wef global water issues - planning for the future

P R E S E N T S

Planning for the FutureA COLLECTION OF PAPERS FROM WEFTEC, THE WORLD'S

PREMIER TECHNICAL EXHIBITION AND CONFERENCE

GLOBAL

ISSUES

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Right Sizing a Cogeneration System

Reducing Operating Costs with Energy Efficient MBR Designs

Is Thermal Oxidation of Biosolids with Energy Recovery Sustainable?

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ABSTRACTJohnson County Wastewater,Overland Park, Kansas, isexpanding its solids processingfacilities at the 14.5 milliongallon per day (mgd) DouglasL. Smith Middle BasinWastewater Treatment Plant(Middle Basin WWTP) tomatch a recent expansion of itsliquid stream capacity, acceptadditional trucked in sludgesfrom other wastewatertreatment plants, and provide areadily accessible disposal sitefor fat, oil, and grease (FOG)wastes in support of theCounty’s aggressive FOG wastemanagement program in thecollection system. The comprehensivecogeneration evaluationconducted during designincluded a payback analysisthat determined the economicviability of cogeneration for theMiddle Basin WWTP and theoptimum size and operatingscenarios for the cogenerationunits. The study evaluated threeengine sizes in five engineconfigurations (with andwithout standby units), and

two operating scenarios (withand without supplementalnatural gas), for a total of ninedifferent engine/operationalalternatives. Engine sizesranged from 633 kW to 1060kW and capital constructioncosts for the nine alternativesranged from $3.6 to $6.7million. Electrical power costsavings were determined fromprojected reductions in facilitiescharges, summer and winterdemand charges, and summerand winter energy charges. The20-year life cycle analysisincluded projected engine downtimes for scheduled andunplanned maintenance andanticipated variability in dailydigester gas production.

INTRODUCTIONJohnson County Wastewater(JCW) is expanding its solidsprocessing facilities at the 14.5million gallon per day (mgd)Douglas L. Smith MiddleBasin Wastewater TreatmentPlant (Middle Basin WWTP)to match a recent expansionof its liquid stream capacity,accept additional trucked in

sludges from other wastewatertreatment plants, and providea readily accessible disposalsite for fat, oil, and grease(FOG) wastes in support ofthe County’s aggressive FOGwaste management programin the collection system.

A key component of the solidsprocessing improvementsproject was the addition of acombined heat and power(CHP) cogeneration systemthat will provide most of theelectricity required by thetreatment plant whilesubstantially reducing thetreatment plant’s carbonfootprint. The carbonfootprint reduction was animportant aspect of theproject as it will enable JCWto contribute to thegreenhouse gas (GHG)reduction goals established bythe Johnson County, KansasBoard of CountyCommissioners in December2007 that included:

• Reduction of GHG emissionsby one third by 2020

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RIGHT-SIZING A COGENERATIONSYSTEM FOR A MIDDLE SIZEDWASTEWATER TREATMENT PLANTDale Gabel, P.E.1*, Doug Nolkemper, P.E.2, Susan Pekarek, P.E.2, and Katie Chamberlain, P.E.1

1 CH2M HILL, INC., Denver, Colorado 2 Johnson County Wastewater, Johnson County, Kansas

A key component of the

solids processing

improvements project

was the addition of a

combined heat and

power (CHP)

cogeneration system


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• Reduction of GHG fromenergy use in Countybuildings to zero by the year2030

• Reduction of county-wideGHG emissions by 80percent by the year 2050.

Both the Middle Basin WWTPand the Blue River MainWWTP are biological nutrientremoval (BNR), activatedsludge treatment plants. Thethickened solids from the BlueRiver Main WWTP will betrucked to the Middle BasinWWTP for processing prior todisposal via land application.The FOG wastes generated inJohnson County andanticipated to be trucked intothe Middle Basin WWTP areprimarily from restaurantgrease traps that are requiredby County ordinance to beperiodically pumped, andfrom four food processingindustries that producemargarine, salad dressing,mayonnaise, and a number ofsauces. The FOG waste will beprocessed at a new FOG waste

receiving station prior to beingfed directly to the anaerobicdigesters.

The variability of volatilesolids loadings to the digestersfrom the two WWTP solidsand the FOG wastes isanticipated to create asignificant range in digestergas production. Acomprehensive life-cycleanalysis determined theoptimal number and size ofcogeneration units based on adetailed analysis of digestergas production variability,capital costs, and costs foroperations and maintenanceof the cogeneration units.

METHODOLOGYAnaerobic digester gasproduction at most WWTPstypically exhibits somevariability. The analysis of thedaily gas produced by theMiddle Basin WWTP digestersduring the period of Junethrough November 2007 alsodisplayed significantvariability. During this time

period, the influentwastewater flow averaged10.5 mgd and the digestersproduced an average 125,200standard cubic feet per day(scf/day) of total digester gaswith a standard deviation of25,400 scf/day. Reliable gasmeasurement data outside ofthis period was not available.A normal distribution of themeasured gas productiondisplayed as a cumulativeprobability plot is shown inFigure 1.

Wastewater Treatment Plant Solids A recent expansion of theliquid treatment processes ofthe Middle Basin WWTPincreased the capacity to 14.5mgd, which could be operatedas a base load due todownstream treatment plantson the same sewer inceptor.The liquid treatmentexpansion also included BNRcapabilities to meet State ofKansas effluent nutrient goalsof 8.0 milligrams per liter(mg/L) total nitrogen and 1.5mg/L total phosphorus. The

Blue River Main WWTP isalso a BNR treatment plantwith the capabilities to meetthese effluent nutrient goals.Average annual daily flow tothe Blue River Main WWTPwas estimated to be 5.3 mgdat the start of cogenerationsystem. The solids loadings tothe Middle Basin WWTPanaerobic digesters from thesetwo treatment plants wereestimated using CH2M HILL’sPro2D™ advanced whole-plant computer simulator,which had been calibrated toMiddle Basin WWTP data.Average daily digester gasproduction from the twotreatment plants wasestimated at 226,000 scf/dayusing the Pro2D simulator.

FOG WastesFOG wastes from restaurantgrease traps and the foodprocessing industries had asignificant impact on theestimated gas production asdetermined by the Pro2Dsimulator. Data from theJohnson County

Figure 2. Cumulative Probability Plots of Middle Basin WWTPAnaerobic Digester Gas Production with Varying Quantities FOGWaste Generated in Johnson County

Figure 1. Cumulative Probability Plot of Middle Basin WWTPAnaerobic Digester Gas Production for the period of June 2007through November 2007


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Environmental Departmentindicated that approximately4.3 million gallons ofrestaurant and industrial FOGis produced annually in theCounty, but it is unknownhow much of this materialwould actually be delivered tothe Middle Basin WWTP.Using the standard deviationcharacteristics of the measureddigester gas production in2007, a series of cumulativeprobability plots weredetermined based on varyingthe FOG waste quantitiesfrom 0 to 100 percent of theweek day average quantity. Asshown in Figure 2 on page 4,the estimated average digestergas production (50 percentcumulative probability) wouldvary from approximately230,000 scf/d for no FOGwaste sent to the Middle Basindigesters to 680,000 scf/d if100 percent of the FOG wastein the County were sent to thedigesters.

Based on the projectedreduction in hauling costdistances for the FOG wastehaulers if they utilized theMiddle Basin WWTP FOGwaste receiving facility, it wasassumed that up to 75 percentof the material generated inthe County would be deliveredto the Middle Basin WWTP,which results in an estimatedaverage digester gasproduction of 550,000 scf/d.

Cogeneration SystemsThree internal combustionengine generator sizes in fiveconfigurations (with andwithout standby units), andtwo operating scenarios (withand without supplemental

natural gas to maximizeelectricity production), for atotal of nine separateengine/operational alternativeswere evaluated. As shown inTable 1, the engine sizesranged from 633 kW to 1060kW with capital constructioncosts for the nine alternativesranging from $3.6 to $6.7million. The cogenerationsystems selected for thisanalysis are all GE Jenbachersystems; however, similarresults would be expected forother manufacturers of similarcogeneration systems.

The operating range of eachcogeneration configurationwas compared to thecumulative probability plot ofdigester gas production todetermine potential need forsupplemental natural gas tomaximize electrical output. Anexample is presented as Figure3 for the two 1060 kWconfiguration with bothcogenerators as duty units.This configuration providedalmost full coverage of theanticipated digester gasproduction, thus nearlyeliminating flaring of excessdigester gas. A small rangewas identified between the 10and 17 percent productionprobability that the digestergas production was sufficientto operate one cogenerationsystem, but not sufficient tooperate both units. It wasassumed that supplementalnatural gas would be added toduring these periods tooperate both units at 50percent of the rated capacityof the cogeneration systems,which is the recommendedminimum.

In addition to producingelectrical power, thecogeneration units would alsoproduce hot water from thejacket and exhaust coolingsystems. The quantity of hotwater produced was assumedsufficient to heat the fourdigesters and the treatmentplant buildings connected tothe plant-wide heating system.

Electrical Power SavingsThe Middle Basin WWTP isdefined as a Large PowerService customer to KCPL, thelocal electric utility. As a LargePower Service customer,KCPL‘s monthly bills reflectthe total of four charges:

Figure 3. Comparison of the Operating Range of the Two 1060 kWConfiguration to the Cumulative Probability Plot of Digester GasProduction with 75 Percent of the FOG Waste Generated inJohnson County

Table 1. Cogeneration Engine Configurations and Capital Costs

The Middle Basin WWTPis defined as a LargePower Service customerto KCPL, the local electricutility. As a Large PowerService customer, KCPL’smonthly bills reflect thetotal of four charges:

• Customer charge• Facilities charge• Demand charge

• Energy charge

• Customer charge• Facilities charge• Demand charge • Energy charge

Table 2 summarizes thecharges and rates for each ofthese four charge categoriesfor the 2007 billing year for aLarge Power Service customersuch as Middle Basin WWTP.The sum of the customer,facilities, demand, and energycharges represents the totalmonthly electric bill beforetaxes. In 2007 the annualelectrical service bill for theMiddle Basin WWTP wasapproximately $500,000.

For all but one of thecogeneration configurations,the estimated annual savingsin electrical utility costs weredetermined to be greatest forthe natural gas operatingscenario that includedutilization of supplementalnatural gas to operate the dutyunits at full capacity. Thesavings more than covered thecost of supplemental naturalgas. The one unit that was notevaluated at with all dutyunits at full capacity was thetwo 1060 kW configurationwith both units as duty units.As shown in Figure 3, thisconfiguration provided nearlycomplete coverage of theestimated digester gasproduction, so could beoperated to match gasproduction and avoid the needto flare unused digester gas.Table 3 summarizes theannual electrical powerproduced by each cogeneratorconfiguration as well as theannual cost for supplementalnatural gas and the net annualsavings in electrical utility

costs based on the rate chargesprovided in Table 2.

RESULTS ANDDISCUSSIONFor each of the nine scenarios,a net present value economicpayback analysis wasperformed based on thefollowing assumptions:

20 year period for economicanalysis, starting in 2010 andending in 2030• Net discount rate of 3% • All electric utility rates and

charges increase by 5%every year from 2007 until2030

• Natural gas rates increase by5% every year from 2007 to2030

• Parasitic loads to thecogeneration engine systems,including the gas cleaningsystem, radiator, etc., have atotal power requirement of35 kW per hour

• An annual gas cleaningequipment service contractestimated at $40,000 in2010

• An annual cogenerationsystem maintenance servicecontract estimated at$0.0140 per maximumannual kW-hours output bythe cogeneration units.

The life-cycle economicanalyses were used to estimatethe payback periods presentedin Table 4.

Each of the nine combinationsof engine configurations andoperating scenarios providedsubstantial savings inpurchased electrical power.The life-cycle analysis revealedthat one duty and one standby


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Table 2. Summary of KCPL Charges and Rates for Large PowerService Customers (2007 Rates)

Table 3. Summary of Electricity Produced, Supplemental NaturalGas Costs, and Savings in Electricity Costs


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1060 kW unit resulted in theshortest payback period of16.6 years. However, JCWselected the two duty 1060kW unit configuration with apayback period of 18.2 yearsto minimize flaring of digestergas and maximize reduction inGHG emissions in support ofthe County’s GHG reductiongoals. At the design quantityof FOG waste delivered to thetreatment plant, traveldistance for waste haulers wasestimated to be reduced byannually. The project isestimated to reduce the GHGemissions of the treatmentplant by approximately 9,700metric tons in carbon dioxideequivalent emissions annually.

CONCLUSIONSThe cogenerationconfiguration of two duty1060 kW units was selectedfor the Middle Basin WWTPbased on a detailed life-cycleanalysis and the potential toreduce the GHG emissions ofthe treatment plant byapproximately 9,700 metrictons of carbon dioxideequivalents annually. Keys torightsizing the cogenerationsystem for JCW included:

• Truck in sludge from otherplants

• Incorporate other highlydegradable organic wastes(e.g., FOG wastes)

• Supplement digester gaswith natural gas to keepengines running

• Minimize facility anddemand charges on electricalbill with dual cogenerationunits

• Accept relatively long pay-back period

• Emphasis on reducingcarbon footprint

The engineering and designfor the solids processingimprovements project havebeen completed. The projecthas been awarded $17.8million in American Recoveryand Reconstruction Act(ARRA) funding as a “green”project and is currently inconstruction with startupscheduled for December 2010.

BIBLIOGRAPHYD. Gabel, S. Pekarek, D.Nolkemper, M. Kalis.Sustainability Incorporatedinto the Solids HandlingImprovements of the DouglasL. Smith Middle BasinWastewater Treatment Plant.Presentation at the WEFResiduals Biosolids SpecialtyConference, Portland, OR.(May 5, 2009)

J. W. Wittwer. Graphing aNormal Distribution in ExcelFrom Vertex42.com,November 1, 2004.http://vertex42.com/ExcelArticles/mc/NormalDistribution-Excel.html

Table 4. Life-Cycle Economic Payback Periods

AbstractThe MBR technology is anattractive, flexible solution forplant expansion/enhancementas well as for greenfieldfacilities. While capital costsof MBRs have become fairlycompetitive with conventionaltreatment systems, theoperating costs, specificallyenergy requirements, requireadditional focus. In order toprovide the most cost effectiveand energy efficient system,enhancements with design,operations, and equipmentselection are required. Thereare several areas within thedesign of an MBR plant whichprovide the opportunity for acost effective design whichbalances capital and operatingcosts. These include use ofprimary clarification, use offlow equalization, adjustingthe balance of the solidsbetween biological treatmentand the membrane basins, andpump configuration. Keyoperational focus areasinclude membrane scour airoperational strategies, use offlux enhancers, optimizationof the number of membranes

in service, and biologicaloperating conditions. Alongwith the operational strategiesto reduce energy, energyefficient equipment must beselected.

Keywords MBR, energy, design,operations, equipment, fluxenhancers, aeration

IntroductionThe MBR technology hasrapidly gained acceptance asan attractive, flexible solutionto plant expansion/enhancement as well as forgreenfield facilities due thefollowing attributes: a smallfootprint which can facilitatenew and retrofit plantobjectives, flexibility toachieve various levels ofnutrient removal, and theexceptional overall organicand solids effluent quality.However, a review of theMBR systems available in themarket identifies significantdifferences both in design andoperation. Plantconfiguration, the range ofoperating conditions and

equipment design all playheavily on the resultingeffluent quality and equallyimportantly on the operatingregime for a given plant. Within the last decade therehas been exponential growthin the MBR field which hassparked an increase in thenumber of manufacturers.The increased competition hasreduced the MBR equipmentcosts, and the escalatingcommodity prices favored thesmall footprint design; hence,the capital costs of an MBRplant became competitive withconventional activated sludgeplants. However, a key areaof focus within the MBRindustry which still needsoptimization is energy.Historically, the energyrequirement for an MBRtypically exceeded that of aconventional activated sludgeplant by a factor of 1.5 to 3.

In order for MBR technologyto reach the next level oftechnological excellence,energy requirements must bereduced. As illustrated inFigure 1, the primary energy


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Energy Efficient MBR DesignsCan Significantly ReduceOperating Costs

1Black & Veatch, 2Black & Veatch,

C.L. Wallis-Lage1*, S.D. Levesque2

Within the last decadethere has beenexponential growth in the MBR field which hassparked an increase inthe number ofmanufacturers. Theincreased competitionhas reduced the MBRequipment costs, and the escalating commodityprices favored the smallfootprint design; hence,the capital costs of anMBR plant becamecompetitive withconventional activatedsludge plants.


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requirements are related toaeration (76%) with pumpinga far second (14%). To thatend, the key opportunities forenergy reduction center onaeration; however, all energyrelated elements should beconsidered. In order toprovide the most cost effectiveand energy efficient system, itis important to look atopportunities related todesign, operations, andequipment.

Design Elements to ReduceEnergyThere are several areas withinthe design of an MBR plantwhich provide the opportunityfor a cost effective designwhich balances capital andoperating costs. These includeuse of primary clarificationahead of the MBR, use of flowequalization, adjusting thebalance of the solids betweenthe aeration basin and themembrane basins, and pumpconfiguration.

Primary ClarificationWith increasingly large MBRs,the natural engineering

tendency is to consider theaddition of primary clarifiersto reduce the load to the MBRsimilar to the benefits whendesigning large conventionalactivated sludge treatmentplants. However, it isimportant to completelyevaluate the impact of addingprimary clarifiers.Traditionally, the principaldriver for using a primaryclarifier has been loadreduction in order to: (1)reduce the power require-ments associated withaeration, and (2) reduce thebiological tank volume. Foran MBR, the aeration powerrequirements are acombination of process airand membrane scour air, withthe volume of scour airapproximately equal to theprocess air requirement. Witha reduction in organic load,process aeration requirementswould reduce; however, thescour air requirements wouldnot change. Consequently, theactual power reduction wouldonly be associated with theprocess air and would be amuch smaller fraction of the

overall aeration powercompared to a conventionalplant. However, there areother energy/O&M relatedbenefits to reducing theorganic load to the MBR.Decreasing the organicloading on the MBR processmeans that for a given flowrate, the MBR process canoperate at lower MLSSconcentration which maydecrease membrane foulingtendency and lead to longercleaning intervals and longermembrane life. The reducedorganic load can lead toincreased oxygen transferefficiency and, consequently,lower aeration blower powerconsumption.

The use of primary clarifiersalso adds some additionaltreatment consideration withrespect to the overall energybalance at a treatment plant.Inclusion of a primary clarifierresults in a two sludge systemwhich makes anaerobicdigestion attractive. The energyassociated with the gasproduction from anaerobicdigestion may be beneficial in

the bigger picture and, therefore,outweigh the marginal reductionin energy savings associatedwith the MBR.

From a design standpoint, theuse of primary clarifiersimpacts other processelements. There is anopportunity to locate the finescreens downstream of theprimary clarifiers which wouldsignificantly reduce screeningsproduction and, therefore,screenings handling. Use ofprimary clarifiers may increasethe plant footprint, and thelarge surface area of primaryclarifiers generates significantodors which must becontrolled. All of these issuesshould be considered incombination with thediscussion above in the finaldecision to use primaryclarification.

Flow EqualizationThe use of the MBRtechnology has rapidlyadvanced in recent years fromsmall, satellite (or scalping)plants to large scale, end-of-the-line facilities. As a result,this newer generation of MBRplants must accommodateflow fluctuations from bothdiurnal variation and stormevents. Because membranesizing is hydraulically driven,alternatives to increasing thenumber of membranes shouldbe considered if the peak flowis more than twice the averageflow, as the economical upperflow limit for membranes inmost MBRs is approximately1.5 to 2 times the average flowrate. Designing membranes toaccommodate higher peakflows typically results in fluxesat the average flow which are

Figure 1. Energy requirements for an MBR. (Hribljan, June 2007)

below the optimized point andsignificantly increasesequipment cost. In addition tocost, there are operationalbenefits associated with aconstant and reasonable flux,e.g., reduced fouling rateshence less frequent intensivecleaning and the opportunityto operate with lower airscour rates. The combinationof a reduction in themembrane surface area andoperating with a lower airscour rate provides theopportunity for a significantenergy reduction. An exampleof the beneficial impact ofequalization and adjusted airscour rates is provided inTable 1 based on using hollowfiber membranes.

There are two options forequalization: external andinternal. External equalizationconsists of separate tankageahead of the biological processtankage. External equalizationeasily satisfies designrequirements, but if the driverbehind the selection of an MBRis footprint, an additionalfacility may be difficult toincorporate on space-constrained sites. Externalequalization can be usedintermittently as an off-line

basin to handle storm flows oras an in-line basin to dampendaily flows as well as handlepeak day flows. Internalequalization, i.e., sidewaterdepth variation within thebiological process tankage, canbe used if the flow variation isnot too large. Most plants willbe limited to approximately 0.5– 1.0 m of sidewater depthvariation before the aerationblowers are significantlyimpacted, unless less efficientpositive displacement blowersare used. Typically, internalequalization is best suited fordampening the diurnal patternbecause the level variationrequired to manage storm flowseffectively tends to besignificant and could adverselyimpact the blower design. Forsome facilities, a combinationof both external and internalflow equalization (see Figure 2)provides a cost effectivesolution, with the externalequalization basin used for off-line storage of storm flows andinternal equalization used tohandle the daily diurnalvariation.

Balance of SolidsTraditionally, MBR systemshave been designed to operateat similar MLSS concen-

trations in both the aerationbasins and the membranetank. The end result is a veryhigh solids recirculation rate,e.g., 4 – 5 times the influentflow. MBR systems also tendto be designed using smallerprocess volumes and higherMLSS concentrations thanconventional biologicalprocesses. The result issuppression of aeration alpha,leading to increased air flowrequirements. While it isn’tfeasible in all MBR designs,under certain circumstances,e.g., the use of primaryclarifiers, there is anopportunity to operate withlower MLSS concentrations,hence less mass, in theaeration basins. This mode ofoperation could reduce thesolids recycle flow rate by50%. The energy reduction istwo-fold: (1) reduction inpumping and (2) dependingon the true impact of MLSSon alpha, a potential increasein alpha which improvesoxygen transfer efficiency.

Pump ConfigurationsThe three key pumpingrequirements for an MBR areas follows: solids return,nutrient recycles, andpermeate. As noted above, the

recycle volume ranges fromthree to five times the influentflow rate depending on theoverall MBR designconfiguration. The pumpingconfiguration can either beforward pumping (i.e., pumpingmixed liquor from the aerationbasins to the membrane tanks)or return pumping (i.e.,pumping mixed liquor from themembranes to the aerationbasins). The preferredconfiguration is a function of themembrane manufacturer andthe membrane tank layout andlocation, for example, whethernew membrane tanks will beconstructed or membranes willbe retrofitted into existing tanks.

Up to two nutrient recyclepumping steps may berequired depending on thenutrient reductionrequirements. Becausemembrane air scour results inelevated dissolved oxygen(DO) concentration in thesolids recycle, an independentrecycle is typically required toreturn nitrates from theaerobic portion of theactivated sludge basins to theanoxic zone whendenitrification is required. Analternative strategy is toprovide a deoxygenation zone


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Table 1. Energy and footprint impact based on equalization.* Figure 2. External and internal flow equalization optimizesmembrane design.

to reduce the DO of the solidsprior to returning them to theupstream biological basins.Should enhanced biologicalphosphorus removal beemployed, a separate recycle isneeded to return solids fromthe anoxic zone to theanaerobic zone to eliminateDO and nitrate inhibition inthe anaerobic zone.Innovative plant config-urations using in-wall pumpsor low head submersiblepumps can minimize theenergy requirements for thenutrient recycle pumps.

Permeate from the membranesmay be pumped or flow bygravity depending on themembrane configuration andhydraulic constraints. Theoptimum configuration tominimize energy is to flow bygravity. This configurationwould require sufficient waterdepth above the membranes tooffset the headloss associatedwith flux variation, fouling ofthe membranes, and anydownstream processes prior to

discharge. If pumping isultimately required to reachthe discharge point, permeatepumps may be the most costeffective selection.

The potential reduction inpumping energy byimplementing the designstrategies discussed above issignificant. For example, at aproject in southern which willretrofit the MBR technologyinto existing conventionalactivated sludge basins, thepumping requirement will bereduced from a traditionallybased MBR design pumpingrate of 9Q down to 4Q. Keydesign features include the useof a deoxygenation zonewhich will allow the return ofthe solids from the membranetank directly to the anoxiczone, hence, serving as aportion the nitrified mixedliquor recycle. The existingnitrified mixed liquor recyclereturn capacity is 2Q whichwas left in operation. Tominimize the solids returncapacity, a higher MLSS

concentration was allowed inthe membrane tank comparedto the biological basins.Insufficient carbon incombination with excess tankcapacity resulted in the use ofa 4-stage Bardenpho processto reach the TIN goal of 10mg/L. Implementing gravityflow in lieu of pumpingpermeate could have provideda further pumping reductionof 1Q.

Operational Elements toReduce EnergyHand in hand with the designelements discussed above, arethe various operationalelements that influence theoverall energy efficiency of theMBR design. Currently thesingle largest energy cost isaeration – both for the biologyand for the maintenance of themembranes. Hence,opportunities to reduceaeration have the potential toreduce the overall energyrequirements significantly. Membrane Air ScourA key factor in the

performance of themembranes in an MBRprocess is the dailymaintenance provided byscour air. Air scour can be oneof, if not the single largest,energy use in the process. Inthe last few years, thedominant membrane suppliershave decreased air scourenergy requirements, andfurther improvement isanticipated. This has been oneof the factors making MBRprocesses increasinglycompetitive with conventionalactivated sludge processes.

Specifically, various membranesuppliers have used thefollowing techniques tominimize energy consumption:

• Intermittent air scour • Lower air scour flow rates at

lower flux

One membrane supplier,ZENON, holds patents for“cyclic” air scour, but othersuppliers have found thatreducing airflow to allmodules also is effective.ZENON cut scour airrequirements by 50% manyyears ago when theyimplemented their patented“cyclic” air scour whichcycled air on and off in 10second intervals. The changein scour air operationsignificantly reduced theirenergy requirements in themembrane tank. Their mostrecent development savesadditional energy by allowingeven longer rest periodsbetween aeration period whenthe flux is below the averagedesign condition. The systemuses “10/10” air scour at highflux and “10/30” air scour at


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Figure 3. MBR design significantly impacts pumping requirements.

lower flux. The “10/30” airscour works as follows: for 10seconds, half of the modules ina given cassette receive airscour. For the next 10 secondsthis cassette does not receive airscour, but air scour is beingused in other cassettes. For thenext 10 seconds, the other halfof the modules in the originalcassette receives air scour. Forthe last 10 seconds of the cycle,the original cassette does notreceive air scour. So, a givencassette receives air ½ the time,and a given module receives air¼ of the time. The air scourblowers meanwhile produce airat a constant flow rate. Whenoperating in the 10/30 mode ofoperation, the energyrequirement associated with thescouring air is reduced to 0.16m3/m2/hr (airflow permembrane surface area).

Another membrane supplier,Kubota, uses continuousaeration but graduates thevolume of air based on theflux, e.g., lower air scour ratesare used with lower flux.Depending on the specificmembrane design, the airscour can be as low as 0.16m3/m2/hr. With the mostrecent Siemens system, apulsed discharge of air is usedto scour the membranes. Thisdesign has reduced their airscour requirements to as lowas 0.11 m3/m2/hr dependingon the overall system design.With the Huber system,intermittent aeration occursbased on the rotation of themembrane panels through theaerated portion of themembrane tank, whichreduces air scourrequirements. The end resultis that the membrane air scour

requirement can vary between0.11 and 0.73 m3/m2/hr whichresults in a significantvariation in the energydemand associated withmembrane maintenance.

Flux EnhancersIntermittent use of membraneperformance enhancers,specifically a polymer basedproduct called MPE 50supplied by Nalco, can beused to reduce the overallmembrane footprint and,therefore, air scour. Theaddition of flux enhancersallows a wider flux operatingrange and has been used todemonstrate performancebenefits both in pilot scale andfull scale plants. There aretwo operating extremes whichappear to benefit from theaddition of the polymer basedflux enhancer. If themembrane quantity is drivenby peak flow, the fluxenhancer allows operation at ahigher peak flux thantraditionally accepted, withoutexcessive or rapid fouling,which results in both an initialcost reduction based on thequantity of membranesinstalled as well as an energysavings based on the reductionin overall air scourrequirements. Extensivetesting was completed within to demonstrate that theKubota membrane couldoperate without rapid foulingat approximately 1.5 timestheir typical peak flux(Enviroquip, 2007). If themembrane quantity is basedon minimum temperaturewhich reduces the design flux,full scale testing in with theKubota membranes indicatedthat the addition of the

polymer based flux enhancersupported operation at a moreaggressive flux at a lowertemperature without adverseimpact on the membraneperformance. By operating ata higher flux, the membranequantity and the associatedenergy requirements can bereduced. Flux enhancingpolymer can also be a meansof increasing the short-termcapacity of the membranesystem which could impactredundancy requirements.

Optimize Membranes in ServiceMatching the number ofmembrane trains in servicewith the plant flow is anoperating strategy that canreduce energy, as themembranes which are not inservice do not require thesame degree of air scour asthose in service.Consequently, takingmembrane tanks out of servicefor portions of the day whenflow is low provides theopportunity to reduce the airscour requirements during therest period. This mode ofoperation also enhancesmembrane performance due toa more consistent flux.Varying the number of basinson line is primarily anopportunity for plants whereequalization is not provided.

Optimize DO within the Bio ProcessIn all wastewater treatmentplants which use aerobictreatment, the biologicalaeration demand is asignificant contributor to theplant energy requirements.With an MBR there are twoopportunities to reduce the


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Matching the number ofmembrane trains inservice with the plant flowis an operating strategythat can reduce energy,as the membranes whichare not in service do notrequire the same degreeof air scour as those inservice.


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total aeration demand in thebiological aeration basins: (1)operate at the minimum DOrequired to achieve completetreatment, and (2) return thesolids from the membranetank to the oxic portion of thebiological basins to utilize theelevated DO which can occurwithin the membrane tankfrom the air scour.Historically, the aerobicportion of the biologicalbasins has been operated witha target DO of 2 mg/L inorder to consistently achieveperformance goals and tominimize the potential forfilamentous growth. By usingmembranes for solidsseparation in lieu of gravitysettling systems, the adverseimpact of filaments issignificantly reduced.Consequently, aerobic basinscould be operated with aresidual DO of 1 mg/L, orpotentially less, in order toreduce aeration airflowrequirements. However, it iscritical to have sufficientsludge age and hydraulicresidence time to achieve therequired performance,especially with a reducedconcentration of DO. Withrespect to the solids recycleline, returning the solids fromthe membrane tank to theanoxic zone (if part of thebiological basins) could bedetrimental to denitrificationdue to the elevated DO.Depending on the membranemanufacturer, the DO in themembrane tank can varybetween 2 mg/L and 6 mg/L.By returning the solids to theoxic zone, there is anopportunity to utilize the DOto offset a portion of theaeration demands.

Equipment Elements toReduce EnergyThe highest powerconsumption at a wastewatertreatment plant is tied to theaeration system; consequently,optimizing oxygenrequirements in addition tooperating efficient equipmentare important elements inkeeping operating costs down.Ancillary equipment such asmixers for anaerobic andanoxic basins in BNR plantsshould also be closelyscrutinized to keep energyrequirements low.

Diffused AerationFine bubble diffusers provideefficient oxygen transfer andhave been proven to bedurable for wastewatertreatment plant applications,thus they are the predominantaeration device used today.There are many types of fine

pore diffused aeration systemsin the marketplace. In generalthese aeration systems aregrouped into porous ceramics,porous plastics and perforatedmembranes. The perforatedmembranes include traditionaldisk and tube membranes aswell as panel and stripmembrane units, which havehigher oxygen transferefficiency. The size of theaeration system in an MBR issignificantly impacted by theoxygen demand, diffuserdepth, and alpha. The oxygentransfer efficiency of a varietyof fine bubble diffusedaeration devices is illustratedin Figure 4.

BlowersFor purposes of energyefficiency, multi-stage,integrally-geared single-stageand gearless single-stage(turbo) centrifugal blowerscould be used. For a given

capacity, single-stage blowerstend to be more expensivethan multi-stage blowers.However, single-stage blowerstend to have greater turndowncapability, which could allowfewer, larger blowers to beused for a given situation. Thenet effect could be a reductionin capital cost. Anotheradvantage of single-stageblowers is that they tend to bemore efficient than multi-stageblowers, reducing electricalpower consumption.

Many multi-stage blowersystems achieve capacityturndown using inletthrottling, which reduces themass airflow at the blowerinlet while keeping volumetricairflow the same. At least onesingle-stage blowermanufacturer uses variableinlet guide vanes and variableoutlet diffuser vanes, bothunder control of a local PLC,

Figure 4. Oxygen transfer efficiency for fine bubble diffusers.


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to achieve turndown andmaximize operating efficiency.Either alone or together withthese approaches, a blowersystem can use variable speeddrives (VSDs). A summary ofblower efficiency andturndown capacity is providedin Table 2.

Turbo blowers are a newertype of single-stage blowerthat operate at very highspeed. These blowers have anumber of advantages,including excellent energyefficiency; however, theycurrently are offered only inrelatively small capacity.

MixingFor the un-aerated portions ofthe activated sludge basins(i.e., anoxic and anaerobiczones), high efficiencymechanical mixing equipmentshould be used. The mostpopular choices includesubmersible propeller bladetype pumps by Flygt, EMUand Landia, and top entering,high efficiency mixers whichoperate with a very low rpmmanufactured by Chemineer,Lightnin and PhiladelphiaMixer. Recent additions tothe mixing marketplaceinclude the INVENT mixer

and the EnerSave mixer, whichuse significantly less energy yetappear to produce similarmixing and performanceresults.

CONCLUSIONThe MBR technology hasrapidly gained acceptance asan attractive and flexiblesolution to plantexpansion/enhancement aswell as for greenfield facilities.While capital costs of MBRshave become fairlycompetitive with conventionaltreatment systems, theoperating costs, specifically asrelated to energyrequirements, requireadditional focus. In order toprovide the most cost effectiveand energy efficient system, itis important to exploreopportunities related todesign, operations, andequipment. There are severalareas within the design of anMBR plant which provide theopportunity for a costeffective design whichbalances CAPEX and OPEX.These include use of primaryclarification ahead of theMBR, use of flowequalization, adjusting thebalance of the solids betweenthe aeration basin and the

membrane basins, and pumpconfiguration.

Hand in hand with the designelements, are the variousoperational elements thatinfluence the overall energyefficiency of the MBR design.Currently the single largestenergy cost is aeration – bothfor the biology and for themaintenance of themembranes. Hence,opportunities to reduceaeration have the potential toreduce the overall energyrequirements significantly.Key areas of focus withrespect to operational energyreduction include membranescour air operationalstrategies, the use of fluxenhancers to allow a widerflux operating range,optimization of the numbermembranes in service and theoxic operating conditionswithin the biological basins.Along with the operationalstrategies to reduce energy,energy efficient equipment,specifically the aerationequipment, the blowers andthe mixers must be selected.

REFERENCESEnviroquip 2007, Flux

Documentation via personalcommunication

Hribljan, Michael J., WEFWebcast “Large MBR Designand Residuals Handling,” June12, 2007

Table 2. Blower efficiency and turndown capacity.


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ABSTRACTMany wastewater utilities aredeveloping or have recentlydeveloped biosolidsmanagement plans (BMP) thatprovide a strategy for 20 to 25years into the future. Indeveloping BMPs, wastewaterutilities are faced with manychallenges, such as satisfyingthe customer base, satisfyinginternal and externalstakeholders and developing aBMP that is affordable,sustainable for the future andmeets regulations.Traditionally, small, mediumand some large-sizedwastewater utilities have landapplied anaerobically digestedClass B biosolids either asliquid or dewatered cake,while several of the largerwastewater utilities havethermally oxidized theirbiosolids. With growing publicpressure to reduce or ceaseland application, there hasbeen renewed interest inthermal oxidation as amanagement strategy, often

raising the question: is itsustainable? If the wastewaterutility has existing digestion, isthermal oxidation compatiblewith digestion or shoulddigestion be stopped?

This paper presents acomparison of six differentthermal oxidation processschemes with respect to theireconomic, energy and carbonfootprints for undigested anddigested solids. The results ofthe comparison providewastewater utility staff andengineers with the positiveanswer to the question “Isthermal oxidation of biosolidswith energy recoverysustainable?”

INTRODUCTIONMany wastewater utilities aredeveloping or have recentlydeveloped biosolidsmanagement plans (BMP) thatprovide a strategy for 20 to 25years into the future. For abiosolids management plan tobe effective, it should be

updated every 5 years or so toreview the progress ofimplementation and to re-planas necessary. In some cases, itis appropriate to reviewtechnologies that may havebeen emerging at the time thatthe BMP was completed, buthave subsequently beencommercialized withdemonstrated municipalwastewater experience and todetermine whether to includeas part of the implementation.

In developing BMPs,wastewater utilities are facedwith many challenges, such assatisfying the customer base,satisfying internal andexternal stakeholders anddeveloping a BMP that isaffordable, sustainable for thefuture and meets regulations.Traditionally, small, mediumand some large-sizedwastewater utilities have landapplied anaerobically digestedbiosolids either as liquid ordewatered cake, while severalof the larger wastewater

utilities have thermallyoxidized their biosolids. Withgrowing public pressure toreduce or cease landapplication, there has beenrenewed interest in thermaloxidation as a managementstrategy, often raising thequestion: is it sustainable?[For this paper, sustainabilityis measured by economicparameters, energyconsumption and greenhousegas emissions.]

BACKGROUNDThermal oxidation has beenpracticed in since the 1930s.Early facilities employedmultiple hearth furnaces,which required generousamounts of fuel for normaloperation. In the early 1970s,fluid bed combustors (FBC) inthe cold windboxconfiguration began to beused. These also requiredgenerous amounts of fuel.Following the first oil crisis in1973, furnace manufacturersdeveloped the hot windbox

Energy Recovery fromThermal Treatment: TODIGEST OR NOT TO DIGEST –Is this Sustainable?Peter Burrowes, CH2M HILL, Tim Constantine, CH2M HILL, Jeremy Kraemer, CH2M HILL, Ky Dangtran, Infilco Degremont

* CH2M HILL, 300 – 72 Victoria Street, South, Kitchener, ON N2G 4Y9


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FBC that recovered energyfrom the oxidation process toreduce the use of fuel (WEF,2009). Together withimprovements to dewateringtechnology (e.g. high solidscentrifuges), FBCs have, for acouple of decades, beencapable of operating withoutthe requirement for fuelduring normal operation whenusing undigested solids.However, there are fewexamples of thermal oxidationfacilities that recover andutilize energy in excess of thatneeded to operate the thermaloxidation process.

This practice of utilizingthermal oxidation as a disposaloption has been perpetuated byconsultants and manufacturerstrying to develop thermaloxidation systems to competefinancially with other methodsof biosolids management, aswell as the lack of regulatory orother drivers with respect toclimate change and loomingenergy shortages. This is incontrast to European practicesthat decades ago began torequire thermal oxidation

facilities to be equipped withenergy recovery. More recently,these facilities have beeneligible for and have receivedcarbon credits. With the recentawareness of global climatechange in North America, itwill become important forwastewater utilities that areconsidering thermal oxidationto seriously look intoincorporating energy recovery.

Fluid Bed Thermal Oxidation VariantsThe majority of fluid bedthermal oxidation systems inNorth America are configuredspecifically for volumereduction/solids disposal asshown in Figure 1. In thisconfiguration, the processtrain consists of a hotwindbox fluidized bed reactor,a primary (fluidizing air/hotgas) heat exchanger (HtX),and the air pollution controltrain. Fluidizing air isprovided by a fluidizing airblower which providessufficient pressure to move allthe incinerator gases throughthe train to the stack. This iscommonly referred to as a

forced draft or push system.The air pollution control trainshown in Figure 1 consists ofa wet venturi/impingementscrubber and a mercuryadsorber, which is required insome states and would likelybe required under proposedMACT rules. The firstmercury adsorber in thisconfiguration installed at amunicipal sludge incinerationplant in North America wasthe one installed at Ypsilanti,MI waste water treatmentplant (Dangtran, 2007). Theventuri scrubber could be aconventional high energyventuri followed by animpingement tray or packedscrubber with a demister or itcould consist of a quenchsection followed by animpingement scrubber, with anintegral multiple venturisection and a demister. Thehot windbox configurationutilizes heat in the exhaust gasto reduce or eliminate theneed for auxiliary fuel.However, there is still asignificant quantity of heat inthe exhaust gas that is wasted.Examples include: Ypsilanti,

MI and Puerto Nuevo, PuertoRico (Mercado, 2010).

The less commonconfiguration in NorthAmerica includes energyrecovery as shown in Figure 2.

In this configuration a wasteheat boiler follows theprimary HtX to recoverenergy, usually as steam.Steam can be used to drive asteam turbine, which in turncould drive a generator toproduce electricity or afluidizing air blower. Forlarger and high pressuresystems, water tube boilers areused and these require aninduced draft (ID) fan so thatthe boiler operates under aslight vacuum to preventleakage of gas. This type ofsystem is commonly referredto as a balanced draft or push-pull system. For smaller orlower pressure systems, a firetube style of boiler is used andthe ID fan is not required.Energy can also be recoveredusing thermal oil in the boiler,although there are no currentapplications in North

Figure 1 - Conventional Fluidized Bed Thermal Oxidation System Figure 2 – Conventional Fluidized Bed Thermal Oxidation Systemwith Waste Heat Recovery


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America. Examples include:Metropolitan WWTP,(Burrowes, 2007a) andCleveland Southerly, Cayuga,OH (Welp et al., 2010), whichis under construction.

The third variation uses theEuropean approach as shownin Figure 3.v

In this configuration,dewatered solids are thermallydewatered (scalped) in asteam-heated scalping dryer toraise the dry solids contentsufficiently to allowautogenous operation. Theprocess train uses a cold orwarm windbox fluid bedreactor, a waste heat boilerand the same air pollutioncontrol system as the previoustwo configurations (Europeansystems usually use dry airpollution control systems). Asfor the previous configuration,a push system or a push-pull

system is used depending onthe boiler type. Some of thesteam generated in the wasteheat boiler is required for thescalping dryer. The remaindercan be used to generateelectricity or drive a fluidizingair blower. In Europe, watertube boilers are arranged sothat one of the sectionsincludes a low temperature airheater for preheatingfluidizing air to improveenergy efficiency. Thisconfiguration has not beenutilized in North America todate, although cold windboxor warm windbox fluid bedreactors were used on systemsprocessing thermallyconditioned solids. Examplesinclude: Beckton, UK;Crossness, UK and Hamburg,Germany.

Air EmissionsAir emissions from fluid bedthermal oxidation systems are

regulated in various waysaround the world. In theUnited States, thermaloxidation is regulated by theClean Air Act and the CleanWater Act. Under the CleanAir Act, New SourcePerformance Standards andPrevention of SignificantDeterioration must becomplied with, as well as theNational Emission Standardsfor Hazardous Air Pollutantsand state air regulations.Pending regulations underSection 129 of the Clean AirAct Amendments of 1990could add emission limits fornine additional aircontaminants. The EuropeanUnion Directive 2000/76/ECon waste incineration providesemission limits for all types ofincinerators in Europe and isthe standard that is used inmost of the Europeancountries that practice thermaloxidation, including the

United Kingdom, Germanyand the Netherlands. InOntario, one of the twoprovinces in Canada thatpractices thermal oxidation ofbiosolids, thermal oxidation isregulated by the Ministry ofthe Environment (MOE)through Regulation 419.Recent Certificates ofApproval for fluid bed thermaloxidation systems containconditions which includespecific stack emission limits.In addition, emissions mustmeet point of impingementconcentrations that aredetermined through airdispersion modelling. Thereare no specific guidelines foremissions from fluid bedthermal oxidation systems.However MOE Guideline A-7Combustion and Air PollutionControl Requirements for newMunicipal Waste Incineratorsprovides air emission limits, aswell as establishesrequirements for controlmonitoring and testing.Although this guidelineexempts incineratorsprocessing wastewater solids,it can provide a good meansof comparing emissions fromfluid bed thermal oxidationsystems. Similarly, EuropeanUnion Directive 2000/76/ECon waste incineration can alsoprovide a good comparison ofemissions performance of fluidbed thermal oxidationsystems. A recent comparisonof emissions measured duringstack tests at the fluid bedthermal oxidation system atthe Duffin Creek WastewaterTreatment Plant (RegionalMunicipality of Durham,Ontario, Canada) indicate theemissions are below thosespecified in Ontario MOE

Figure 3 - European Style of Fluidized Bed Thermal Oxidation System with Waste Heat Recovery


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Guideline A-7 and EuropeanUnion Directive 2000/76/EC(Burrowes, 2009).

The IssueOne of the questionswastewater utilities thatpractice anaerobic digestionand are considering thermaloxidation are faced with is:should they continue digestionif they adopt thermal

oxidation? Another questionis, given there is an additionalcost to install and operate,should they include energyrecovery with thermaloxidation?

METHODOLOGYThe analysis which followsbelow assumes thermaloxidation is applied to awastewater treatment plant

that already processes theirsolids through anaerobicdigestion and dewatering todetermine whether it would bepreferable to continuedigestion after thermaloxidation is added ordiscontinue thermal oxidationand process all untreatedsolids through thermaloxidation. As this scenario isonly likely for larger plants, a

hypothetical wastewatertreatment plant with a futurewastewater flow of 450 MLDis evaluated. The assumedsolids productions throughdewatering for both digestedand undigested solids for thisanalysis are presented in Table1, and are typical of WWTPstreating primarily municipalwastewater.

Approximately 85,000 kg/d ofcombined primary and wasteactivated solids with aconcentration of about 4.2%and a volatile solids content ofabout 77% will be processedeither through digestion,dewatering and thermaloxidation or throughdewatering and thermaloxidation without digestion.In the case of the undigestedsolids, approximately 83,600kg/d of dewatered solids witha solids content of 28.5% anda volatile solids content of77% are to be thermallyoxidized. In the case of thedewatered digested solids,approximately 54,750 kg/d ofdewatered solids with a solidscontent of 25% and a volatilesolids concentration of 65%are to be thermally oxidized.The digestion processgenerates approximately34,430 m3/d of biogas. In asecond digestion scenario, thesolids will be thermallyhydrolyzed (assumed to beCambi THP for illustrativepurposes) prior to digestion.Approximately 40,660 kg/d ofdewatered digested solids witha solids content of 35% and avolatile solids concentration of55% are to be thermallyoxidized. In this scenario,approximately 36,860 m3/d ofbiogas will be generated.

Table 1 - Solids Production Summary


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The thermal oxidationvariants presented earlier wereevaluated for the undigestedand digested solids cases, asfollows:

The following are the stepsperformed in the analysis:Mass and energy balanceswere developed for eachalternative to determineprocess parameters, quantitiesand equipment sizingrequirements.

• Equipment quantity andsizes were selected andconstruction costs developedfor each alternative.

• Annual quantities ofmaterials, resources andutilities were developed andcosts associated with thesewere developed into annualoperating and maintenancecosts.

• Annualized costs weredeveloped, as well as unitcosts based on annual drytonnes processed.

• Carbon footprints weredetermined for eachalternative.

Mass and Energy BalancesThe solids production anddigestion operation valuespresented in Table 1 were

developed usingCH2M HILL’s process designmodel Pro2D®. In addition tothe solids numbers presentedin Table 1, peak week andpeak day quantities were alsodeveloped by the Pro2D®model. Peak month quantitiesare typically used for sizingsolids processing equipment.

In its simplest form, thecombustion of wastewatersolids can be represented bythe equation in Table B:

The first two terms in theequation represent theorganics and water in thedewatered cake. The third and

Alternative

Undigested 1: (Undig1)

Undigested 2: (Undig2)

Digested 1: (Dig 1)

Digested 2: (Dig 2)

Digested 3: (Dig 3)

Digested 4: (Dig 4)

Description

Conventional third bed thermal oxidation system, using undigested solids

Conventional fluid bed thermal oxidation system with waste heat recovery and electricity generation, usingundigested solids

Conventional fluid bed thermal oxidation system, using digested solids

Conventional fluid bed thermal oxidation system with waste heat recovery andelectricity generation, using digested solids

European style of thermal oxidation system with waste heat reecovery andelectriciy generation, using digested solids

Conventional fluild bed thermal oxidation system with heat recovery andelectricity generation, using digested solids, with raw solids pre-treated bythermal hydrolysis

C12H10O2SsludgeFuel + 43H2Osludge +

21O2Air + 45N2Air + Inerts

12CO2 + 6H2OFuel + SO2 + 43H2Osludge +

6O2Excess + 45N2Air+ Inerts

Table A

Table B

Mass and energybalances weredeveloped for eachalternative to determineprocess parameters,quantities and equipmentsizing requirements.

fourth terms represent the airrequired for combustion plus40% excess air. The last termrepresents the inerts, whichafter combustion, become ash.The right hand side of theequation represents theproducts of combustion,together with the water fromthe dewatered cake, ash andexcess air components. Thisequation is the basis of themass balance for the thermaloxidation system.

The energy balance is basedon the concept that the energyor heat entering a componentis equal to the heat exiting thecomponent. The equationshown in Table C representsthe energy balance for thethermal oxidation system:

The mass and energy balancesfor the thermal oxidationsystems were developed usingCH2M HILL heat and massbalance model for fluid bedthermal oxidation systems(HAM©). The modelcalculates the quantities of allsolids, liquid and gaseousquantities at each of the mainitems of equipment in theprocessing train and carriesout an energy balance at eachof these nodes. Figure 4presents an example of theenergy balance expressed askWh/dt for alternativeUndigested 2.

For each of the alternatives,two model runs wereperformed: for annual averagesolids produced and peakmonth solids. The annualaverage values were utilized indetermining annual quantitiesand peak month values wereused for equipment sizing.

Sizing and OtherConsiderationsThe following assumptionsabout existing equipment inthe wastewater plant are:

• The existing digesters haveadequate capacity forprojected solids quantities,but some will requireupgrading

• The existing cogenerationsystem has reached itscapacity and will beexpanded

• The existing centrifugedewatering system hasadequate capacity fordewatering digested solids,but not sufficient capacityfor the undigested solids.

Equipment is sized based onpeak month solids production.For redundancy, one sparecentrifuge is provided at peakmonth solids production.Thermal oxidation capacity isprovided for processing peakmonth solids production withall units operating.Redundancy is provided bybypassing and haulingdewatered cake to landfill. Ashis hauled to landfill.

Process sizing and selection ofequipment, capital costs andoperating and maintenancecosts were developed usingCH2M HILL’s solids processsizing and cost model,Technomic©.

Table 2 on page 21 provides asummary of process operatingfactors and cost factors thatwere used in developingannual quantities and annualcosts.

Carbon FootprintsCarbon footprints wereestimated using proceduresdeveloped by theIntergovernmental Panel onClimate Change (IPCC, 2006).The methodology consists ofmultiplying activity data foreach of the greenhouse gascomponents by theappropriate emission factorand the global warmingpotential (Burrowes, 2007).Appropriate emission factors


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Figure 4 - Example of Energy Balance for Alternative Undigested 2

Table C

energy released by organics +

sensible heat in fluidizing air +

energy released by auxiliary fuel

sensible heat in exhaust gases +

latent heat in exhaust gases+ sensible heat in

inerts + heat losses

were selected from varioussources (IPCC, 2006; EC,2008; WRI, 2005).

RESULTSTable 3 presents a summary ofdigested and undigested solidsproduced annually, thequantity processed throughthermal oxidation, bypassedduring maintenance, togetherwith ash trucked to landfill.Approximately 85% of thesolids produced are thermallyoxidized and 15% landfilleddue to maintenance activities.

Table 4 presents a summary ofthermal oxidation equipmentcapacity, sizes, number ofunits and performance foreach of the alternatives. Forthe undigested solidsalternatives, a single fluidizedbed reactor is required sizedfor peak month solidsproduction at 4.4 dt/h. ForUndig 1 and 2, the reactorfreeboard diameter is 10,500mm diameter. Approximately9,000 kg/h of steam isproduced from Undig 2 and itis used to produce 1,850 kWof electricity.

For the digested alternatives, asingle fluidized bed reactor isrequired, each sized for peakmonth solids production at2.9 dt/h (Dig 1 and 2) and 2.3dt/h (Dig 3). For Dig 1 and 2,each reactor freeboarddiameter is 8,400 mmdiameter and 7,600 diameterrespectively. For Dig 3, thereactor freeboard diameter is6,200 mm diameter. For Dig3, the scalping dryer has anevaporation rate of 2,840 kg/hof steam. Dig 2, 3 and 4alternatives produce 4,800,7,150 and 3,500 kg/h steam

respectively. Some of thesteam from Dig 3 is used forthermal dewatering, theremainder for generatingelectricity. Electricityproduction for Dig 2, 3 and 4is 980, 990 and 880 kWrespectively. It should be notedthat the cogeneration systemfor the digestion alternativesproduces about 2,600 kW ofelectricity (Dig2 and 3) and3,300 kW for Dig 4.

Table 5 presents the estimatedcapital costs for each of thealternatives. The undigestedsolids alternatives vary in costfrom about $87.5 million toabout $122 million. Thedigested solids alternativesvary in cost from about $98.5

million to about $152 million.For the undigested solidsalternatives, costs includeadditional centrifuges andthermal oxidation equipment.For the digested solidsalternatives, these costsinclude upgrades to digesters,biogas treatment and newcogeneration equipment andthermal oxidation systemswith energy recovery andelectricity generation.

Table 6 presents the estimatedannual owning, operating andmaintenance (O&M) costs foreach of the alternatives. Forthe undigested solidsalternatives, the total annualcosts vary from about $15million to $18 million.

Including the electricity offsetsdue to electricity generationfrom thermal oxidation forUndig 2, these costs arereduced to about $17m. Forthe digested solids alternatives,the total annual costs varyfrom about $15 million to $20million. Including theelectricity offsets due toelectricity generation fromcogeneration with digester gasand thermal oxidation, thesecosts are marginally reducedto about $12.5 million to $17million. For all alternatives,O&M costs include for allprocessing including thermalhydrolysis pretreatment,digestion, cogeneration,dewatering and thermaloxidation, as appropriate.


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Table 2 - Summary of Process and Cost Factors

Figure 5 shows the unitoperating and maintenance(O&M) cost, the unit owningcost and the total unit costwith and without off-setcredits for electricitygeneration for eachalternative. Unit costs arebased on combined solidsquantities prior to processing.For the undigestedalternatives, O&M costs rangebetween $256/dt and $277/dt,annual owning costs varybetween $216/dt and $302/dt.For the digested alternatives,O&M costs range between$239/dt and $260/dt, withannual owning costs varyingbetween $243/dt and $376/dt.

Figure 6 presents the netannual energy consumption,normalized to equivalentnatural gas. The net annualenergy consumption comprisesenergy consumption lessenergy off-sets. Thecomponents that contribute toenergy consumption arenatural gas usage for processheating (digesters, thermal

hydrolysis and thermaloxidation – both start-up fueland auxiliary fuel for some ofthe digested alternatives). Thecomponents that contribute toenergy off-sets are electricitygenerated from cogenerationusing biogas and electricitygenerated from the energyrecovered from thermaloxidation. Electrical energywas normalized to equivalentnatural gas using average heatrates for prime movers andenergy sources (EIA, 2008).

For all alternatives, theequivalent energyconsumption is between about50 GWh/year and 100GWh/year. For the undigestedalternatives, the net energyconsumption varies betweenabout 11 and 50 GWh/year.For the digested alternatives,the net energy consumptionvaries between about -10GWh/year and -38 GWh/year.

Figure 7 presents a summary ofgreenhouse gas (GHG) emissionsand off-sets (credits) that make

up the carbon footprint of eachalternative, expressed as tonnesof carbon dioxide equivalentsper year (tCO2e/year). Thecomponents that make up thecarbon footprint includeemissions of carbon dioxide(CO2) from fixed carbon in thesolids (anthropogenic), methane(CH4) process and comfortheating, CO2e from diesel fuelused in the transportation ofsolids and ash to landfill andCO2e from electricity used in theprocessing of the solids. Thecarbon footprint also includesthe CO2e offset credits due togeneration of electricity frombiogas cogeneration and fromthermal oxidation energyrecovery.

For all alternatives, CO2eemissions are between about9,000 and 11,000 tCO2e/year.However, when includingoffset credits, the net carbonfootprint for the undigestedalternatives ranges betweenabout 9,000 tCO2e/year toabout 11,000 tCO2e/year. Forthe digested alternatives, the

net carbon footprint rangesbetween about 3,000tCO2e/year to about 6,300tCO2e/year.

DISCUSSIONThe foregoing analysiscompared the addition ofthermal oxidation to anexisting wastewater treatmentplant that anaerobicallydigests wastewater solids andcogenerates the biogas versusmothballing anaerobicdigestion and replacing it withthermal oxidation of raw(undigested) solids. For theundigested solids, one scenarioincluded a conventionalthermal oxidation system withno energy recovery (Undig 1),while the other includedenergy recovery withelectricity production (Undig2). For the digested solids, allscenarios include utilizing thebiogas for cogeneration. Thecogeneration system produceselectricity and heat for heatingthe digesters. Treatment of thebiogas to remove H2S andsiloxanes is assumed. The first


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Figure 5 - Summary of Unit Cost for Owning, Operatingand Maintaining

Figure 6

scenario (Dig 1) includesthermal oxidation ofdewatered solids withoutenergy recovery and electricitygeneration. The secondscenario (Dig 2) includesthermal oxidation ofdewatered solids with energyrecovery and electricitygeneration, while the thirdscenario (Dig 3) utilizesthermal dewatering (scalping)of the dewatered solids toachieve autogenous operationwith energy recovery. Some ofthe steam produced in a wasteheat boiler is used for thethermal dewatering, theremainder is used to generateelectricity in a steam turbinegenerator set. The fourthscenario (Dig 4) incorporatesthermal hydrolysis (THP)ahead of digestion. Undigestedsolids are dewatered to about18% dry solids prior to theTHP. Steam required for THPis generated by thecogeneration system and asteam boiler. Energy recoveredduring thermal oxidation is

used to generate electricity.Table 4 summarizes thenumber of equipment trains,capacity and performance foreach of the alternatives. Forthe undigested solidsalternatives, larger diameterreactor freeboards arerequired than for the digestedalternatives. The freeboarddiameters for the digestedsolids decrease in size basedon additional dewatering(scalping) of the dewateredsolids or with thermalhydrolysis, which reduces thetotal solids to half that of theundigested alternatives.

It can be seen that while thedigested alternatives generateless solids for thermaloxidation, the lower heatingvalue does impact operations.The use of auxiliary fuel in thefluidized bed is required forDig 1 and Dig 2. For Dig 3,the lower heating value iscompensated for by thethermal dewatering, whichmakes the operation

autogenous. For Dig 4, thedigested solids, following THPand dewatering, areautogenous due to higherdewaterability. However, someauxiliary fuel is used in asteam boiler for THP.Electricity generated throughthermal oxidation is less forthe digested scenarios.However, this is balanced bythe electricity generatedthrough biogas cogeneration.When electricity consumptionis combined with electricityproduction, the digestionalternatives clearly have anadvantage, due to theelectricity from biogascogeneration. If there is nocogeneration, the undigestedalternatives have an advantagefrom an overall energyperspective, includingauxiliary fuel.

Table 5 summarizes the capitalcosts of each alternative. Thedigestion alternatives have alower cost than the undigestedalternatives for the thermaloxidation systems, reflectingthe smaller equipment due tofewer solids or dryer solids tobe processed. However, theoverall capital cost is higherdue to the digester upgrades,biogas treatment, cogenerationequipment and in the case ofDig 3 and 4, for the THP andthermal dewatering equipment.The alternatives that do notinclude thermal oxidation withenergy recovery and electricitygeneration (Undig 1 and Dig 1)have the lowest capital costs,reflecting the additional costsrequired to provide energyrecovery and electricitygeneration for thermaloxidation.

Table 6 and Figure 5summarize the annual costsfor each alternative. Table 6provides the annual costs andFigure 5 presents the costs perdry tonnes processed. Theannual O&M costs for thedigested alternatives are aboutthe same or less than thecorresponding undigestedalternatives, reflecting that thedigestion alternatives includeO&M costs for digestion andcogeneration, as well asdewatering and thermaloxidation, as in the undigestedalternatives. When the owningcosts are combined with theO&M costs, the digestedalternatives have a higherannual cost. But, the digestedalternatives have a lower netannual cost with the additionof the off-set credits due toenergy recovery. It should benoted that if the digesters didnot already exist, the cost toprovide these wouldsubstantially increase the costof the digested alternatives. Insummary, Dig 1 has the lowestnet annual cost. However, ofthe digested alternatives thatinclude thermal oxidationwith energy recovery andelectricity generation, Dig 4has a net annual cost of about6% more, which at the level ofdetail used in the analysis isabout the same. The otherdigested alternatives have netannual costs of about 30%and 35% more than Dig 1.The two undigestedalternatives have net annualcosts of about 18% and 35%more than Dig 1.

Figure 6 summarizes theannual energy consumption ofeach alternative. Normalizing


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Figure 7 - Carbon Footprint Summary


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to equivalent natural gasprovides a basis for comparingfossil fuel consumption. Undig1 has the highest energyconsumption of about 100GWh/year of equivalentnatural gas. The otheralternatives require about 50GWh/year each. When off-setcredits from electricitygenerated from biogas andthermal oxidation are included,the digested alternatives havesignificantly less energyconsumption than theundigested alternatives. Whencompared to Dig 1, the otherdigested alternatives thatinclude thermal oxidation withenergy recovery and electricitygeneration have 3 – 4 timeslower net annual energyconsumption. The undigestedalternatives have 2 – 6 timeshigher net annual energyconsumption than Dig 1.

Figure 7 summarizes thecarbon footprint for eachalternative. The gross GHGemissions are similar for all ofthe alternatives, with thedigested alternatives slightlylower. However, the net GHGemissions for the digestedalternatives are much lowerthan the undigestedalternatives. Using Dig 1 as abasis for comparison, netGHG emissions from theother digested alternatives areabout 80%, 50% and 65% ofDig 1, respectively. The twoundigested alternatives areabout 70% and 45% morethan Dig 1, respectively.

CONCLUSIONSThe analysis clearly indicatesthat, given an existing WWTPwith digestion andcogeneration, it is more cost

effective to continue digestionand add thermal oxidation forthe digested biosolids than todiscontinue the digestion andcogeneration process andinstead thermally oxidize allof the solids. However, ifdigestion did not exist, itwould be more costly toinstall digestion, dewateringand thermal oxidation than toinstall dewatering and thermaloxidation for raw solids alone.When considering whether toinclude thermal oxidationwith energy recovery andelectricity generation withexisting digestion, the analysisindicates that net annual costsmay increase from about 6%to 34% depending on whattype of process scheme ischosen. Installing THP toimprove digestion andincrease biosolidsdewaterability provides themost cost effective thermaloxidation with energyrecovery and electricitygeneration scenario. While thecosts of thermal oxidationhave not been compared toother management practiceshere, other studies have shownthat thermal oxidation is costeffective compared to otherpractices.

From an energy perspective,thermal oxidation with energyrecovery and electricitygeneration combined withexisting digestion provides themost benefits.

From an environmentalperspective, carbon footprintsare similar. However, thebenefits from the GHG offsetsdue to biogas cogeneration aregreater, with the greatestbenefits realized from

alternatives that includethermal oxidation with energyrecovery and electricitygeneration.

Thermal oxidation withenergy recovery and electricitygeneration when coupled withexisting digestion and biogascogeneration is a sustainablepractice for largermunicipalities and should beconsidered whenmunicipalities are determiningtheir long-term strategies forbiosolids management.

BIBLIOGRAPHYBurrowes, P., Bauer, T. (2004),Energy Considerations withThermal Processing ofBiosolids, Presented at theBioenergy Workshop –Permitting, Safety, PlantOperations, Unit ProcessOptimization, EnergyRecovery and ProductDevelopment, WaterEnvironment Federation,Cincinnati, OH – August 11and 12, 2004

Burrowes, P., Borghesi, J.,Quast, D. (2007a) The TwinCities Sludge-to-Energy (StE)Plant Reduces GreenhouseGas Emissions, Presented atthe 80th Annual TechnicalExhibition and Conference,Water EnvironmentFederation, San Diego, CA –October 2007

Burrowes, P. (2007b) ThermalOxidation for ElectricityProduction, Presented inWorkshop A: EnergyBalancing in BioenergyUtilization Systems, Residualsand Biosolids Conference2007 – Innovation andSustainability, Water

Environment Federation,Dallas TX, April 2007

Burrowes, P. (2009) Personalcommunications with , Regionof Durham with respect toemissions performance of theDuffin Creek ThermalOxidation Systems comparedto the proposed operatinglimits for the new Region ofDurham energy from wastefacility.

Dangtran et al. (2007),“Replacement of a MultipleHearth By a Fluid BedIncinerator – The YpsilantiSludge Disposal Case History.”Presented at WECTEC, SanDiego, October 2007.

Energy InformationAdministration (EIA, 2008),Electric Power Annual 2008[DOE/EIA-0348(2008)], U.S.Energy InformationAdministration Office of Coal,Nuclear, Electric and AlternateFuels, U.S. Department ofEnergy, Washington, DC20585

Environment Canada (EC,2008), National InventoryReport 1990 - 2006,Greenhouse Gas Sources andSinks in Canada, theCanadian Government’sSubmission to the UNFramework Convention onClimate Change, May 2008

Intergovernmental Panel onClimate Change (IPCC,2001), Climate Change 2001:The Scientific Basis.Contribution of WorkingGroup I to the ThirdAssessment Report of theIntergovernmental Panel onClimate Change [Houghton,


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J.T., Y. Ding, D.J. Griggs, M.Noguer, P.J. van der Linden,X. Dai, K. Maskell, and C.A.Johnson (eds.)]. CambridgeUniversity Press, Cambridge,United Kingdom and NewYork, NY, USA.

Intergovernmental Panel onClimate Change (IPCC, 2006)2006 IPCC Guidelines forNational Greenhouse GasInventories, Prepared by theNational Greenhouse GasInventories Programme,Eggleston H.S., Buendia L.,Miwa K., Ngara T. andTanabe K. (eds). Published:IGES, Japan.

Mercado, A. (2010),“Incinerator De LechoFluidizado Pas PuertoNuevo.” Presented at theWater & EnvironmentAssociation AnnualConference 2010.

WEF Manual of Practice No.30 (WEF, 2009), WastewaterSolids Incineration Systems,Water EnvironmentFederation, Mc Graw Hill,2009.

Welp, J., Mault, L., Rowan, J.,Wilson, M., Stone, L. andDominak, R. (2010),NEORSD Experiences thePower of Renewable Energy.Residuals and Biosolids 2010.

World Resources Institute(WRI, 2005) GHG Protocol –Guide (03/21/05) ver. v1.3

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