The overall aim of this study is to gain a deeper understanding of anaerobic microbial degradation processes of organic solid substrates. A number of commercial anaerobic in-vessel digestion technologies have been developed for the treatment of the organic fraction of municipal solid wastes (OFMSW) with the growing awareness and expectations of the community in relationship to environmental care. One of the major problems associated with in-vessel anaerobic digestion of MSW is the high capital cost for large-scale reactors and conveyor systems. Therefore, any advances that decrease the degradation time for MSW and thereby increase the potential throughput of anaerobic digesters are significant. This thesis, for the first time, made detailed microscopic observations of hydrolysing bacteria and biofilms on the surface of organic particles under anaerobic conditions. Also, this study investigates the influence of solid residence time (SRT) on the solubilisation of particles under a range of well-defined conditions.
The experiments were performed in batch and semi-continuous mode. A mixture of leachate and cellulose was used as a medium. The performance of the reactors was evaluated by measuring the pH, soluble chemical oxygen demand (sCOD), total volatile fatty acid (TVFA), NH4-N and methane production rate, and calculating cumulative methane production and the extent of cellulose solubilisation. The microbial biofilm architecture was examined by using light and electron microscopy. Fluorescent in situ hybridisation with confocal laser scanning microscopy (FISH-CLSM) was used to investigate the microbial biofihn population structure.
The batch experiments were initiated by inoculating with leachate and strained rumen contents. The complete anaerobic biodegradation of cellulose occurred in the reactor where the adherent cells had developed into a mature biofilm on the surface of cellulose. In contrast, only a few attached cells were observed on the particle surfaces in the soured reactors. This indicates that the formation of biofilms on the particle surface is necessary for the complete anaerobic degradation of organic matter. The results of rumen experiments suggest that the difference in the rate of solubilisation between the rumen and leachate systems was due to a difference in the rate of microbial attachment and a difference in the microbial populations. The light and electron microscopic examinations show that hydrolysing bacteria were anchored to the particle surfaces by extracellular polysaccharide (ECP) and the hydrolytic enzyme complexes were cell-associated. The FISH-CLSM observations suggest that the microbial communities involved in the complete anaerobic biodegradation processes exist entirely within the biofilm. The FISH-CLSM and transmission electron microscopy (TEM) biofilm observations found for the first time that the predominant methanogens in an anaerobic digestion environment are present in the form of striking ball-shaped colonies within biofilm.
The effect of SRT on the solubilisation of cellulose was investigated in a semicontinuous mode under the six different SRTs. The scanning electron microscopy (SEM) images, showing that cellulose is only solubilised beneath adherent bacteria on the surface of the particles, suggest that a more fundamental measurement of the hydrolysis rate is the production of sCOD per unit surface area per unit time. The attachment of hydrolysing bacteria to the solid surface was limited at SRTs less than 5 days which leads a reduction in the extent and rate of cellulose solubilisation. The constant values of specific hydrolysis rate at longer SRTs suggest that the microbial solubilisation rate is constant regardless of SRTs, when the particle surface is covered with microorganisms. However, the investigation of the extent of cellulose solubilisation shows that the microbial pathways that lead to biomass and ECP are associated with longer SRTs. The model that was used to derive the surface specific hydrolysis rate assumed exponential microbial growth across the surface of the cellulose particles. The model did not consider surface limitations, but this is not a significant shortcoming because the particles are almost fully degraded once their surfaces are fully covered by biomass.
The results in this thesis provide fundamental information on microbial hydrolysis processes and have important implications for improvement of the performance of in-vessel MSW anaerobic digestion processes.