380 © IWA Publishing 2012 Water Science & Technology | 65.2 | 2012

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Specific energy consumption of membrane bioreactor

(MBR) for sewage treatment

Pawel Krzeminski, Jaap H. J. M. van der Graaf and Jules B. van Lier

ABSTRACT

This paper provides an overview of current electric energy consumption of full-scale municipal MBR

installations based on literature review and case studies. Energy requirements of several MBRs were

linked to operational parameters and reactor performance. Total and specific energy consumption

data were analysed on a long-term basis with special attention given to treated flow, design capacity,

membrane area and effluent quality. The specific energy consumption of an MBR system is

dependent on many factors, such as system design and layout, volume of treated flow, membrane

utilization and operational strategy. Operation at optimal flow conditions results in a low specific

energy consumption and energy efficient process. Energy consumption of membrane related

modules was in the range of 0.5–0.7 kWh/m3 and specific energy consumption for membrane

aeration in flat sheet (FS) was 33–37% higher than in a hollow fibre (HF) system. Aeration is a major

energy consumer, often exceeding 50% share of total energy consumption. In consequence, coarse

bubble aeration applied for continuous membrane cleaning remains the main target for energy

saving actions. Also, a certain potential for energy optimization without immediate danger of

affecting the quality of the produced effluent was observed.

doi: 10.2166/wst.2012.861

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er 2018

Pawel Krzeminski (corresponding author)Jules B. van LierDepartment of Water Management,Section Sanitary Engineering,Delft University of Technology,Stevinweg 1,PO Box 5048,2600 GA Delft,The NetherlandsE-mail: p.krzeminski@tudelft.nl

Jaap H. J. M. van der GraafWitteveenþ Bos,van Twickelostraat 2,PO Box 233,7400 AE Deventer,The Netherlands

Keywords | energy consumption, energy efficiency, full-scale, membrane bioreactor (MBR), operation,

performance

INTRODUCTION

A membrane bioreactor (MBR) combines biological waste-water treatment with a membrane separation step. MBR

technology is rapidly developing with an increasingnumber of applications and increasing capacity. At presentthe number of MBR installations exceeds 800 installations

in Europe alone. The MBR technology is now regarded asmature and various authors denominate MBR as the bestavailable technology for industrial but also municipal waste-water treatment (Kraume & Drews ; Lesjean et al. ).However, despite these developments, energy demand andrelated costs issues are, together with the membrane foulingissues, major drawbacks that restrict further expansion.

High aeration rates for frequent membrane cleaningremain a challenge in terms of energy consumption andoptimization of MBRs (Judd ; Verrecht et al. ).

To research the specific energy requirements of MBRsand elucidate where possible future energy consumptionreduction can be achieved, extensive research on thespecific energy consumption in several full-scale MBR

plants was performed. This paper provides an overview ofcurrent electric energy consumption of full-scale municipal

MBR installations based on literature review and four casestudies. Moreover, operational processes associated withaspects of energy are also investigated in this study.

Literature review

In the past 50 years, developments in MBR technologyresulted in an energy demand reduction from about

5.0 kWh/m3, needed for the first side-stream MBRs,to 1.0 kWh/m3 in 2001–2005 and very recently to about0.5 kWh/m3 for the present Zenon submerged MBRs (Buer &Cumin ). The energy requirement of the first tubular

side-stream MBR installations was reported to be typically6.0–8.0 kWh/m3 (Van Dijk & Roncken ), mainly due toenergy intensive cross-flow pumping of the liquid. The intro-

duction of the submerged membranes concept reduces thepumping energy requirement to 0.007 kWh/m3 of permeate

381 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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compared with values exceeding 3.0 kWh/m3 required for

the side-stream mode (Visvanathan et al. ). The sub-merged concept allows reduction of average powerconsumption to 2.0 kWh/m3 of treated water (Ueda et al.) compared with 3.0–4.0 kWh/m3 for a side-streamMBR.

In 2003, Cornel et al. () investigated the energy con-sumption of two full-scale municipal MBRs with andwithout a separate membrane tank. The one with mem-

branes submerged in the aeration tank consumed about1.0 kWh/m3 and the one with separate membrane tankabout 2.5 kWh/m3. In 2005, STOWA and Global Water

Research Coalition published the State of the ScienceReport (STOWA ) on MBRs for municipal wastewatertreatment in which energy consumption was reported to

be in the range of 1.5–2.5 kWh/m3. Also Krause ()reported the specific energy consumption of MBR plantsto be in the range of 0.8–2.2 kWh/m3. During the periodof 2001–2006 the energy consumption of European MBRs

was notably reduced from 2.0 to less than 1.0 kWh/m3,mainly due to membrane module development and optimiz-ations in process operation (Giesen et al. ). Other

authors (Van der Roest et al. ; Lesjean & Luck )also observed improvement in energy efficiency andreported the energy demand for full-scale municipal MBR

installations to be about 0.9–1.0 kWh/m3. Further improve-ment is possible, as the theoretical energy consumption fora municipal MBR with a separate membrane tank was esti-

mated to be 0.8 kWh/m3 (Krause & Cornel ).Information on energy demand of full-scale MBR plants

published in peer-reviewed journals is limited. However, aconsiderable number of references can be found in other

non-peer-reviewed publications. Typical energy demandvalues for MBR systems are reported to be in the range of0.8–1.4 kWh/m3, but a wide range of energy consumption

figures are reported in the literature (Lazarova et al. ).For example, the energy usage of seven German full-scalemunicipal MBRs was reported to be: 0.7, 0.8, 1.0, 1.0, 1.2,

1.6 and 1.8 kWh/m3 (Palmowski et al. ). A summaryof the energy requirements for various municipal MBRs isprovided in Table 1 while Figure 1 presents histograms sep-

arated on the basis of membrane configuration (Figure 1(a))and flow rate (Figure 1(b)).

Conventional activated sludge systems vs. membranebioreactors

The energy consumption of membrane bioreactors is often

compared with conventional activated sludge (CAS) waste-water treatment systems and is reported to be 30–50%

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(STOWA ; Lazarova et al. ), 75–90% (Van Bentem

et al. , ) or 10 to 100% superior to CAS energy con-sumption (Livingstone et al. ). The difference arises fromthe fact that the authors compared different MBR concepts

and CAS plants with specific design and operational charac-teristics. For example, Mizuta & Shimada () analysedelectric energy consumption at 985 Japanese municipalwastewater treatment plants (WWTPs) and reported con-

sumption of CAS system to be between 0.3 and 1.9 kWh/m3. Whereas the former value is beyond the potential of cur-rent MBRs, the latter one is easily achievable in most well-

operated full-scale MBRs. However, also, much lowerenergy consumption values for CAS systems are reported.The CAS energy demand, expressed per volume of treated

wastewater, widely ranges, being 0.1–0.2 kWh/m3 (Gnirss& Dittrich ), 0.2–0.3 kWh/m3 (Ueda et al. ),0.3 kWh/m3 (Yang et al. ), 0.4 kWh/m3 (Van Bentemet al. ), 0.5 kWh/m3 (Judd ), 0.4–0.6 kWh/m3

(Cornel et al. ) and 0.9–2.9 kWh/m3 for industrial appli-cations (Cummings & Frenkel ).

Due to intensive membrane aeration rates required to

manage membrane fouling and clogging, MBR energy con-sumption was three times higher even when comparedwith CAS systems combined with advanced treatment tech-

niques (Gnirss & Dittrich ). However, the gap wassignificantly reduced in recent years. Nowadays, the MBRenergy requirement is comparable with CAS with tertiary

treatment (Brepols et al. ), yet still 10–30% higher(Van Bentem et al. , ). It should be noted, however,that a fair comparison of MBR systems with CAS systems isonly possible when similar effluent quality is produced.

Meaning, a direct comparison between MBR and evenCAS with sand filtration is not appropriate.

Nevertheless, Krause & Dickerson () and Krause

et al. () clearly stated that operation of a full-scalemunicipal MBR, with a total energy demand at the samerange as a CAS process having an energy requirement of

0.5 kWh/m3, is possible provided a new mechanicalcleaning process (MCP) and optimized PLC programmingare used.

MATERIALS AND METHODS

MBR plant description

Four full-scale MBR installations treating mainly municipal

wastewater in The Netherlands were investigated andassessed. The selected MBRs include plants equipped with

Table 1 | Energy consumption of various municipal MBR installations

InstallationMembranetype

Capacity[P.E.]

Dry weatherflow [m3/d]

Rain weatherflow [m3/d]

Start ofoperation

Period ofanalysis

Energyconsumption[kWh/m3] Reference

Schwagalp (DE) FS/Hubert 780 100 156 2003 N.A. 1.40 (Judd )

Park Place (US) HF/Memcor N.A. 610 890 2003 N.A. 1.10 (Fatone et al. )

METU Ankara (TR) FS/Hubert 2,000 144 N.A. 2005 N.A. 1.0–2.0 (∼1.4) (Komesli & Gokcay )

Grasse Roumiguières (FR) HF/Zenon 24,000 6,250 N.A. 2007 N.A. 0.47–2.2 (Lazarova et al. )

Glessen (DE) HF/Zenon 9,000 2,000 6,500 2008 N.A. 0.90 (Brepols et al. )

Rodingen (DE) HF/Zenon 3,000 300 3,200 1999 2001 2.0–2.4 (Cornel et al. ; Brepols et al. )

Markranstadt (DE) HF/Zenon 12,000 2,700 4,320 2000 2001–2003 0.8–1.5(∼1.36)

(Giesen et al. ; Cornel & Krause; Pinnekamp )

Knautnaundorf (DE) FS/Hubert 900 113 432 2002 2002–2003 1.3–2.0 (Judd ; Giesen et al. ; Fatoneet al. )

Cauley Creek (US) HF/Zenon N.A. 9,464 18,930 2002 2003 1.59 (Pellegrin & Kinnear )

Brescia-Verziano (IT) HF/Zenon 46,000 12,000 42,500 2002 2003–2005 0.85 (Giesen et al. ; Fatone et al. ;Wallis-Lage and Levesque )

Monheim (DE) HF/Zenon 9,700 1,820 6,900 2003 2003–2005 1.00 (Giesen et al. )

Viareggio (IT) HF/Zenon 24,000 5,250 6,000 2005 2006 <0,60 (Fatone et al. )

Nordkanal-Kaarst (DE) HF/Zenon 80,000 16,000 45,000 2004 2004–2005 0.4–0.9 (∼0.9) (Judd ; Giesen et al. ; Fatoneet al. ; Wallis-Lage & Levesque; Brepols et al. ; Engelhard &Lindner ; Judd )

Seelscheid (DE) FS/Kubota 11,500 8,544 11,000 2004 2004–2005 0.9–1.7 (∼1.5) (Giesen et al. ; Pinnekamp ;Wallis-Lage & Levesque )

Pooler (US) HF/Zenon N.A. N.A. 11,400 2004 2005 1.74 (Pellegrin & Kinnear )

Schilde (BE) HF/Zenon 10,000 5,520 8,500 2004 2005–2006 0.62–0.64 (Wallis-Lage & Levesque ; Fenuet al. ; Garcés et al. )

Fowler (US) HF/Zenon N.A. N.A. 9,500 2004 2005–2007 4.23 (Pellegrin & Kinnear )

Varsseveld (NL) HF/Zenon 23,150 6,000 18,120 2005 2005–2009 0.75–1.0 (Giesen et al. ; Van Bentem et al.; Van Bentem et al. )

Westbury (UK) FS/Kubota 4,700 4,150 5,008 2002 2006–2007 1.98 (Ryan )

Dundee (US) FS/Kubota N.A. 2,990 5,700 2005 2006–2007 0.66–1.23 (Stone & Livingston )

Heenvliet (NL) FS/Toray 3,300 912 2,400 2006 2006–2009 0.7–1.2 (Mulder ; Mulder et al. ;Mulder et al. )

(continued)

382P.

Krzeminskiet

al. |Energy

consumption

offull-scale

mem

branebioreactors

(MBRs)

Water

Science

&Tech

nology

|65.2

|2012

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Table

1|co

ntinue

d

Installation

Membra

ne

type

Cap

acity

[P.E.]

Dry

weat

her

flow

[m3/d

]Rainweat

her

flow

[m3/d

]Sta

rtof

operation

Periodof

analys

is

Energ

yco

nsu

mption

[kW

h/m

3]

Refe

rence

Ulu

Pan

dan(SG)

HF/Z

enon

N.A.

23,000

23,000

2006

2007

0.54

–0.55

(Wallis-Lag

e&

Levesqu

e;Tao

etal.)

Delph

os(U

S)

FS/K

ubota

50,000

5,70

045

,500

2006

2007

–200

91.59

–1.95

(Livingstone)

Hea

ldsbrug(U

S)

HF/M

emco

rN.A.

6,05

715

,142

2004

2008

–200

91.82

(Pellegrin

&Kinnea

r)

LOTT(U

S)

HF/M

emco

rN.A.

N.A.

7,60

020

0620

08–2

009

1.61

(Pellegrin

&Kinnea

r)

Bon

itaSprings

(US)

HF/Z

enon

N.A.

15,250

N.A.

2007

2008

–201

01.43

(Pellegrin

&Kinnea

r)

Run

ningSprings

(US)

FS/K

ubota

5,00

02,30

04,50

020

0320

09–2

010

1.3–

3.0(∼

0.7)

(Jud

d)

Sab

adell-R

iuSec

(ES)

FS/K

ubota

200,00

035

,000

62,880

2008

2010

0.8–

1.0

(Jud

d)

San

taPau

la(U

S)

HF/K

och

42,500

12,900

27,000

2010

2010

1.16

(Koc

h)

Lege

nd:HF–ho

llow

fibre;

FS–flat

shee

t;N.A.–no

tav

ailable.

383 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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flat sheet (FS) and hollow fibre (HF) membranes submerged

in the separate filtration tank along with plant equippedwith side-stream externally placed tubular (MT) membranes.A description of the investigated plants is presented in

Table 2.Heenvliet, Varsseveld and Ootmarsum MBRs were

monitored in respect to energy consumption, operationand performance. However, for Ootmarsum WWTP, the

presented energy consumption values are for the entiretreatment plant, the MBR plus the CAS with sand filter.Due to lack of installed electricity measurement devices,

available energy data are limited to the total energy for theWWTP and energy for aeration purposes only. Hence, theextensive analysis of the Ootmarsum MBR system is not

feasible and only total plant consumption can be con-sidered. Therefore, Ootmarsum MBR will not be discussedin terms of energy consumption and for the comparisonstudies another tubular installation, namely MBR Terneu-

zen, will be used.

Data collection, processing and analysis

This analysis was performed based on the data collected bythe Waterboards at each location. The energy consumptiondata, reported as kWh, are based on the electric power con-sumed at each investigated location. The specific energy

consumption data are reported as specific electricity con-sumption per volume of treated wastewater and expressedas kWh/m3. Additionally, parallel to the energy consump-

tion study, plant performances were monitored andanalysed in respect of their potential indirect relation withenergy consumption. The performance of the MBR plants

was evaluated in environmental and economic termsbased on major performance indicators as proposed byBenedetti et al. () and Yang et al. ():

• effluent concentration of pollutants (mg/L),

• removal efficiencies of pollutants expressed as % of

incoming load, and

• energy consumption per volume of treated wastewater(kWh/m3).

RESULTS AND DISCUSSION

MBR performance

Good removal efficiencies of chemical oxygen demand(COD), biological oxygen demand (BOD) and Total Kjeldahl

Figure 1 | Energy consumption histograms on the basis of: (a) membrane configuration and (b) flow capacity.

Table 2 | Characteristics of MBRs

Location Heenvliet Varsseveld Ootmarsum Terneuzen

WWTP configuration CASþMBR MBR CASþMBR CASþMBR

Membrane configuration Submerged Submerged Sidestream Sidestream

Membrane location Separate filtration tank Separate filtration tank External External

Membrane type Flat sheet (FS) Hollow fibre (HF) Tubular (MT) Tubular (MT)

Membrane supplier Toray Zenon-GE Norit Norit

Total membrane area [m2] 4,115 20,160 2,436 13,860

Biological Capacity [P.E.] 3,333 23,150 7,000 15,500

Hydraulic capacity (DWF) [m3/h] 50 250–300 75 400

Hydraulic capacity (RWF) [m3/h] 100 755 150 620

Average Flux (DWF) [LMH] 12–24 15–25 26–40 25–40

SADm [Nm3/m2 h] 0.3 0.2 0.3 0.6

SADp [m3/m3] 12.9 12.3 6.0 13.3

Legend: DWF – dry weather flow; RWF – rain weather flow; P.E. – person equivalent; SADm – specific aeration demand per membrane area; SADp – specific aeration demand per permeate

volume.

384 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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Nitrogen (TKN) were achieved in all of the plants. All MBRsremoved COD to about 25 mg/L with removal efficiency

between 92 and 96%. BOD was removed far below the10 mg/L requirement with efficiencies of about 99%. BODconcentrations lower than 1.0 mg/L, in Ootmarsum and

Varsseveld, and 1.7 mg/L in Heenvliet were accomplished.Total Kjeldahl Nitrogen was removed with 96–98%efficiency, to concentrations of about 2.0 mg/L. The bestnitrogen removal was achieved in Heenvliet where the aver-

age value of total nitrogen (n-Total) was 3.0 mg/L for the year2008. In all cases, biological removal of phosphorus was lim-ited due to insufficient anaerobic conditions in the bioreactor

and/or low sludge loading levels. Phosphorus removal effi-ciency was in the range of 67–96% and 73–94% in 2008and 2009, respectively. Nonetheless, phosphorus removal

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of 94–96% reaching total phosphorus (P-Total) concen-trations of 0.4–0.7 mg/L was attained in MBR Varsseveld in

combination with the dosage of iron chloride sulfate. InMBRs of Heenvliet and Ootmarsum, chemicals were notadded, which resulted in phosphorus removal of 67–74%

and higher concentrations in the effluent, namely 1.7–2.2 mg/L. The summary of overall performance of the inves-tigated MBRs, in terms of pollutants removal efficiency, withminimal, average and maximal values, is presented in

Table 3.

Total and specific energy consumption

Detailed energy consumption data for three MBR installa-tions are summarized and presented in Table 4.

Table 3 | Influent and effluent characteristics and removal efficiency

Influent Effluent Removal efficiencyCOD BOD P-Total TKN COD BOD N-Total P-Total TKN COD BOD P-Total TKN

MBR Performance [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [%] [%] [%] [%]

Heenvliet (FS) 2008 Min 61 23 2 10 8 1.0 0.0 0.3 0.5 92 98.5 67 98Mean 366 156 7 44 25 1.7 3.0 2.2 1.0Max 665 300 12 71 94 44 13 7.6 3.0

2009 Min 41 19 1 9 6 1.0 0.7 0.2 0.3 93 99.0 73 97Mean 374 171 8 48 24 1.3 4.2 1.9 1.1Max 716 310 17 85 58 3.9 8.6 5.8 3.0

Varsseveld (HF) 2008 Min 400 140 6 32 16 0.5 1.4 0.1 0.8 96 99.7 96 96Mean 693 297 12 60 25 0.9 3.9 0.4 2.1Max 870 460 16 82 33 1.9 15 1.2 13

2009 Min 280 93 5 24 12 0.5 2.3 0.1 1.0 96 99.7 94 96Mean 752 306 13 59 25 0.8 5.8 0.7 1.8Max 1,250 700 35 81 36 1.6 20 3.2 3.8

Ootmarsum (MT) 2008 Min 199 58 3 13 51 0.5 1.4 0.1 0.6 95 99.5 72 97Mean 513 212 7 47 23 0.9 3.6 2.0 1.5Max 865 400 11 78 46 2.2 8.7 11 7.7

2009 Min 190 63 2 15 15 0.5 1.5 0.1 0.5 95 99.6 74 96Mean 514 222 7 45 24 0.8 3.6 1.7 1.7Max 1,030 390 13 70 39 2.1 8.8 5 7.3

Table 4 | Summary of energy consumption data of investigated MBR installations

Location Heenvliet MBR Varsseveld MBR Terneuzen MBR

Period of study 2008–2010 2005–2010 2010

Design dry weather flow [m3/month] 36,000 180,000 288,000

Treated flow [m3/month] 27,826 132,054 169,984

Monthly power requirement [kWh]

Max 33,869 146,051 166,332

Average 22,700 110,486 154,636

Min 14,165 58,408 146,581

Daily power requirement [kWh] 1,788 N.A. 5,888

Yearly power requirement [kWh] 227,001 1,325,833 N.A.

Specific energy consumption [kWh/m3]

Max 1.82 1.44 1.28

Average 1.06 0.84 0.97

Min 0.77 0.60 0.76

Specific energy consumption in 2008 [kWh/PEremoved] 89 67 N.A.

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The specific energy consumption for each MBR ana-

lysed on a long-term scale is presented in Figure 2.The specific energy consumption of the Heenvliet MBR

varied between 0.8 and 1.8 and was on average 1.1 kWh/m3

(Figure 2(a)). For the total plant, thus for combined MBR

and CAS systems at Heenvliet, the specific energy

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consumption ranged between 0.3 and 1.1 and was on aver-

age 0.6 kWh/m3.The specific energy consumption of the Varsseveld

MBR, presented in Figure 2(b), varied between 0.6 and 1.4and was on average 0.8 kWh/m3. The total energy consump-

tion was reduced from the initial value 1.1 kWh/m3 after the

Figure 2 | Specific energy consumption per volume of treated wastewater for:

(a) Heenvliet (FS), (b) Varsseveld (HF), (c) Terneuzen (MT) MBRs.

386 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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start-up of the plant to 0.8 kWh/m3 after 6 months of oper-ation (Giesen et al. ). For the next 3 years, energy

consumption was slowly but steadily reduced (VanBentem et al. ). After 5 years of operational experience,further energy reduction is expected with a goal to reach

0.7 kWh/m3 during normal MBR operation (Van Bentemet al. ).

Figure 2(c) shows the energy results, based on dailyvalues, from the first operational period of the Terneuzen

MBR. The energy consumption of the not yet optimized

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installation varied between 0.8 and 1.3 with an average con-

sumption of 0.97 kWh/m3. It is important to stress thatmajor problems are usually visible during the plant start-up but also with long-term experience. Hence, comparison

between Terneuzen MBR and other MBRs already operatedfor many years should be done carefully. Nevertheless, it isexpected that after start-up and the optimization period,energy consumption will be reduced to the design values

of 0.5–0.6 kWh/m3 (Mulder ). Typical specific energyconsumption values for a tubular airlift MBR are reportedto be in the range of 0.4–1.0 kWh/m3 (Judd ; Van ’t

Oever ; Helble & Mobius ). In 2008, specificenergy consumption of only ultrafiltration installation, i.e.sludge circulation, membrane aeration, permeate and back-

wash pumps, in Ootmarsum was reported to be lower than0.4 kWh/m3 (Borgerink & Schonewille ) and in 2009in the range of 0.2–0.3 kWh/m3 (Futselaar et al. ).This is lower than the currently achieved 0.7–0.8 kWh/m3

in Terneuzen. Detailed distribution of energy consumptioncomponents for each MBR is presented in Figure 3.

The specific energy consumption of Heenvliet MBR

increased from 2008 to 2010 (Figure 3(a)). This can beexplained twofold. Firstly, the volume of treated flow inthe MBR decreased leading to higher specific energy con-

sumption. Secondly, the 2008 data do not include datafrom January to April, a period when heating WWTP build-ings and offices is significantly contributing to higher

specific energy consumption.Figure 3(b) shows an increase in the specific energy con-

sumption in the year 2008, very likely due to maintenanceworks that were performed in the membrane tanks. At

that time, in order to prevent membrane fouling or clogging,process settings for the MBR operation were much moreconservative, i.e. higher aeration rates and increased recircu-

lation. In 2009 the settings were optimized again, resultingin lower energy consumption.

In the case of WWTPs connected to the combined sewer

system, such as Varsseveld, the specific energy consumptionstrongly depends on the weather conditions and amounts oftreated flow. The volume of treated flow in the MBR Varsse-

veld was 10 and 15% lower in 2008 and 2009, respectively,compared with the previous years. Hence, as Varsseveldexperienced very dry months, the energy consumption perm3 was higher, leading to a high yearly average in 2009

and 2010. However, compared with other dry months inthe past, the plant was actually performing much better interms of total energy consumption. The energy consumption

of the blowers producing air for membrane scouring wasbased on cyclic aeration: 15 s on and 15 s off.

Figure 3 | Specific energy consumption distribution of equipment for: (a) Heenvliet (FS),

(b) Varsseveld (HF), (c) Terneuzen (MT) MBRs.

Figure 4 | Energy consumption distribution of MBR equipment for: (a) Heenvliet (FS), (b)

Varsseveld (HF), (c) Terneuzen (MT) MBRs.

387 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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Implementation of more economical aeration strategy, so

called ‘eco-aeration’, developed by the membrane supplier(Zenon-GE) and based on 10/30 intervals, could potentiallysave 50% of currently consumed energy (Buer & Cumin

). The total specific energy consumption of the MBRcould then be about 0.7 kWh/m3 and as such reach the setgoals (Van Bentem et al. ; Van Bentem et al. ).Nevertheless, as reported by Judd (), certain over aera-

tion might be beneficial from an operational point of view,

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as it reduces the chances of unscheduled manual interven-

tion at the plant caused by fouling or clogging.

Specific energy components

Figure 4(a) shows the percentage distribution of the energyconsumption in the full-scale flat sheet MBR in Heenvliet.

Aeration is the major component of energy consumptionas the blowers providing air for the membrane scouring

388 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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and the biological process contribute to nearly 70% of the

total energy demand. The coarse bubble aeration is the lar-gest consumer being 56% and 0.48 kWh/m3; processaeration energy demand is 11%; mixers and recirculation

pumps consumed 9 and 6%, respectively. The rest, 17%, ismainly associated with the pumping, i.e. recirculation,permeate extraction and sludge discharge, the pre-treatment,the mixers and the heaters during winter months.

Figure 4(b) shows the percentage distribution of theenergy consumption in the full-scale hollow fibre MBRin Varsseveld. The results show that blowers providing

air for the membrane scouring and the biological processcontribute to more than 50% of the total energy demand.The coarse bubble aeration is the largest consumer, being

36% and 0.3 kWh/m3; process aeration energy demand is17%; permeate and feed pumps consumed 15 and 11%,respectively. Energy consumption related with the mem-brane operation, i.e. membrane air scouring, feed and

permeate pumps, required about 0.5–0.6 kWh/m3 of trea-ted wastewater. The rest (16%) represents energyconsumed by the other installed equipment. The three

main contributors are: the pump for internal recirculationfrom the oxic to the anoxic zone, about 0.03 kWh/m3; themixers in the anoxic tank, about 0.025 kWh/m3; and the

recirculation pump that pumps sludge from the oxiczone to the fine screens, about 0.02 kWh/m3. Other indi-vidual components, with energy usage less than

0.01 kWh/m3, are the chemical dosing pumps, the wastesludge pumps, the gravity thickener, the thickened

Figure 5 | Specific energy consumption as a function of treated wastewater.

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sludge pumps, the process water pumps and the heating

of the buildings (Van Bentem ).Figure 4(c) shows the percentage distribution of the

energy consumption in the full-scale tubular MBR in Ter-

neuzen. Membrane aeration, doubled at the time due to aclogging problem of the aerators, is responsible for con-sumption of 35% of total energy. The airlift system, i.e.feed and permeate pumps, contributes to 46% of total

energy consumption, mainly due to the high recirculationrate of activated sludge. The rest, 11%, is representingother smaller contributors such as: waste sludge pump,

iron-chloride dosing pump, online measurements, lightsand computers at offices.

Flow dependency

Operation at optimal flow conditions, i.e. close to design

flow at dry weather conditions (DWF), results in low specificenergy consumption of about 0.7–0.8 kWh/m3 (Figure 5).Under these high utilization conditions, reduction in

energy consumption was, depending on the plant, between5 and 20% compared with the average energy consumption.This is due to the fact that required membrane aeration rates

are not proportional to the volumes of the treated flow. Thisphenomenon is also partially explained by operation of theprocess equipment, e.g. pumps and blowers, at or neartheir best efficient points when the flow increases. Although

total energy consumption increased as the flow increases, animprovement in energy efficiency was observed with

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increase in the volume of treated wastewater. Contrarily,

sub-optimal operation below the design flow leads tohigher specific energy consumption values.

Plant capacity and membrane area

Figure 6 presents specific energy consumption as a functionof plant capacity for Dutch and German municipal MBR

plants (adopted from Pinnekamp ). Although, the smal-lest installations are the least energy efficient, the biggest arenot the most efficient ones either. Hence, the capacity of the

plant does not determine the energy efficiency of the instal-lation. Furthermore, all of the compared MBRs were moreenergy demanding than the average CAS treatment plantin The Netherlands, represented by the benchmark value.

General improvement in the range of 11–19% in energy effi-ciency for Dutch MBRs was observed during the 2008–2009period.

The specific energy consumption per area of the mem-branes installed was lower for hollow fibre installation(Figure 7). The observed improvement for Heenvliet MBR

is a logical consequence, also reported in the literature byJudd (), of an operational concept change from serial toparallel where only a small fraction, i.e. 25%, of the influent

is treated in the MBR. As a result, since March 2009, twomembrane lines were operated alternately to increase mem-brane utilization and to reduce energy demand formembrane air-scouring. Obviously, the operational power

demand increases with the amount of membranes installedin a submerged system. However, when energy usage is

Figure 6 | Energy consumption as a function of plant design capacity (Pinnekamp 2008).

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normalized for the membrane area, their specific energy con-

sumption decreases. Thus, big MBR installations are moreenergy efficient, in terms of membrane surface specificenergy consumption (in kWh/m2), compared with the small

ones. Additionally, operation of side-stream membranes isthe most energy demanding. However, because side-streamsystems can apply higher fluxes, it needs less membranesthan submerged systems and thus requires lower capital

costs. When results are compared for similar capacity,side-stream systems require 60–70% less membranes.

Effluent quality

Analysis was performed based on the Heenvliet MBR due tothe availability of a large energy and effluent data set. No

direct relation between total and specific energy consump-tion and concentration of total suspended solids (TSS),COD, BOD, P-Total, N-Total and TKN in the effluent was

observed. Also when accounting for the specific energyrequirements for process and membrane aeration rates noclear dependency on effluent quality could be determined.

Hence, certain potential energy savings will not have adirect impact on effluent quality. This observation is inagreement with Verrecht et al. () who reported a

reduction in energy consumption in a small-scale decentra-lized MBR by 23% without compromising effluent quality,represented by COD and NO3-N data. However, for amore accurate assessment of the potential energy reduction

in a full-scale MBR, more specific measurements anddetailed analysis is required. It was also observed that

Figure 7 | Energy consumption as a function of installed membrane area.

390 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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effluent concentrations of analysed parameters, i.e. COD,

BOD and TKN, were not dependent on the influent concen-trations. Only the effluent P-Total was slightly affected bythe influent concentration.

CONCLUSIONS

Specific energy requirements of several MBRs were linkedto operational parameters and reactor performance. Based

on the results presented in this paper, the following con-clusions can be made:

• The municipal MBRs are well operated, with good per-formance, without major problems and, despite oftensub-optimal operation, consume on average 0.8–

1.1 kWh/m3; values similar to other comparableinstallations.

• Investigated full-scale MBRs have a potential for further

improvement in energy efficiency.

• Operation at optimal flow conditions, i.e. close to designflow at dry weather conditions (DWF), results in a low

specific energy consumption of about 0.7 kWh/m3. Alsoincrease in the applied flux results in low energyconsumption.

• The specific energy consumption of an MBR system is

dependent on many factors, such as system design andlayout, volume of treated flow, membrane utilizationand operational strategy.

• Lack of clear correlation between total and specificenergy consumption and TSS, COD, BOD, N-Total and

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TKN concentrations in the effluent indicate a potential

for energy optimization studies without immediatedanger of affecting the quality of the produced effluent.

• Aeration is a major energy consumer, often exceeding50% share of total energy consumption, with a minimum

of 35% for membrane aeration. In consequence, coarsebubble aeration applied for continuous membrane clean-ing remains the main target for energy saving actions.

• Specific energy consumption for membrane aeration inflat sheet MBR was 33–37% higher than in hollow fibresystem whereas total specific energy consumption differs

only 0.2 kWh/m3.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the MBR2þ projectconsortium: Evides, WitteveenþBos and Hollandse Delta

Water Board for their financial support of this research.The authors also like to thank Hollandse Delta, Regge&Din-kel and Rijn&Ijssel Water Boards for their support and

cooperation within the work described in this article. Theauthors also want to thank the European Commission forits financial support through the MBR-Train project. MBR-Train is a Marie Curie Host Fellowship for Early Stage

Research Training supported by the European Commissionunder the 6th Framework Programme (Structuring the Euro-pean Research Area – Marie Curie Actions). The authors

would like to thank Wilfred Langhorst (Heenvliet); NielsNijman, Philip Schyns and André van Bentem (Varsseveld);

391 P. Krzeminski et al. | Energy consumption of full-scale membrane bioreactors (MBRs) Water Science & Technology | 65.2 | 2012

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Jeroen Buitenweg and Rob Borgerink (Ootmarsum); Han

van den Griek and Jan Willem Mulder (Terneuzen) who col-laborated in this research.

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First received 14 July 2011; accepted in revised form 9 September 2011