Microbial Community Profiling of Biodegradable Municipal Solid Waste Treatments : Aerobic Composting and Anaerobic Digestion

An enormous quantity of solid waste is generated annually all over the world. Solid waste can be divided into three main categories: municipal waste, industrial waste and agricultural waste. The focus of the research presented in this thesis was on the biodegradable fraction of municipal solid waste (MSW), and particularly on the biowaste and sewage sludge generated in the Nordic countries. In general, there are two major options for processing biodegradable MSW in a sustainable manner: aerobic treatment (e.g. composting) and anaerobic treatment (e.g. anaerobic digestion). The key interest of this study was in analysing the microbial community composition in composting and anaerobic co-digestion using various types of biodegradable MSW as feedstock. In addition, the aims of this study were to: 1) understand the connections between the microbial communities and the capacities of disease-suppressive composts against the soilborne plant pathogens Pythium and Fusarium; 2) investigate the effects of pH and microbial composition on odour emission in biowaste composting in Nordic countries; 3) study the connections between microbial communities and the key methanogenesis intermediates under various conditions (e.g. temperature, OLR) in the anaerobic digestion process; and 4) find a functional compromise between the waste treatment capacity of anaerobic digestion, biogas production and a stable microbial community. To achieve the above-mentioned goals, DNAbased microbiological techniques (e.g. DNA extraction, PCR, qPCR, DGGE and cloning) and sequencing techniques (i.e. Sanger sequencing and high-throughput pyrosequencing) were applied. The results are summarized in five articles/manuscripts enclosed with this thesis. Studies on composting (articles I–IV) illustrated that the microbial communities were abundant and diverse, with Proteobacteria and Ascomycota as the dominant bacterial and fungal candidates, respectively. The presence of bacterial Acidobacteria Gp14 and fungal Cystobasidiomycetes in Pythium-suppressive composts indicated their possible roles in the suppression of Pythium wilt disease (article I). Actinobacterium and non-pathogenic relatives of the pathogen Fusarium may suppress Fusarium disease on tomato plants (article II). In food waste composting (articles III and IV), results indicated that LAB (e.g. Lactobacillus) together with Clostridia were responsible for the high odour emission in the studied composts. The results suggest that a potential odour reduction strategy would be to rapidly overcome the low pH phase through high initial aeration rates and the use of additives such as bulking material, as well as pre-treatment of the composting feedstock in anaerobic digestion. A study on anaerobic digestion (article V) demonstrated a rather limited methanogenic archaeal community that was dominated by Methanobacteriales and Methanosarcina in both mesoand thermophilic processes, even with Methanothermobacter as an additional abundant genus in the thermophilic production cycle. Key factors such as the acetate concentration and OLR, as well as substrates such as propionate apparently contributed to the dominance of Methanosarcina. Biogas production was greater in the thermophilic process; when the OLR increased to 5 kg VS m d, the efficiency maximum was reached. The microbial groups within the microbial community usually remain largely unexplored. In this thesis, the key microbial candidates that are involved in plant disease suppression, composting odour emission and anaerobic co-digestion were revealed. As the rapid progress of high-throughput sequencing approaches continues, better coverage of the microbial community will be achieved.