Early in the life cycle of a wastewatertreatment project, there will be facilitymanagers, operators, and designers

who select a treatment process alternativeamong competing feasible technologies. Theprocess selection decision process balancestrade-offs, such as capital and operating costs,with multiple required performance factors,such as treatment effectiveness to meet efflu-ent permit limits, ease of operation, etc.Proven technologies, such as conventional ac-tivated sludge (CAS) treatment, offer decadesof operating data on which to base life cyclecost estimates, providing confidence in them.Newer technologies, such as membranebioreactors (MBRs), lack the depth of avail-able capital and operation cost data on whichto base project lifecycle costs.

The technology of MBRs has evolvedover the last decade to include improvementsintended to reduce energy consumption.Published energy consumption values showconsiderable variability. Attempts to recon-cile energy consumption discrepancies in op-erating MBR facilities are discussed andshould provide decision makers with in-

creased confidence in operating cost esti-mates for this technology in order to developaccurate life cycle costs.

Objective and Method

Energy consumption of operating MBRwastewater treatment plants (WWTPs), mostlyin North America, were summarized and eval-uated. Data were gathered from the literatureand directly from WWTP records to compareunit energy consumption in kilowatt hours permillion gallons (kWh/MG) of wastewatertreated, and was compared on a plant-by-plantbasis. The literature reports MBR energy con-sumption ranges from 1,170 to 20,300kWh/MG. This represents a factor of twentyand demonstrates the difficulty in developingaccurate estimates (Gellner and Riddell, 2008;Hribljan and Reardon, 2007; Pawloski et al.,2007; Stone and Livingstone, 2008; van Bentemet al., 2007; Williams et al., 2008). Variabilityin system size, treatment process units used be-sides MBR, design variations, and individualoperation techniques accounts for this widerange of unit energy consumption.

For comparison, the National Associa-tion of Clean Water Agencies (NACWA) re-ported an average consumption of 1,766kWh/MG for wastewater treatment facilitiesin its 2008 financial survey summary. Thesurvey included 101 agencies representingover 67 million people, primarily from sec-ondary treatment CAS facilities, and consid-ered in-plant energy consumption only.Figure 1 shows the distribution of energyconsumption for 54 of the surveyed WWTPs.The data show that 90 percent of the sur-veyed facilities operate at energy levels of3,000 kWh/MG or less and 50 percent oper-ate at less than 1,500 kWh/MG.

This analysis included, when available, abreakdown of energy consumption rates forindividual unit operations and unitprocesses. Normalized unit energy consump-tion was analyzed to evaluate MBR energydemands as a function of flow rate. The ef-fect of flow rate on MBR energy consump-tion depends on the number of membranestanks in service and the scour air require-ments. Matching flow rate to membranetanks in service can become challengingwhen pronounced diurnal and seasonal vari-ation occurs.

Results and Conclusions

The nine plants that were investigatedused membranes from three vendors:GE/Zenon, Siemens/Memcor and Enviro-quip/Kubota. The plants investigated usingpublished literature data included Fowler,Pooler, and Cauley Creek in Georgia; Dundeein Michigan; and Varsseveld in the Nether-lands. Data investigated directly from the fa-cility operating data included Healdsburg in

Membrane Bioreactor Energy Consumption: Helping Utilities

Understand And Manage Cost SavingsMarie-Laure Pellegrin and David J. Kinnear

Marie-Laure Pellegrin, Ph.D., is membranebioreactor practice leader, and David J.Kinnear, Ph.D., P.E., is east region processleader at HDR in Tampa.

F W R J

Figure 1. Distribution of Unit Energy Consumption (kWh) Per Million Gallons Treated Source: 2008 NACWA Financial Survey (54 Agencies Reporting Data) Continued on page 82

80 May 2012 • Florida Water Resources Journal

California; Delphos in Ohio; Lacey, Olympia,Turnwater, and Thurston (LOTT) in Wash-ington; and Bonita Springs in Florida.

Fowler: Georgia MBR Wastewater Treatment Plant (Cooper et al., 2006;Williams et al., 2008)

The Fowler plant has been in operationsince 2004, with a rated treatment capacity of2.5 million gallons per day (mgd), averagedaily flow (ADF), and serves southernForsyth County, located approximately 20miles north of Atlanta. Effluent is reused forirrigation and disposal at dedicated drip irri-gation fields. GE/Zenon provided the mem-branes for this facility. The plant consists of aheadworks with fine screens, anoxic reactors,aerobic reactors, membrane tanks, ultravio-let (UV) disinfection, side stream screens,aerobic digesters, and a biosolids dewateringsystem (see Figure 2).

Dundee: Michigan MBR Wastewater Treat-ment Plant (Stone and Livingston, 2008)

The Dundee plant has been in operationsince 2005, with a rated treatment capacity of1.5 mgd, ADF, and 3.0 mgd sustained peakday flow (PDF). Enviroquip/Kubota providedthe membranes for this facility. The plantconsists of a headworks with fine screens,anoxic reactors, aerobic reactors, membranetanks, aerobic digesters, and membranethickeners (see Figure 3).

Pooler: Georgia MBR Wastewater Treatment Plant (Pawloski et al., 2007)

The Pooler plant has been in operationsince 2004, with a rated treatment capacity of3.0 mgd, ADF, and 4 mgd PDF, and alsoserves the city of Bloomingdale. Effluent isdischarged to Hardin Canal, a tributary to theOgeechee River. GE/Zenon provided themembranes for this facility. The plant con-sists of an equalization (EQ) tank, and aheadworks with coarse and fine screens,anoxic reactors, aerobic reactors, membranetanks, and UV disinfection (see Figure 4).

Varsseveld: The Netherlands MBR Wastewater Treatment Plant (van Bentemet al., 2007)

The Varsseveld plant has been in opera-tion since 2004, with a rated treatment capac-ity of 1.3 mgd, ADF, and 4.8 PDF, and was usedas a research tool to advance MBR technologyin Europe. GE/Zenon provided the mem-branes for this facility. The plant consists of aheadworks with coarse and fine screens,anoxic zones and aerobic zones in a circuit sys-tem, and membrane tanks (see Figure 5).

Figure 2. Fowler MBR Process Schematic

Figure 3. Dundee MBR Process Schematic

Figure 4. Pooler MBR Process Schematic

Figure 5. Varsseveld MBR Process Schematic

Continued from page 80

82 May 2012 • Florida Water Resources Journal

Continued on page 84

Figure 6. Cauley Creek MBR Process Schematic

Cauley Creek: Georgia MBR WastewaterTreatment Plant (Williams et al., 2008)

The Cauley Creek scalping plant has beenin operation since 2002, with a rated treatmentcapacity of 5.0 mgd, ADF. Effluent is reused for

the reuse distribution system or is dischargeddirectly to Cauley Creek. GE/Zenon providedthe membranes for this facility. The plant con-sists of a headworks with fine screens, five-stageBardenpho biological treatment, membranetanks, UV disinfection, solids dewatering usingcentrifuges, and odor control (see Figure 6).

Healdsburg: California MBR WastewaterTreatment Plant

The Healdsburg plant has been in opera-tion since 2004, with a rated treatment capacityof 1.6 mgd, ADF, and 4.0 mgd sustained PDF.Siemens/Memcor provided the membranes forthis facility. The plant consists of a headworkswith coarse and fine screens, pre anoxic reac-tors, anoxic reactors, aerobic reactors, mem-brane tanks, UV disinfection, the Cannibalsludge management process, and centrifuge forbiosolids dewatering (see Figure 7).

Delphos: Ohio MBR WastewaterTreatment Plant

The Delphos plant has been in operationsince 2006, with a rated treatment capacity of3.8 mgd, ADF, and 12.0 mgd PDF. Enviro-quip/Kubota provided the membranes for thisfacility. The plant consists of a headworks withfine screens, anoxic reactors, aerobic reactors,membrane tanks, UV disinfection, post aera-tion, autothermal thermophilic aerobic diges-tion (ATAD), and belt filter presses forbiosolids dewatering (see Figure 8).

LOTT: Washington MBR Wastewater Treatment Plant

The LOTT Martin Way scalping plant hasbeen in operation since 2006, with a ratedtreatment capacity of 2.0 mgd, ADF.Siemens/Memcor provided the membranesfor this facility. The plant consists of a head-works with fine screens, anoxic reactors, aero-bic reactors, membrane tanks, and chlorinecontact chamber (see Figure 9).

Bonita Springs: Florida MBR WastewaterTreatment Plant

The Bonita Springs scalping plant hasbeen in operation since 2007, with a ratedtreatment capacity of 4.1 mgd, ADF. GE/Zenonprovided the membranes for this facility. Theplant consists of a headworks with fine screens,EQ tank, anoxic reactors, aerobic reactors,membrane tanks, chlorine contact chamber,rotary drum thickener, centrifuge, and dryerfor biosolids management (see Figure 10).

Energy Consumption Summary Results

The process aeration and air scour blow-ers consumed the most energy at all the facil-ities evaluated (Figures 11-14 summarize theenergy consumption at four of the facilitiesevaluated). Based on energy consumption ob-servations at MBR facilities, recent research ef-forts have focused on reducing air scourenergy requirements without increasing mem-brane fouling.

Figure 8. Delphos MBR Process Schematic

Figure 9. LOTT MBR Process Schematic

Figure 10. Bonita Springs MBR Process Schematic

Figure 7. Healdsburg MBR Process Schematic

Continued from page 82

84 May 2012 • Florida Water Resources Journal

Unit flow energy consumption for the nineplants evaluated decreased as the flow increases.Reported unit energy consumption at these fa-cilities is shown in Figure 15. As additionalmembrane trains come into service, and as flowincreases closer to the design flow rates, the en-ergy consumption decreases. This phenomenonis also due to recent energy reduction improve-ments that were made by MBR manufacturersthat include the 10/10 or 10/30 aeration strategyby GE/Zenon, the change in air scouring flowrate based on flux from Enviroquip/Kubota, andthe Mempulse strategy from Siemens/Memcor.

The MBR energy consumption based onFigure 15 ranged from 2,500 kWh/MG to25,000 kWh/MG for MBR WWTPs operatingbetween 1 and 5 mgd. Published operating datafrom NACWA presented earlier in Figure 1 in-dicates that 85 percent of the 54 CAS WWTPsconsume 2,500 kWh/MG or less. AlthoughMBRs still consume more power than CAStreatment systems, recent design improvements

have decreased unit energy consumption rates.When considering sustainability, decision mak-ers must consider the benefit of the superior ef-fluent quality compared to increased energyconsumption and carbon dioxide emissions.

Figure 16 presents cumulative energy con-sumed in kWh as a function of cumulative flowrate treated for eight of the facilities evaluated.The plant with the highest overall energy con-sumption is Fowler, which consumed 16,000kWh/MG on average, based on power cost datafrom January 2005 to July 2007. As previouslyshown in Figure 11 for the Fowler plant, 34 per-cent of the overall energy consumption is due tomembrane processes and 20 percent is due toactivated sludge process. Based on Williams etal. (2008), the Fowler plant has not yet imple-mented the newest air scouring strategy. The fa-cility is using the 10/10 Zenon strategy and isoperating at low flux of 5.8 gallons per squarefoot of membrane surface area per day (gfd).Additional reductions in energy consumption

may be achieved by implementing the Zenon10/30 strategy and increasing operating flux ifmembrane performance would allow.

Of the remaining plants evaluated, theDelphos plant reported an average unit en-ergy consumption of 9,900 kWh/MG fol-lowed by Healdsburg, with 6,900 kWh/MG,using power cost data from December 2007 toSeptember 2009 for Delphos and May 2008 toAugust 2009 for Healdsburg. There was no re-ported individual power consumption dataavailable from the membrane process itself.Both of these plants operate solids streamprocess that increase overall power consump-tion rates (i.e., ATAD for Delphos and Canni-bal for Healdsburg). Additionally, both plantsoperate at low fluxes (10 gfd for Delphos and8 gfd for Healdsburg). Additional reductionon power cost could be achieved by increas-ing operating flux if membrane performanceallows.

Figure 11. Fowler MBR Unit Energy Consumption by Unit Operation and Process (Williams et al., 2008)

Figure 12. Dundee MBR Unit Energy Consumption by UnitOperation and Process (Stone and Livingstone, 2008)

Figure 13. Pooler MBR Unit Energy Consumption by Unit Operation and Process (Williams et al., 2008)

Figure 14. Varsseveld MBR Unit Energy Consumption byUnit Operation and Process (van Bentem et al., 2007)

Florida Water Resources Journal • May 2012 85

Continued on page 86

The Pooler facility used 6,600 kWh/MGon average, using power cost data from Janu-ary to November 2005. As previously shown inFigure 13, 42 percent of the power is con-sumed by the membrane air scouring processand 30 percent is due to process air. Williamset al. (2008) reported that Fowler implementedthe Zenon 10/30 strategy, which helped reducetheir overall power costs.

The Dundee facility consumes 6,300kWh/MG on average. As previously shown inFigure 12, 43 percent of the power consumedis due to membrane air scouring and 9 percentis due to permeate pumping. This plant is alsousing membranes to thicken sludge and ac-counts for another 14 percent of the powercosts for the membrane blower and 14 percentfor the digester blower.

The lowest unit power consumption ratesof the nine plants evaluation were the scalpingplants. Each of these plants was designed tooperate at a relatively constant flow rate. TheLOTT facility consumes 6,100 kWh/MG onaverage, using power cost data from Decem-ber 2008 to December 2009. The Cauley Creekplant consumes 6,000 kWh/MG on average,using power cost data from June 2003 to De-cember 2003. The Bonita Spings plant con-sumes 5,400 kWh/MG on average, usingpower cost data from January 2008 to January2010. No breakdown of the costs from indi-vidual unit operations and unit processes wasavailable to understand how much of the over-all energy is due to membrane process. TheLOTT information does not include biosolidsprocessing at its facility, but operates at lowflux of 6.2 gfd. Williams et al. (2008) reportedthat at Cauley Creek, 80 percent of the overallplant power accounts for the MBR system.Bonita Springs includes a pelletizer facility. Abreakdown of this plant would be useful toidentify which portion of the total power costis due to the MBR system.

Conclusion and Perspective

From the nine MBR plants investigated,average unit energy consumption ranged from5,400 kWh/MG to 16,000 kWh/MG. Selectedplants of those evaluated could reduce energyconsumption by implementing the new airscour strategies, or by operating their plantcloser to design fluxes if the membrane per-formances allow. Power consumption data re-ported was for the overall plant. Since thetreatment process elements varied significantlyamong the nine plants evaluated, a more de-tailed evaluation would be useful that is basedon individual unit process and unit operationsto further quantify energy consumed for themembrane bioreactor only.

Figure 15. MBR Energy Usage in kWh/MG as a Function of Flow fromFowler, Dundee, Pooler, Varsseveld, Delphos, Cauley Creek, Healdsburg,LOTT, and Bonita Springs. S = Siemens, Z = Zenon, K = Kubota

Figure 16. MBR Energy Usage Trend in kWh as a Function of Wastewater Treated from Fowler, Dundee, Pooler, Varsseveld,Delphos, Cauley Creek, Healdsburg, LOTT and Bonita Springs. S = Siemens, Z = Zenon, K = Kubota

Continued from page 85

86 May 2012 • Florida Water Resources Journal

Facility managers must also consider otherfactors that influence process selection, includingmeeting local regulatory effluent limits and/or ef-fluent quality requirements for reuse applications.

A unit energy consumption rate of 5,400kWh/MG, on average, was the lowest rate ob-served for the nine plants evaluated. 2,500kWh/MG was the lowest monthly average re-ported (Figure 15). All of the 54 CAS WWTPsreported by NACWA consumed less than5,400 kWh/MG and 85 percent consumed2,500 kWh/MG or less.

Increased energy consumption can occurdue to limitations on air scour blower turn-down capability. Optimal energy consumptionoccurs when an MBR operates close to the de-sign flow based on the number of membranetrains in operation.

To optimize energy consumption forMBR systems, adequate turndown capabilityand modular construction with multipletrains should be provided. Operating strate-gies should attempt to optimize the numberof trains in operation to maximize membraneutilization. Influent flow equalization could beprovided to decrease the amount of mem-brane surface area provided, thereby decreas-ing the energy demands for air scouring. Thisrequires close operator attention, since as the

flow increases through the membranes, theopportunity for fouling increases.

References

• N.B. Cooper, J.W. Marshall, K. Hunt, J.G.Reidy. Energy Usage and Control at a Mem-brane Bioreactor Facility. Water EnvironmentFederation Technology and Exhibition Con-ference. Dallas, Texas. 2006. pp 2518-2526.

• T. Gellner, K. Riddell. Membrane Bioreac-tors: Innovative Operations: A CompanionProgram to W102. Preconference WorksopW207. Water Environment Federation Tech-nology and Exhibition Conference. Chicago,Illinois. 2008.

• M.J. Hribljan, R. Reardon. Membranes inMunicipal Wastewater Treatment: What YouNeed to Know Before Embarking on YourMBR Project. Preconference WorksopW202. Water Environment Federation Tech-nology and Exhibition Conference. SanDiego, California. 2007.

• National Association of Clean Water Agen-cies. Highlighting Challenges in Utility Fi-nancing and Management. 2008 NACWAFinancial Survey Summary.

• J. Pawloski, J. Peeters, B. Ginzburg, J. Winn.A New Approach to Minimizing Membrane

Aeration Energy Costs. Water EnvironmentFederation Technology and Exhibition Con-ference. San Diego, California. 2007. pp8462-8469.

• M. Stone, D. Livingston. Flat Plate MBR En-ergy Consumption - Village of Dundee, MI.Water Environment Federation MembraneTechnology Conference. Atlanta, Georgia.2008. pp 525-547.

• A.G.N van Bentem, C.P. Petri, P.F.T. Schyns,H.F. van der Roest. Membrane Bioreactors.Operation and Results of an MBR Waste-water Treatment Plant. STOWA Report. IWAPublishing. 2007.

• R. Williams, P. Schuler, K. Comstock, R. Pope.Large Membrane Bioreactors of Georgia: AGuide and Comparison. Water EnvironmentFederation Membrane Technology Confer-ence. Atlanta Georgia. 2008. pp 548-561.

Acknowledgment

The authors wish to thank Jim Flugumfrom Healdsburg (California), Cliff Morrisfrom Bonita Springs (Florida), Kim Riddellfrom Delphos (Ohio), and Wayne Robinsonfrom LOTT (Washington) for the many ques-tions and the data collections that were nec-essary to prepare this article. ��

Florida Water Resources Journal • May 2012 87