Characterization of Fast Anaerobic Digestion in a Novel Reactor Design with Immobilized Biofilms

Abstract

Anaerobic Digestion (AD) has become a prioritized technology for energy production from diverse waste streams from industry and agriculture, especially from enzymatically hydrolyzed Organic Fraction of Municipal Solid Waste (OFMSW). Through this practice, it is possible to produce a gas mixture of methane and carbon dioxide that is popularly referred to as biogas. This product can then be used to sustain different energy intensive activities through electricity or heat production. Despite a lot of research in the field, the reactor designs available cannot handle substrates with high organic loads and particulate material. In this project, we characterize the Fast Anaerobic Digester(FAD), a novel reactor design that could potentially improve the digestion process in particular of complex biomass. Amongst the main feature of the FAD there is a particular compartmentalization and immobilization of active microorganisms in fixed biofilm carrier. Within the scope and hypotheses of the thesis, the most crucial factors during the degradation of enzymatically liquefied OFMSW have been characterized in laboratory and pilot scale, the main microbial populations involved in the AD conversion have been unraveled using high throughput DNA sequencing and the influence of high concentrations of cat-ions have been examined. The performance of the FAD reactors studied in the project has revealed that the reactor design did deliver significant improvements in efficiency and high degradation of COD at low HRTs. The compartmentalization design provided improved retention of the convertible material, while the flow regimes inside the reactor secured improved contact between the digesting media and the immobilized microorganisms. During more than 6 months in operation, the FAD reactor expressed more than 70% of the methane potential in the substrate up to an OLR of 20.8gCOD*lreactor-1*day-1 at HRTs lower than 5 days. The yields between 0.2 and 0.3lCH4*gCOD-1 and productivities of 2-6lCH4*lreactor-1*day-1 achieved below 10 days HRT were higher than any other anaerobic digestion data reported in literature for OFMSW. Despite hypothesizing that there would be a selective pressure to form distinct populations in response to substrate composition and compartmentalization, it was found the there were dense and diverse microbial communities in the biofilm layers and no compartment specialization. Remarkably, it was shown that the methanogenic archaeal populations are less prone to changes in composition compared to the bacterial communities regardless of whether they are in the biofilm or in the digesting media. During mesophilic conversion, Methanosarcina spp. and Methanoculleus spp. constitutes almost two thirds of the total methanogenic population. Increasing the temperature to the thermophilic range shifted these groups to be dominated by Methanoculleus thermophilus and other known thermophiles. In regard to the bacterial populations, unclassified Cloacamonas and unclassified Bacteroidetes were the most abundant until unclassified Firmicutes and even Clostridia increased in relative abundance when the temperature regime shifted. In a separate study on the effects of the inoculum composition on the microbial diversity in AD of enzymatically liquefied OFMSW, it was possible to show that regardless of the seed material, the archaeal populations quickly establish to constitute between 30% to 50% of the total populations. In addition, the data obtained strongly indicates that the enzymatic treatment to generate the liquefied OFMSW enhances the AD by circumventing the rate limiting hydrolysis step in the overall biomethanation process. A limitation of a fast anaerobic conversion process is determined by a high level of cat-ions in the digesting media. The inhibitory effects of Na and Mg were systematically assessed in batch reactors to determine the repercussion on the methanogenesis. It was possible to show that these cat-ions retarded methane production in a dose dependent manner using ethanol and methanol as pure substrates. Within the substrate levels examined, it was found that the methane yields in cat-ion inhibited reactors is highly dependent on the organic load as well as the inhibitor concentration. However,the initial methane production rates mainly dependent on the cat-ion concentration. Addition of EDTA or crown ethers were able to abolish the inhibitory effects, indicating that cat-ion inhibition is a reversible process that could potentially be controlled during fast anaerobic digestion processes.