  in  a conventional activated
sludge system (Scott Blair, personal communica-
tion, 2006). These higher operating costs  are
often  partially  offset by  the lower costs  for
sludge disposal  associated with running at longer
sludge  residence  times  and with  membrane
thickening/dewatering of wasted sludge.

Fleischer et al. (2005) compared operating costs.
They  estimated the operating costs of an MBR
system including activated  carbon adsorption at
$1.77 per 1,000 gallons treated. These costs were
of the same order of magnitude as those of alter-
native processes, and they compared favorably to
those of processes  that are chemical-intensive,
such as lime treatment.

ACKNOWLEDGMENTS
The  authors  acknowledge  Dr. Venkat Mahen-
draker,  GE/Zenon,  Mr.  John Irwin, Siemens/
U.S. Filter, and Mr. Scott Blair and Mr. Leroy
Bonkoski of the  Traverse  City WWTP  for
their assistance in  obtaining  data  and  system
information.  EPA  acknowledges external  peer

-------
reviewers Pat Brooks, Alan Cooper, and Glenn
Daigger for their contribution.

PRODUCT LITERATURE USED
Enviroquip/Kubota. Sales literature.

Siemens. Product literature.
   .

Zenon. Case studies: Cauley Creek, Georgia.
   .

Zenon. Case studies: Traverse City, Michigan.
   .

REFERENCES
Crawford, G., G. Daigger, J. Fisher, S. Blair, and
   R. Lewis. 2005. Parallel Operation of Large
   Membrane Bioreactors at Traverse City. In
   Proceedings of the Water Environment Fed-
   eration 78th Annual Conference &
   Exposition, Washington,  DC, CD-ROM,
   October 29-Nov 2, 2005.

Crawford, G., A. Fernandez, A. Shawwa, and G.
   Daigger. 2002 Competitive Bidding and
   Evaluation of Membrane Bioreactor Equip-
   ment—Three Large Plant Case Studies. In
   Proceedings of the Water Environment Fed-
   eration 75th Annual Conference &
   Exposition, Chicago, IL,  CD-ROM, Septem-
   ber 28-Oct 2, 2002.

Crawford, G., D. Thompson, J. Lozier, G. Daigger,
    and E. Fleischer. 2000. Membrane
    Bioreactors—A Designer's Perspective. In
    Proceedings of the Water Environment
    Federation 73rd Annual Conference &
    Exposition on Water Quality and
    Wastewater Treatment, Anaheim, CA,
    CD-ROM, October 14-18, 2000.
Fleischer, E.J., T.A. Broderick, G.T. Daigger, A.
   D. Fonseca, R.D. Holbrook, and S.N. Murthy.
   2005. Evaluation of Membrane Bioreactor
   Process Capabilities to Meet Stringent Efflu-
   ent Nutrient Discharge Requirements. Water
   Environment Research 77:162-178.

Fleischer, E. J., T. A. Broderick, G. T. Daigger,
   J. C. Lozier, A. M. Wollmann, and A. D.
   Fonseca. 2001. Evaluating the Next Genera-
   tion of Water Reclamation Processes. In
   Proceedings of the Water Environment Fed-
   eration 74th Annual Conference & Exposition,
   Atlanta, GA, CD-ROM, October 13-17, 2001.
Hermanowicz, S.W., D. Jenkins, R.P. Merlo, and
   R.S. Trussell. 2006. Effects of Biomass Prop-
   erties on Submerged Membrane Bioreactor
   (SMBR) Performance and Solids Processing.
   Document no. 01-CTS-19UR. Water Envi-
   ronment Federation.

Metcalf & Eddy. 2003. Wastewater Engineering,
   Treatment and Reuse. 4th ed. McGraw-Hill,
   New York.

Wallis-Lage, C., B. Hemken, et al. 2006. MBR
   Plants: Larger and More Complicated. Pre-
   sented at the Water Reuse Association's 21st
   Annual Water Reuse Symposium, Holly-
   wood, CA, September 2006.
    vvEPA
                                                         United States
                                                         Environmental Protection
                                                         Agency
                                                        Septemb
               er 2007

-------