Changes in the Microbial Succession During Sewage Sludge Composting and its Correlation with Physico-Chemical Properties

Sewage sludge composting is a process entailing a continuous succession of microorganisms. To understand the microbial mechanisms involved in sewage sludge composting, we performed an aerobic static composting of sewage sludge and sawdust (ratio = 3:1 m/m) in medium-scale bioreactor systems. The associated changes in physico-chemical parameters (i.e., temperature, organic matter, pH, ammonium nitrogen) were studied parallelly to those in the microbial (i.e., bacteria, fungi, archaea) succession. Additionally, we discussed correlations between these physico-chemical parameters and the microbial communities. The results showed that the pile temperature went through mesophilic phase, thermophilic phase, and cooling phase. The pile temperature reached a maximum of 78.68°C by day 3 and remained above 55°C for more than 6 days, complying with the harmless composting requirements. The organic matter content decreased gradually, the pH increased after a first decrease and the NH4 +-N content showed a consistent trend. The dominant bacteria during composting were Ureibacillus, Bacillus, Sphaerobacter, and Thermobifida, while the dominant fungi were unclassified_f_ Trichocomaceae, unclassified_d_Eukaryota, Hypocrea and Thysanophora; finally, the dominant archaea were Methanobrevibacter, Methanosaeta, Methanobacterium, and unclassified_k_norank. The composting stages were characterized by different microbial compositions. The mesophilic phase presented a relatively uniform proportion of bacterial genera, while the thermophilic and cooling phases were dominated by Ureibacillus and Bacillus, respectively. The fungus unclassified_f_Trichocomaceae played a major role during the mesophilic, thermophilic, and cooling phases, while unclassified_d_Eukaryota played a major role during the mesophilic and thermophilic phases. For what concerns the archaea, Methanobrevibacter played a major role in the mesophilic, thermophilic, and cooling phases, Methanosaeta during the mesophilic and thermophilic phases, and Methanobacterium during the thermophilic and cooling phases. Additionally, the bacterium Ureibacillus and the archaea Methanospirillum were positively correlated with temperature, while the bacteria norank_Pem15, norank_JG30-KF-CM45 and the archaea Methanosphaera were negatively correlated with temperature. The fungi Thysanophora, unclassified_d_ Eukaryota, and unclassified_p_Ascomycota were negatively correlated with pH. Moreover, the bacterium norank_c_1-20, the fungi Trichosporon, norank_o_Saccharomycetales, unclassified_o_Pleosporales, and the archaea Methanosaeta, Methanomethylovorans were positively correlated with organic matter. On the other hand, the bacteria Bacillus, Thermobifida, the fungus unclassified_f_Trichocomaceae, and the archaea Methanobrevibacter were negatively correlated with organic matter. Finally, the bacteria Bacillus, Thermobifida, the fungus unclassified_f_Trichocomaceae, and the archaea Methanobrevibacter were positively correlated with ammonium nitrogen, while the bacterium norank_c_1-20, the fungi Trichosporon, norank_o_Saccharomycetales, unclassified_o_Pleosporales, and the archaea Methanosaeta, Methanomethylovorans were negatively correlated with ammonium nitrogen. This paper provides new solid bases to understand changes in microbial composition and their correlation with physico-chemical parameters during sewage sludge composting.