EPA &EPA June - NREL

Residuals Use and Energy CanserVationThe DOE studies found that WWTPmanagers' primary concern is to meetdischarge requirements. Energyconservation, when considered at all, isoften of secondary importance. Now,many WWTP managers are finding thatenergy conservation and use of residualsas fuels can actually enhanceenvironmental compliance. Theexperiences of some of these facilities arepresented as examples to other agenciesconsidering whether to implement suchtechnologies.L cal objectives nd conditions, however,will decide the use made of biogas at aparticular plant.In-plant uses are those that result in thebiogas being consumed completely withinthe wastewater treatment plant, either asprimary or backup fbel. Uses includeheling boilers in process heatingoperations and space heating and cooling,engine-driven machinery, enginegenerators for electricity generation,solids incinerators, boilers forpasteurization of digested biosolids, gasfired biosolids dryers, and generation ofelectricity by steam turbines and fuel cells.Figure 1 provides a schematic of in-plantuses. These uses are described in detail inthe next section.Basics of Biogas Generation and UseAnaerobic digestion is one of the mostwidely used processes of wastewaterbiosolids stabilization. The processinvolves bacterial decomposition of theorganic constituents of the biosolids in theabsence of oxygen. The products ofanaerobic digestion, apart fiom solids,include water and a gas composed ofmethane, carbon dioxide, hydrogensulfide, and other minor gaseouscompounds. This "biogas" has a heatvalue of approximately 550 Btu/ft3, about60 percent of the heat value of naturalUse ofwaste heat recovery increasesenergy efficiency in the system, and is ofparticular value whenever in-plant useinvolves the operation of equipment notprimarily designed to produce heat (Le.,engines, incinerators, turbines, etc.). Asthe case histories in this studydemonstrate, he1 energy efficiency can beincreased fiom 30 to 70 percent byrecovering heat for process or spaceheatinglcooling requirements. Recoveryof biogas should always be supplementedwith waste gas burners, or flares,toensure that excess gas is controlled withthe smallest environmental impact.gas-Biogas may be used either off-site orwithin the plant to improve energyefficiency ofwastewater treatmentprocesses. Both possibilities should beconsidered when designing new treatmentfacilities or upgrading existing ones.2

Residuals Use and Energy ConservationThe case study presented below of SeattleMetro's Renton Reclamation Plantdescribes one such use. Generally, it isless practical to process biogas for offsiteuses if the gas can be used in the plant.Offsite, biogas can be used to createeither energy or chemicals that are soldfor use external to the plant. There aremany potential offsite uses for biogas, asindicated in the schematic in Figure 2.Figure 1: Onsite uses for biogas3

Residuals Use and Energy Conservationheat for space heating and cooling, (3)powering engines used to drive equipmentdirectly, (4) powering engines used withgenerators to drive remote equipment,and (5) powering engines used withgenerators to produce general purposeelectrical power.In-Plant Applications for BiogasBiogas use can result in significant energysavings. Production depends on plantwastewater flows and suspended solidsloading, rather than on warm weather orother outside variables, as long as thedigester environment is uniform.The five most adaptable in-plant uses forbiogas are as a he1 for (1) generating heatfor treatment processes, (2) generating621B-02Figure 2: Offsite uses for biogas5

Residuals Use and Energy Consemationthe need for standby electric power tooperate this equipment during periods ofpeak load. The electric power company,in turn, can make this peaking poweravailable to someone else. Any type oftreatment plant can use direct enginedriven equipment.Process HeatingA plant that uses anaerobic digestion forbiosolids stabilktion should include aprocess-heating system that can maintainthe contents of the digesters at theiroptimum temperature (usually 95" F).Such a system should maintain boilertemperatures above 212' F, and hot waterin the biosolids heat exchanger should notbe allowed to rise above 160' F. Attemperatures more than 160' F thebiosolids heat exchanger may cake withbiosolids, which quickly ruins the system'sheat transfer coefficient. Other uses ofprocess heat include chlorine and s u bdioxide evaporation and raw biosolids andscum preheating.Indirect Engine DrivesIndirect engine-driven equipment providesthe designer with an exceptionally flexiblesystem. It can be used (1) to reduce peakdemands of major equipment that isremote fkom the source of fuel andmaintenance, (2) to drive both local andremote equipment, (3) to achieveoperational speed variability of remotemajor equipment, and (4) to use enginegenerators as both indirect engine driversand general-purpose electrical generators.The extra flexibility obtained by usingindirect engine-driven equipment may bethe difference between efficient andinefficient use of biogas.Space Heating.The use of space heating can be expandedeffectively to include space cooling.When combined with absorptiverefigeration units, the hot waterproduced with the biogas can be arrangedto produce chilled water, which can thenbe piped around the plant for space andequipment cooling. Often such spacecooling can increase savings byeliminating the need for excessiveventilation.General Purpose Power GenerationAs more plants are modified or enlargedto include secondary treatment processes,efficient use of biogas will require greateruse of in-plant, general-purpose powergeneration. Biogas production fiomplants involving secondary treatment canbe sufficient to provide up to 60 to 80percent of the plant's total power needs,depending on the actual treatmentprocesses involved. In those plants withminimal process pumping, biogas mayprovide nearly all of the power needs.Engines for generating plant powerusually operate at slower speeds andDirect Engine DrivesDirect engine-driven equipment usually isemployed in plants whose majorhorsepower demands are required onlyduring peak flow or load conditions, forexample, raw wastewater pumps, effluentpumps, and aeration blowers. The use ofdirect engine-driven equipment eliminates7

Residuals Use and Energy Conservationscrubbing. Any boiler or engine usingunscrubbed biogas must be operated attemperatures above 212"F. Unless thecombustion temperature is maintained at ahigh level, exhaust temperatures will notbe sufficient to maintain non-condensingconditions Within the collection anddischarge conduits. The carbon dioxideand hydrogen sulfide in the spent biogasbecomes acidic and extremely corrosivewhen combined with water. Exhaustcondensation must be eliminated fromequipment heled by unscrubbed biogas.Blending biogas with a gas having lowerhydrogen sulfide content can reduce thecorrosivity concerns associated withunscrubbed biogas.lower mean effkctive pressures. Suchheavy-duty engines can generate powerreliably for many years.Precautions for Use of UnscrubbedBiogasBiogas contains 60 to 70 percentmethane, 30 to 40 percent carbon dioxide,up to % percent hydrogen sulfide andother inert gases and water vapor. ManyWWTPs clean up the biogas before use toremove contaminants. Sunnyvale, forinstance, uses simple baffle platecondensers to remove moisture fiombiogas. Biogas fiom Hyperion'sanaerobic digesters contains 60 to 100ppm of hydrogen sulfide, which wouldproduce unacceptable emissions when thegas is burned. Therefore, Hyperion treatsthe biogas in a Stretford unit to reducethe suhr content to less than 40 ppm ofhydrogen sulfide. Seattle Metro removescarbon dioxide fiom biogas produced atthe Renton WWTP before sale to thelocal gas utility for offsite use. Biogaswhich does not meet the standard of 99percent purity is rejected by the utility.Biogas heat recovefy systems must beisolated from each other. The upsets(production rate changes) of one systemmust never be allowed to affect theoperation of another. This isolation canbest be accomplished by using separatesteam condensers to transfer the boiler orengine heat into a common hot-watercirculation system. The system provides aflexible method of transferring heatthroughout the plant. Using individualsecondary parallel heat loops to points ofneed assures that the final suppiy of hotwater is at optimum temperature.Depending on local factors and the finaluse intended for the biogas, scrubbing isnot always necessary. However, certainprecautions should be considered in theevent that biogas is used without8

Residuals Use and Energy CansexvationCounty SanitationDistricts of OrangeCountyThis section discusses the energyThe 2020 Vision Plan incorporates avariety of energy consewation activities,including lighting, building heating andcooling, and generation of electricityonsite.programs implemented at the twowastewater treatment plants operated bythe County Sanitation Districts of OrangeCounty.Facility DescriptionThe County Sanitation Districts ofOrange County (CSDOC) provideswastewater treatment for a population ofabout 2.1 million people. CSDOCoperates two treatment plants, with acombined average wastewater flow ofabout 235 MGD. Each plant usesadvanced primary treatment with ferricchloride and anionic polymer addition inthe primary basins. About 50 percent ofthe plants' flow receives secondarytreatment. The plants discharge to theocean through a common outfa11 whichhas a 301(h) waiver.'In June 1993 CSDOC put the CentralPower Generation System (Central Gen)on-line. Central Gen incorporates stateof-the-art techniques to reclaim energyfiom biogas. This system has beeninstalled at both treatment plants.Currently, CSDOC does not purchase anyelectricity, as all of its electricity needs aresupplied by onsite manufacture of energyfiom a combination of biogas and naturalgas. CSDOC projects that by the year2010 enough biogas will be produced tocompletely fie1 all the generators.Other aspects of CSDOC's energyconservation program include improvingoperator skills, motivating and trainingoperators to be "energy aware," providingcomputerized power management data,optimizing equipment for maximumefficiency, and providing managementtechnical skills, support, and funding.CSDOC has an energy conservationcommittee to review existing measuresand propose new possibilities for savings.Operation of processes at the treatmentplants is aggressive. CSDOC hasimplemented a lighting conservationprogram and a summer peak savings program.CSDOC has carried out various energyconservation techniques for several years.For instance, the facility uses biogas toheat the digesters and to fuel someengines that run pumps and blowers.However, the recovery system did nothave the capacity to use all the gasproduced by the digesters, and the excesswas burned off In 1989, CSDOCcodified formal energy conservation plansin the "2020Vision Plan."9

Residuals Use and Energy ConservationDescription of the Technologies:Central Power Generation SystemCentral Gen consists of a total of eightinternal combustion engines fbeled byboth biogas and natural gas. The enginesdrive generators to produce electricitythat is then used to operate the treatmentplants. These engines were specificallydesigned to reduce emissions from theengine exhaust and to use all the gasproduced by the digesters. Power outputis 5 megawatts at the Fountain Valleyplant (Plant 1) and 7 megawatts at theHuntington Beach plant (Plant 2).Plant 2 has the greater energy demand (8megawatts), due mainly to the presence ofthe outfd pumping station at this plant.Plant 1 uses about 4 megawatts. Now,allbiogas fiom Plant 1 is exported viapipeline to Plant 2 for use, and the Plant 1Central Gen operates entirely on naturalgas*The three engine generators instalIed atCSDOC's Plant 1 are Cooper BessemerModel LSVB-12SGC. The five enginegenerators installed at CSDOC's Plant 2are Cooper Bessemer Model LSVB16SGC. Plant 1 engines are rated at2,500 kilowatts each, and those at Plant 2are rated at 3,000kilowatts. At 7,200Btuhorsepower, the engines are highlyefficient.The engine units consist of an electricalgenerator, a spark ignition gas-fbeledinternal combustion engine, enginecooling equipment with automatic andmanual controls, and engine exhaust andjacket water heat recovery equipment andcontrols. AU engines are the stratifiedcombustion charge type, with separateprecombustion chambers designed toreduce exhaust pollutant emissions. Thegenerators' design efficiency is rated at aminimum of 96.5 percent at ratedconditions.Each engine has a fuel-injection systemsuitable for accommodating biogas andnatural gas. A fie1 gas cutoffvalve andtotalizing flowmeter are provided for bothfkels and each engine. The engines canuse either biogas, natural gas, or anycombination of the two he1 types. Theengine he1 control system can rapidly andautomatically adjust the heyair ratio inresponse to changes in engine load or fuelheating value. The engine design enablesthe fuel control system to accomplishthese adjustments in a manner that doesnot reduce engine efficiency or result ingreater pollutant emissions, even at a fie1value fluctuation rate ofup to plus orminus 100 Btu per cubic foot per minute.Three-stage biogas filters to remove oil,water mist, and solids are installed on theengine fbel supply piping. The threestages consist of: (1) mechanicalcentrifbgal separation, (2) separation bycoalescing and entrainment, and (3) finalfiltration through a porous-fiberglassmedium. These filters are designed toremove 99 percent of all dispersed liquid,five microns and larger, and a minimum of98 percent of all solids, one micron andlarger. A differential pressure gauge ispresent to indicate when cleaning orreplacement of the filters is necessary.

Residuals Use and Energy Consemationand a consulting firm provided operatortraining for Central Gen.Each engine generator unit has anelectronic governing system for automaticsynchronization, load sharing, and loadregulation. An air fuel ratio controller isalso present on each engine tocontinuously monitor the air fuel ratio.Systems that use exhaust sensors can besusceptible to damage by components ofbiogas, so CSDOC specified that controlof the air fbel ratio must be maintained bymonitoring air manifold temperature andpressure and engine load instead. Enginesare also supplied with various protectiveand safety devices and monitoring andmeasuring devices to ensure safe andefficient operation. Equipment vendorsDescription of the Technologies:Waste Heat RecoveryThe facility uses engine heat to heat thedigesters and for some heating andcooling needs of buildings. The ability torecover and use "waste" heat givesCentral Gen greater thermal efficiencythan that of Southern California Edison(60% compared to 30%). Each enginegenerator has a minimum recoverablethermal output at rated load as foliows:Engine exhaust4.395.27Engine jacket water1.902.30primaries. This resulted in an increase inbiogas production, because the energycontent of the solids recovered from theprimaries is greater than that for solidsrecovered from secondq treatment. InAPT,chemicals are added to the primarysettling facilities. Currently, ferricchloride and polymer are added for about12 to 13 hours daily. The facility hasconducted experiments with chemicaladdition on a continuous 24-hour per daybasis, and found it to be a cost-effectivemeans to increase biogas production.Central Gen has more than adequatecapacity to use all the biogas produced byThe jacket water heat recovery systemtransfers heat to a plant-wide circulatingpressurized hot water system. Theexhaust heat recovery system is designedto reduce the engine exhaust gastemperature to a minimum of 380' Fwhile generating 125 psig dry saturatedsteam.Process Modifications: AdvancedPrimary TreatmentApplication of advanced primarytreatment (APT)at both plants hasincreased solids and BOD removal in theIt

Residuals Use and Energy Conservationthe facility, and as more biogas isproduced, less natural gas needs to bepurchased. Plant s t d estimates thatbiogas production increases between 12and 18 percent because of APT. Thelower figure of 12 percent is gained with16 hours per day APT at 20 mgL femcchloride and 0.15 mg/L polymer. Thehigher figure of 18 percent is obtainedwith increased chemical addition (femcchloride at 30 mg/L and polymer at 0.22mg/L)*a0aDissolved air flotation (DAF)process reductionsDAF fan turned offTransformer turned offReduced operation of aeratorsDewatering fatl turned offElimination of scrubberrecirculation pumps associatedwith obsolete scrubbers.Lighting energy consemtionUse of gravity feed reduced the need forpumping, and the facility realizedsubstantial energy savings by insulatingthe digester domes.APT has reduced the need for secondarytreatment, resulting in energy savings.Before APT, the primary treatmentprocess removed about 65 percent oftotal suspended solids; with APT theplants achieve 80 percent removal.Increasing the amount of primary solidssent to the anaerobic digesters results inincreased biogas production, equivalent to3,000 kilowatts.Pretreatment Program Effects omEnergy ConservationImposing mass-based Iimits on BODdischarges fiom industrid users hascontributed to the Districts' ability toreduce its energy use. In the past, theplant observed dramatic increases ininfluent BOD during the food processingseason. One industry alone discharged upto 70,000 pounds of BOD per day overthe two to three month season. CSDOCnow firnits discharges from foodprocessing industries to 10,000 poundsper day average, and 15,000 pounds perday maximum BOD. Plant staff hascalculated the total reduction in BODdischarged by industry to be equivalent to12 MGD of secondary treatment onaverage, peaking at up to 50 MGD ofsecondary treatment for several weeks ata time. The staff estimates that energyuse is reduced by 500 kilowatts per yearby these efforts.Another benefit achieved through APT isreduction of the amount of biosolids thatmust be disposed offsite. Less biomass isproduced in the secondary process.Therefore, less biosolids must be hauledoffsite, resulting in reduced vehicularemissions and conservation ofnonrenewable fuels.Other ModificationsBesides advanced primary treatment,CSDOC has implemented other processchanges designed to reduce energyconsumption. These include thefollowing:12

Residuals Use and Energy CoaSenationBenefits of the Energy ConservationProgram: Air Emissions Reductionsallowances on the results of the riskassessments.CSDOC cites concerns with meeting airemissions requirements as one of twofactors driving their energy conservationefforts. Southern California airregulations are among the most stringentin the country. Both CSDOC andHypenon are subject to local regulationspromulgated by the South Coast AirQuality Management District(SCAQMD). SCAQMD regulatesemissions of sulfur dioxides fromstationary source internal combustionengines, and sets limits on the allowablecontent of sulfur in gaseous fuels.SCAQMD also requires wastewatertreatment plants to develop riskassessments, and bases influent volumeSubstitution of biogas for natural gas hasenhanced the CSDOC plants' ability tomeet air quality requirements. Becausebiogas has a heat value approximatelyone-half that of natural ga

United States U.S. EPA EPA 832-R-95-003 &EPA Environmental Protection 401 M Street SW June 1995 Agency Washington, DC 20460 A U.S. Department National Renewable NREL/TP- 430-7974 of Energy Energy Laboratory DE95009216 Golden, CO 80401 Case Studies in Residual Use and Energy Conservation at Wastewater Treatment Plants Interagency