Fuel Gas Developments

Transcription

1 Fuel Gs Developments Editor Donld L. ise, Ph.D. Vice President Dyntech R/D Compny Cmbridge, Msschusetts CRC Series in Bioenergy Systems Series Editor-in-Chief Donld L. ise, Ph.D. /1193 CRC Press, Inc. Boc Rton, Florid

2 73. Chpter 4 BICVERSI F CLASSIFIED MUICIPAL SLID ASTES : STATE-F-THE-ART REVIE AD RECET ADVACES Adm S. g, Dnnv'Y. ong, Michel K. Stenstrom, Lory Lrson, nd Robert A. Mh TABLE F CTETS 1. Introduction II. Stte-of-the-Art Review A. Anerobic Digestion Process B. Technicl Evlution Sources. untity, nd Composition of MS Physicl Properties of MS Moisture Content b. Prticle Size c. Prticle Density d. Chemicl Composition e. Mechnicl Properties Preprocessing Unit pertions for Resource Recovery Shredders b. Screens c. Air Clssifiction d. Mgnetic Seprtion Preprocessing Systems for Energy Recovery MS Pretretment MS Digestion Performnce Prmeters utrient Requirements b. rgnic Loding Rte nd Hydrulic Retention Time c. MS Feed Slurry Concentrtion d. Temperture e. Mixing f. Gs ulity nd untity g. Dewtering Chrcteristics nd Residue Disposl h. Rector Design C. Economic Studies III. Recent Advnces A. Bench-Scle Studies B. Pilot-Scle Studies C. Results D. Discussion Source of MS Preprocessing System Process Design Prmeters IV. Concluding Remrks References

3 74 Fuel Gus Developments I. ITRDUCTI Rising energy costs hve creted fresh interest in novel, renewble. but heretofore uneconomicl. energy sources. A number of lternte technologies for using these resources hve been described, including solr heting. wind nd wve energy genertion, nd biomss conversion. The importnce of biomss is recognized by the mny energy conversion processes under current scrutiny such s direct combustion. novel pyrolysis techniques, nd fermenttion to produce solvents nd gses. Anerobic digestion to produce methne from wstewterderived sludges is prticulrly well-known biomss energy recovery technique in use for over 50 yers. The process ws trditionlly imed t the stbiliztion nd decomposition of orgnic wste mterils from domestic sewge by conversion to crbon dioxide nd methne. In the complete nerobic dissimiltion of orgnic mtter, only C: nd CH, re formed ; bout 90% of the originl energy in the substrte is retined in the resultnt methne gs.' This fct, coupled with current interest in renewble energy resources, hs refocused ttention on nerobic digestion becuse of primry interest in energy production nd secondry interest in wste tretment or biomss decomposition. The use of nerobic digestion for fermenting orgnic mtters other thn wstewter-derived sludges is not commercilly widespred. A number of frm wstes such s cttle nd swine mnures hve been treted in nerobic lgoons for mny yers, but this ppliction ws strictly for the purpose of wste disposl nd not CH, production. nly reltively recently hs the use of nerobic digestion for energy production from orgnic substrtes been seriously considered. f the mny types of orgnic products generted ntionlly s wste mteril (including municipl sewge sludge, industril wstes, griculturl mnures, crops nd forestry residues), municipl 'solid wste (MS) hs gined recognition s potentilly vluble resource, contrry to its customry problemtic reputtion ssocited with lnd fills nd incinertion. The energy nd resource potentil of MS is demonstrted by its het vlue (bout 4700 Btu/lb s discrded wstes) ;' by the presence of recyclble metls, glss, nd fibers ; nd by its cyclic genertion. ith the cost of energy continuing to rise nd the obvious need for resource conservtion, such potentils give MS incresed vlue. Municipl solid wste production is mjor concern in urbn res. Incresed wste production is ccompnied by decresed lnd-fill sites for wste disposl. The ongoing legl, finncil, environmentl, nd societl problems of developing new lnd-fill sites dely nd restrict future uses of lnd for this purpose. Thus. nerobic digestion nd recycling of MS s n lternte mens of disposl is finding greter interest becuse of this need. Anerobic digestion of clssified MS (grbge nd refuse which hs been shredded nd sorted) hs the dvntges of reducing the fermentble MS frction from 40 to 55% nd producing clen-burning, medium Btu gs (550 to 650 Btu/ft'), thus voiding ny ir qulity problems ssocited with direct MS incinertion. In mny urbn res. this cn be mjor dvntge. The nerobic digestion of MS for the purpose of methne formtion (methnogenesis) is technology relly in its dolescence. Much of the bsic knowledge of nerobic digestion ws developed by wstewter tretment engineers ; fermenttion of MS hs consequently relied on the trditionl nerobic digestion technology commonly employed in the tretment of sewge nd relted wste sludges. In this sense, the methnogenic digestion of MS hd some predictble bsis s vible process. This chpter will describe nd evlute the stte-of-the-rt of methnogenic MS digestion nd present recent dvnces in lbortorynd pilot-scle studies of this developing technology. ne prime trget is to estblish the conditions for mximum conversion of MS to methne.

4 7s II. STATE-F-THE-ART REVIE Anerobic digestion hs been used for wstewter sludge volume reduction nd stbiliztion for mny yers. The production of low-to-medium Btu gs from this process provides portion of the het nd power requirements for the tretment plnt. Successful ppliction of nerobic digestion of wstewter sludges led Bbitt et l.' to ferment domestic solid wstes by the nerobic digestion process. However, these erly results were generlly unsuccessful nd consequently ppliction of this process to wstes other thn those from wstewter ws not pursued for mny yers. Interest in the digestion of MS ws revived in the lte 1960s, principlly becuse of its potentil for energy recovery. As prt of 5-yer comprehensive study on solid wste mngement. Golueke exmined the fesibility of digesting "synthetic" MS with sewge sludge nd niml mnure. Results showed high percentge of the orgnic refuse ws nerobiclly digested to methne nd crbon dioxide. Klein-' nd McFrlnd et l. b investigted MS "s received" (minimlly pretreted by shredding only) by fermenting it in 400-gl, pilot-scle digester. The effect of the solid wstes on the digestion process nd its potentil for reducing the bulk of the input mteril were evluted. They confirmed Golueke's initil lbortory findings nd further demonstrted tht high proportion of the "s received" MS could be digested with sewge sludge over n extended period of time. The digestion of processed MS with limited ddition of sewge sludge ws evluted economiclly to determine the cost of methne production using multistep process of pretretment. digestion, gs seprtion, sludge dewtering, nd residue disposl.' The rector ws 100-gl thermophifc digester fed processed MS from 3 geogrphicl loctions. Dt on the effects on digestion of effluent sludge recycling, nutrient ddition, custic pretretment of substrte. nd the settling nd dewtering chrcteristics of the resultnt sludge were used in computer simultion study. The simultion showed tht the overll process ws cost-effective ; credit llowed for refuse disposl ws the most sensitive economic fctor. Some privte industries nd governmentl gencies currently working on reserch in this field include Biogs of Colordo, Cl Recovery Systems, Institute of Gs Technology, Dyntech, Systems Technology Corportion. Southern Cliforni Edison, Cliforni Energy Commission. nd the U.S. Deprtment of Energy (DE). Presently, the only lrge-scle MS digester in opertion is the DE-sponsored Refuse Conversion to Methne (RefCom) project in Pompno Bech, Fl. This process involves primry shredding, ferrous metls recovery, secondry shredding, trommeling, nd ir clssifiction. The resulting clssified MS is premixed with rw sludge, nutrients, nd wter nd pneumticlly trnsferred into two mechniclly gitted nerobic digesters. The RefCom fcility is designed to process up to 100 ton of MS per dy, nd its primry objective is the demonstrtion of relible nd cost-effective conversion of MS to methne gs. o dt hve yet been reported. A. Anerobic Digestion Process Anerobic digestion is widely employed t most mjor municipl wstewter tretment plnts but is used less frequently in the tretment of industril nd griculturl wstes. Consequently. there exists n bundnce of literture pertining to empiricl pplictions in the sewge sludge tretment field. But, the fundmentl microbiologicl determinnts for opertionl process control were overly simplified minly becuse of poor bsic understnding of the microbiology of the fermenttion. An understnding of the bsic biology of the methnogenic bcteri nd their complex microbil interctions will lmost certinly yield more relible engineering designs nd opertionl criteri for methne production. The bsis of the methnogenic digestion process is n interdependent metbolism of

5 76 Fuel Gus Developments 0 HYDRLYSIS (EXTRACELLUAR EZYMES) CHEMHETERTRPHICFERMETATIVEBACTERIA (-METHAGES) FATTY ACID-XIDIZIG BLIGATE PRT REDUCERS (-METHAGES) ACETICLASTIC METHAGES HZ-XIDIZIG AD FRMATS USIG METHAGES FIGURE 1. Methnogenic degrdtion of orgnic mtter. complex substrtes medited by mixture of fculttive nd strictly nerobic bcteri which minerlize orgnic mtter in noxic environments. The end-products of complete dissimiltion of crbohydtes, lipids, or nitrogenous orgnic compounds re methne nd crbon dioxide ; with only smll yield of microbil cells. The bsic elements involve conversion of complex substrtes through intermedite compounds to the terminl products. methne. nd crbon dioxide. Figure 1 summrizes the current conception of this fermenttion s series of 3 simultneous events comprised of 3 interdependent rections. First, complex orgnic compounds re hydrolyzed by extrcellulr enzymes nd fermented to hydrogen, crbon dioxide, nd smller orgnic products, primrily ftty cids. Secondly, these orgnic intermedites re metbolized by cetic cid producing (cetogenic) bcteri including obligte hydrogen-producing bcteri." Finlly, the methnogenic bcteri rpidly metbolize hydrogen nd crbon dioxide to methne nd by doing so mintin low enough hydrogen concentrtion to mke thermodynmiclly fesible" continued oxidtion of low moleculr weight compounds to cette. HZ, nd C,.' The cette generted in these rections is metbolized by specilized group of ceticlstic methnogens to CH, nd C,. Acette is quntittively the most significnt methnogenic substrte in these sludge fermenttions. About 70% of the totl methne is produced directly from clevge of the methyl group of cette to form methne. In properly operting digester. these bcteri interct in conjoint metbolic conversion of the strting substrtes to CH, nd C:. Creful control of the digestive process is required to mintin proper physiologic blnce between the cid-forming, cetogenic. nd the methne-producing bc-

6 77 Tble 1 TYPICAL PERATIAL CRITERIA FR AAERBIC DIGESTI, AT MESPHILIC TEMPERATURE Criteri ' Stndrd rte digestion High rte digestion Solids. detention time. dys Solids loding rte Ib VS ft'/dy kg VS/m'/dy Voltile solids reduction percent Gs production ft'/ib VS destroyed m'/kg VS destroyed ft'/lb VS dded m'/kg VS dded (Adpted from U.S. EPA, Process Design Mnul for Sludge Tretment nd Disposl. Municipl Environmentl Reserch Lbortory. Center for Environmentl Reserch Informtion Trnsfer. EPA 6251, ith permission.) teri. Stble digestion is imed t mintining fvorble environmentl conditions for blnced microbil popultions. Control prmeters include orgnic loding rte, feed qulity nd concentrtion, temperture, ph nd solids, nd hydrulic retention time. Control nd optimiztion of the methne fermenttion in digester systems will depend on further elucidtion of the metbolic interctions nd interdependencies mong the microbil constituents. In the future, it my be possible to chieve physicl seprtion between some of these microbil couplets in the form of two-stge or three-stge fermenttion systems. A closed rector tnk mde of concrete or steel is commonly used for nerobic digestion. Either one of two methods of opertion my be employed : stndrd- or high-rte digestion. In the stndrd-rte process, the digester is usully unheted nd unmixed nd retention times my vry from 30 to 60 dys. In the high-rte process, the digester is heted nd completely mixed, the retention time is usully 10 to 30 dys. A combintion of these two processes is known s two-stge digestion process. In such two-stge system, the second stge, which is essentilly stndrd-rte digester, functions to seprte the digested solids from the superntnt liquor. U.S. Environmentl Protection Agency (EPA)" design criteri for sewge sludge digesters re shown in Tble 1. B. Technicl Evlution The process prmeters used in digestion of MS re identicl to those used in sewge sludge fermenttion. Four mjor technicl elements re importnt : preprocessing, digestion, gs recovery, nd residue disposl. In the following sections. considertions essentil for estblishing the digestion process design criteri will be discussed. These include the sources nd chrcteristics of MS, MS preprocessing nd pretretment. nd digester control prmeters such s nutrient requirements, orgnic loding rte. retention time. feed slurry concentrtion. temperture, mixing, gs qulity, nd quntity. Residue dewtering chrcteristics. ultimte disposl, nd rector design will lso be considered. t'. Sources. untirv. nd Composition of MS The sources. quntity, nd composition of MS my vry significntly with seson nd

7 78 Fuel Gus Developments Tble 2 PRJECTI F MS GEERATI RATES Yer Million tonsiyer Totl gross discrd' et wste` Cpitl/dy (Ib) ' Residentil nd commercil sector genertion only. et wste is referred to totl gross discrds minus recovered. (Adpted from U.S. EPA, A Technicl, Environmentl. nd Economic Evlution of the et Processing System for the Recovery nd Disposl of Municipl Solid ste, S-109C, ith permission.) geogrphicl loction. These chrcteristics determine the use of ny one of severl mechnicl preprocessing configurtions to produce the feed qulity required by the digestion process. Hence, their considertion is of primry importnce. A definition of solid wste is essentil for the estimtion of quntities nd composition of solid wste. "Post-consumer solid wste" includes the solid refuse nd grbge generted from privte houses nd prtments, smll commercil businesses, nd office buildings. This definition excludes mining wstes, griculturl wstes, industril processing wstes, demolition nd construction debris, nd wstewter-derived sludges. Post-consumer wste is commonly referred to s Municipl Solid ste (MS). In generl. this is the wste "tht grbge trucks tke wy." In 1975 the Environmentl Protection Agency (EPA)" using government nd trde ssocition sttistics, estimted tht n nnul solid wste production of 136 million tons re generted by residentil nd commercil sections in the U.S. This is equivlent to 3.4 Ib of solid wste per cpit per dy. The projected solid wste genertion for the yers 1980, 1985, nd 1990 using gency-generted dt re summrized in Tble 2. By 1985, the predicted MS genertion rte will increse 37% to 4.67 Ib per cpit per dy. The vrition inms composition due to geogrphicl loction, sesons, nd type of dwelling is illustrted in Tbles 3 nd 4." Tble 3 gives dt from vrious U.S., loctions. The composition dt were generted by methodologicl smpling, segregtion, nd weighing of MS. Tble 4 shows the sesonl vrition of municipl solid wste composition for the southern U.S. The discrded solid wste mixed with other refuse mterils my either lose or gin moisture. For exmple, food wstes trnsfer significnt mount of moisture to pper nd textiles. The choice of wstes "s discrded" is useful for true reltive mgnitude of wste genertion of vrious ctegories for estimting grbge potentil nd forecsting refuse genertion rtes. Consequently, the dt (Tble 4) were djusted for moisture content corresponding to the originl vlue s the mteril entered the refuse storge bins on n "s discrded" bsis nd prior to moisture equilibrtion. The composition nd projected quntities of MS in Southern Cliforni" re shown in Tbles 5 nd 6. The composition dt for Los Angeles County were bsed on weight verging survey in the City of Los Angeles nd n estimtion of the commercil wste generted in the county. The ntionl verge composition of MS" obtined from is lso included in Tble 6.

8 79 F 8 ' = C A V E 9 3 Y oo.x w1 n - ~+ n tw1!n - - e1 o n 7 ew1 t1 b F wy C u w m F-! Z i.r Y v,o f'1 ~ L r Y V ~ ~J n 7 9 h Y 2 b wt n1 fwl o ~o ~ n o0 v~ ewf twl o0 0 "r V1 In ~ e 1 r en z n h h o ~ x eo H U 0 Y V T V1 P1 v, T n In h 00 v, n T 'J v, (n `? ew1 h 9 r %'~ ; rw1 T r! ` v 00 "i ri ~ of r r C 0 Z 7 7 u 00 c u v v n u > o E u L 3'. y V h A C A V.~.n '` ~ ~ > e o ~p " u L 7 o E u - :, c 7 e. ~ 7 '` ~ d _n.y C v T u J z C ~ 7 C.v 9, _C "~" ` v 7 v ` s G ~C _ U c x C.n 7 " L ~ '` ~ r E n F o v 'C = A c _~ ~ C S cl n 7 7? D A L.E ~ Y ' d E Z J _ e v e 'c E E E E o c v v u o - ~. = E - c.o F > d e E U < v 'v v. w Y ; ^. c E t E E 5 E n >- m & U Umzm V ~C7 'n u ~k ;o c,k,j~ uik ;o u ik Z o e

9 80 Fuel Gs Developments w F e s ee v E 00 e oo-., _ SEE 3 L X F C h. V h '.0 Ic 0 r 00 - ~D f~1 Ic 00 r r r P1 r C _ /+ r r - e v ~ r C r ~ +l P) ~ r v 0 r r Y r Y r r I-o o r C V - 0 V ~ ~ ' 'S, L -? 0 C nf 00 H1 < ~0 l+1 m yr R1 o ~ P'1 - Pf r A1 yr? _ yr ~D C r t yr v? fn ~D v r? _ v r r0 0-1 %0 00 v v r r 3 y~ C co r U., u F - r-: 0, _ I 10 r r n n ~o 00 v n o e~ r < o ~+ 00 rr en I to r 3 h r 'o ~C i'.1 00 ` ~ M n ~ ~G r ~ A C t 0o " _ e~ v _ ~ r C r x _ C v r r f7 P1 H u Z e 0 `ou c u. Z A c Z u = U _.` m Z o cm e U t,~ o m. U c co E F U U U U.2 Z 00 4 U 2 S Z. ~ 3 U y

10 4 52 o00m g x I I I I I I I I I I r u 9 ~ ~ - v I Y A1 ~ K I "G IT n - V1 L m =' I fn - V1 - C H o r n - V1 - L? r; - ti vi 3 G L V ' ' r? ~D [~ et D r *7 ~ of - r n, n n nvvre T C n = od ? CC D - ~ - y1 T r_ r Y C vi 0 r 3 u e%i Z c.c r r - U C C I ~ I ~ z C n I I I I I I I =' n n Lt n Y ~ ~o n - C- 00 vi - 10 r - _ U C H Y s ~ n = ' w A ~' 3 L 3 G T 7 A ~ d _U C,V, L ~ T r 3 e V H L L y V U U V '` I U L V L. I~ Y `u ~ = u u v u u u u u e 'c ~ ~ Z ~ m m <<<<<Lt =, _u e D v Y >. v 7 V V nu y Ali u 3 2 A - c e E < E Z <c',i-_. V ti

11 82 Fuel Gs Developments Tble 4 ESTIMATED AVERAGE MUICIPAL SLID ASTE CMPSITI, SUTHER STATES, 1970 EIGHT % AS DISCARDED The refuse composition in winter in southern sttes in similr to tht shown in fll. Averge As As Ctegory Summer Fil inter* Spring discrded mixed Pper Yrd wste Food wste Glss Metl ood Textiles Lether nd rubber Plstics Miscellneous Totl too (From ilson. D. G., Hndbook of Solid ste Mngement. Vn ostrnd Reinhold. ew York, 1977, 17. ith permission.) _ Tble 5 UATITIES AD PERCETAGES F MUICIPAL REFUSE I SUTHER CALIFRIA County untity (ton/dy) Percent of totl untity (ton/dy) Percent of totl untity (ton/dy) Percent of totl Los Angeles , , rnge , , Riverside' , ,200 8 Sn Bernrdino, , Ventur , Totl 28, , (Tken from Brown nd Cldwell Consulting Engineers. Energy Recovery from ste nd Biomss. Site Specific Studies. prepred for Southern Cliforni Edison. Rosemed. Clif , 2.) The following trends in MS composition (Tble 6) were noted in Los Angeles during 6-yer period. 1. There ws substntil decline in the proportion of mixed pper. This ws ttributed to smll reduction in the newspper component nd n increse in yrd trimmings. The net effect ws n pproximte 20% decline in totl digestible orgnic mteril. 2. Plstic incresed by bout 50%, textiles by more thn 80%, nd lumber by more thn 200%. verll totl undigestible orgnics incresed by more thn 120%. 3. Cermics nd stone incresed threefold nd dirt nd miscellneous doubled. Totl inorgnics incresed bout 10%. Compring the county dt with the ntionl dt reveled the following :'

12 83 Tble 6 LS AGELES MUICIPAL REFUSE CMPSITI, PERCET BY EIGHT City of Los Angeles Los Angeles tionl County verge Item 72' 74" 77 Commercil' verge Digestible rgnics Pper Crdbord ~ ewspper Mixed pper Food wstes Yrd Trimmings Subtotl Undigestible rgnics Plstics Textiles Lether nd rubber Lumber Subtotl lnorgnics Metls Ferrous Aluminum ther Glss Cermics nd stone A Dirt nd miscellneous Subtotl TTAL ' Source : Envirogenics Systems Co., Systems Engineering nlysis of Solid ste Mngement in the SCAG Region. June 1973 Source : Alpern. R. M.. s reported in Zinder et l.. untity nd Composition of rgnic Solid stes in Southern Cliforni nd Their Potentil s Substrtes for Microbil Methne Production, Source : Alpern, R. M.. interdeprtmentl correspondence to illim Guber. Assistnt Director. Bureu of Snittion. City of Los Angeles. July Source : Huitric. R. personl communiction, Los Angeles County Snittion Districts. Februry ' eighted verge of " City of Los Angeles" nd "Commercil." 'tionl Center for Resource Recovery. Inc. Municipl Solid ste. Its Volume, Composition nd Vlue, CRR Bulletin. Volume Ill. o. 2. Spring (Tken from Brown nd Cldwell Consulting Engineers, Energy Recovery from ste nd Biomss. Site Specific Economic Studies. 1979, 2.) I. The proportion of totl digestible orgnic mtter for the county, 50.5% ws significntly less thn the ntionl 69.6% verge. 2. The proportion of totl indigestible orgnic mtter for the county, 24.3%, ws greter thn the ntionl 8.4% verge. 3. The proportion of totl inorgnic mtter for the county, 25.1 %, ws slightly higher thn the ntionl 22.8% verge. These genertion rte nd MS composition dt for Los Angeles County nd the ntion re indictive of the vrition of MS with time nd loction. Thus. extrpoltion of results from one loction to nother my not be vlid nd should be done with cution. Becuse

13 84 Fuel Gs Developments of these vribilities. the design of solid wste mngement systems must hve high mrgin of opertions flexibility nd must be designed for extrordinry contingencies. These requirements often result in overdesign in order to ccommodte the vrition in MS composition. 2. Physicl Properties ofms For the design of resource recovery equipment, knowledge of the physicl, chemicl, nd biologicl properties of mixed refuse is essentil. The most importnt fctors re moisture content, prticle size, prticle density, chemicl composition. nd mechnicl properties.. Moisture Content Moisture content of vrious components of refuse chnges with time becuse time-dependent movement of moisture occurs during trnsfer nd storge. The moisture content is very importnt in the design of storge nd conveying equipment becuse mteril chrcteristics such s size, density, nd brsiveness re ltered with chnges in wter content. 's Furthermore, moisture content of MS influences shredding energy requirements ; energy requirements re lest t moisture content of 35 to 40%.' 6 An incresed moisture content lso exerts deleterious effect on the purity of nd recovery of ferrous metls by mgnetic systems. However, ferrous metls recovery increses with incresed moisture content when electronic seprtors re used." A lower MS moisture content lso reduces the overll energy requirements for pyrolysis since less het is required to rech operting tempertures. b. Prticle Size Prticle size is n importnt fctor in resource recovery becuse most seprtion processes -require specific nd reltively uniform prticles for efficient opertion. Prticle size mesurement of MS is difficult becuse of the presence of odd shpes nd the common use of sieving s method of mesurement. Eddy current processes used in resource recovery re lso ffected by prticle size." Prticle size lso hs significnt effect on lnd-fill gs production. A 10-fold decrese in refuse prticle size increses lnd-fill gs production by 4.4 times." Digester efficiency nd mixing requirements my lso be function of prticle size. Conversely, the energy consumption of MS preprocessing systems is inversely relted to prticle size; smller prticles require more energy. c. Prticle Density Mteril density is n importnt fctor in ny resource recovery opertion. The low initil density nd poor compction property (t moderte pressures) of municipl solid wste decrese crrying cpcity nd contribute to high cost collection nd huling. Density of shredded refuse is lso importnt for the design of storge tnks nd retrievl systems becuse of the effect of grvity during long-term storge. Also, in lnd fill opertions, the greter the refuse density the less methne produced." This effect my be due to reduction of exposed surfce re vilble for microbil degrdtion.' d. Chemicl Composition The economic recovery of mterils nd energy ultimtely depends on the chemicl composition of the refuse which determines its potentil heting vlue. Toxic nd custic substnces in municipl refuse my cuse corrosion of refuse-processing equipment, my be potentil source of ir nd wter pollution, nd my inhibit eventul digester performnce. Hydrochloric cid formed from plstics such s polyvinyl or vinylidene chloride is commonly found when refuse is combusted. The proportion nd type of orgnic mtter in the refuse my lso be correlted to its energy vlue. Fts nd lipids yield more methne thn proteinceous substrtes.'-' Grbge hs typicl heting vlue of 8484 Btullb, but

14 pper is lower (7572 Btu/Ib) nd fts re much higher (16,700 Btu/lb.)" Soluble nd simpler orgnic compounds re ssimilted nd metbolized more redily by microbes ; consequently, the presence of lower moleculr weight substrtes my give rise to methne very rpidly. Such compounds my be metbolized t higher substrte loding rtes nd lower retention times. Reclcitrnt orgnic mtter ; such s lignins or celluloses, my be pretreted by hydrolysis with enzymes, cids, lkli, or het. The crbon to nitrogen (C:) rtio is often reported in evluting digester substrte composition. A certin C: rtio in the feed is necessry for optiml microbil metbolism, but n exct rtio for methne production hs not been estblished. In the co-digestion of urbn refuse nd sewge sludge, C: rtio of 50:1 to 70 :1 ws recommended by Klss et l.' A reported C: rtio up to 90 :1 did not significntly ffect methne production. Digestion filure ws predicted if the D: rtio exceeded 45 :1 during fermenttion of pper pulp nd sewge sludge.' At C: rtios below 25 :1, norml digestion occurred, nd, t C: rtios greter thn 25 :1, cid-forming bcteri predominted." e. Mechnicl Properties Informtion on mechnicl properties of mterils is importnt for identifying the requirements for comminution of heterogeneous mterils in MS. Stress strin dt re especilly importnt for the design of shredding equipment. 3. Preprocessing Unit pertions for Resource Recovery Vrious commercil systems hve been developed for processing MS to recover vluble mterils (luminum, glss, etc.) nd to produce frction commonly referred to s refusederived fuel (RDF). These systems were developed for specific requirements of mrket conditions nd other fctors unique to the prticulr project. However, despite the commercil vilbility of unit processing modules, stndrd preprocessing system does not exist." Lck of sufficient operting informtion nd experience, the vrible nture of MS, nd diverse site-specific objectives ctully preclude ny stndrdiztion. A typicl mteril processing system employs shredders, trommels, ir clssifiers, mgnetic seprtors, nd glss extrctors ; these components re combined in vriety of sequences. The RDF production plnts presently in opertion re representtive of fst-genertion fcilities under continuous modifiction. Among existing RDF-producing fcilities some re opertionl nd some re in vrious sttes of chnge.' In the present summry, the following sections provide briefdescription of unit opertions utilized in mteril processing systems. A more complete nlysis on function nd design of unit modules is given by Vesilind nd Reimer.". Shredders Size reduction of MS is importnt in energy conversion of solid wste. Shredded refuse is reltively uniform in size, more homogeneous, nd more compcted thn unshredded refuse.' Shredding reduces lnd-fill volume requirements nd my be justified solely on this bsis. Shredding technology, borrowed lrgely from mining industries, is difficult to pply in resource recovery becuse of the nonhomogeneity of MS. Most of the mchines used for shredding MS reofthe hmmermill type. These include verticl xis hmmermills (Tolle-Mche Ltd., London, Englnd nd Heil, Inc., Milwukee, is..). horizontl xis hmmermilis (Broyeurs Gonrd, Pris, Frnce nd Jeffrey Mnufcturing Co., Pennsylvni), verticl xis grinders (Eidl-Interntionl Corp.), nd horizontl xis impctors (Hzemg Co., ew York,.Y.). The hmmermill consists of centrl rotor with rdil hmmers which re free to move on pins nd re enclosed in hevy-duty csing. In horizontl hmmermill, the rotor is supported on both ends, nd feed is introduced on conveyor by grvity. A grte below 85

15 86 Fuel Gs Developments the rotor permits only prticles smller thn the grte opening to escpe from the csing. The verticl hmmermill consists of verticl shft with hevy-duty csing. Verticl clernce of the mill nd csing is reduced grdully nd thus prticle size is reduced s it moves through the mchine. The size reduction of refuse is ffected by refuse flow rte, moisture content, residence time. nd physicl size of the shredder. The product prticle size distribution is function of feed prticle size nd men residence time, while energy requirement is function of holdup nd moisture content of the refuse." Higher flow rtes produce finer prticle sizes but require more energy. However, even t higher speeds. energy consumption decreses with incresing moisture content ; the minimum mount of energy is required t moisture content of 35 to 40%. 16 b. Screens The objective of screening is size seprtion. Screens re used t the beginning of resource recovery system for rough sorting ; towrd the end of the process, they re used for recliming orgnic mterils nd glss. There re two types of screens commonly used for resource recovery: (1) vibrting screens. or (2) revolving screens (lso referred to s trommels). Trommel screens re superior to vibrting screens becuse of lower cpitl cost nd lower power consumption.' A high flow rte reduces the efficiency of ny screens, but the rte of reduction is higher for vibrting screens thn for trommels. A trommel screen requires bout 12% of the energy needed by comprble vibrting screen. Furthermore, trommel screens hve n overll efficiency of bout 90% vs. 72% for vibrting screens. c. }fir Clssifiction The ir clssifiction process employs strem of ir to seprte light orgnic mtter from hevier inorgnic mtter. Shredded MS is introduced ner the mid-point of verticl shft, nd ir is introduced t the bottom of the shft t high rte. The dense prticles move downwrd nd the light prticles rise. The lighter prticles re usully seprted from the ir strem by creting cyclone effect. Air clssifiction geometry cn hve significnt effect on process performnce. orrel nd Vesilind" introduced "totl efficiency" term (the product of light nd hevy frction recovered under specific set of conditions) for evluting ir clssifier performnce. Mteril recovery is function of ir speed, nd is pprently mximum t 1500 ft/min ir speed. Moisture content hs little effect on light products recovery; the efficiency decreses only by bout 5% if moisture content is doubled." At higher feed rtes, lower quntity of light products is recovered. d. Mgnetic Seprtion Mgnetic seprtors re used primrily for seprting ferrous mtter from MS. The recovery of slble ferrous mterils nd the increse in het content nd biodegrdbility of the RDF re two importnt resons for using this process component. Mgnetic seprtors lso improve the life spn of downstrem processing equipment by reducing the mount of brsive wer. The mgnetic seprtor is usully locted fter the primry shredder nd ir clssifiction units. Two types of mgnetic seprtors frequently used for resource recovery re holding-type nd suspended-type seprtors. The shredded MS is fed directly onto the collecting surfce ofthe holding type seprtor but is loded onto conveyor belt below the mgnetic collecting surfce in the suspended type seprtor. 4. Preprocessing Systemsfor Energy Recovery The physiochemicl processes for energy conversion of MS involve vrious combustion

16 87 Tble 7 CMPARATIVE RDF CHARACTERISTICS Course Fine Densified Powdered et-pulped Higher heting vlue s re- - 5,319 5,610 6,000 7,740 3,600 ceived (Btu/Ib) Ash (90) Moisture (96) A ominl prticle size (in.) x A (<I) Bulk Density (ib/ft') A (Est. ) Hndling/storge Poor Poor Good Good A chrcteristics ote : A : ot vilble. (Tken from Mitre Corp., Resource Recovery Reserch nd Demonstrtion Pln. U.S. Deprtment of Energy Contrct o. EM. 78-C-01-42(41, Bedford, Mss., 1979.) nd incinertion methods while biologicl processes include nerobic methne or lcohol fermenttions. The prcticl ppliction of either process requires preliminry seprtion of MS components into combustible nd noncombustible frctions nd degrdble nd nondegrdble components. Preprocessing systems my be ctegorized ccording to five resultnt products : corse RDF, fine RDF, densified RDF, powdered RDF, nd wet pulped RDF.22 Corse RDF is produced by single pssge of MS through shredder to yield smller prticles of combustible mterils. Fine RDF is produced by second shredding to reduce the prticle size even further. Densified RDF is formed by processing fine RDF through pellet mill. Powdered RDF is produced by mechnicl, chemicl, nd therml ction on MS; it is dry nd free-flowing. et pulped RDF is formed by grinding MS in wet pulper using wter medium ; prticles lrger thn 1 in. re rejected from this system. Comprtive RDF chrcteristics re shown in Tble 7. Ptented preprocessing systems re vilble from Combustion Equipment Assocites (Bridgeport. Conn.) which mrkets powdered RDF known s EC-Fuel -II nd from Prsons nd hittermore, Inc. (Hempsted,.Y.) which produces pulped fiber RDF using the Blck Clwson process. These systems produce n RDF for energy recovery by physicochemicl mens. Preprocessing systems used in physiochemicl conversion do not necessrily yield redily digestible product for biologicl energy conversion. Severl systems re designed to produce RDF specificlly for digestion. These re schemticlly illustrted in Figure 2. f the schemes in Figure 2, the Cl Recovery System is the most promising. In this process, n orgnic digestible portion is seprted from the "hevies" nd the fibrous "light" frctions of RDF. Recent studies on the nerobic digestion of this product re reported in lter section of this pper. The results suggest tht certin key innovtive fetures my mke methne gs production from MS economiclly fesible. A detiled review of the Cl Recovery System is reported by Svge, Diz. nd Trezek." S. MS Pretretment The digestible frction of MS contins lignocelluloses which re not esily fermented ; probbly less thn 50% is degrdble. Lignin itself my not be redily broken down under nerobic conditions. nd it seems likely tht 50% or more of the MS frction remins undigested over extended time." The undegrded lignin/cellulose tends to ccumulte in the digester nd crete mixing nd scum problems (see lter). Thus, in ddition to the prepro-

17 88 Fuel Gus Developments J m F- V_ J C7 J I 2 _V m v V J J > f _u 2 r. r J C V ~i ~ > S 2 S - x V 2 g U x E J H y L U. 2 T H ~ C 2 H I If7 J!- 0 m K E ~ U v ~8 2 z J. ~- 2 ~ ~ -~ > F L6 c > m J U 2 x 2 f V '^ G o~q d f 'o H 2 CA u D 2 tt C w y 2 2 w -~ r P j H U x Z H 1L V U C 2 Z J x y C V V f H U tij U C _ t to V J ~ Li L x U > V X 17 s J H x 1 2 J V d? 2 C H H 2 G x y,u > V V _ y m f J 2 > d f- V C ~ 0 H s d V U P! J J 0

18 89 Tble 8 METHDS FR TREATMET F CELLULSE T ICREASE DIGESTIBILITY Physicl Bll milling Hmmer milling ethering Boiling High pressure steel Electron irrdition Photo-oxidtion etting Gmm rdition Chemicl Sodium hydroxide Ammoni (liquid) Ammoni (gs) Hydrochloric cid Acetic cid Sulfuric cid Sodium sulfide Sulfur dioxide (gs) itrogen dioxide (gs) Potssium hydroxide Phosphoric cid Combintions Hot bll mixing H nd bll mixing, nd irrdition (Tken from Brown nd Cldwell Consulting Engineers, Microbil Production of Methne from Refuse. Report prepred for So. Cliforni Edison Co., Reserch nd Development ) cessing stges in MS bioconversion, pretretment of the frctionted MS is lso importnt, primrily since MS contins mostly cellulosic pper products (75% crbohydrte) nd lignocelluloses. The Cl Recovery process tends to by-pss this problem by removing some of the undigestible cellulosic mteril s RDF. Pretretment of the MS or some removl of undigestible cellulosic mtter prior to digestion (Cl Recovery process) re two possible lterntives to del with this problem. Pretretment processes my be grouped into three bsic methods : physicl, chemicl-therml. or combintion of both. Vrious pretretment methods for cellulose=6 re listed in Tble 8. In physicl pretretment processes, size reduction nd seprtion of inorgnic compounds re pprently the most significnt fctors ffecting the economics of biologicl conversion processes. Ghosh nd Klus' showed tht refuse prticle size hs significnt effect on digestibilty. Lbortory experiments using refuse with medin size prticles of 10.1 to 5.1 mm demonstrted tht lower gs yields nd production rtes were obtined with corser refuse. Unfortuntely, the cost-effectiveness of process vritions for size reduction nd seprtion is not well estblished. The effect of prticle size nd seprtion efficiency is under study by ste Mngement, Inc. t the Pompno Bech, Fl. fcility. This effort should yield vluble informtion on the reltionship between prticle size nd biodegrdbility nd the degree of size reduction nd seprtion necessry for mteril hndling of solid wste slurries. Severl chemicl nd therml lbortory scle pretretment methods hve been investigted for bioconversion of MS. The most promising is het tretment under lkline conditions. Alkli pretretment pprently increses digestibility by swelling the substrte nd incresing the pore size of the cellulose mtrix. Lignin is solubilized to phenols nd crbohydrtes nd consequently subject to microbil enzyme degrdtion. The rupture of chemicl bonds t

19 90 Fuel Gs Developments higher tempertures nd pressures produces products susceptible to hydrolytic ctivity by microbil digestion. Pek biodegrdbility of lkline-heted MS occurs t ph 13 nd 392." Het pretretment of newsppers under lkline conditions incresed methne production by 25 to 47%.'e A custic dose of 3g H or lime per 100 g of refuse heted to 266 F incresed biogs production in lbortory digesters by over 30%.' In ddition, n incresed rte in substrte utiliztion implied potentilly shorter retention times for digestion. Alkli pretretment t elevted tempertures is pprently effective for incresing MS digestibility ; however. scle-up prmeters, disposl of nonbiodegrdble residues. formtion of soluble lignins in the digester effluent. nd the cost effectiveness of such tretment hve not been evluted. 6. MS Digestion Performnce Prmeters After the MS hs been crefully evluted nd preprocessed to yield n orgnic frction for digestion. severl design fctors for optiml gs production nd voltile solids destruction my be estblished ; mong these re nutrient requirements, orgnic loding rte, hydrulic retention time, feed slurry concentrtion. temperture, nd mixing.. utrient Requirements Typicl MS is deficient in both nitrogen nd phosphorus. The quntity of nitrogen required for supplementing nerobiclly digesting refuse is lb/ton of voltile solids fed." Less informtion is vilble on the phosphorus requirement lthough it is much less thn nitrogen. Any nutrient requirements my be stisfied by chemicl ddition of synthetic or refined chemicls or by introduction of orgnic wste mterils rich in the deficient nutrients. Addition of refined chemicls my be excessively costly nd nutrient supplementtion by ddition of orgnic wstes such s sewge or niml wstes my be more costeffective pproch. The digestion of clssified MS supplemented with rw sewge sludge hs been investigted by numerous workers ptimum gs production ws chieved with mixture of 80% refuse nd 20% sludge.' The present work demonstrtes incresed rtes nd yields of methne from MS by supplementtion with niml mnure nd sewge sludge. b. rgnic Loding Rte nd Hydrulic Retention Time The orgnic loding rte (LR) is the quntity of orgnic mtter fed per unit volume of digester liquid per unit time (e.g., lb voltile solids [VS) per ft'/dy). LR influences the stbility of the process. A sudden increse in feed rte my elicit n increse in cid-forming bcteri ; if overloding occurs, unstble conditions my result becuse of the inbility to keep pce with end-product conversion to methne by the slower growing methnogenic bcteri. LR nd hydrulic retention time (HRT), the theoreticl time incoming liquid remins in the digester, re mutully dependent vribles. LR nd the substrte concentrtion define the retention time for given volume : t ny LR, retention time is chnged only by chnging the substrte concentrtion. At long retention times (>30 dys), biodegrdtion of orgnic solids will be essentilly complete ; however, prcticl retention times re usully shorter to chieve more cost efficient process. A typicl LR nd retention time for sewge sludge digesters is 0.2 to 0.4 lb VS per ft'/dy nd 10 to 15 dys, respectively. rgnic loding rtes nd retention times for MS digestion my rnge from 0.07 to 0.35 lb VS per ft'/dy nd from 10 to 30 dys, respectively. In most reports of MS digestion. co-digestion of rw sewge sludge is included.'-"-' At 15-dy retention time, n LR higher thn 0.3 lb VS per ft'/dy ws not stisfctory.' The optiml loding rtes nd retention times for methnogenic digestion will depend on the qulity of the feedstock nd the opertionl objectives of the overll process. For the most cost effective opertions, the highest loding rtes nd shortest retention times require the lest cost.

20 91 c. MS Feed Slurry Concentrtion The MS feed concentrtion is importnt in the opertionl design nd performnce of nerobic digesters. In sewge sludge digesters, more concentrted influent feed yielded better digester stbility nd performnce, ' presumbly becuse of higher stedy-stte concentrtion of microorgnisms. Feed concentrtion lso determines the hndling nd pumping properties of the influent mteril nd the degree of mixing required for efficient opertion. Little informtion is vilble on n optiml MS feed slurry concentrtion ; it usully is estblished by determining orgnic loding rte, retention time, nd physicl processing limittions. An upper limit for totl influent solids bsed on pumping nd mixing limittions is pprently round 8%. d. Temperture Temperture is n importnt vrible in estblishing the rte nd determining the stbility of the digestion process. Digesters my be operted t two temperture rnges : mesophilic nd thermophilic. Thermophilic digesters operte in the rnge of 120 F to 135 F with n optimum t bout 130 F. Mesophilic digesters operte between 85 F nd 100 F with n optimum t 98 F. Although higher gs yields nd gs rtes re obtined in thermophilic digestion, it is not commonly used in municipl sludge fermenttions becuse of incresed heting (nd thus energy) requirements, greter fermenttion instbility, nd generl inexperience with the process. In either temperture rnge, 5 to 10 C fluctution in temperture my result in n imblnce in the microbil fermenttion nd led to digester instbility. Thermophilic digestion is generlly more sensitive to such fluctutions. In contrst to sewge sludge digestion, little informtion is vilble on optiml tempertures for lrge-scle MS digestion. Lbortory digester experiments on MS t vrious retention times nd tempertures gve n optimum mesophilic temperture of 107 F nd minimum thermophilic temperture of 140 C.`2 Also, thermophdic digestion yielded higher gs production rtes. Slightly different results were obtined by Ghosh et l." using 12- dy retention time. These studies gve n optimum mesophilic digestion of refuse-sludge mixtures between 95 F to 104 F t low loding rte (0.07 Ib VS per ft'/dy). At higher loding rte (0.14 lb VS per ft'/dy), the optimum mesophilic temperture ws 95 F. The optimum thermophilic temperture ws 131 F. At given loding rte nd retention time, both higher gs yield nd better effluent qulity were obtined t the mesophilic optimum compred to the thermophilic optimum. e. Mixing Complete mixing of digester contents is essentil for optiml digester performnce, especilly in sewge digesters where high-volume mixing ws found beneficil."-" Most digesters re mixed by gs recircultion. but other methods include mechnicl mixing nd liquid recircultion. ne mjor problem with MS digestion is the formtion of thick scum lyers becuse of indequte mixing; this reduces the efficiency of the digester nd hs been reported by severl investigtors.'-''0-`3 ith hydropulped MS s substrte, fibrous mr's formed in the upper region of 55-gl rector; cellulose tended to flot on the surfce, dhered together during mixing, nd formed lrge mts of fibrous scum. Two methods of mixing were tested in 100,000-gl rector' by using either mechnicl gittor or gs mixing system. The effect of vrious feed rtios of MS to sewge sludge, orgnic loding rtes. nd feed solid concentrtions were exmined. The following conclusions were mde from this study. l. A 4% solids slurry cn be digested on short-term bsis. 2. Both the gs mixing system nd the mechnicl gittor mintin firly uniform solids distribution in the lower nd middle levels of the rector.

21 92 Fuel Gs Developments 3. In both mixing systems build-up of I- to 3-ft fibrous scum lyer of 20 to 25% totl solids occurred with I month of opertion. 4. Grit content in the feedstock must be lowered to void using brsion-resistnt slurry pumps. 5. MS differs enough from sewge sludge to mke unfesible direct ppliction of sewge sludge digestion prctices. Mixing of RDF slurries still remins significnt opertionl problem becuse of strtifiction of the liquid nd binding of mixing shfts nd impellers by fibrous mtrices which develop. This is the mjor opertionl difficulty which must be solved before further lrgescle MS digestion is fesible. Fctors influencing mixing such s feedstock preprtion, MS size, impeller nd rector design, nd shft speed must lso be crefully evluted. f. Gs ulity nd untity Severl importnt technicl nd economic considertions re relted to the gseous endproducts of nerobic digestion. These include gs quntity, qulity, processing, nd mrketing potentil. In generl, the volume of gs produced in digester will depend on the feedstock chrcteristics nd digester opertionl prmeters. For mesophilic domestic sewge sludge digesters, the gs produced verges between 16 nd 18 ft'ab VS destroyed (bout 10 ft'/ib VS dded). Gs yields for MS digestion re substntilly less. MS gs production rtes re not only strongly influenced by temperture, retention time, nd loding rte, but MS sources nd preprocessing schemes my lso ccount for lrge vrinces in gs yields.'-"-" There is little doubt tht nutrients from dded sewge sludge enhnce decomposition nd gsproduction rtes in MS digestion. In two seprte studies '31.1 gs yields from vrious mixtures of highly processed MS nd rw sludge verged bout 7.8 nd 9.3 ft'/ib VS dded, respectively. The vrition my be due to severl fctors including qulity of feedstock, rw sludge rtio, retention time, nd orgnic loding rte. An upper limit of gs production from MS hs not yet been reported. Digester gs consists primrily of methne nd crbon dioxide with trce mounts of hydrogen sulfide nd nitrogen. Typicl gs composition for sewge sludge digesters rnge from 60 to 70% nd 30 to 40% crbon dioxide with heting vlue of pproximtely 600 Btu/ft. '. e In contrst, vrious studies on MS digestion yielded vlues closer to 50 to 60% methne nd 40 to 50% crbon dioxide with heting vlue of bout 550 Btu/ft.'.s.".'." Unscrubbed digester gs is wet, mildly corrosive, nd contins bout hlf the heting vlue of nturl gs. Any clenup of the product gs will depend on its intended use. In wstewter tretment plnts with proper piping nd storge fcilities, tretment is miniml or unnecessry if the gs is used s fuel for boiler nd internl combustion engines. If the gs is used s nturl gs substitute, it must be upgrded to high-btu equivlent of pipeline qulity by removing the crbon dioxide nd hydrogen sulfide. Prticultes in the gs my be removed by lrge sedimenttion trps, nd wter my be removed by use of pipeline trps. A review by Ashre et l.' of gs purifiction systems indicted tht commercilly vilble methods for tretment of digester gs include physicl nd chemicl bsorption, dsorption, nd membrne seprtion processes. Experience with lrge-scle MS digester gs tretment is limited t this time; however, current gs purifiction processes my be pplied to MS product gs without nticipted problems. g. Dewtering Chrcteristics nd Residue Disposl The.dewtering chrcteristics of MS digester effluent re n economic considertion in the overll bioconversion process. The solid residue must be seprted nd dewtered to the mximum extent for economicl disposl. A low moisture content in the residue is desirble to ccomplish the following objectives :

22 93 1. Mintin n odorless, biologiclly stble sludge ' 2. Reduce fuel requirements if incinertion is finl disposl mechnism 3. Reduce huling costs to lndfills or drying fields 4. Reduce lechte production t lnd-fill site. Vrious technologies hve emerged for processing digester effluent nd for ultimte disposl of process wste sludges These re discussed in comprehensive report prepred by the Los Angeles/rnge County Municipl Assocition, LA/MA." For digested sewge sludges, vilble dewtering processes include vcuum filtrtion, centrifugtion, filter presses, horizontl belt filtrtion. snd drying beds, nd lgoons. Vcuum filtrtion nd centrifugtion for dewtering MS sludges nd the effects of recycling nd chemicl conditioning prior to dewtering were exmined by Pfeffer nd Liebmn." Buchner funnels nd filter test lef technique demonstrted tht vcuum filtrtion of digested MS sludge (5 to 6% TS) my yield 20 to 25% solid cke with solids cpture of 90 to 95%. Cke solid could be incresed to over 30% with solid cpture of 90 to 95% if chemicl pretretment were pplied; however, the cost of pretretment ws not offset by the svings in the overll processing costs. Recycling of filtrte resulted in buildup of fine prticles tht eventully reduced the filter rte nd solid cke content. In centrifugtion tests, cke solid concentrtions vried between 27 to 40% with solids cpture of 61 to 88%, depending on the type of centrifuge used. Thus, centrifugtion is lower totl cost system thn vcuum filtrtion provided the solids lost in the centrte re not importnt nd tht the resulting cke solid is incinerted.", Existing technologies for dewtering domestic sludges cn be successfully pplied to digested MS sludge without significnt modifictions. Filter presses probbly my be used to dewter high solids concentrtion MS slurries with sizble quntities of fine prticles= however, supporting experimentl evidence is not yet vilble. Ultimte disposl of digested sludge fter tretment depends on site-specific economics nd governmentl regultions. Common disposl methods include lnd-filling, incinertion, pyrolysis. solr drying fields, nd sludge storge bsins. The dvntges nd disdvntges of vrious disposl methods for MS sludges re not well described ; however, combustion methods with het recovery my provide more beneficil effects thn lnd-filling, especilly since cceptble lnd-filling sites re becoming very scrce in mjor prts of the country. 16 h. Rector Design ne mjor drwbck of conventionl digester design is the lrge volume needed. Severl design vritions hve been proposed to reduce cpitl nd operting costs by digester volume reduction. The success of n innovtive design could hve significnt impct on the economics of MS bioconversion since the digestion process my represent 28% of the energy consumption nd 35% of the cpitl cost in the overll digestion systems' Alternte design concepts hve been reviewed nd discussed in report by MITRE Corportion. -" They re summrized here in Tble 9. Severl models my be superior to the conventionl digester. However, the technology of these novel concepts re t n erly stge of development nd re still unproven t the prcticl level. Detiled studies. including economic nlysis of these concepts, should be mde in order to incorporte ny dvntges into "estblished." conventionl systems. C. Economic Studies umerous studies on the economics of MS bioconversion to methne hve been reported A computer model ws developed" for sizing process equipment t the lowest cost bsed on the 1000 ton/dy Dyntech system shown in Figure 3. Anlysis of cpitl nd operting costs, credits for hndling MS (tipping fee) nd sewge sludge. nd

23 94 Fuel Gus Developments Tble 9 ALTERATE REACTR DESIGS Process description Fixed or fluidized bed: pcked bed digester composed of continment vessel, inert bed mteril which support biologicl growth, circulting fluid, substrte (i.e., MS) 2-Stge digestion : incorportes high-rte digestion nd stndrd unmixed digester for solids removl 2-Phse digestion: 2 biologiclly ctive digesters in series functioning to optimize conditions for cid-forming nd methneproducing bcteri, 2-phse seprtion cn be ccomplished by kinetic control of both groups of bcteri through djustment of orgnic loding rte nd cell retention time Plug flow digestion : conicl cylinder lying on its side through which digester substrte continuously moves. Feedstock continuously loded from one end nd dischrged from the other end. Virtully no blending or mixing solids Potentil dvntges Specific gs production expressed s Vol. CH,/Vol Rector is up to 7 times tht of conventionl systems. Ability to tret 2 to 3 times the solids concentrtion of conventionl systems. Less energy requirements Lower retention time Incresed process control, lower overll retention time, improved digester efficiency nd hence methne yields, less energy requirements for mixing, nd less digester volume required o energy requirements for mixing, lower cpitl cost, more efficient conversion Technicl sttus Pilot scle ppliction to MS ; plnt biomss. o detiled conceptul design or cost nlysis of lrge scle bioconversion fcility s yet ell estblished for sewge sludge tretment Lbortory scle kinetic control demonstrted using glucose, sewge sludge nd cellulose s substrte ; 2-phse sewge sludge digestion plnt design developed Economic fesibility demonstrted with frm nd griculturl wstes (pilot scle). MS/ rw sludge cid digestion by plug flow demonstrtion on bench scle penlties for the disposl of the effluent were included. To ccount for uncertin vlues in the nlyticl description of the system, sensitivity study ws incorported to evlute technologicl dvnces or economicl chnges on the process. The process economics were most sensitive to the digestible frction of MS nd tipping fees. ther mjor economic fctors included digester operting conditions, dewtering costs, nd finncing options (i.e., public vs. privte). Bsed on these fctors, the cost of methne production ws economiclly cceptble compred with projected costs of nturl or synthetic gs. Additionl considertions such s cost of disposl fcilities for trommel screen "unders" (trommel rejects), ir clssifiction of "hevies," dewtered cke, incresed electric power requirements, nd operting personnel were not considered. Inclusion of these fctors into the nlysis gve different picture. It showed tht the economic fesibility of the process ws not encourging nd tht tipping fee of $15.60/ton (without incinertion) to $19 to $22/ton (with incinertion) ws required s the mjor source of revenue to offset Cost. 23 The technicl nd economic fesibility ws lso exmined for vrious MS bioconversion processes bsed on four mjor process steps : feedstock preprtion, feedstock pretretment, digestion nd gs production, nd residue disposl." Four process were selected on this bsis (Tble 10), nd cost estimtes were prepred for production plnt cpcities of 1000, 2000, nd 3000 ton/dy of MS. A number of process vribles ffected the cost of gs production for the four lterntive processes. The most influentil were the mnner of finncing, feedstock preprtion costs, nd tipping fees. The following conclusions were reched in the nlysis :

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25 96 Fuel Gs Developments Tble 10 FUR ALTERATIVE MUICIPAL SLID ASTES DIGESTI PRCESSES Alterntive I- Mesophilic digestion of fluff RDF: residue disposl to lndfill Alterntive2- Mesophilic digestion of fluff RDF: therml processing of residue Alterntive3- Thermophilic digestion of fluff RDF: therml processing of residue Alterntive4- Mesophilic digestion of Cl Recovery System RDF: therml processing of residue (Tken from Brown nd Cldwell Consulting Engineers. Microbil Production of Methne from Refuse. Report Prepred for So. Cliforni Edison Co.. Reserch nd Development Minimum cost gs would be produced t mximum loding rtes of MS subjected only to shredding nd ferrous metls removl. 2. Feed concentrtion should be the mximum, depending on vilble mixing. 3. The dewtered residue should be thermlly processed, nd the het recovered for plnt opertion; excess stem should be sold. 4. The Cl Recovery process is pprently the most cost-effective t bout 1000 ton/dy if excess stem is sold nd orgnic loding rtes re restricted to the lower rnge. 5. The economics of fuel gs production re less ffected by process vrition thn by externl fctors such s tipping fees, sewge sludge disposl credits, nd the method nd cost of digested residue disposl. Site-specific economic studies" on the digestion of MS with therml processing of the residue disclosed five importnt fctors for estblishing the economic fesibility of ny MS conversion process : l. Cpitl cost of the fcility 2. pertion nd mintennce cost 3. Mrket for recovered stem from therml processing of the nondigestible frction or gs from pyrolytic therml processing of nondigestible frction 4. Lnd cquisition nd development of cost 5. Tipping fee revenue The first three fctors re function of process selection nd fcility design ; two re site specific. Among the mjor conclusions were the remining 1. A fcility designed for nerobic digestion nd therml processing of MS in Southern Cliforni cn produce medium Btu nd/or stem product t cost s low s $6.00/ 10 Btu. 2. The most cost-effective option pprently involves mesophilic digestion of the noncellulose orgnic frction, with therml processing of the nondigestible frction. 3. Energy recovery from the therml processing step is necessry for the economic fesibility of the process. A generl cost evlution of the co-digestion of MS nd sewge sludge ws recently undertken" with the following ssumptions : resource recovery plnt sized t 2500 metric

26 tons MS per dy; use of Cl Recovery feedstock; digester operting conditions s specified by Pfeffer nd Liebmnn," vcuum filtrtion for dewtering ; nd lnd-filling for the residue disposl. The lrgest projected expense ws the vcuum filter dewtering equipment. The difference in cost between the system with nd without dewtering ws $25.7 million/yer without ccounting for the tipping fee. Thus, lthough lower disposl fees were expected for dewtered solids, the svings were not sufficient to ugment the cpitl nd operting costs of the filtering equipment RECET ADVACES Much work (Tble 11), mostly t mesophilic tempertures, hs been reported on methne production from nerobic digestion of vrious orgnic wstes. The feedstocks included primry nd ctivted sewge sludge, processed municipl solid wste, diry nd beef cttle wstes, wter hycinths, kelp, nd pet. The orgnic loding rte rnged from 0.05 lb VS (voltile solids) per cubic foot rector to s much s 1.7 lb/ft'/dy. The hydrulic retention time (HRT) lso vried widely, from 3 dys for cttle feedlot wste t the thermophilic 60 C to 60 dys for pet t 35 nd 55 C. The "methne conversion" vlues (cubic foot CH, per pound VS dded) indicte the efficiency of the digestion process to convert the orgnic substrte into methne gs. The mximum biogs yield from protein is 12 ft'/lb nd 20 ft'/ib for ft. 22 This is equivlent to rnge of 5 to 9 ft'/ib of orgnic substrte dded, ssuming the presence of n undigestible component in the substrte. The mximum theoreticl yield of 8.4 ft' CH, per pound VS dded for pet' should be decresed to 6.7 ft' CH, per pound VS dded if bcteriologicl growth yield of 0.2 were used in clculting the methne yield. The mximum theoreticl yields of CH, clculted for kelp nd corrected for cellulr mintennce requirements" re between 5.84 nd 6.77 ft' CH, per pound VS dded. ne pound of ny digestible substrte (such s MS) yields mximum of 6.65 ft' CH, t stndrd temperture nd pressure. 65 Methne conversion efficiencies of 4 ft' CH, per pound VS dded re typicl for the digestion of vrious orgnic wstes (Tble 11). However, the digestion of sewge sludge commonly produces 5.5 ft' CH, per pound VS dded, depending on the proportion of methnogenic substrtes such s fts nd other lipids. The digestion of MS lone or supplemented with rw sewge sludge yielded methne conversion vlues in cubic feet CH, per pound VS dded of 3.80,' 5.16,"-' 3.85,' nd 2.73." hen the methne conversion vlue is divided by the hydrulic retention time, vlue clled the "specific methne production rte" (cubic feet CH, per pound VS dded per dy) results ; this figure is useful for evluting gs production for orgnic loding rtes nd retention times required to chieve tht conversion. High reltive vlues for the specific methne rte indicte good methne conversion efficiency without excessive retention time. In the sme MS digestion studies cited bove, the respective specific CH, production rtes were 0.38, 0.17, 0.13, nd 0.27 ft' CH, per pound VS per dy. Although n intermedite CH, conversion efficiency of 3.80 ft'/ib VS ws reported by Ghosh et l." the specific CH, production rte ws high t 0.38 ft'/lb/dy becuse of the low retention time of 10 dys. Since the specific CH, production rte depends on conversion efficiency nd retention time, it is useful for evluting optiml methnogenic digestion nd process design. Dt on the digestion of diry nd beef cttle wstes support the following conclusions : I. The homogeneous nture of the substrte nd its dptbility to thermophilic digestion fcilitte the use of high loding rtes. 2. Digestion t high loding rtes results in reltively high volume CH, per volume digester-dy production vlues. 3. Methne conversion efficiency nd percent VS reduction vlues re typiclly low.