The bio-disposal of lignocellulose substances with activated sludge

Qi, Bing Cui (2001-03)

Thesis (PhD)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: Lignocellulose is the principal form of biomass in the biosphere and therefore the predominant renewable source in the environment. However, owing to the chemical and structural complexity of lignocellulose substrates, the effective and sustainable utilization of lignocellulose wastes is limited. Many environments where lignocellulose residues are ordinarily stored can be highly acidic (e.g. landfills), and under these circumstances biodegradation of the lignocellulose is slow and unhygienic. Owing to the metabolic activities of the micro-organisms, the initially acidified habitats rapidly undergoes self-neutralization. A number of pathogenic bacteria (coliforms and Salmonella sp.) are present during this slow degradation process and it is therefore imperative to improve the efficiency and hygienic effects of the biodegradation of the lignocellulose. Although the fundamentals of biodegradation of lignocellulose have been widely investigated, many issues still need to be resolved in order to develop commercially viable technology for the exploitation of these waste products. For example, owing to the complex, heterogeneous structure of lignocellulose, the degree of solubilization, modification and conversion of the different components are not clear. Likewise, the overall anaerobic degradation of lignocellulose is not understood well as yet. In this study, the emphasis was on the promotion of solid anaerobic digestion of lignocellulose wastes for environmental beneficiation and waste reutilization. The degradation of lignocellulose in landfill environments was first simulated experimentally. Once the microbial populations and the degradation products of the system were characterized, the promotion of anaerobic digestion by use of activated sludge was studied. This included acidogenic fermentation, as well as recovery of the methanogenic phase. Moreover, special attention was given to the further disposal of humic acids or humic acid bearing leachates formed in the digestive system, since these acids pose a major problem in the digestion of the lingocellulose. With ultrasonication, approximately 50% of the lower molecular weight fraction of humic acids could be decomposed into volatile forms, but the higher molecular weight fraction tended to aggregate into a colloidal form, which could only be removed from the system by making use of ultrasonically assisted adsorption on preformed aluminium hydroxide floes. This was followed by an investigation of the microbial degradation of humic acids and the toxicity of these acids to anaerobic consortia. Further experimental work was conducted to optimize the biological and abiological treatment of lignocellulose in an upflow anaerobic sludge blanket (DASB) reactor fed with glucose substrate. The humic acids could be partially hydrolysed and decomposed by the acid fermentative consortia of the granules in the DASB reactor. Finally, solid mesothermophilic lignocellulose anaerobic digestive sludge can be viewed as a humus-rich hygienic product that can improve the fertility and water-holding capacity of agricultural soil, nourish plants and immobilize heavy metals in the environment as a bioabsorbent.

AFRIKAANSE OPSOMMING: Lignosellulose is die hoofbron van biomassa in die biosfeer en is daarom ook die belangrikste hernubare bron in die omgewing. As gevolg van die chemiese en strukturele kompleksiteit van lignosellulose substrate, is die doeltreffende en volhoubare benutting van lignosellulose afval egter beperk. Die suurgehalte van die omgewings waar lignosellulose reste gewoonlik gestoor word, soos opvullingsterreine, kan hoog wees en onder hierdie omstandighede is die biodegradasie van die lignosellulose stadig en onhigiënies. As gevolg van die metaboliese aktiwiteite van die mikro-organismes ondergaan die aanvanklik aangesuurde habitatte vinnig self-neutralisasie. 'n Aantal patogeniese bakterieë (koliforme en Salmonella sp.) is deurgaans gedurende dié stadige natuurlike proses teenwoordig en dit is dus van die grootste belang om die effektiwiteit en die higiëne van die bioafbreking van die lignosellulose-substraat te verhoog. Alhoewel die grondbeginsels van die bioafbreking van lignosellulose reeds wyd ondersoek is, moet verskeie probleme nog opgelos word ten einde kommersieel haalbare tegnologie te ontwikkel vir die ontginning van afvalprodukte. Byvoorbeeld, as gevolg van die komplekse, heterogene struktuur van lignosellulose, is die graad van solubilisering en die modifikasie en omskakeling van verskillende komponente nog onduidelik. Net so word die algehele anaerobiese afbreking van lignosellulose ook nog nie ten volle verstaan nie. In hierdie ondersoek het die klem geval op die bevordering van soliede anaerobiese digestie van lignosellulose afval vir omgewingsverbetering en die benutting van die afval. Die afbreking van lignosellulose in opvullingsterreine is eers eksperimenteel gesimuleer. Nadat die mikrobiese populasies en die afbrekingsprodukte gekarakteriseer is, is die bevordering van anaerobiese digestie deur die gebruik van geaktiveerde slyk bestudeer. Dit het asidogeniese fermentasie ingesluit, sowel as herwinning van die metanogeniese fase. Spesiale aandag is gegee aan die verdere verwerking van humus sure en humussuurbevattende legate wat in die digestiewe stelsel gegenereer is, aangesien die sure probleme veroorsaak het met die vertering van die lignosellulose. Met ultrasoniese straling is nagenoeg 50% van die lae-molekulêre massafraksie van die humussure ontbind in vlugtige vorm, maar die hoë-molekulêre massafraksie het geneig om in 'n kolloïdale vorm te aggregeer, wat slegs uit die stelsel verwyder kon word deur middel van ultrasonies ondersteunde adsorpsie op voorafgevormde aluminiumhidroksiedvlokkies. Dit is gevolg deur 'n ondersoek na die mikrobiese afbreking van humus sure en die toksisiteit van die sure ten opsigte van anaerobiese konsortia. Verdere eksperimentele werk is gedoen ten opsigte van die biologiese en abiologiese behandeling van lignosellulose in 'n opwaartsvloeiende anaerobiese slikkombersreaktor (OASK) gevoer met glukosesubstrate. Die humus sure kon gedeeltelik gehidroliseer en ontbind word deur die suurgistende konsortia van die granules in die OASK reactor. Ten slotte kan die vaste termofiliese-mesofiliese anaerobiese lignosellulose verteringslik ook gesien word as 'n humusryke higiëniese produk wat die vrugbaarheid en die waterhoudende vermoë van landbougrond kan verhoog, plante kan voed en kan funksioneer as bioabsorbeerder van swaarmetale in die omgewing.

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