Optimization of Anaerobic Digestion of Sewage Sludge Using Thermophilic Anaerobic Pre-Treatment

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1 Downloaded fom obi.du.dk on: Jun 18, 2018 Opimizaion of Anaeobic Digesion of Sewage Sludge Using Themophilic Anaeobic Pe-Teamen Lu, Jingquan; Ahing, Bigie Kiæ Publicaion dae: 2007 Documen Vesion Publishe's PDF, also known as Vesion of ecod Link back o DTU Obi Ciaion (APA): Lu, J., & Ahing, B. K. (2007). Opimizaion of Anaeobic Digesion of Sewage Sludge Using Themophilic Anaeobic Pe-Teamen. Geneal ighs Copyigh and moal ighs fo he publicaions made accessible in he public poal ae eained by he auhos and/o ohe copyigh ownes and i is a condiion of accessing publicaions ha uses ecognise and abide by he legal equiemens associaed wih hese ighs. Uses may download and pin one copy of any publicaion fom he public poal fo he pupose of pivae sudy o eseach. You may no fuhe disibue he maeial o use i fo any pofi-making aciviy o commecial gain You may feely disibue he URL idenifying he publicaion in he public poal If you believe ha his documen beaches copyigh please conac us poviding deails, and we will emove access o he wok immediaely and invesigae you claim.

2 Opimizaion of anaeobic digesion of sewage sludge using hemophilic anaeobic pe-eamen Jingquan Lu BioScience and Technology, BioCenum-DTU Technical Univesiy of Denmak DK-2800 Lyngby Denmak Submied in paial fulfillmen of he equiemens fo he Ph.D degee a he Technology Univesiy of Denmak Apil, 2006

3 Conens Conens...1 Tile page...4 Peface...5 Acknowledgemen...6 Summay (English) Inoducion Issues on sewage sludge Geneaion and composiion Envionmenal impac Teamen and disposal mehods Pinciples in anaeobic digesion of sewage sludge The 4-sep anaeobic digesion model Hydolysis Acidogenesis Aceogenesis Mehanogenesis Facos affecing anaeobic digesion pocess Specific chaaceisics of sewage sludge Tempeaue ph Maco- and miconuiens Inhibiion Toxiciy Reenion ime Agiaion Feeding saegy Opimizaion of anaeobic digesion fo sewage sludge eamen Themophilic anaeobic pe-eamen Summay of he pe-eamen mehods Raionale fo conducing pe-eamen unde hemophilic anaeobic condiions Exploiaion of he anaeobic hemopiles fo hydolysis and acidogenesis Simulaneous achievemen of hemal pe-eamen effecs Simulaneous achievemen of paseuizaion Easy fo opeaion Inenal-supplied enegy fo heaing Deeminaion of opimal empeaue and eenion ime Raionale fo deeminaion of opimal empeaue and eenion ime Maix of empeaue and eenion ime combinaions Expeimens The opimal combinaion of empeaue and eenion ime

4 4.4 Diffeeniaion of he effecs on he hydolysis of he oganic paiculaes The diffeen chaaceisics of pimay sludge and wase acivaed sludge Expeimens Diffeeniaion of he biological and hemal effecs Two-sep anaeobic digesion Theoeical backgound of wo-phase AD pocess Compaison of he wo-phase pocess wih he single-phase pocess Opeaion a nomal RT Opeaion a educed RT Peubaion ess Summay of he wo phase pocess Conclusion...49 Refeences...51 Papes: Pape-I: Themophilic anaeobic pe-eamen fo hydolysis and hygienizaion of sewage sludge Pape-II: Compaison of he wo-phase anaeobic digesion (73 o C/55 o C) he single-phase anaeobic digesion (55 o C) in eaing sewage sludge Pape-III: Biological and hemal effecs of hemophilic anaeobic pe-eamen on he hydolysis of oganic solids in sewage sludge Pape-IV: Themal pe-eamen of pimay sludge and seconday sludge a 70 o C pio o anaeobic digesion Pape-V: Effec of hype-hemophilic pe-eamen on hemophilic anaeobic digesion of pimay sludge Confeence pesenaion Effecs of empeaue and hydaulic eenion ime on hemophilic anaeobic pe-eamen of sewage sludge - 3 -

5 Pojec ile: Developmen of an innovaive and cos-efficien mehod fo opimal biogas poducion and euse of sewage sludge Thesis ile: Opimizaion of anaeobic digesion of sewage sludge using hemophilic anaeobic pe-eamen Suden: Jingquan Lu BioScience and Technology, BioCenum-DTU Technical Univesiy of Denmak, DK-2800 Lyngby, Denmak Supeviso: Bigie Kiæ Ahing BioScience and Technology, BioCenum-DTU Technical Univesiy of Denmak, DK-2800 Lyngby, Denmak Submied in paial fulfillmen of he equiemens fo he Ph.D degee a Technical Univesiy of Denmak Apil,

6 PREFACE This hesis pesens he esuls fom he wok done fo my Ph.D sudy a BioScience and Technology, BioCenum-DTU, he Technical Univesiy of Denmak duing he peiod fom Febuay 1, 2003 o Apil 30, 2006 wih Pofesso Bigie Kiæ Ahing as supeviso. The hesis is oganized in wo pas. Pa one includes he lieaue suvey of he sewage sludge poblems, he heoeical backgound of anaeobic digesion, he mehods fo pe-eamen, and he lab wok esuls of he sudies on he effecs and mechanisms of hemophilic anaeobic pe-eamen, compaison of he wo-phase pocess wih he single-phase pocess, and he consideaions of he sa-up, opeaion as well as he economic assessmen of he wo-phase pocess. Pa wo conains five papes submied o inenaional jounals and one oal pesenaion a an inenaional confeence. The papes and he pesenaion ae: Pape I: Pape II: Pape III: Pape IV: Pape V: Lu, J. and Ahing, B.K. (2006) Themophilic anaeobic pe-eamen fo hydolysis and hygienizaion of sewage sludge. Manuscip submied o Jounal of Envionmenal Engineeing. Lu, J. and Ahing, B.K. (2006) Compaison of he wo-phase anaeobic digesion (73 o C/55 o C) wih he single-phase anaeobic digesion (55 o C) in eaing sewage sludge. Manuscip submied o Wae Science and Technology. Lu, J. and Ahing, B.K. (2006) Biological and hemal effecs of hemophilic anaeobic pe-eamen on he hydolysis of oganic solids in sewage sludge. Manuscip submied o Wae Reseach. Skiadas, I.V, Gavala, H.N., Lu, J. and Ahing, B.K. (2005) Themal pe-eamen of pimay sludge and seconday sludge a 70 o C pio o anaeobic digesion. Wase Science and Technology, Vol. 52, No. 1-2, pp Lu, J., Gavala, H.N., Skiadas, I.V., Mladenovska, Z. and Ahing, B.K. (2007) Effec of hype-hemophilic pe-eamen on hemophilic anaeobic digesion of pimay sludge. Acceped by Jounal of Envionmenal Managemen (Ref. No.: JEMA-D ). Confeence Pesenaion: Lu, J. and Ahing, B.K. (2005) Effecs of empeaue and hydaulic eenion ime on hemophilic anaeobic pe-eamen of sewage sludge. In: Confeence Poceedings of 4 h Inenaional Symposium on Anaeobic Digesion of Solid Wase. Copenhagen, Vol. 1, Secion 3a, pp

7 ACKNOWLEDGEMENT I would like o acknowledge my supeviso, Pofesso Bigie Kiæ Ahing, fo he kind supevision, suppo and guidance duing my Ph.D sudy. I would like o acknowledge all of he people who have eve helped me duing my Ph.D sudy. Special acknowledgemen is given o Zuzana Mladenovska, Hinich Hamann, and he echnicians, Thomas Andesen, Gie Hinz-Beg, Anee H. Løh, Kain M. Due fo hei help wih my expeimens and he analysis of he expeimenal esuls. The happy and fiendship woking amosphee ceaed by all of my colleagues in he Goup of BST will be one of he bes pas ha ae woh easuing fo me in my life. I am vey gaeful o he Sabina Lind and he es of he saff of Lundofe Wasewae Teamen Plan, Lyngby, Denmak, fo hei paien help wih aking he sewage sludge samples used in he sudy, and fo hei povision of useful daa egading he geneaion and eamen of sludge in he wasewae eamen plan. I would like o expess my hankfulness o my fome colleagues in Fushun EPA in China, my family and my Danish and Chinese fiends fo hei suppo and encouagemen duing his sudy. The pojec was financed by DTU. Besides, Daloon Fonden and he Sino-Danish Scienific and Technological Coopeaion Commiee also povided sipend fo my say in Denmak duing his sudy. Jingquan Lu Copenhagen, Denmak Apil 30,

8 SUMMARY The pupose of my Ph.D sudy is o develop an innovaive and cos-efficien mehod fo opimal biogas poducion and euse of sewage sludge. Topics including hemophilic anaeobic pe-eamen, enhancemen of hydolysis of oganic paiculaes, pahogen educion effec, and efficiency and sabiliy of wo-phase anaeobic digesion ae heefoe focused on. Afe he opimal empeaue and eenion ime fo he pe-eamen had been deemined by manipulaing coninuous sied ank eacos unning a empeaues in he ange of o C fo he eenion imes in he ange of days (Pape I), a wo-phase anaeobic pocess wih a pe-eamen phase unde opimal condiions befoe he mehane phase was esed in ems of eaco oganic educion efficiency, pahogen educion effec and pocess sabiliy by using a single-phase pocess as conol (Pape II). To fuhe invesigae he pe-eamen mechanisms, a sudy on disinguishing diffeen effec mechanisms was caied ou (Pape III). In addiion, in ode o idenify he micobial aciviies, enegy balance fo he wo-phase eaco sysem, wo ses of wo-phase pocess (70 o C/55 o C) wee sudied, in which pimay and wase acivae sludge was used as subsae, especively (Pape IV and Pape V). I was idenified ha pe-eamen unde hemophilic anaeobic condiions, wih he opimal combinaion of empeaue and eenion ime of 73 o C and 2 days, simulaneous enhancemen of hydolysis of he oganic paiculaes in he sludge, high degee of acidificaion of he hydolysis poducs and achievemen of saisfacoy pahogen educion effec wee obained. The enhanced hydolysis and acidificaion wee conibued by hemal and biological effecs as he majo mechanisms fo boh pimay sludge and wase acivaed sludge. Fo wase acivaed sludge, an addiional mechanism, micobial cell lysis, was also involved. Themophilic anaeobic pe-eamen unning a he idenified opimal condiion, i.e., a he empeaue of 73 o C fo a eenion ime of 2 days, was employed as he acidphase of he wo-phase anaeobic digesion sysem. By unning he wo-phase CSTR pocess (73 o C/55 o C) wih a paallel single-phase CSTR pocess (55 o C) as conol, i - 7 -

9 was veified ha he wo-phase pocess could keep no only he saisfacoy pahogen educion effec ha he single-phase pocess could no achieve, bu also possessed supeioiies ove he single-phase pocess such as inceased efficiency in conveing wase oganic maeial ino biogas and enhanced pocess sabiliy due o he effec of pe-eamen. Micobial aciviies of he wo-phase pocess wee highe han hose of he single-phase pocess. I was concluded fom his sudy ha hemophilic anaeobic pe-eamen can be used o opimize anaeobic digesion pocess fo sewage sludge eamen. The opimized wo-phase pocess is high-ae, efficien and cos-effecive, and possesses he capabiliy o eliminae he pahogens. The significance of implemening he opimized wo-phase anaeobic digesion pocess lies in he following aspecs: 1. The envionmenal poblem caused by sewage sludge, which is a global one and geing moe and moe sevee, can be solved in a susainable way; 2. The enegy saved in he oganic maeial of he sewage sludge can be exaced in he fom of biogas, which is CO 2 neual and enewable, and can be used o poduce eleciciy and hea; 3. The hoough eliminaion of pahogens makes i possible o ecycle he plan nuiens and ine oganic maeial in he digesed effluen back o he famland as feilize and soil condiione wihou any fea of speading of epidemic disease; 4. The hea needed o keep he pocess empeaue can be obained by buning he biogas poduced by he pocess iself, so hee is no dependency on he exenal enegy supply

10 1. INTRODUCTION The developmen of wasewae eamen echnology ogehe wih he implemenaion of singen envionmenal legislaion has successfully poeced he aquaic sysem fom polluion in many of he counies of he wold. Howeve, sewage sludge, as he by-poduc of he wasewae eamen plan, is also geneaed a he same ime. Unlike he ohe kinds of wase, he geneaion of which can be educed by clean-poducion echnology, sewage sludge is no eviable and is geneaion will, on he conay, incease along wih he incease of wasewae dischage and eamen ae (Wang, 1997). Sewage sludge is now becoming a woldwide envionmenal poblem because of is inceasing poducion and is high conens of oganic wase and pahogens, as well as xenobioics and heavy meals. If no being eaed o disposed popely, his dangeous wase may cause he envionmen and human as well as animal healh exposed o emendous hea (Ahing, 2003). Anaeobic digesion is a biological pocess ha can degade wase oganic maeial by he conceed acion of a wide ange of micooganisms in he absence of oxygen. The pocess consiss of a complex seies of eacions ha conve a wide aay of polymeic subsances such as cabohydaes, poeins, and lipids, having cabon aoms a vaious oxidaion and/o educion saes, o one-cabon molecules in is mos oxidized sae (CO 2 ) and is mos educed sae (CH 4 ). In a vaiey of anaeobic envionmens, such as he inesinal ac of animals, maine and fesh wase sedimen, paddy fields, sewage sludge, wae logged soils, and in he egion of volcanic ho spings and deep-sea hydohemal vens, naually exiss his pocess (Wesemann, 1996; Madigan, e al., 2003). Anaeobic digesion has been manipulaed by man fo many yeas o ea sewage sludge (Hamzawi, e al., 1998). Befoe anaeobic digesion, he oganic maeial in he sludge also auomaically decay due o he biological aciviies of he exensive exisence of micooganisms in he sludge, poducing offensive, odoous and educed end poducs such as fay acids, mecapans and amines. Afe anaeobic digesion, - 9 -

11 he digesae consiss of an odo fee esidue wih appeaance simila o pea. Mehane poduced by he anaeobic digesion pocess is a clean, CO 2 neual and enewable enegy ha can be used o poduce hea and eleciciy. Fuhe moe, anaeobic digesion seems o be he only cos-effecive mehod ha makes i possible fo sewage sludge o use famland as a safe and pemanen oule desinaion wih posiive effec, i.e., he digesae, which has eained plan nuiens such as N and P, can be ecycled as feilize and soil condiione back o he famland and hus keeps hese naual nuiens ecycled wihin a closed loop ecosysem, and emain o impove he soil sucue of he famland. The unique feaues of anaeobic digesion have made i supeio o any ohe mehods such as landfill, incineaion, aeobic eamen and ec., which have eve been and ae sill being used fo he eamen and disposal of sewage sludge in some pas of he wold. Howeve, he advanages of anaeobic digesion in he eamen of sewage have no been bough ino full play. This pocess is sill fa fom opimizaion. When conduced a mesophilic empeaues, he pocess eenion ime nomally has o be se as long as 30 days. When conduced a hemophilic empeaues, alhough he eenion ime can be educed o 15 days, special cae fo opeaion has o be aken fo he sabiliy of he pocess. No mae unde which empeaue condiions, only aound 50% o 60% of he oganic maeial can be degaded, leaving a lage poenial o incease he biogas poducion. Besides, even hough i is has been widely believed ha anaeobic digesion pocess is of he capabiliy of educing he pahogens (Bendixen, 1999; Nielsen & Peesen, 2000), he popeies of he digesed effluen is sill hadly meeing he singen equiemen by legislaions in elaion o land applicaion of sewage sludge in agiculue (Vesilind, 2000). In ecen yeas, due o he diecive of minimizaion of landfill and calling fo euse and ecycle of he wase by he new wase managemen policies (EU, 2000), and he eageness fo exacion enegy fom wase including sewage sludge o ease up he dependence on enegy fom fossil fuels (Chynoweh, e al., 2001), emendous eseach has been caied o opimize he anaeobic digesion pocess. Bee undesanding of he basic mechanisms occuing in he anaeobic pocess, conducing he pocess a hemophilic empeaues, applicaion of diffeen kinds of mehods fo

12 pe-eamen, developmen in pocess monioing and conol, and phase sepaaion have conibued o he impovemen of anaeobic digesion pocess fom diffeen aspecs. Howeve, simulaneous achievemen of boh biogas incease and digesae paseuizaion has neve been obained by using a cos-effecive and single effo. This sudy is o opimize he anaeobic digesion pocess using hemophilic anaeobic pe-eamen. I is hoped ha he pe-eamen could enhance he hydolysis of he oganic paiculaes in he sludge so as o incease he efficiency of he oganic maeial convesion o mehane, and a he same ime, pahogens in he digesed sludge can be oally eliminaed duing his pe-eamen sep so as o he digesed effluen can mee he equiemen on euse of he digesed sludge in agiculue

13 2. ISSUES ON SEWAGE SLDUGE 2.1 Geneaion and composiion Sewage sludge is an unwaned and ineviable by-poduc fom wasewae eamen plans, he pupose of which is o claify wasewae. Sewage sludge is geneaed by sedimenaion boh befoe and afe he bio-eamen pocess, named as pimay sludge and wase acivaed sludge (o seconday sludge), especively. In some wasewae eamen plans, eiay sludge may be poduced as well due o he eiay o polishing eamen of he biologically eaed wasewae. Howeve, he quaniy of his kind sludge is elaively small and he oganic facion in he solids is sligh, so his facion is ofen ignoed. Pimay sludge is of a non-homogeneous naue because of some ahe coase consiuens in i. Afe hickening in he pimay seling ank, he solids conen is abou 5-10%, of which abou 70% consiss of oganic mae. Wasewae nomally conains housands of diffeen oganics, so he composiion of pimay sludge is vey complex. Seconday sludge consiss of wase acivaed sludge fom he aeaion anks o humus fom he ickling files, boh of which ae composed of micooganisms and ohe life foms, which ae wihdawn o flushed ou of he sysem. In addiion, small popoions of adsobed suspended solids and colloids deived fom he wasewae ae included. By nomal hickening and/o dewaeing echnologies, he solids concenaion in seconday sludge is abou 1-6%, in which he oganic facion is also aound 70%. In geneal, pimay sludge and seconday sludge accoun fo 60-80% and 20-40% of he oal sewage sludge based on he volaile solids (VS) conens, especively, and he pecenage depends no only on he echnique of he wasewae eamen pocess, bu also depends on he souces of he wasewae. (Mudack & Kuns, 1986; Henze, e al., 2000). The poducion of sewage sludge is huge and has been a woldwide poblem. In 2000, he sewage sludge poducion in Denmak and in he EU membe saes was abou 160,000 and 8,500,000 ons of dy weigh, especively (Magoaou, 2000; Jensen and Jepsen, 2005). To he global pespecive, especially in he developing couies, along wih he incease in populaion, developmen in ubanizaion and he implemenaion

14 of he singen legislaion and egulaion elaed o wasewae and sewage sludge, hee will be moe and moe wasewae o be eaed and hus moe and moe sewage sludge o be geneaed and eaed (Hame, e al., 1985). 2.2 Envionmenal impac Wasewae eaed in he wasewae eamen plan consiss of domesic sewage, indusial effluens and som-wae unoff fom oads and ohe paved aeas, and heefoe, sewage sludge conains no only wase oganic maeial, bu also pahogenic micooganisms, as well as ace amoun of polluans such as xenobioics and heavy meals. Sewage sludge is a good subsae fo baceia and heefoe puefies vey apidly (Mudack & Knus, 1986), causing offensive emission, gowh of flies o pahogens, as well as conaminaion of soil and aquaic sysem. The pahogens ha ae concenaed ino he sewage sludge include viuses, baceia, poozoans and lage paasies such as human oundwoms, apewoms and live flukes. These pahogens ae capable of independen exisence. They will muliply unde suiable condiions. Unde condiions unsuiable fo gowh, hey ae no necessaily lehal. Some of hem may fuhe evolve hei sucues such as cyss o spoes as a means of suvival in advese condiions. Wihou pope eamen, hey may spead diseases. Heavy meals such as cadmium, mecuy, zinc, ec., ae oxic. They may ene he sewage sludge via indusial dischages, and also in some cases fom galvanized wae pipes. Xenobioics such as PCBs, hebicides, insecicides and ec, ae chemical subsances ha ae foeign o he biological sysem and may accumulae in human body hough food chain. So, if sewage sludge is no popely eaed o disposed, i can make he envionmen and he healh of human and animals exposed o a emendous hea (Hänel, 1988; Mudack & Kuns, 1986; Caingon, 2001; Ahing, 2003). 2.3 Teamen and disposal mehods Many mehods have been developed and used in he eamen and/o disposal of sewage sludge, bu he opeaion of he eamen o disposal is usually expensive and/o easy o conaminae he envionmen. Fo mos of he wasewae eamen plans, he eamen and disposal of sewage sludge accouns fo 50% o moe of he oal capial and opeaion cos (Benefield & Randall, 1985). So he pocessing and disposal of sewage sludge ae inceasingly being he opics of envionmenal,

15 financial and echnological concens (Woodad & Wukasch, 1994). The following ae he mehods ha can be efeed o fom lieaues up o oday: o o o o o o o o Speading on wild land Discading ino he ocean Diec applicaion on famland Landfill Incineaion Aeobic composing Anaeobic digesion Injecion ino old oil wells Among hese mehods, speading on wild land, discading ino he ocean and diec applicaion on famland ae he mos pimiive disposal mehods. Due o he obvious conaminaion o he envionmen, hese mehods have been in pacice fobidden in many of he counies and egions. Landfill only pospones he poblem since he leakages can conaminae he gound wae and ae subjeced o advanced eamen in mos Euopean counies involving high puificaion cos. Fuhe moe, expendiue fo developmen of landfill aeas is geing highe due o he decease of space in highly populaed counies. So, disposal of sewage sludge by landfill is o be phased ou, even hough 35-40% of he sludge in Euope is sill deposied in landfill oday (Ødegaad e al., 2002). Incineaion seems o be a good mehod because, afe dewaeing and buning, hee is almos nohing lef compaed o he oiginal huge sludge volume. Howeve, i coss enegy fo dewaeing and nomally needs inpu of exenal fuel. Also, he disposal of ash and he eamen of exhaus gas ae expensive (Ahing, 2003). Aeobic composing canno ecove he enegy of he biomass in he sludge, and may cause odo poblem. Injecion ino old oil wells is no applicable fo counies and egions whee hee exiss no oil field. So, i has been geneally acceped ha, among all he eamen and disposal mehods ha have been used up o now, anaeobic digesion is univesal and susainable. This is because anaeobic digesion can sabilize he oganic maeials, educe he numbe of pahogens (Bendixen, 1999; Nielsen and Peesen, 2000), degade xenobioics (Hamann and Ahing, 2003), ecove CO 2 neual enegy in he fom of CH 4, as well

16 as make i possible o ecycle he plan nuiens in i back o he famland. And heefoe, anaeobic digesion complies wih oday s sludge managemen policies, i.e., o educe he wase seam o landfill and o ecycle he oganic maeial and he plan nuiens back o he agiculual soil (EU, 2000; Ahing, 2003). So, anaeobic digesion of sewage sludge is woh of fuhe sudying

17 3. PRINCIPLES IN ANAEROBIC DIGESTION OF SEWAGE SLUDGE 3.1 The 4-sep anaeobic digesion model Anaeobic digesion is a muliple bio-pocess, of which fou main seps can be idenified (Ghos 1975; Kaspe and Wuhmann, 1978; Guje and Zehnd, 1982; Madigan, e al., 2003), namely hydolysis, acidogenisis, aceogenisis and mehanogenisis, involving six majo disinc pocesses (Figue 3-1). Hydolysis Acidogenesis 100% COD Paiculae oganic maeial Poeins Cabohydaes Lipids ~39% 1A 1B 1C ~21% ~40% ~5% ~34% Amino acids, sugas Fay Acids ~35% 2 ~66% ~34% 3 Anaeobic Oxidaion ~20% ~0% ~20% Inemediay poducs, e.g., ~34% popionae buyae ~11% Aceogenesis ~23% 4 ~11% ~12% ~8% Aceae Hydogen ~?% 5 6 ~70% Mehanogenesis ~30% Mehane 100% COD Figue 3-1. Poposed eacion scheme fo he anaeobic digesion of sewage sludge. Adaped fom Guje and Zehnde (1983). Pecenages indicae subsae flow in he fom of COD o CH 4 equivalens. Only he ne flow of subsaes hough exenal pools is indicaed. Numbes in he cicles idenify diffeen pocess Hydolysis In his sep, complex oganic polymes ae hydolyzed ino smalle unies such as sugas, long-chain fay acids and amino acids. This is caied ou by diffeen goups of obligae o faculaive femenaive baceia hough exceing exacellula enzymes (Kaseng e al., 1992). The poeolyic baceia poduce poeases ha caalyze he hydolysis of poeins ino amino acids (Figue 3-1, 1A); he cellolyic and xylanolyic baceia poduce cellulases and/o xylanases ha degade cellulose and xylan (boh ae cabohydaes) o glucose and xylose, especively (Figue 3-1, 1B); and he liplyic baceia poduce lipases ha degade lipids o glyceol and long chain fay acids (Figue 3-1, 1C)

18 3.1.2 Acidogenesis The dissolved sugas, long-chain fay acids and amino acids poduced by hydolysis ae used in his sep eihe by femenaive baceia (Figue 3-1, 2) o by anaeobic oxidizes (Figue 3-1, 3) (Guje and Zehnde, 1983), foming aceae and ohe shochain fay acids, alcohols, hydogen and cabon dioxide. Acidogenesis is a obus and ofen he fases sep in he whole anaeobic digesion pocess. When poons ae used as elecon accepo wih concuen hydogen poducion, he oxidaion of subsae by femenaive baceia povides he lages amoun of enegy. In a well-opeaed anaeobic eaco, abou 70-80% of he hydolysis poducs will be ansfomed diecly o mehanogenic subsaes i.e., hydogen, cabon dioxide and aceae, wih he emaining 20-30% ansfomed ino ohe inemediae poducs, such as volaile fay acids (VFAs) and alcohols (Guje and Zehnd, 1983; Schink, 1997; Ahing 2003). Howeve, when he poduced hydogen canno be consumed simulaneously by he hydogen-uilizing mehanogens, he inceasing concenaion of hydogen is inhibioy o he hydogen poducion. In his case, he femenaive baceia will swich meabolism so ha hey can sill obain some enegy, and hen abou 50-70% of he hydolysis poducs will be ansfomed ino he inemediae poducs (Byan, 1979; Klass, 1984; Ahing, 2003). The inemediae poducs poduced in acidogenesis sep canno be uilized by he mehanogens, and mus be fuhe degaded in he aceogenesis sep by aceogens Aceogenesis Since he inemediae poducs mus be fuhe oxidized o aceae, H 2 and CO 2 befoe hey ae used by mehanogens, he aceogenesis sep is cucial fo he successful poducion of biogas (Figue 3-1, 4). In conas o he femenaive baceia, he aceogens ae obligaely symbioic baceia (McInemey, e al., 1980; Boone & Byan, 1980; Wesemann, 1996). Unde sandad condiions, hese oxidaion pocesses ae endohemic, i.e., enegy demanding and canno gow when he H 2 paial pessue is high. As shown in Figue 3-2, only when H 2 paial pessue is lowe han a ceain level, he oxidaion of popionae and buyae can be possible. Also in conas o he femenaive baceia, aceogens canno swich hei meabolic

19 pahway bu educe of H + o H 2. Theefoe, he poceeding of aceogenesis is elying on he pesence of hydogen-uilizing mehanogens o emove H Mehanogenesis -40 Bu ya e oxida ion 0 40 Popionae oxidaion H 2 paia l pe s s ue (-lo g a m) Table 3-2. Fee enegy change of aceogenesis and mehanogenesis fom H 2 as a funcion of he P. Daa deived fom Ache (1983). H 2 Calcualion based on sandad values fo fee enegies a ph 7, 25 o C, 34 mm HCO, 1mM VFAs and 0.7 am. 3 PCH Mehanogenesis The final sep of he anaeobic digesion is called mehanogenesis ha minealizes he femenaive poducs o mehane. This sep is caied ou by wo main goups of mehanogens: he aceiclasic mehanogens, which degade aceae, belonging o he genea Mehanosacina and Mehanosaea (Figue 3-1, 5), and he hydogen-uilizing mehanogens (Figue 3-1, 6), of which an aay of genea exis. Mehanogens belong o Achae, a unique goup of micooganisms, phylogeneically diffeen fom he main goup of pokayoic micooganisms (Madigan, e al., 2003). I is esimaed ha, unde sabilized condiions, abou 70% of mehane is poduced by he aceaeuilizing mehanogens and 30% by he hydogen consuming mehanogens (Smih e al., 1980; Klass, 1984). Mehanogenesis is egaded as he moive foce of he whole anaeobic degadaion. In conas o some of he aceogens, mehanogenesis is an enegy poducing pocess unde sandad condiions (Figue 3-2). Only if he pesence of he hydogen-uilizing mehanogens keeping he paial pessue low, can he aceogens pefom a caabolic oxidaion which would no be enegy yielding if he hydogen-consuming baceia had

20 no been pesen (Ache e al., 1986; Kaspa and Wuhmann, 1978; Byan, 1979). This biological phenomenon is called inespecies hydogen ansfe. As i can be seen fom daa shown in Table 3-1, due o he consumpion of H 2 and aceae by he mehanogens, he oxidaion of popionae and buyae is exohemic and possible unde sandad condiions. Table 3-1. Gibbs fee enegy change by synophic convesion Reacion Aceogenesis (1) + CH 3CH 2COO + 3H 2O CH3COO + HCO3 + H + 3H (2) + CH CH CH COO + 2H O 2CH COO + HCO + H + H Mehanogenesis (3) + 4 H 2 + HCO3 + H CH4 + 3H 2O (4) CH3COO + H 2O CH4 + HCO Synophic convesion (5) CH 3CH 2COO + H 4 2O HCO H + CH (6) CH 3CH 2CH 2COO + H O HCO + H + CH o Δ G (kj/eacion) o Values fo Gibbs fee enegy ( Δ G ) ae given unde sandad condiions, i.e., 1M, 1am, ph=7.0, T=25 o C. Values of aceogenesis and mehanogenesis ae adaped fom Wesemann (1996). Also, i has o be noiced ha, fom Figue 3-2 and Equaion 3 in Table 3-1, when H 2 paial pessue is lowed, he enegy yield of H 2 -uilizing mehanogens will be educed. This implies ha hee is a ceain naow H 2 paial pessue ange whee oxidaion of fay acids wih H 2 as a poduc, and mehanogenesis wih H 2 as a subsae is possible (Dolfing, 1988; Wesemann, 1996). Alhough he main poducs of he anaeobic pocess ae cabon dioxide and mehane, mino quaniies of niogen, hydogen, ammonia and hydogen sulfide (usually less han 1% of he oal gas volume) ae also geneaed (McIneney e al., 1980). The mixue of he gaseous poducs is emed as biogas. 3.2 Facos affecing anaeobic digesion pocess As menioned above, anaeobic digesion is a vey complicaed pocess depending upon he synegy ineacions beween he vaious goups of micooganisms involved. A fine balance beween hese micobial communiies is necessay fo

21 successful digesion giving he lages convesion ae of oganic maeial o mehane. When anaeobic pocess is used fo he digesion of sewage sludge, i migh be affeced by he specific chaaceisics of he sewage sludge o be eaed, envionmenal facos such as empeaue, ph, pesence of inhibioy o oxic subsances, ec., and opeaional facos such as hydaulic and solid eenion ime, mixing and feeding saegies and so on Specific chaaceisics of sewage sludge The specific chaaceisics of sewage sludge include he ype of sludge (pimay sludge, seconday sludge o he mixue of hese wo ypes of sludge), composiion of he complex polymeic oganics (pecenage of cabohydaes, poeins and lipids), solid concenaion and size disibuion. Nomally, vaiaions in composiion and in size disibuion of pimay sludge ae bigge han ha of seconday sludge. When sewage sludge wih diffeen specific chaaceisics is used as feedsock, he eaco pefomance may change, especially when CSTR ype eaco is used (Chyi & Dague, 1994; Feeio & Soo, 2003; Palmowski and Mulle, 2003). The degadaion ae of seconday sludge is only one-half of ha of pimay sludge (Ghosh, 1995). Also, fo he seconday sludge, he degadaion ae is affeced by he sludge age. Fo example, by using he same anaeobic digesion pocess, he degadaion ae is only 14% fo he feed wih a sludge age of 30 days, and 31% fo he feed wih a sludge age of 5 days (Gosse, J. & Belsee, 1982). When anaeobic digesion is sudied a lab scale, especially when small volume of eaco is used, he specific chaaceisics of he sludge o be used should be idenified. Meanwhile, as he oganics in he sludge may decay even a 4 o C, pope mehods o soe he sludge should be used so ha he vaiaions of eaco pefomance caused by he influen flucuaion can be avoided (pape I and II) Tempeaue Tempeaue is one of he mos impoan envionmenal paamees fo anaeobic digesion. Biologically speaking, empeaue deemines if a ceain kind of micooganism can suvive o gow in he eaco and if hey ae living hee wih hei highes aciviies. Pacically speaking, highe empeaue means high

22 consumpion of enegy. So, he choice of empeaue and conol of he level in quesion ae of cucial significance fo anaeobic digesion. A diffeen empeaue anges, he micobial consoia ae diffeen. Anaeobic digesion can be caied ou by anaeobic psychophiles, mesophiles, hemophiles and exeme hemophiles, he opimum woking empeaue anges of which ae o C, o C, o C and above 65 o C, especively [Madigan e al. 2000]. A consan empeaue is vey impoan fo a micobial consoium because once i has adaped o a ceain empeaue, i can olean a vey small changes in empeaue. Especially an incease of he empeaue jus above he opimum can soon lead o a dasic decease of he gowh ae of he micobes. Figue 3-3 shows he elaions beween empeaue and gowh ae fo diffeen baceia consoia. Themophile Example: Bacillus Seaohemop -hililus Hypehemophile Example: Themococcus cele Hypehemophil Example: Pyolobus Fumaii Gowh ae Psychophile Example: Polaomonas vacuolaa Mesophili;e Example: Escheichia Coli Tempeaue ( o C) Figue 3-3. Relaionship beween baceial gowh ae and empeaue [Madigan e al. 2003] When anaeobic digesion is caied ou a hemophilic empeaues, many advanages such as highe convesion ae, bee pahogen educion effec, and shoe eenion ime ae obseved han when i is caied ou a mesophilic empeaue. Bu i is nomally pefomed a empeaues close o he uppe limi of some of he oganisms involved in he pocess. This is why he pocess is especially sensiive o empeaue flucuaions (Ahing, e al., 2001). Nomally, he empeaues ae kep below o C, which coesponds o he uppe empeaue limi of he hemophilic sains of Mehanosacina (Zinde e al. 1984; Soensen 1996)

23 3.2.3 ph Each goup of micooganisms has hei opimum ph ange. Apa fom he influence on he gowh of micooganisms, ph can affec ohe facos such as dissociaion of compounds (ammonia, sulfide, oganic acids, and ec.) of gea impoance fo he whole pocess of anaeobic digesion. ph in anaeobic digeses is mainly conolled by he bicabonae buffe sysem (Rozzi, 1991; Peoius, 1994). Theefoe, ph in biogas plans depends on he paial pessue of CO 2 and he concenaion of alkaline and acid componens in he liquid phase. Expeimenal esuls fom many sudies shows ha when empeaue and eenion ime have been deemined, he ph in a pocess fed wih idenical sludge will be sabilized a a ceain value ha benefis he dominan micooganisms (Easman & Feguson, 1981; Mion, e al., 2000). In wasewae eamen plan, he ph value in he anaeobic digesion eaco is nomally in he ange of , which is beneficial o he gowh of mehanogens Maco- and miconuiens Maconuiens ae he elemens ha he cellula maeial of he anaeobic micooganisms compises, including hydogen, niogen, oxygen, cabon, sulfu, phosphous, poassium, calcium, magnesium and ion. Nomally, anaeobic micooganisms equie hese elemens pesened wih a concenaion aound 10-4 M. In addiion o he miconuiens, a numbe of ohe elemens, such as Ni and Co mus be pesen in small amoun, i.e. below10-4 M. This is because ha hese elemens ae impoan fo he gowh of anaeobic oganisms. Fo example, Ni is necessay fo acivaing faco F 430, which is a co-faco involved in mehanogenesis. Bu i can be inhibioy fo femenaive as well as mehanogens if i is pesen in high concenaion. Fo anaeobic eamen of mixed wase, such as sewage sludge, i is ofen assumed ha he necessay nuiens ae available and in non-limiing amouns. Howeve, a eamen of single wase o wasewae facion, he degadaion can be limied by he availabiliy of nuiens. Thee ae examples ha supplemenaion of Ni and Co simulaes anaeobic pocess (Speece, 1983; Fosell, 1985) Inhibiion Inhibiion by ammonia

24 Ammonia (NH 3 /NH 4 ) can be oxic o anaeobic digesion and he acive componen esponsible fo ammonia inhibiion is he unionized fom of ammonia, i.e. fee NH 3. The fee NH 3 concenaion can be calculaed by he following equilibium equaion: [ NH ] 3 = (Eq. 3-1) [-NH3 ] (1 [H + + ]/ k A ) whee [ ], [- ] and [ H NH 3 NH 3 + ], ae he fee ammonia, oal ammonia and poon concenaion, especively, and k A is he dissociaion consan, which inceases wih empeaue. So, ammonia inhibiion is highe unde high empeaues and high ph values. Among he micooganisms involved in he anaeobic pocess, mehanogens ae especially sensiive o ammonia inhibiion. Thus, when an anaeobic pocess is inhibied by ammonia, he concenaion of VFAs will incease and his will lead o a decease of ph. The decease of ph will paly couneac he effec of ammonia due o a decease in he fee ammonia concenaion. This phenomenon is called inhibied seady sae Inhibiion by niogen oxides I is epoed ha hydogen-uilizing mehanogens can be inhibied by niogen oxides, such as niae, niie and niic oxide; boh ae pesen in naual maine sedimens and in man-made envionmens wih whole-cell of Mehanobaceium hemoauoophicum and Mehanobaceium fomicicum suspensions. And expeimenal esuls suggesed ha he inhibioy effec was no due o edox change o subsae compeiion, bu due o he inhibiion of he aciviy of some componen of he mehanogenic enzyme complex iself (Baldeson & Payne, 1976). Hydogenuilizing mehanogenesis is one of he majo pahways in anaeobic digesion pocess, especially when anaeobic digesion is conduced a hemophilic empeaues, and aceae-uilizing mehanogenesis is inhibied by elevaed empeaue, aceae oxidaion followed by hydogen-uilizing mehanogenesis is he only way o conve aceae (Ahing, 2003). So, sewage sludge fom a wasewae eamen plan wihou deniificaion pocess may bing he dange of posing inhibiion o he anaeobic

25 sludge eamen pocess when wasewae conains high concenaion of hese niic compounds Inhibiion by shock loading of subsaes A sudden inceased load of poeins will esul in fomaion of ammonia, so subsae may cause inhibiion as well. Besides poeins, a sudden addiion of lipids o he eaco can cause inhibiion of he anaeobic pocess, since he hydolyic, acidogenic and mehanogenic baceia can be inhibied by accumulaion of long chain fay acids poduced duing hydolysis of lipids. Geneally, i akes long peiod fo he eaco o adap o he oveload of his kind of subsaes. Anohe case of subsae inhibiion is he shock oveload of easily degadable subsaes, which can be quickly hydolyzed and femened by he acidogens esuling in a sudden decease in ph of he eaco and finally inhibi he mehanogens Toxiciy Besides ammonia and niae/niie, heavy meals, such as Zn, Cu and Cd can be oxic o acidogenic baceia [Ahing & Wesemann, 1983; Zinde e al. 1984]. Howeve, many of hese elemens and compounds can be oleaed in elaively high concenaion due o absopion in ine maeial conained in he eaco Reenion ime Fo he CSTR eacos, which ae he mos pevailingly used ypes of eacos, hydaulic and solid eenion ime is he same. Reenion ime is an impoan opeaional paamee ha is easy o opeae and conol. Temendous effos has bee been pu ino he eseach of he effec of eenion ime on anaeobic digesion (Bouzas, e al., 2002; Elefsiniois & Oldham, 1994 a, b; Peo, e al., 1988; Zhang & Noike, 1994). Biologically, only hose whose doubling ime ae shoe han he eenion ime can be kep in he eaco, so eenion ime is one of he bes paamee o be manipulaed fo sepaaing and eniching diffeen goups of he micobes involved in he anaeobic pocess. Also, eenion ime deemines he ime ha

26 subsaes can be aacked by he enzymes in he eaco. In pacice, longe HRT means bigge woking volume of eaco and highe invesmen and opeaion coss Agiaion saegy I is nomally believed ha agiaion is necessay o help he diffusion of subsae and incease hei conacs wih he micobes, especially when aw sludge is inemienly fed ino he eaco. Agiaion saegy can affec anaeobic digesion of sewage sludge and opimum agiaion saegy should be found (Peo, e al., 1988). Howeve, Banise & Peoius (1998) found ha, when using pimay sludge fo VFA poducion, vigoous mixing was no helpful. Also, a sudy caied ou by Soo, e al. (2001) using mixue wases of oganic facion of municipal solid wase, pimay sludge and seconday sludge show ha coninuous mixing was no necessay fo good pefomance of he anaeobic pocess and was inhibioy a highe loading aes. In addiion, i was also found ha mixing levels migh be used as an opeaional ool o sabilize unsable anaeobic eaco Feeding saegy Pacically, anaeobic eacos eaing sewage sludge in wasewae eamen plans ae fed semi-coninuously insead of coninuously. Fo example, in Lundofe Wasewae Teamen Plan, he fequency of feeding of aw sludge o he eaco is once pe hou. Feeding fequency deemines he aio of food o micobe (F/M) when he eenion ime and he woking volume have been fixed. Nomally, he aio can be saisfied so ha hee is no negaive effec on he sabiliy and on he pefomance of he anaeobic eacos. Howeve, when hygienic popey of he digesae is consideed, he minimum guaaneed eenion ime (MGRT) of he sludge in he eaco should be aken ino consideaion (Faell, e al., 1988). Then, he F/M aio and MGRT should be compomised by he adjusmen of feeding fequency. 3.3 Opimizaion of anaeobic digesion fo sewage sludge eamen Even hough anaeobic digesion is a good biological pocess and has a long hisoy and a woldwide applicaion in eaing sewage sludge, his bioechnology is sill fa fom being opimized. The oganic maeial in sewage sludge is in he fom of

27 paiculaes, and his makes hydolysis of hese paiculaes he obsacle of he whole anaeobic digesion (Ghosh, 1975; Easman & Fegusson, 1981; Li & Noike, 1992). Meanwhile, some of he micobial goups involved ae slowly gowing and sensiive o changes in opeaing condiions and o vaiaions of influen sludge composiion and concenaion, which ae caused by he inhomogeneiy of he naue of he sludge. These usually causes insabiliy of he eaco pefomance (Huysman e al., 1983; Gijzen e al., 1988; Rozzi and DiPino, 1994), leading o he decease in biogas poducion. Consequenly, only aound 50% of oganic maeial in sewage is degaded and conveed o biogas by convenional anaeobic digesion pocess. Figue 3-4 shows an example fom ou lab expeimen. 1=55% 2=90% 3 =98% Oganic paiculaes d-cod -VFAs CH % 3.72% 0.93% 54.49% Figue 3-4. Schemaic diagam of he convesion ae in each sep and he final COD disibuion of mixed sewage sludge afe anaeobic digesion in a single-phase eaco (55 o C, 15 days). As i can be seen in Figue 3-4 (daa adaped fom ou lab expeimen), when he aw sludge, a mixue of pimay sludge and seconday sludge wih a solid concenaion of 34.6g/l, was digesed in he convenional single-phase hemophilic anaeobic eaco unning a 55 o C fo a eenion ime of 15 days, he emaining solid concenaion in he effluen was 20.5g/l. No accumulaion of d-cod o VFA was found. The convesion ae of oganic maeial fom he fom of paiculaes o hydolysis poducs, measued as dissolved COD is only abou half of he ae fom hydolysis poducs o acidogenisis poducs, measued as VFA, o he ae fom acidogenisis poducs o mehanogenisis poducs, measued as CH 4. So, hee exiss a lage poenial o opimize he pocess fo exploiaion of enegy fom sewage sludge. The inees in inceasing he convesion of he oganic maeial is fuhe linked o

28 he educion in he final amoun of solid, which has o be disposed afe he eamen (Ahing, 2003). The ohe aspec o opimize he anaeobic digesion pocess is o make i possible o ecycle he digesae, he effluen fom anaeobic digesion pocess, back o he famland. The digesae consiss of an odo fee esidue wih appeaance simila o pea. The plan nuiens such as N, P and ec. ae eained in he digesae afe anaeobic digesion. Alhough he applicaion of he digesae o famland is he bes final disposal opion and also encouaged by he EU diecives (EU, 2000), commimen o he elevan cieia can seldom be held by he convenional anaeobic digesion pocess. Besides xenobioics and heavy meals, he concenaions of which in sewage sludge can be educed by conol a souce (Jensen & Jepsen, 2005), pahogens ae nomally he mos ciical paamee limied by he cieia and he agiculue seco concens vey much abou he isk of speading disease. Sudies have shown ha anaeobic digesion can kill pahogens, especially when i is conduced a hemophilic empeaues (Bendixen 1999; Nielsen and Peesen, 2000). I will be significan if i is possible fo sewage sludge o use famland as a safe and pemanen oule desinaion wih only posiive effec, i.e., he digesae can be ecycled as feilize and soil condiione back o he famland so as o keep hese naual nuiens wihin a closed loop sysem, and emain o impove he soil sucue of he famland. So, effos mus be made o enhance he hydolysis and impove he pocess sabiliy fo biogas poducion and o eliminae he pahogens

29 4. THERMOPHILIC ANAEROBIC PRE-TREATMENT 4.1 Summay of he pe-eamen mehods The pe-eamen mehods ha ae mos fequenly cied in lieaue ae summaized in Table 4-1. Table 4-1 Mehods of pe-eamen fo anaeobic digesion of sewage sludge Mehod Sludge Impovemens Refeence Sied ball-mill 23% hydolysis, 14% incease in VS WAS (22Wh/l) emoval Mülle, 2004 Jeing-smashing (50ba, 5 imes) WAS 15% incease in VS emoval Choi e al., 1997 Mechanical Ulasound (40Hz, 375W/l, 1h) WAS 20% in VS emoval Kim e al., % incease in hydolysis and app. Elecon beam ( Shin and Kang, WAS 90% in VFA, RT can be educed fom 20 kgy) 2003 days o 10 days when digesion in CSTR Chemical Themal Combined (Mech.+ chem., he.+ chem.) NaOH (40 meq/l, 24 h) WAS 36.3% hydolysis of oal COD Chiu e al., 1997 Ozone (0.05g/g TS) Low empeaue ( o C, min) High empeaue ( o C, min) Ulasound + NaOH (40meq/l, 375W, 24 h) 175 o C+NaOH (30meq/l, 121 o C, 1h) WAS 37% hydolysis, 28% incease in VS emoval Goel e al., 2003 WAS 30-50% incease in biogas poducion Hiaoka e al, 1984 WAS 60% incease in VS emoval, and 100% incease in biogas poducion Li and Noike, 1992 WAS 89% hydolysis Chiu e al., 1997 WAS 55% hydolysis, 58% incease in CH 4 poducion Haug e al., 1978 Feezing and hawing WAS 7% incease in CH 4 poducion Wang e al, 1999 Biological Themophilic aeobic (70 o C, 5 h) Mico-aeaion (37 o C, 4d) Anaeobic (25 o C, 3d) Anaeobic (37 o C, 3d) Anaeobic (49.8 o C, 3.1d) Auoclaved WAS PS PS PS WAS 30% VS emoval, 150% incease in biogas poducion 60% incease in hydolysis 13% incease in hydolysis, 50% incease in VFA 17% incease in hydolysis, 64% incease in VFA 20% incease in hydolysis, 80% incease in VFA Hasegawa e al., 2000 Johansen and Bakke, 2005 Mion e al., 2000 Easman and Feguson, 1981 Ghosh e al., 1995 Diffeen mehods have diffeen mechanisms. Theoeically, mechanical disinegaion povides enegy necessay fo he disupion of oganic paiculaes o micobial cells by pessue, ansacional o oaional enegy, ceaing ensions on he suface of he paicles o cells. Sied ball mill disinegaes paicles by cushing. High-pessue homogenise disinegaes paicles by caviaion effecs due o sudden pessue elease. Ulasonic homogenise leads o sludge disinegaion by vibaion. Mechanical je echnique has he simila pinciple as dissolved ai floaion, by which

30 sludge seam is pessuized o 5-50 bas, he pessue is subsequenly eleased acoss a nozzle and he sludge seam impinges on a splash plae. Elecon beam uses elecical pulses o induce shock waves in solid and liquid media. When he powe supply is swiched on, he volage acoss he elecodes is abou seveal 10 kv. Evenually, i comes o a beakhough beween he elecodes and hus beaks he oganic maeials. The disinegaion of he oganic paicles will incease he suface aea of he paicles fo he accessibiliy of he enzymes in he eaco. The disupion of cells will elease he inecellula conen ha is moe soluble. Because he essence of hydolysis is he decomposiion of oganic polymes involving he spliing of bond and he addiion of he hydogen caion and he hydoxide anion of wae, addiion of acids o alkalis can incease he concenaion of hese ions and hus incease he hydolysis eacion ae. The aim of ozone pe-eamen is a paial oxidaion and a hydolysis of he oganic mae. A complee oxidaion is avoided and in sead lage molecules ae cacked ino smalle ones and hadly degadable compounds ae ansfeed ino moe easily degadable ones. Themal pe-eamen is nomally used o eamen wase acivaed sludge in he empeaue ange fom o C. In his empeaue ange, he cell walls can be desoyed and his makes he poeins accessible fo biological degadaion. Because mechanical, chemical and hemal mehods belong o diffeen mechanisms, so hese mehods can be combined and diffeen effecs fom each mehod can be obained simulaneously. By feezing and hawing acivaed sludge, he size of he floc sucue will ievesibly educed and he sludge will be moe compac. This mehod is no commonly used, alhough i migh be of poenial applicaion in he cold weahe egions. In biological pe-eamen, he ae of hydolysis is a funcion of he souce of hydolyic enzymes, and hus he aciviy and populaion of he micobial populaion

31 in he eaco. Sudies on he diec addiion of enzymes, eihe by addiion of complex enzymes o by addiion of a mixue of cabohydolases, pepidases and lipases, confimed he enhancemen of hydolysis. As i can be seen fom Table 4-1, all of he pe-eamen mehods have posiive effec on he enhancemen of hydolysis and hus can impove he anaeobic digesion pocess. Bu we have o noice ha due o he diffeence in enegy inpu, dosage of chemical eagen, eamen ime and he ype of sludge wih diffeen solid concenaions used in diffeen sudies, he effecs on hydolysis enhancemen, volaile solid emoval and biogas poducion ae no compaable in ems of eamen cos vesus eamen efficiency. In geneal, mechanical, chemical, hemal and feezing and hawing mehods need sho eamen ime, fom minues o hous. Compaed wih hese pe-eamen mehods, biological eamen mehods need longe eamen ime, fom hous o days. Nomally, i is egaded ha he inpu of enegy fo mechanical pe-eamen is lowe han fo hemal pe-eamen, bu i needs invesmen of mechanical insallaions and coss fo mainenance opeaion. Themal pe-eamen uses hea enegy, which is cheap and nomally can be obained fom he buning of biogas on he eamen sie. Since hemal pe-eamen a highe empeaues nomally poduces inhibioy subsances o he anaeobic digesion pocess, empeaues lowe han 100 o C is nomally used o pe-ea wase acivaed sludge. Chemical mehods by addiion of alkali and acids migh be expensive and have o inoduce foeign ions o he digesion sysem. Among he chemical mehods, addiion of ozone is he bes, because i does no inoduce foeign ions and only consume eleciciy fo geneaing ozone. A significan advanage of biological anaeobic mehods is he elaively highe poducion of volaile fay acids han he ohe pe-eamen mehods Raionale fo conducing pe-eamen unde hemophilic anaeobic condiions Exploiaion of he anaeobic hemophiles fo hydolysis and acidogenesis Themophilic anaeobic baceia widely exis in naual and man-made envionmens, such as ho sping and food pocessing indusy (Kisjansson and See, 1992), and