Anaerobic Membrane Bio Reactors Applications and Research Directions

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Anaerobic Membrane Bioreactors: Applications and Research DirectionsBao-Qiang Liaoa; Jeremy T. Kraemerb; David M. Bagleyb

a Department of Chemical Engineering, Lakehead University, Thunder Bay, Ontario, Canada b

Department of Civil Engineering, University of Toronto, Toronto, Ontario, Canada

To cite this Article Liao, Bao-Qiang , Kraemer, Jeremy T. and Bagley, David M.(2006) 'Anaerobic Membrane Bioreactors:Applications and Research Directions', Critical Reviews in Environmental Science and Technology, 36: 6, 489 — 530To link to this Article: DOI: 10.1080/10643380600678146URL: http://dx.doi.org/10.1080/10643380600678146

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Critical Reviews in Environmental Science and Technology, 36:489–530, 2006Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643380600678146

Anaerobic Membrane Bioreactors: Applicationsand Research Directions

BAO-QIANG LIAODepartment of Chemical Engineering, Lakehead University, Thunder Bay, Ontario, Canada

JEREMY T. KRAEMER and DAVID M. BAGLEYDepartment of Civil Engineering, University of Toronto, Toronto, Ontario, Canada

Membranes provide exceptional suspended solids removal and com-plete biomass retention that can improve the biological treatmentprocess, but their commercial application to anaerobic treatmenthas been limited. This review summarizes the state of the art withrespect to anaerobic membrane bioreactors (AnMBRs), determinesthe types of wastewaters for which AnMBRs would be best suited,and identifies the research required to increase implementation.AnMBRs have been tested with synthetic, food processing, indus-trial, high solids content, and municipal wastewaters at labora-tory, pilot, and full scale. Chemical oxygen demand removal rangesfrom 56% to 99%, while the reported design membrane fluxes rangefrom 10 to 40 L/m2/h. AnMBRs should be immediately applicableto highly concentrated, particulate waste streams like municipalsludges where the membrane can decouple the solids and hydraulicretention times. Opportunity for application to dilute wastewatersalso appears strong, while application to highly concentrated solu-ble wastewaters is likely limited. Greater assessment of vacuum-driven immersed membranes, combining external or immersedmembranes with retained biomass reactor designs, control of mem-brane fouling, and economic feasibility are the key research areasto be addressed.

KEY WORDS: anaerobic wastewater treatment, biogas production,membrane configuration, membrane flux, membrane fouling

This work was supported by the Natural Sciences and Engineering Research Council ofCanada.

Address correspondence to David M. Bagley, Department of Civil Engineering, Universityof Toronto, Toronto, ON, Canada M5S 1A4. E-mail: [email protected]

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490 B.-Q. Liao et al.

I. INTRODUCTION

Anaerobic processes have been successfully used to treat industrial, foodprocessing, and agricultural wastewaters for more than a century. Currently,there are nearly 1600 commercial anaerobic wastewater treatment systemsin operation in the world.59 Anaerobic processes are also widely used totreat sludges from municipal wastewater treatment plants and successfullytreat animal manures and the organic fraction of municipal solid waste.59,101

Anaerobic processes have been less widely applied to dilute wastewaters,such as municipal wastewater, but such applications do exist, in particular incountries with warmer climates.98

Anaerobic processes offer several widely known advantages over con-ventional aerobic processes. First, no oxygen is required, so the challenge,expense, and energy required to dissolve oxygen into water are eliminated.Second, methane is produced and serves as a renewable energy source.Where the economics are favorable, this methane may be combusted toproduce electricity and heat. Finally, less biomass is produced. In the ab-sence of oxygen as an electron acceptor, anaerobic microbial systems dis-card the electrons onto methane instead of using them to grow more mi-croorganisms. These advantages are offset by the slow growth rates ofthe methanogenic organisms and the microbial complexity of the systems.Biomass retention is critical to provide sufficient solids retention time (SRT)for the methanogens, but even so the low effluent concentrations achievedby aerobic processes are difficult to achieve. Consequently, anaerobic pro-cesses are almost exclusively applied to the concentrated waste streams men-tioned earlier, with aerobic processes used primarily for more dilute wastestreams.

The application of anaerobic processes to more dilute waste streamsmay nevertheless be appropriate. Recently, Shizas and Bagley90 measuredthe potential energy in the organics of municipal wastewater to be upto nine times greater than the electricity needed to operate a municipalwastewater treatment plant. The methane-rich biogas produced from anaer-obic sludge digestion can be combusted to produce a significant fractionof the electricity needed to run the plant. To achieve energy sustainabil-ity, however, anaerobic treatment must be applied to the wastewater di-rectly. This should increase the fraction of the potential energy recoveredas biogas, and will decrease the oxygen requirement. The latter could leadto reduced energy expenditures, further improving the energy sustainabil-ity calculation. Technical and economic challenges exist, however, with thefirst and likely most important being the identification of suitable anaerobicprocesses.

In recent years, membrane technologies have been successfully incorpo-rated into the aerobic biological wastewater treatment process. These mem-brane bioreactors (MBRs) have been proven for municipal and industrial


Anaerobic Membrane Bioreactors 491

wastewater treatment.92 A key advantage of these MBR systems is thecomplete retention of biomass in the aerobic reactor. This eliminates theimpact of biomass separation problems that could deteriorate the perfor-mance of conventional aerobic biological treatment and provides very lowsuspended solids concentrations in the treated effluent. Indeed, the con-cept of effluent suspended solids is almost meaningless when the mem-brane pore size is smaller than the pore size of the filter used to measuresuspended solids. Equally important, complete biomass retention thor-oughly decouples the SRT from the hydraulic retention time (HRT), allow-ing biomass concentrations to increase in the reaction basin, thus facilitat-ing relatively smaller reactors, if desired, and higher organic loading rates(OLR).

If membranes work well with aerobic processes, why not with anaero-bic processes? Would the anaerobic membrane bioreactor (AnMBR) com-bine the advantages of the MBR and anaerobic processes? In particular,would the complete retention of slow-growing methanogenic organisms im-prove the applicability of anaerobic processes to more waste streams? Howwould AnMBRs compare to existing high-rate anaerobic processes, for exam-ple, the upflow anaerobic sludge blanket (UASB) reactor? What waste streamswould be most suitable to treat with AnMBRs? What design and operationalchallenges exist?

The answers to these questions and others are not readily available ina single source. The purpose of this literature review, then, is to summarizeand critically evaluate the work that has been conducted on AnMBRs andthen use this evaluation to answer these questions. In addition,the needsfor future research and development to more fully utilize AnMBRs will beidentified.

II. CONFIGURATIONS AND HISTORICAL DEVELOPMENT

A. Configurations

An AnMBR can be simply defined as a biological treatment process operatedwithout oxygen and using a membrane to provide complete solid-liquid sep-aration. This definition is too broad for discussion, however, because a num-ber of alternatives exist for both the anaerobic process and the membraneprocess. The performance and operating characteristics of the AnMBR candepend significantly on which alternatives are selected, so a brief descriptionof the key options is merited.

The defining characteristic of different anaerobic processes is whetherthere is biomass retention. Commercial high-rate anaerobic reactors are fea-sible because biomass is retained, either by the formation of granular sludgeor by attachment to a fixed or mobile support material, thereby decoupling



the SRT from the HRT.59 The most common reactor designs that providebiomass retention are the upflow anaerobic sludge bed (UASB), hybrid UASB,anaerobic filter (AF), expanded granular sludge bed (EGSB), and fluidizedbed (FB). Descriptions of these technologies are provided by Kleerebezemand Macarie59 and Speece.91 When biomass is retained, the effluent sus-pended solids concentration is significantly lower than the biomass concen-tration in the reaction zone. A UASB, for example, has a biomass concentra-tion of 20–30 g/L59 but < 1 g/L of suspended solids in the effluent. Reactordesigns that do not provide biomass retention are the completely stirred tankreactor (CSTR) and plug-flow reactor with suspended biomass. In this case,the SRT is equal to the HRT and the effluent suspended solids concentration(soluble substrate 4–6 g/L59, particulate substrate 20–50 g/L68) is equal tothe bulk reaction zone solids concentration. This reactor design is used fortreatment of high-solids wastes, such as municipal wastewater sludges, andvariations include two-stage, temperature-phased, and acid-gas phased.4,28,68

In acid-gas phased anaerobic digestion, the fermentation/acidificationreactions are physically separated from the acetogenic/methanogenicreactions.28

There are two principle approaches to membrane design and opera-tion. The membrane may be operated under pressure or it may be operatedunder vacuum (Figure 1). In the first approach, the membrane is separatefrom the bioreactor and a pump is required to push bioreactor effluent intothe membrane unit and permeate through the membrane (Figure 1a). Thisconfiguration is often called an external cross-flow membrane, although thiscan occasionally lead to confusion, as noted later. The cross-flow velocity ofthe liquid across the membrane surface serves as the principle mechanismto disrupt cake formation on the membrane.

When the membrane is operated under a vacuum, instead of direct pres-sure, the configuration is often called submerged or immersed because themembrane is placed directly into the liquid. A pump or gravity is used topull the permeate through the membrane. Because the velocity of the liq-uid across the membrane cannot be as readily controlled, cake formationcan be disrupted by vigorously bubbling gas across the membrane surface.For aerobic MBRs, the air used also provides aeration, while for AnMBRsbiogas must be used. The vacuum-driven immersed membrane approachmay be used in two configurations. The membrane may be immersed di-rectly into the bioreactor (Figure 1b) or immersed in a separate chamber(Figure 1c). The latter configuration now looks like an external membrane,and will likely require a pump to return retentate to the bioreactor. However,unlike the external cross-flow membrane, the membrane here is operatedunder a vacuum instead of under pressure. The external chamber configura-tion (Figure 1c) is used for full-scale aerobic wastewater treatment plants be-cause it provides for easier cleaning of the submerged membranes, because



FIGURE 1. Schematic of anaerobic membrane bioreactor configurations. (a) Pressure-drivenexternal cross-flow membrane. (b) Vacuum-driven submerged membrane with the membraneimmersed directly into the reactor. (c) Vacuum-driven submerged membrane with the mem-brane immersed in an external chamber.



the chambers can be isolated instead of the membranes being physicallyremoved.20

B. Historical Development

The first test of the concept of using membrane filtration with anaerobictreatment of wastewater appears to have been reported by Grethlein35 in1978. The external cross-flow membrane treated septic tank effluent andresulted in an increased biomass concentration, 85–95% biochemical oxygendemand (BOD) reduction, 72% nitrate removal, and 24–85% orthophosphatereduction.

The first commercially-available AnMBR was developed by Dorr-Oliverin the early 1980s for high-strength whey processing wastewater treatmentand was known as the Membrane Anaerobic Reactor System (MARS).63,94

The MARS system was comprised of a completely mixed suspended growthanaerobic reactor for biodegradation and an external cross-flow membranemodule for biomass separation. The MARS process was tested at pilotscalebut was not applied at full scale,93 possibly due to high membrane costs.

A significant research effort was Japan’s Aqua Renaissance ’90 project.This 6-year research and development program was carried out startingfrom 1985 to develop a variety of different configurations of AnMBRsfor industrial wastewater and sewage treatment.49,56,69,70 Various mem-brane configurations, including polymeric and ceramic membranes in cap-illary, hollow fiber, tubular, and plate and frame modules, were testedfor biomass retention in both suspended and attached growth anaerobicbioreactors. Wastewaters containing sewage, fats/oils, wheat starch, pulp andpaper mill effluent, alcohol fermentation effluent, and night soil were suc-cessfully treated (chemical oxygen demand [COD] removal generally >90%)in pilot-scale AnMBRs.

Commencing in 1987, a system known as ADUF (anaerobic diges-tion ultrafiltration) was developed in South Africa for industrial wastewa-ter treatment.82 The ADUF completely mixed process configuration was thesame as for the MARS process, although the locally manufactured membranedid not require a high-pressure support system.82 A number of pilot- andfull-scale ADUF systems are in operation,93 including the BIOREK process,which is intended for manure treatment with nutrient and biogas recovery.74

The BIOREK process contains six unit operations: preseparation, the ADUFprocess, ammonia stripping process, reverse osmosis, gas purification, andpower generation. Pilot- and full-scale testing results indicated that the CODremoval efficiency was over 90%.

Through the 1990s, AnMBR research activity increased with investi-gations into different membrane materials,47,84,88 characterization of mem-brane foulants,16,17 and development of strategies for membrane cleaning



and fouling management.8,13,15,108 Also, immersed membranes,39 non-CSTRreactor designs,6 and phylogenetic analytical techniques67 such as fluorescentin-situ hybridization (FISH)106 started to be used.

III. APPLICATIONS OF ANAEROBIC MEMBRANE BIOREACTORS

A. Synthetic Wastewaters

Synthetic wastewaters are typically used to test new concepts such as theAnMBR. The results of a number of studies are summarized in Table 1.The substrates used included volatile fatty acids, starch, glucose, molasses,peptone, yeast, and cellulose. Although the COD removal was generally>95%, only a few studies had an OLR of >10 kg COD/m3/d and onlytwo had a maximum OLR of 20 kg COD/m3/d or higher.12,29 These gen-erally low OLRs for such readily biodegradable substrates are surprising,considering that OLRs for high-rate anaerobic reactors are typically in therange of 10–20 kg COD/m3/d.91,100 In addition, none of the reportedstudies achieved high COD removals at HRTs <10 h. For example, Cadiet al.12 observed a large drop in COD removal efficiency from 91% to 78%when the HRT was decreased from 11 to 6 h at a constant OLR of 2 kgCOD/m3/d.

B. Food Processing Wastewaters

Food processing wastewaters are characterized by high organic strengths(1000–85,000 mg COD/L) with a wide range of suspended solids concen-trations (50–17,000 mg/L) (Table 2). Food processing wastewaters are read-ily biodegradable, so anaerobic treatment is well established; approximately76% of all the anaerobic reactor installations worldwide are for the food andrelated industries.59

Many AnMBR studies have assessed food-processing wastewaters(Table 2). Both pilot and full-scale AnMBR systems have been used to fa-cilitate the retention of biomass and improve effluent quality.11,14,56,63,94,107

AnMBRs have been used for the treatment of effluents from field crop pro-cessing (sauerkraut, wheat, maize, soybean, palm oil), the dairy industry(whey), and the beverage industry (winery, brewery, distillery). High CODremoval efficiencies (usually >90%) were achieved, but the organic load-ing rates, generally in the range 2–15 kg COD/m3/d, were low in com-parison to existing high-rate anaerobic systems, which can achieve OLRsof 5–40 kg/m3/d.59 Most studies used completely mixed reactors with ex-ternal cross-flow membranes, though several studies investigated the useof two-phase systems with and without a membrane after the acidificationreactor.56,107,109 This compares with traditional high-rate anaerobic treatment


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dic

ates

the

mem

bra

ne

pro

duce

dth

efinal

effluen

t).

c—

Indic

ates

valu

enotre

ported

.

498



of food-processing wastes, which predominantly uses the UASB reactorconfiguration.59

C. Industrial Wastewaters

Non-food-processing industrial wastewaters include effluents from the pulpand paper, chemical, pharmaceutical, petroleum, and textile industries. Thecharacteristics of industrial wastewaters are sector specific although, in gen-eral, they have the potential to have a high organic strength and containsynthetic and natural chemicals that may be slowly degradable or non-biodegradable anaerobically, and/or toxic. Traditionally, industrial wastew-aters are treated by a combination of physical, chemical and biologicalprocesses because no single method can achieve complete treatment. Animportant concern associated with the biological treatment of such wastewa-ters is toxicity to the microorganisms. However, wastewaters containing toxiccompounds can still be anaerobically degraded provided that appropriateprecautions are taken.91 This can include pretreatment to remove the in-hibitors prior to anaerobic treatment,69 acclimation of the biomass by gradualincrease of inhibitor concentration, and provision of a sufficiently high SRT(safety factor).91 Membrane bioreactors may have an advantage over otheranaerobic systems because the biomass can be retained even if an inhibitorupsets the treatment system. Because toxics rarely cause cell death, treatmentwould only be temporarily impaired.91

To date, AnMBRs have only been applied to pulp and paper and textilewastewaters (Table 3). The source and composition of the different pulp andpaper effluents and whether each is amenable to anaerobic treatment werereviewed by Rintala and Puhakka.81 Anaerobic treatment of pulp and paperwastewaters has become more common; approximately 9% of all anaerobicinstallations are for the pulp and paper industry.59 The use of AnMBRs forpulp and paper wastewaters has been reported five times, as summarizedin Table 3. Evaporator condensates (EC) from pulp and paper plants arecharacterized by high soluble CODs of 10–42 g/L, due mainly to methanol,low suspended solids (<3 mg/L), plus inhibitory turpene oils and sulfurcompounds.69,70 Pretreatment of the condensate by microfiltration and bio-gas stripping was used to remove the inhibitory turpene oils and sulfur com-pounds and the pH was adjusted to neutral. The pretreated condensate wasthen amenable to treatment in a thermophilic attached-growth ultrafiltrationAnMBR that provided a biochemical oxygen demand (BOD) removal effi-ciency of >93%.69 The ultrafiltration membrane used for cell retention andrecycling in the AnMBR allowed the OLR to be more than doubled to 35.5 kgBOD/m3/day, from 15 kg BOD/m3/d without the membrane. The use of anAnMBR to treat segregated kraft bleach plant wastewater provided a mod-est increase in the adsorbable organic halogen removal efficiency, from 48%to 61% in comparison to a UASB without membrane.36 Economic analyses


TAB

LE3

.Su

mm

ary

ofA

nM

BR

Per

form

ance

for

Tre

atm

entofIn

dust

rial

Was

tew

ater

Typ

eof

was

tew

ater

Scal

ea

Typ

eof

reac

torb

Rea

ctor

volu

me

(m3)

Tem

p.

(◦ C)

HRT

(d)

OLR

(kg

CO

Dm

−3d

−1)

MLS

S(g

L−1)

Feed

CO

D(g

L−1)

Feed

TSS

(gL−1

)

Effl

uen

tCO

D(g

L−1)

CO

Dre

mova

lef

fici

ency

Ref

eren

ce

Kra

ftble

ach

pla

nt

effluen

tL

CST

R0.

015

351.

00.

04e

7.6–

15.7

∼0.0

4e—

∼0.0

16e

61%

e36

Kra

ftpulp

effluen

tP

UFA

F5

—0.

535

d9.

419

.2d

—1.

5d93

%d

76Pulp

and

pap

eref

fluen

tP

FB7

—c

——

—28

151.

196

%56

Eva

pora

tor

conden

sate

(met

han

ol)

PU

FAF

553

0.5

35.5

d7.

617

.8d

<0.

003

1.2d

93%

d69

,70

Woolsc

ouring

PU

FAF

4.5

376.

815

—10

2.4

30.5

5150

%40

aL

=la

bora

tory

/ben

chsc

ale,

P=

pilo

tsc

ale.

bCST

R=

com

ple

tely

stirre

dta

nk

reac

tor,

FB=

fluid

ized

bed

,U

FAF

=upflow

anae

robic

filte

r.c—

Indic

ates

valu

enotre

ported

.dU

nits

are

BO

Din

stea

dofCO

D.

eU

nits

are

AO

X(a

dso

rbab

leorg

anic

hal

oge

n).

500



indicated that the total cost of AnMBR treatment of kraft mill effluent wassignificantly lower than for aerobic treatment and only slightly higher thanfor high-rate anaerobic treatment (although the AnMBR had higher effluentquality).69,70,76

The sources and characteristics of wastewaters generated by textile pro-cessing are discussed in a recent review by Bisschops and Spanjers.9 Only1% of commercial anaerobic installations are for the textile industry.59 AnAnMBR treating wool-scouring wastewater achieved a 50% COD removal atan OLR of 15 kg/m3/d.40 The addition of membrane filtration approximatelydoubled the biomass concentration and increased the total organic carbon(TOC) and grease removal efficiencies from 45 to 90% and from 33 to 99%,respectively.

Membranes without biological treatment have been used in the tex-tile industry to allow reuse of electrolyte solutions60,96 and chemicals,97

and anaerobic treatment without membranes has been applied successfullyto both petrochemical86 and textile wastewaters. The reader is referred torecent reviews that discuss the anaerobic degradation of petroleum com-pounds, including the BTEX compounds (benzene, toluene, ethylbenzene,and xylene),65 hydrocarbons,111 aromatic and aliphatic compounds,59 andhalogenated compounds.27 Reviews are also available for the biologicaltreatment of textile mill effluents97 and the decolorization of textile dyingeffluents.37,78 As long as a wastewater is amenable to anaerobic treatment,in theory an AnMBR could be used to treat it.

D. High-Solids-Content Waste Streams

Waste streams that contain a high proportion of particulates include wastew-ater treatment plant sludges, the organic fraction of municipal solid waste,animal processing plant effluents, and manures. Anaerobic digestion is a com-mon technology for treating such high-solids waste streams, as discussed inrecent reviews and books.48,66,85,101 Digestion is usually performed in com-pletely mixed reactors at low organic loadings of 1–3 kg COD/m3/d.101 Theslow hydrolysis/solubilization of particulates is often rate limiting.101 There-fore, in completely mixed reactors that do not decouple SRT from hydraulicretention time (HRT), the long retention time required for hydrolysis leadsto large reactor volumes and lower OLRs.

In recent years, the AnMBR technology has been successfully testedin both pilot- and full-scale plants for treatment of high solids wastes, assummarized in Table 4. AnMBRs have been tested with wastewater treatmentplant sludges, pig manure, and chicken slaughterhouse effluent. Relativelyhigh OLRs of 3–5 kg COD/m3/d were achieved with high COD removals(80% or higher) for the manure and slaughterhouse wastewaters as comparedwith the usual loadings of 1–3 kg COD/m3/d for high-solids wastes. The


TAB

LE4

.Su

mm

ary

ofA

nM

BR

Per

form

ance

for

Tre

atm

entofH

igh

Solid

sConte

ntW

aste

s

Typ

eof

was

tew

ater

Scal

ea

Typ

eof

reac

torb

Rea

ctor

volu

me

(m3)

Tem

p.

(◦ C)

HRT

(d)

SRT

(d)

OLR

(kg

CO

Dm

−3d

−1)

MLS

S(g

L−1)

Feed

CO

D(g

L−1)

Feed TS

(gL−1

)

Effl

uen

tCO

D(g

L−1)

CO

Dre

mova

lef

fici

ency

Ref

eren

ce

Prim

ary

sludge

PU

pflow

mix

ed0.

1235

20—

c1.

0622

–35

40.2

44.4

1854

%33

Prim

ary

sludge

PCST

R0.

535

8.4

335

0.93

d—

0.24

0.16

0.03

d79

%d

71Prim

ary

sludge

PCST

R0.

555

7.8

197

1.16

d—

0.24

0.16

0.03

d78

%d

71Coag

ula

ted

raw

sludge

LV

FAfe

rmen

ter

CST

R0.

076

350.

510

4.6e

342.

3e6.

81.

3e43

%e

53

Scre

ened

sludge

PSe

mi

contin

uous

CST

R

1.8

—14

26—

55—

——

—80

Sew

age

sludge

LCST

R0.

004

25–5

06.

7–20

—0.

17–1

.35d

20–4

0—

—<

0.3

—3

Pig

man

ure

PCST

R0.

135

6—

5—

3020

390

%74

Pig

man

ure

FCST

R20

035

10—

3—

3020

2.4

92%

74Pig

man

ure

P2

phas

eCST

R+M

/H

ybrid

3/3

20/3

51–

2/1–

2—

/—2.

8–5.

5/—

—/—

5.5

0.6

1.1

80%

61

Chic

ken

slau

ghte

rhouse

LCST

R0.

007

301.

2—

4.3

225.

22.

4–4.

7<

0.5

90%

29

aL

=la

bora

tory

/ben

chsc

ale,

P=

pilo

tsc

ale,

F=

full

scal

e.bCST

R=

com

ple

tely

-stir

red

tank

reac

tor,

Hyb

rid

=U

ASB

with

anae

robic

filte

rin

stea

dof

aso

lids/

liquid

/gas

separ

ator,

Mdes

ignat

esth

elo

catio

nof

the

mem

bra

ne

(no

Min

dic

ates

the

mem

bra

ne

pro

duce

dth

efinal

effluen

t).

c—

Indic

ates

valu

enotre

ported

.dU

nits

are

VSS

inst

ead

ofCO

D.

eU

nits

are

TO

Cin

stea

dofCO

D.

502



COD removals were generally lower for sludge treatment possibly becauseof a larger portion of inert solids. Nevertheless, Aya and Namiki3 found thatorganic substances in the sludge were almost all converted into biogas, andinorganic solids were dissolved into the liquid phase by biological reaction.

All the studies considered here used a CSTR reactor configuration. With-out a membrane, the SRT would have been equal to the HRT. In membranesludge digesters, however, the complete retention of solids in the reactordecoupled the SRT from the HRT.79,80 Pillay et al.80 were able to increasethe reactor solids concentration from 2.6% to 5.5% using a membrane andas a result decreased the HRT by almost half (to 14 d) while the SRT wasmaintained at 26 d. Pierkiel and Lanting79 used HRTs of 1–3 d with SRTs of8–12 d.

Another proposed advantage of complete retention of particulates andbiomass is an increase in the rate of hydrolysis/solubilization of solids. Thishas been tested several times,29,53,61 although none of the studies evaluatedthe hydrolysis/solubilization without a membrane. Therefore, the advantageobserved may be due to either increased SRT or increased solids concen-trations and not necessarily an inherent increase in hydrolysis/solubilizationrate.

Based on the results from pilot-scale trials, Pillay et al.80 conducted aneconomic evaluation of a full-scale anaerobic digester for sludge treatmentwith and without membrane filtration. The results indicated that the AnMBRprocess was technically and economically feasible, and offered significantadvantages over the conventional anaerobic digestion process. Comparedto the conventional anaerobic digester, the capital and total project costsavings for an AnMBR using their low-cost membrane were 27% and 12%,respectively.

E. Municipal Wastewater

Municipal wastewater is characterized by low organic strength (250–800 mgCOD/L) and low suspended solids concentrations (120–400 mg/L).68 Theaerobic activated sludge process is the dominant technology for treating mu-nicipal wastewater and in recent years the aerobic membrane bioreactor hasbeen widely used.20,102 Anaerobic treatment of sewage is not widespread, tra-ditionally being performed in UASB reactors in warm climate regions,59,98,101

but it is technically feasible even for temperate climates as discussed in recentreviews.46,87,110 Conventional UASB sewage treatment usually has an HRT of0.25–0.33 d and results in a BOD removal efficiency of 80%, effluent COD of100–220 mg/L, and effluent total suspended solids (TSS) of 30–70 mg/L.101

In general, sewage has a larger portion of refractory COD compared to foodand beverage wastewaters. For example, Elmitwalli et al.24 found the anaer-obic biodegradability of domestic sewage to be 71–74% at 30◦C whereas



most studies for food processing wastewaters (Table 2) had COD removalsof >90%.

Table 5 summarizes the studies on the use of AnMBRs for sewagetreatment. In general, AnMBR sewage treatment had lower effluent COD(<100 mg/L) and suspended solids concentrations compared to conventionalUASB treatment. This is expected because the membrane can provide ap-proximately 100% removal of suspended solids.5,95 In addition, the COD orBOD removal efficiency was comparable to UASB treatment and very highSRTs could be maintained (e.g., 150 days).105 AnMBRs also provided highCOD removals for the treatment of night soil56 and sludge heat treat liquor.50

A comparison between conventional activated sludge, aerobic MBR,UASB, and AnMBR for municipal wastewater treatment is shown in Table 6.For both aerobic and anaerobic systems, a membrane dramatically improvesTSS removal, although the treated effluent quality in terms of COD is betterfrom the aerobic systems than the anaerobic systems. Because of this fact,aerobic posttreatment of anaerobic effluent has been suggested as a methodto further improve COD and nutrient removals.101 The anaerobic HRT ap-pears to be generally longer than 8 h, compared to 4–8 h for aerobic.

On the other hand, the anaerobic processes had lower energy require-ments than their aerobic counterparts. The electricity use of the UASB is thelowest, well below that of activated sludge. The electricity use of the AnMBRis slightly lower but nevertheless near that of the aerobic MBR when bothsystems used immersed membranes. However, the electricity use of bothanaerobic systems can be offset by use of the produced methane, so thenet energy consumption by both anaerobic systems should be less than theaerobic ones. In all cases, electricity consumption will be higher if externalcross-flow membranes are used instead of immersed membranes, because asuction pump operates at lower pressure and less water is pumped.1

IV. FACTORS AFFECTING THE TREATMENT PERFORMANCEOF AnMBRs

A. Microbial Activity

Pressure-driven external cross-flow membranes use high liquid velocities tominimize fouling and maintain high fluxes. This requires large volumes ofliquid to be pumped; on the order of 25–80 m3 of liquid must pass over themembrane for each 1 m3 of permeate in anaerobic systems.10,20 Liquid recir-culation through the membrane pump has resulted in a substantial decreasein the observed mean particle size by a factor of 3–5.6,13,18,23 Circulation ofthe biomass through the membrane pump has in some instances resultedin a decrease in microbial activity7,10,32,33 but not in others.8,16,45 This dis-agreement may be due to the use of different types of pumps. Kim et al.,55


TAB

LE5

.Su

mm

ary

ofA

nM

BR

Per

form

ance

for

Tre

atm

entofM

unic

ipal

Was

tew

ater

Typ

eof

was

tew

ater

Scal

ea

Typ

eof

reac

torb

Rea

ctor

volu

me

(m3)

Tem

p.

(◦C)

HRT

(d)

SRT

(d)

OLR

(kg

CO

Dm

−3d

−1)

MLS

S(g

L−1)

Feed

CO

D(g

L−1)

Feed

TSS

(gL−

1)

Effl

uen

tCO

D(g

L−1)

CO

Dre

mova

lef

fici

ency

Ref

eren

ce

Nig

htso

il(h

eat-trea

ted

and

hyd

roly

zed)

PU

ASB

0.4

—c

——

——

25.5

2.6

2.0

92%

56

Hea

t-trea

tliq

uor

LCST

R0.

237

0.6

—15

.421

.410

.30.

32.

081

%50

Prim

ary

effluen

tL

CST

R0.

0132

0.5

217

1.6

70.

080.

120.

0268

%5

Sew

age

LSe

ptic

tank

0.10

6—

5.6–

9.6

—0.

03–0

.05d

—0.

27d

—<

0.04

d>

85%

d35

Sew

age

PCST

R3

10–2

20.

4-0.

8—

0.2d

—0.

1–0.

2d0.

03–0

.1<

0.06

d56

%d

95Se

wag

eP

Hyd

rol

CST

R+M

/U

ASB

—/5

.4—

——

——

1.1

0.5

0.07

94%

56

Sew

age

PH

ydro

lCST

R/

UASB

+M2.

0/5.

435

/—3/

0.27

—5.

77/

400.

490.

30.

0883

%49

Sew

age

PH

ydro

lCST

R+M

/FB

+M

0.5/

1.0

30/—

5/0.

09—

1.8

30/9

0.35

0.3

0.04

90%

49

Sew

age

PH

ydro

lCST

R+M

/U

ASB

8.9/

7725

/28

—/0

.3—

—/0

.97

—0.

4∼0

.25

0.1

73%

57

Sew

age

PH

ydro

lCST

R+M

/U

ASB

8.9/

7725

/12

—/0

.3—

—/0

.65

—0.

3∼0

.25

0.1

58%

57

Sew

age

PH

ydro

lCST

R+M

/U

ASB

8.9/

7725

/18

3.7/

0.3

25/—

2e/0.

4d—

/10

0.17

d0.

20.

07d

61%

d58

Dom

estic

was

tew

ater

LH

ybrid

0.01

820

0.25

150

0.4–

1016

0.1–

2.6

0.1–

0.8

<0.

03>

92%

105

Dom

estic

was

tew

ater

LH

ybrid

0.01

820

0.17

150

0.7–

1022

0.1–

1.8

0.1–

1.0

<0.

03>

92%

105

aL

=la

bora

tory

/ben

chsc

ale,

P=

pilo

tsc

ale.

bCST

R=

com

ple

tely

-stir

red

tank

reac

tor,

FB=

fluid

ized

bed

,H

ybrid

=U

ASB

with

anae

robic

filte

rin

stea

dofa

solid

s/liq

uid

/gas

separ

ator,

Hyd

rol=

side-

stre

amsu

spen

ded

solid

shyd

roly

sis

reac

torplu

sm

ethan

oge

nic

reac

torfo

rco

mbin

edhyd

roly

sate

and

prim

ary

clar

ifier

effluen

t,U

ASB

=upflow

anae

robic

sludge

bla

nke

t,M

des

ignat

esth

elo

catio

nofth

em

embra

ne

(no

Min

dic

ates

the

mem

bra

ne

pro

duce

dth

efinal

effluen

t).

c—

Indic

ates

valu

enotre

ported

.dU

nits

are

BO

Din

stea

dofCO

D.

eU

nits

are

VSS

inst

ead

ofCO

D.

505



TABLE 6. Performance Comparison Between Aerobic and Anaerobic Technologies forMunicipal Wastewater Treatment at 15◦C

Parameter UnitsActivatedsludgea

AerobicMBRb UASB

AnaerobicMBRc

Effluent COD mg L−1 <30d <30d 100–220e <100Effluent TSS mg L−1 <30 <1 30–70e <1OLR kg COD

m−3 d−10.3–0.7d 1.2–3.2d 0.6–3.0 f 0.4–6.0

HRT hr 4–8 4–6 8–14a, f 8–12SRT d 3–15 5–20 30–100g >100MLSS in

bioreactorg L−1 1–3 5–20 15–30h 10–40

Electricity usei kWh m−3 0.2–0.4 0.3–0.6 j 0.11h 0.25–1.0b, j

Issues — Possiblesludgebulking

Membranefouling

RequiresVSS/CODsoluble

< 0.1e

Membranefouling

aReference 68.bReferences 92 and 103.cValues chosen as representative from Table 5.dUnits are BOD instead of COD.eReference 101.f Reference 62.gReference 34.hReference 110.iElectricity use by bioreactor and membrane/secondary clarifier only.j Membrane energy use based on immersed modules.

for example, found that a vane-type rotary (positive displacement) pumpimposed greater shear stress to activated sludge flocs than did a turbine-type centrifugal (kinetic) pump. If the same was true for anaerobic flocsor granules, interspecies hydrogen transfer could be disrupted. Hydrogen-producing bacteria must be in very close proximity to the hydrogen-consuming methanogens in order to keep the hydrogen partial pressure low,thereby allowing otherwise thermodynamically unfavorable reactions to pro-ceed (e.g. acetogenesis of propionic acid, β-oxidation of fatty acids, etc.).28

Because most AnMBR research conducted to date has used externalcross-flow membranes, it is possible that, on average, AnMBRs have hada lower microbial activity compared to nonmembrane high-rate anaerobicsystems. Furthermore, because the liquid velocity in the cross-flow mem-brane is independent of the reactor size, the frequency of liquid recirculationthrough the pump increases for smaller reactors, and is more of an issue forlaboratory-scale systems.10 The use of external cross-flow filtration could beone reason why the maximum attained OLRs have been lower than for ex-isting high-rate anaerobic reactors, at a least at laboratory scale. On the otherhand, in principle this pump-induced shear stress may increase the break-down of complex particulates. This could account for the improved methaneconversion observed by Yushina and Hasegawa109 when a membrane wasplaced after an acid-phase reactor treating particulate soybean wastewater.



Reports of vacuum-driven immersed membranes in AnMBR studies havebeen limited. 39,50,103,105 Although the effluent TSS was higher from immersedmembrane systems,39,103 the microbial activity should remain higher thanis possible with external cross-flow membranes because the biomass doesnot pass through a pump.39 Hernandez et al.39 observed a homogenizationof the size of granules and an increase in activity for a UASB with an im-mersed membrane, although there was granule breakup at an OLR >13 kgCOD/m3/d, possibly due to gas mixing.

B. Operational Temperature, SRT, and HRT

Anaerobic processes are often operated at mesophilic (35◦C) and ther-mophilic (55◦C) temperatures. These temperatures are particularly importantfor the treatment of high-solids-content material such as municipal wastew-ater sludges, where the SRT and HRT are equal and the increased reactionrates at the higher temperatures lead to decreased reactor sizes. For exam-ple, at a loading rate of 2 kg VSS/m3/d, Murata et al.71 observed a volatilesuspended solids (VSS) reduction of 67% at 55◦C versus 56% at 35◦C. Increas-ing the temperature also increases the attainable membrane flux because theliquid viscosity decreases at higher temperatures, as discussed later.

Often, the heating requirement for treating these streams can be metby the produced methane. However, for wastewaters with a low organiccontent (e.g., municipal wastewater) the methane production cannot coverthe heating requirement and operation must be at ambient temperatures.Thus, anaerobic sewage treatment has traditionally only been conducted inwarm climates.87 Several recent reviews have summarized the challengesfor low-temperature operation of anaerobic processes in cool and temper-ate climates.48,62,87 Operation at ambient temperatures appears technicallyfeasible, although SRTs as much as double those for mesophilic operationmay be required,48 and hydrolysis of solids slows significantly.87 Operationbecomes governed by the hydraulic loading rate and not the organic loadingrate because the loss of viable biomass must be minimized.62 Membranesmay alleviate many of these challenges because of their high solids reten-tion capability. An AnMBR can maintain solids retention across a range oftemperatures105 which can allow an increase in the SRT and a decrease in theHRT62 because biomass is retained. For example, Wen et al.105 maintainedan SRT of 150 d with an HRT of 4–6 h at 20◦C treating screened munici-pal wastewater, and Baek and Pagilla5 achieved SRTs >200 d with HRTs of12–24 h at 32◦C treating primary clarifier effluent. However, Merkel et al.67

used fluorescent in situ hybridization (FISH) to observe that tripling the re-actor VSS did not triple the reaction rate because the active biomass did notincrease in the same proportion as the VSS.

The HRT of a reactor significantly influences the capital cost. For a giveninfluent composition, a higher OLR allows a shorter HRT and a smaller



reactor. For example, Ross et al.82 found the OLR of a completely mixedreactor could be increased from 4 to 12 kg COD/m3/d when a membranewas used. In general, the HRTs used with AnMBRs have been higher thanfor nonmembrane high-rate anaerobic reactors. AnMBR HRTs as low as 10 hhave been used for soybean-processing wastewater and sewage, but high-rate anaerobic reactors typically have HRTs of 4–8 h. The complete solidsretention possible in AnMBRs has not yet led to the expected decrease inHRTs.

C. Reactor Design and Membrane Location

By far the most common anaerobic membrane bioreactor design has beenthe CSTR. From Tables 1–5, approximately 67% of the membrane reactorshave been CSTRs, 15% anaerobic filters, 10% UASB or UASB hybrid, 7%fluidized bed, and 2% septic tank. In part, this may be due to the ease ofuse and construction of a CSTR reactor, but it is also likely due to the almostexclusive use of external cross-flow membranes. The high liquid turnoverrate required for external cross-flow membranes10,20 will create a well-mixedflow regime unless the membrane intake and return are specifically locatedso that well-mixed conditions are not created. For example, some studieshave been able to use UASB reactors while still utilizing external cross-flowmembranes.6,13,39

Although Fuchs et al.29 observed a shorter startup period with a CSTRin comparison to those typical of high-rate reactor configurations, the useof completely-mixed reactors is less attractive than a high-rate configurationfor three reasons. First, single-stage reactor configurations have lower CODremoval efficiency than UASB and multistage CSTR reactor configurationsregardless of substrate complexity.4 Second, the CSTR configuration exposesthe membrane to the reactor bulk mixed liquor suspended solids (MLSS)whereas reactor configurations designed to retain biomass (e.g. UASB) ex-pose the membrane only to the residual effluent TSS. For example, the efflu-ent from a UASB AnMBR was 300–550 mg TSS/L,13 while the MLSS of a CSTRAnMBR reactor is commonly >10,000 mg/L (see Tables 1–5). Exposure tohigher solids concentrations usually leads to lower fluxes, as discussed later.Finally, membrane use can reduce the capital cost of high-rate reactor de-signs, for example, by eliminating the need for a solids/liquid/gas separatorin a UASB.6

Two-phase membrane bioreactors have also been examined. In this con-figuration, the membrane may be placed after the second-phase methanereactor,49,109 after the first-phase acid reactor,19,30,56−58,61,107,109 or after bothreactors.2,49,56 The location of the membrane unit within a two-phase sys-tem can have a significant impact on performance. For a two-phase systemtreating soybean wastewater, Yushina and Hasegawa109 observed a CODremoval efficiency of 52% when no membranes were used, compared to



78% when a membrane was used after the methane-phase reactor and 92%when a membrane was used after the acid-phase reactor. The effluent sus-pended solids decreased from 361 mg/L with no membranes to 0 mg/Lwith a membrane after the methane phase and 4 mg/L with a membraneafter the acid-phase. The authors supported placing the membrane afterthe acid-phase reactor for wastewaters containing organic suspended solids.Membrane-coupled hydrolysis/acidification reactors had a 98% solids reten-tion and prevented biomass washout19 and increased biomass concentrationand COD removal.30 Membrane-coupled reactors for hydrolysis of suspendedsolids removed from raw municipal wastewater had relatively long HRTs of3–5 d49,56−58 and with the membrane the SRT would have been even longer(it was not reported in the studies). Such a design retains substances untilthey are converted into smaller products and pass through the membrane orare removed with wasted sludge.

The classic acid reactor is a CSTR with a short SRT (= HRT) operatedat low pH. Not all substrates are amenable to degradation in this classicdesign. For example, the degradation of long-chain fatty acids, aromatics,and some proteins is not thermodynamically favorable in normal acid-phasereactors because the syntrophic relationships needed to consume reducingequivalents have been eliminated.28 A membrane-coupled acid reactor, onthe other hand, can decouple the SRT from the HRT to allow the growth ofhydrogenotrophic methanogens at a short HRT and low pH. In this case, be-cause hydrogen consumption could occur to consume reducing equivalents,compounds may be acidified that otherwise would not.

V. MEMBRANE PERFORMANCE OF AnMBRs

A. Flux

The performance of a membrane is synonymous with flux. Membrane flux isone of the most important parameters that determine the economy of mem-brane bioreactors. A larger membrane flux allows for a smaller membranesurface area for a given hydraulic treatment capacity. However, there exists acritical flux for membrane filtration. Ideally, operation of the membrane be-low the critical flux allows a constant transmembrane pressure (TMP) with-out fouling, while operation above the critical flux causes a rapid increase inTMP.92 Nevertheless, perfect non-fouling operation cannot be expected, andoperation below the nominal critical flux has been observed to cause a slowlinear increase in TMP.13 The critical flux is a function of sludge characteris-tics and concentration and can be determined by three methods.13 The mostconvenient is to measure TMP against a step increase in flux. For example,by this method Cho and Fane13 found the critical flux of an AnMBR to be 30–50 L/m2/h using a cross-flow velocity of 0.93 m/s (Re = 5600). In addition,the critical flux will decrease over time as the membrane becomes fouled.



The characteristics and performance of the membranes used in AnMBRsare summarized in Tables 7 and 8. The reported membrane flux in the lit-erature was from 4 to 250 L/m2/h for external cross-flow membrane mod-ules, and from 3 to 80 L/m2/h for submerged membrane modules. The fluxfor external cross-flow membranes is higher than immersed membranes inaerobic systems as well.20 The typical design membrane flux for AnMBRsis 10–40 L/m2/h at a temperature of 20–50◦C.14,40,74,83,103 The design mem-brane flux is case specific, depending not only on membrane properties(composition, pore size, porosity, hydrophobicity, and surface charge) butalso on anaerobic sludge properties, and operational and environmentalconditions.

B. Membrane Pore Size and Materials

The membrane pore size or molecular weight cutoff has a significant effect onthe membrane flux. Larger pore size or higher molecular weight cutoff usuallyleads to increased flux. For example, Hernandez et al.39 found that the steady-state membrane flux was about 7 times higher for a pore size of 100 µm thanfor 10 µm using immersed membranes with a granular sludge. However,Imasaka et al.41 observed a slight decrease in flux with an increase in poresize from 0.2 µm to 0.57 µm when using external cross-flow membranesto filter a concentrated broth. This decrease occurred because the permeateresistance due to plugging increased more than the decrease in permeateresistance of the membrane from a larger pore size, thereby causing an overalldecrease in flux.41

Microfiltration and ultrafiltration membranes are the most common forMBRs. Microfiltration membranes generally have a pore size >0.05 µm, whileultrafiltration membranes have a pore size between 0.002 and 0.05 µm.68

Both classifications of membranes retain particulates, but ultrafiltration willretain more macromolecules and colloids. To minimize energy use and max-imize flux, the membrane with the largest pore size that will achieve therequired separation should be used.

The membrane material also plays an important role in determining per-formance. Ghyoot and Verstraete33 reported that the flux of a ceramic micro-filtration membrane reached 200–250 L/m2/h, which was 10-fold higher thanthe flux achieved with polymer ultrafiltration, with both membranes produc-ing permeate of similar quality. Decreasing the hydrophobicity of polypropy-lene membranes by graft polymerization with hydroxyethyl methacrylateincreased the long-term achievable flux.15,84,104 An interesting membranedesign used by Pillay et al.80 was a woven-fiber cross-flow microfiltrationmembrane that intentionally required the deposition of a fouling layer toimprove performance and filter particles smaller than the large pore size.Shimizu et al.88 found that negatively charged membranes had a higher fluxthan noncharged and positively charged membranes.


TAB

LE7

.M

embra

ne

Per

form

ance

ofA

nM

BRs

for

Synth

etic

,In

dust

rial

,Fo

od-P

roce

ssin

g,an

dH

igh

Solid

sConte

ntW

aste

wat

erTre

atm

enta

Typ

eof

was

tew

ater

Scal

ebTe

mp.

(◦C)

Mem

bra

ne

mat

eria

lPore

size

c

Mem

bra

ne

area

(m2)

Tra

ns

mem

bra

ne

pre

ssure

(kPa)

Mem

bra

ne

linea

rve

loci

ty(m

s−1)

Initi

alm

embra

ne

flux

(Lm

−2h

−1)

Final

mem

bra

ne

flux

(Lm

−2h

−1)

Ref

eren

ce

Whea

tst

arch

P40

—18

,000

D14

469

0—

d—

14–2

511

,14

Bre

wer

yef

fluen

tP

35Poly

-eth

ersu

lfone

40,0

00D

0.44

140–

340

1.5–

2.6

—7–

5093

Mai

zepro

cess

ing

P—

poly

-eth

ersu

lfone

20,0

00–8

0,00

0D

668

450

1.6

—8–

3783

Woolsc

ouring

P40

–47

poly

-acr

ylonitr

ile13

,000

D3.

12–

2.2e

—30

–45

17–2

540

Glu

cose

L—

Cer

amic

0.2–

0.8

µm

0.00

5514

–83

0.8–

1.7

110–

250

57–6

075

Glu

cose

,pep

tone

L35

–38

Cer

amic

0.2

µm

0.4

30–2

000.

5–4

—12

.5–1

2589

Kra

ftm

illef

fluen

tP

48.4

Cer

amic

,al

um

inum

oxi

de

0.16

µm

1×

2460

1.75

5027

42

Ace

tate

L35

Cer

amic

0.2

µm

0.20

25–1

500–

3.5

—18

–126

8Ace

tate

L35

Inorg

anic

com

posi

te,

zirc

onia

oxi

de

0.14

–0.2

µm

,0.

005–

0.08

µm

—50

——

40–7

023

Mola

sses

fL

20Poly

pro

pyl

ene

10µ

m0.

051

——

100–

160

10–8

039

Mola

sses

fL

20Fi

ber

glas

s10

0µ

m0.

056

——

100–

160

70–1

0039

—L

—H

ydro

phili

zed

PVD

F0.

22µ

m—

—0.

93—

30–5

013

Sew

age

sludge

L25

–50

—15

,000

–20,

000

D0.

0177

——

—5–

103

Sew

age

sludge

L30

–35

Poly

-eth

ersu

lfone

60,0

00D

0.3

375

0.75

3119

33Se

wag

esl

udge

L22

–50

Cer

amic

0.1

µm

0.05

200

4.5

—20

0–25

033

aA

llm

embra

nes

wer

eex

tern

alcr

oss

-flow

unle

ssoth

erw

ise

note

d.

bL

=la

bora

tory

/ben

chsc

ale,

P=

pilo

tsc

ale.

cD

=D

alto

ns

(mole

cula

rw

eigh

tcu

toff).

d—

Indic

ates

valu

enotre

ported

.ePre

ssure

reported

askg

/cm

2.

fSu

bm

erge

dm

embra

ne.

511


TAB

LE8

.M

embra

ne

Per

form

ance

ofA

nM

BRs

for

Munic

ipal

Was

tew

ater

Tre

atm

ent

Typ

eof

was

tew

ater

Scal

eaTe

mp.

(◦ C)

Mem

bra

ne

mat

eria

lPore

size

b

Mem

bra

ne

area

(m2)

Tra

ns

mem

bra

ne

pre

ssure

(kPa)

Mem

bra

ne

linea

rve

loci

ty(m

s−1)

Initi

alm

embra

ne

flux

(Lm

−2h

−1)

Final

mem

bra

ne

flux

(Lm

−2h

−1)

Ref

eren

ce

Dom

estic

was

tew

ater

L—

c—

—0.

0066

7—

0.78

–3.9

339

535

Sew

age

P10

–28

Cer

amic

13,0

00D

13.6

1-2e

2—

15–2

095

Hea

t-trea

ted

liquor

from

sew

age

sludge

f

P35

–38

Cer

amic

0.1

µm

1.06

200d

0.2–

0.3

8–13

3–8

50

Sew

age

P26

Poly

sulfone

and

poly

vinyl

alco

hol

15,0

00D

100

1.5e

0.7

—16

49,56

Sew

age

P26

Poly

ethyl

ene

0.1

µm

541.

1e1.

0—

2449

,56

Dom

estic

was

tew

ater

fL

12.5

–28

Poly

ethyl

ene

0.03

µm

0.3

10–6

0—

—5–

1010

5

Not

e.A

llm

embra

nes

wer

eex

tern

alcr

oss

-flow

unle

ssoth

erw

ise

note

d.

aL

=la

bora

tory

/ben

chsc

ale,

P=

pilo

tsc

ale.

bD

=D

alto

ns

(mole

cula

rw

eigh

tcu

toff).

c—

Indic

ates

valu

enotre

ported

.dU

nits

are

mm

Hg

inst

ead

ofkP

a.ePre

ssure

reported

askg

/cm

2.

fSu

bm

erge

dm

embra

ne.

512



Different membrane materials also lead to different fouling mechanisms.For example, inorganic membranes were found to be fouled primarily by stru-vite (MgNH4PO4·6H2O), whereas organic membranes were fouled by bothbiomass and struvite.47

C. Operational Pressure and Temperature

The function relating permeate flux to the transmembrane pressure (TMP)has two distinct zones. At low TMP the flux is proportional to pressure whileat high TMP the flux is independent of pressure,102 and this phenomenonhas been observed for AnMBRs.8,33 The transition point between the tworegimes could be called the critical pressure and has been reported to bebetween 80 and 260 kPa. Operation above the critical pressure is pointlessbecause of the flux independence and therefore Beaubien et al.8 suggestedlow-pressure operation.

The flux to TMP relationship is complicated by fouling. Using a higherTMP increases the fouling rate, even below the critical flux, and thereforeflux will decrease with time.8 Consequently, there is an optimal choice ofTMP for a given membrane and application that balances these competingfactors to give a maximum permeate flow.

Increasing the temperature also increases the membrane flux becausethe flux is inversely proportional to the fluid viscosity.92 For example, Rosset al.82 found that the flux increased by 2% for each 1◦C rise in temperature.Higher membrane fluxes have been observed for thermophilic operation(55◦C) compared to mesophilic (35◦C).40,82 Temperature is not a practicalcontrol variable like operating pressure, however, because it will usually bedictated by the influent wastewater temperature or the bioreactor operatingtemperature.

D. Hydrodynamics

The flow velocity parallel to the membrane surface can be controlled inpressure-driven, external membrane systems. A cross-flow velocity of 2–3m/s has been used to minimize cake formation on the membrane surface inexternal cross-flow AnMBRs.93,103 Generally, a higher flow velocity results in ahigher shear stress on membrane surfaces, which should reduce membranefouling. Many researchers have observed that an increase in feed velocityled to an increase in flux.8,89,93,103 In particular, Beaubien et al.8 observed asubstantial flux increase from 15 L/m2/h to 35 L/m2/h when the flow velocityincreased from 1.1 m/s to 2.2 m/s. On the other hand, Ghyoot and Verstraete33

observed only a minor flux increase because sludge activity can decreaseat high sludge flow velocities due to the pumping shear stress, as alreadydiscussed. In addition, the introduction of bubbles to create two-phase flow



inside cross-flow membranes can induce greater surface shear and reducefouling.21

The concept of flow velocity is not applicable to vacuum-driven, im-mersed membranes. The analogous concept, however, is the hydrodynamiccondition at the membrane surface. For example, air sparging is used to in-duce shear at the surface of submerged membranes in aerobic MBRs anddisrupt the formation of a cake layer.20,21,102 Such a method can be appliedto anaerobic MBRs with immersed membranes by using the produced biogasfor sparging99; however, there appear to be no reports that have tested thisapproach.

E. Mixed Liquor Suspended Solids

In general, an increase in the MLSS leads to a decrease in membraneflux.8,38,51,63,75,80,82,93 The principle reason is an increased opportunity forcake formation and fouling. This is consistent with observations from aerobicMBRs.64 However, some authors have observed that the flux may be con-stant up to some “critical” MLSS. For example, in batch studies Ross et al.82

observed that a constant membrane flux of 1000 L/m2/h was maintained forMLSS up to 40 g/L, after which the flux decreased rapidly to 400 L/m2/h atan MLSS of 60 g/L. Similarly, Strohwald and Ross93 observed a rapid declinein flux for MLSS concentrations >20 g/L.

The MLSS relationship to flux will be a function of reactor design. Reac-tors without biomass retention, such as the CSTR, expose the membrane tothe full bulk MLSS concentration. Increases in SRT to improve reactor per-formance will increase MLSS but lead to decreased membrane flux. This isthe design that has been examined in most AnMBR studies. In these sys-tems, there is likely an optimal MLSS that achieves maximum reaction rateswhile still retaining satisfactory membrane flux. However, retained biomassdesigns that separate the reactor MLSS from the membrane process shoulddecouple the MLSS to flux relationship. Once more studies with these systemshave been conducted, the general relationship between MLSS and flux thathas been reported may require modification. One possibility is that MLSS andflux may be specified independently to each provide maximum performance.

F. Comparison of Anaerobic and Aerobic MBRs

A comparison of the membrane performance between anaerobic and aerobicMBRs is presented in Table 9. In general, the membrane flux of pressure-driven (external) MBRs is larger than that of vacuum-driven (immersed) MBRsdue to the high liquid velocity in external cross-flow MBRs used to controlmembrane fouling. The membrane flux of AnMBRs has been lower than thatof aerobic MBRs at the same temperature because AnMBRs have a higher



TABLE 9. Comparison of Filtration Conditions Between External and Submerged Membranesfor AnMBRs and Aerobic MBRs

Anaerobic MBRa Aerobic MBRb

Parameter UnitsExternal

cross-felow SubmergedExternal

cross-flow Submerged

Design flux L m−2 h−1 10–40 15 50–100 20–50Applied pressure kPa 150–450 15–50 400 20–50Cross-flow velocity m s−1 2–5 n/ac 3–5 n/aEnergy for filtration kWh m−3 3–7.3 0.25–1.0 4–12 0.3–0.6Temperature ◦C 20–50 20–50 20–30 20–30

aBased on references 11, 14, 40, 42, 83, and 103.bBased on reference 20.cNot applicable.

MLSS, higher residual COD, and lack gas scouring. The increased operatingtemperature for many AnMBRs compared to aerobic MBRs only partiallycompensates.

The energy consumption of pressure-driven, external cross-flow MBRs issignificantly higher than that of the vacuum-driven, immersed MBRs.20,31,92,103

In addition, the energy consumption of aerobic and anaerobic membranesis comparable for each of the external cross-flow and submerged configura-tions. Therefore, the performance of aerobic MBRs can be used as an initialguide to estimate AnMBR performance. In terms of a net energy balance,however, the energy for filtration in anaerobic systems can be partially orwholly offset by the produced methane.

VI. MECHANISMS OF MEMBRANE FOULINGAND ITS CONTROL IN AnMBRs

Membrane fouling in AnMBRs is similar to that in aerobic MBRs, althoughanaerobic systems present certain unique challenges. Membrane fouling inanaerobic MBRs is composite fouling, including biofouling, organic and in-organic fouling. All three fouling mechanisms are usually observed simulta-neously, although the relative contribution of each mechanism depends onmembrane characteristics, sludge characteristics, environmental conditions,reactor design, and the operating strategy.13,16,17,47

A. Biofouling

Biofouling is the result of interactions between membrane surfaces and com-ponents of the biological treatment broth. Biofouling mechanisms can beclassified under three categories: pore clogging, sludge cake formation, andadsorption of extracellular polymeric substances (EPS).64



Cell debris and colloidal particles cause pore clogging. During the per-meation process, these particles with a size dimension comparable to thepore size will accumulate in pores and reduce the surface area for filtra-tion. Choo and Lee17 found that colloids, and not dissolved and cellularfractions, were the main foulant of both MF and UF membranes. Increasedfouling has been associated with the use of pressure-driven, external cross-flow filtration because pump-induced shear stresses decreased the aver-age particle size and liberated colloids that can lead to pore clogging.18,55

The improvement in membrane flux after backwashing15,19,38,50,56,61,108 pro-vides further evidence that pore clogging is involved in membrane foul-ing, and also provides a mechanism to mitigate flux losses due to poreclogging.

If the shear stress at the membrane surface is not adequate to removesolids, sludge cake formation occurs. Choo and Lee16 and Kang et al.47 founda thick cake layer composed of biomass and struvite formed on polymericmembrane surfaces causing major hydraulic resistance. A theoretical modelto predict flux decline was developed by Choo and Lee18 by considering thesolids transport mechanism based on hydrodynamic and surface interactions.The improvement of membrane flux by increased shear forces through theuse of gas circulation and gas–liquid two-phase flow in cross-flow mem-branes 21,42,50 provides evidence that sludge cake formation is involved inmembrane fouling. The extent of biofouling due to cake deposition will de-pend in part on the concentration of suspended material that is broughtinto the membrane. In the CSTR configuration with either pressure-drivenor vacuum-driven membranes, the high concentration of solids presentedto the membrane will exacerbate cake deposition. Retained biomass reactordesigns that do not present the full MLSS concentration to the membraneshould be less challenged by cake deposition.

The third mechanism of biofouling is caused by the accumulation andadsorption of extracellular polymeric substances (EPS) and soluble microbialproducts (SMP) on membrane and pore surfaces. Cho and Fane13 observedthat a lower membrane flux was associated with a larger quantity of EPS perunit membrane surface area and a strong link existed between EPS deposi-tion load and fouling resistance. Membrane autopsy also revealed significantfouling by EPS and an uneven distribution of EPS.

B. Organic and Inorganic Fouling

Organic fouling is due to the accumulation and adsorption of organic con-stituents on the membrane surfaces. Fouling due to EPS could also be con-sidered a subset of organic fouling, but was discussed earlier with biofoulingto emphasize the biological source of EPS and SMP.

The relatively higher effluent COD concentrations in AnMBR systemscompared to aerobic MBRs may increase the relative contribution of organic



fouling in AnMBRs. In general, higher OLRs will lead to higher residual CODsand lower membrane fluxes.39 Furthermore, it is the absolute residual CODand not the COD removal efficiency that affects fouling. For example, a reac-tor with only 70% COD removal with a feed COD of 0.6 g/L will have a lowerresidual COD and lower fouling propensity than a reactor with a 95% CODremoval and feed COD of 20 g/L. Therefore, operating at higher SRT mayhelp decrease organic fouling by decreasing the effluent COD concentration.Powdered activated carbon and zeolites have been added into AnMBRs toadsorb soluble organic compounds and thus reduce organic fouling and en-hance membrane flux,15,77 although this approach would likely be impracticalat full scale.

Inorganic fouling is caused by inorganic colloids and crystals on mem-brane and pore surfaces. Struvite (MgNH4PO4·6H2O) precipitation appearsto be the most common inorganic foulant,16,74,108 and it can occur on bothorganic and inorganic membranes.47 Other inorganic foulants can includeK2NH4PO4 and CaCO3.73,74 AnMBRs may be more susceptible to inorganicfouling than aerobic MBRs, in part because of greater opportunity for pHshifts due to carbon dioxide partial pressure changes and the productionof high ammonia and phosphate concentrations, especially during sludgedigestion.

C. Fouling Management

Because membrane fouling directly affects flux, fouling management hasreceived extensive attention by researchers.64 In general, managing foulingin AnMBRs can follow the general two-pronged approach used to managefouling in aerobic MBRs: (1) reducing the rate of fouling, and (2) cleaning afouled membrane.

Reducing the rate of fouling can prolong the length of time betweencleanings. The fouling rate can be reduced by operating a membrane be-low the critical flux and by maintaining a high shear across the membranesurface, either by velocity gradient or gas sparging. For example, Pierkieland Lanting79 used torsion shear to vibrate a polymeric Teflon membraneand only performed chemical cleaning every 30 d. Backwashing the mem-brane using permeate has been used extensively to disrupt the pore cloggingand cake formation components of fouling.15,19,38,50,56,61,75,108 Similarly, inter-mittent backwashing with air has also been used in an AnMBR,61 althoughbackwashing with biogas would be better so as to eliminate safety concerns,changes in redox potential, and toxicity to obligate anaerobic organisms.

Proper reactor design is crucial to reducing the fouling rate, especiallyfor AnMBRs. As already discussed, CSTR reactor designs will expose themembrane to higher MLSS concentrations, increasing the rate of pore clog-ging and cake formation compared to reactor designs that retain biomass.



The more intimate exposure of the membranes to biomass may also increasefouling due to EPS production associated with biomass. Operating the reactorat higher SRT to minimize the COD concentration exposed to the membraneshould decrease the rate of organic fouling.

Decreasing the rate of membrane fouling will not stop fouling, how-ever, and at some point the flux decline or TMP increase will be suffi-ciently great that the membrane must be cleaned. Because some fouling,especially due to adsorption of organic and inorganic constituents, is irre-versible, the membrane cannot be cleaned to its original state. Chemicalagents have been widely used for cleaning membranes in AnMBRs. For ex-ample, acidic cleaning (HCl, H2SO4) has been extensively used to removeinorganic foulants.15,61,83,108 Alkaline cleaning (NaOH) has been used to re-move biological fouling,61 while caustic hypochlorite83 and ozone aeration54

have been used to dissolve organic foulants.

VII. OPPORTUNITIES FOR ANAEROBIC MEMBRANE BIOREACTORS

Although AnMBRs have not received as much attention as their aerobic coun-terparts in the past decade, more research has been conducted than is sug-gested by recent reviews.92,100,102 As demonstrated by this review, AnMBRshave already been tested with a large variety of wastewaters and half of theresearch has been conducted at pilot or full scale. This information can allowpredictions of the conditions under which AnMBRs may be most successfullyused, as well as identify research needs to improve the technology.

A. Application to Different Wastewaters

The principle characteristics of wastewater can be conceptualized along twoaxes, one describing the concentration of the constituents, and the other de-scribing the particulate nature of the constituents (Figure 2). Sludges frommunicipal wastewater treatment plants, for example, have high concentra-tions of particulate constituents and fall in quadrant (b). Examples of wastew-aters in the other quadrants are included in Table 10. The information sum-marized in Figure 2 and Table 10 can be used to answer the question aboutAnMBR application.

Wastewaters containing high concentrations of soluble constituents[quadrant (a)] are currently treated effectively with a variety of high-rateanaerobic reactors designs, but especially UASB and expanded granularsludge bed (EGSB) reactors.59 These retained biomass processes already de-couple the SRT from the HRT, allowing for high organic loading rates andCOD removal efficiencies with a minimum of solids loss in the effluent. Be-cause most AnMBRs have been CSTR configurations with pressure-driven



FIGURE 2. Applicability of AnMBRs to different types of wastewaters. Quadrant descriptionsare given in Table 10.

external membranes, the OLR achievable by AnMBRs for these wastewatersis well below that already achieved. There appears to be minimal opportu-nity for AnMBRs to be applied to these wastewaters, although relatively littleresearch has been done combining membranes with retained biomass reac-tor configurations. The use of the membrane may, for example, allow higherhydraulic loadings for upgrades of existing or underdesigned systems.39 Cer-tainly, the membrane will produce effluent with a negligible suspended solidsconcentration, but with increased operating concerns and higher costs formembrane purchase and maintenance. Application of AnMBRs to quadrant(a) wastewaters would be necessary only if very low suspended solids con-centrations were required in the effluent.

In contrast to quadrant (a) wastewaters, high-strength particulatewastewaters (quadrant (b)) may be particularly well suited for treatment us-ing AnMBRs. For example, in anaerobic sludge digestion the SRT and HRTare not typically decoupled and the capital cost is high because a long re-tention time (large volume) is required for sufficient hydrolysis. Pierkiel and


TAB

LE1

0.

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nta

ges

ofM

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nes

for

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iffe

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aste

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ers

(a)

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(b)

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tren

gth

par

ticula

te

(c)

Low

-stren

gth

solu

ble

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Low

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gth

par

ticula

te

Was

tew

ater

char

acte

rist

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mple

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age

(prim

ary

effluen

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unic

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age

(raw

sew

age)

Tem

per

ature

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mW

arm

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cold

Cold

Cold

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stin

gte

chnolo

gyA

nae

robic

Anae

robic

Aer

obic

Aer

obic

Anae

robic

ally

trea

table

?Yes

Yes

Yes

Yes

Anae

robic

trea

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tA

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configu

ratio

n(s

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Yes

No

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BR

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tofa

mem

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uen

tTSS

rem

ova

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RT

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nIm

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impro

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lPoss

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RT

dec

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=an

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baf

fled

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aero

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=an

aero

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hyb

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(UASB

with

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liquid

/gas

separ

ator)

,CST

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com

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stirre

dta

nk

reac

tor,

EG

SB=

expan

ded

gran

ula

rsl

udge

bed

,FB

=fluid

ized

bed

,U

ASB

=upflow

anae

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sludge

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.

520



Lanting79 and Pillay et al.80 used a membrane to decouple the SRT and HRT.The present value cost of the membrane digester system of Pillay et al.80 wassignificantly lower than for the conventional digester design. Furthermore,complete retention of particulates may allow greater treatment efficiency byallowing more complete hydrolysis of slowly degraded compounds. On theother hand, Ghyoot and Verstraete33 found that the sludge digester OLR couldnot be increased even though the solids concentration was increased from22 g/L to 35 g/L with the addition of a membrane. This was attributed tothe shear stress during pumping through the external membrane, causinga decrease in microbial activity. These conflicting reports notwithstanding,there appears to be extensive opportunity to apply AnMBRs for quadrant(b) wastes to reduce reactor volumes and capital costs.

The remaining two wastewaters are low-strength soluble [quadrant(c)] and low-strength particulate [quadrant (d)]. Currently, these are treatedaerobically, except in warm climates.98 Preliminary research has beenconducted, however, assessing the feasibility of anaerobic municipal wastew-ater treatment in cold and temperate climates.48,62,87 Complete retention ofthe biomass is critical for low-temperature treatment because “little if anyviable biomass can be allowed to wash out from the reactor.”62 Membranesare ideally suited for anaerobic treatment at low temperature because ofthe extremely high solids retention capability. This means that higher hy-draulic loadings could be used without fear of biomass washout. For ex-ample, Ince et al.45 observed a factor of 50 increase in methanogens basedon most probable number (MPN) measurements in an AnMBR using an ex-ternal cross-flow membrane, indicating the membrane successfully retainedthese slow-growing organisms. Thus, AnMBRs could be well applied to thetreatment of low temperature municipal wastewater. Nevertheless, while op-portunity to treat these wastewaters with AnMBRs appears solid, the maincompetitive technology is the aerobic MBR. Baek and Pagilla5 found thatboth aerobic and anaerobic MBRs achieved similar soluble COD removalswhen treating primary clarifier effluent at the same HRT, so AnMBRs appearto be capable of similar treatment performance as aerobic MBRs. However,for dilute wastewater such as primary effluent there may not be any biogasproduction,5 though the cost of aeration will still be reduced or eliminated.

B. Research Needs

This review has uncovered knowledge gaps that are impeding the use ofAnMBRs as a viable wastewater treatment technology. To fill these gapsand allow greater utilization of AnMBRs, more research is needed to as-sess the feasibility of AnMBR treatment of each of the four wastewater types(Figure 2), assess in greater depth the use of immersed membranes, assessstrategies for membrane fouling control, assess in greater depth combining



membranes with retained biomass reactor designs, assess the impact of mem-branes on biological activity, and determine the conditions under whichAnMBR systems will be economically feasible.

There appears to be little reason to pursue research on AnMBRs fortreatment of high-strength soluble wastewaters except where membranescould prevent biomass washout under toxicity or overload events. Mem-brane sludge digesters, on the other hand, appear to have great potential,and research is needed to determine which membrane configuration (ex-ternal cross-flow or immersed) would be best suited to such high solidsconcentrations as well as to develop appropriate reactor designs that canminimize the solids concentration that contacts the membrane to minimizefouling. In particular, special consideration must be given to inorganic fouling(e.g., struvite) because of the high concentrations of ammonia and phosphatereleased during sludge digestion. For low-strength wastewaters such as mu-nicipal wastewater, there appears to be promise for AnMBRs, but more workis needed at low temperatures in addition to combining membranes withexisting high-rate reactor configurations already determined to be suitablefor dilute wastewaters (i.e., EGSB reactor62).

More engineering research needs to be directed toward the membranesthemselves. Determination of the conditions under which each membraneconfiguration (external cross-flow or immersed) and type of material (e.g. ce-ramic, polymer, etc.) is most appropriate would be beneficial for practical ap-plication. However, there have been a very limited number of AnMBR studiesthat have used the vacuum-driven immersed membrane technology. Giventheir significantly lower energy consumption in comparison to pressure-driven, external cross-flow membranes, immersed membranes have tremen-dous promise for anaerobic membrane bioreactors. Research is needed todetermine the optimal operation of immersed AnMBR configurations withrespect to TMP, backwashing frequency, and time between cleanings, withthe knowledge gained from immersed aerobic MBR operation acting as astarting point. In addition, the use of biogas sparging of immersed mem-branes should be investigated. This can include proper reactor design toutilize produced gas bubbles for natural scouring of membrane modules andthe augmentation of this with biogas sparging if the gas production is toolow.

An important area of future research is in reactor–membrane integration,in particular, the combination of membranes with retained biomass reactordesigns in order to decouple the active biomass concentration from the solidsconcentration applied to the membrane. In this way, the membrane does notconduct all of the solids/liquid separation and fouling should be reduced.Higher SRTs and consequently higher loading rates can be achieved withretained biomass reactor designs without necessarily increasing the solidsconcentration to the membrane. This may help with the low OLRs and HRTsseen in many AnMBR systems to date in comparison to existing high-rate



anaerobic reactors. Some interesting questions arise when retained biomasssystems are used. Can organic fouling be reduced by reducing the effluentCOD concentration at higher SRT (i.e., higher biomass concentration)? Can allbiological parameters be specified independently of the membrane process?Since aerobic systems do not employ biomass retention, and consequentlythe solids concentration applied to the membrane is equal to the biomassconcentration, can membrane fluxes be higher in retained biomass anaerobicMBRs than in aerobic MBRs?

Another important area that needs further research is the control of mem-brane fouling in AnMBRs. The membrane fouling rate and cleaning frequencyimpact the economy of the AnMBR processes. A starting point is to determinethe foulants under different environmental and operating conditions. Variousmicroscopy techniques, including transmission electron microscopy, scan-ning electron miscroscopy, confocal scanning laser microscopy, and Fouriertransform infrared analysis, can be used to identify the nature of foulants.Once the knowledge of foulants is obtained, strategies can be developed tocontrol membrane fouling. Manipulation of hydrodynamic conditions aroundmembrane surfaces and measurement of the physical and chemical propertiesof granular sludge in AnMBRs should be conducted. Development of novelmembrane materials, cleaning strategies, and agents, based on the nature offoulants, will benefit the control of membrane fouling in AnMBRs.

For those applications where external cross-flow membranes are de-termined to be more applicable than immersed membranes, the riddle ofpotentially lower biomass activity due to pumping shear stress needs to besolved. Biomass activity can be assessed using traditional activity assays inaddition to phylogenetic analytical techniques. 67,106 These latter techniqueswould also be useful for determining the extent to which the use of a mem-brane alters the microbial community structure and if this alters the treatmentperformance of the system.

For practical application, AnMBRs must be both technically and eco-nomically feasible. In addition to the technical feasibility research alreadysuggested, economic analyses are required. External cross-flow membraneshave higher fluxes20 and lower effluent concentrations103 but higher energyconsumption20 in comparison to immersed membranes, so life-cycle costingcould be used to determine which has the lower overall costs for a givenapplication. Economic analyses can be used to determine if reductions inaeration energy and methane production can offset electricity consumptionfor membrane filtration in addition to the capital and operating expenses forthe membrane and energy conversion technologies.

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