Microbial consortia dynamics during thermophilic and mesophilic anaerobic digestion of faecal sludge

Abstract

Globally, water scarcity and sanitation have for a long time been of great concern; therefore, water needs to be preserved, but the demands for safe and adequate water are rapidly increasing. Although sanitation in urban areas of South Africa is of the highest standard, some populations such as rural villages lack adequate sanitation and thus use pit latrines as their primary source. This leads to problematic situations due to pit latrines filling up faster than expected, leading to overflowing or leaching of faecal sludge into water systems. Anaerobic digestion is one of the methods that can be used to treat faecal sludge and holds many beneficial properties, such as pathogen reduction and energy production, which can be used as an alternative energy source and for composting. The microbial consortia taking part in these processes consist of hydrolytic, acidogenic, acetogenic and methanogenic microbes. The study aimed to determine the biogas yield and methane production during two anaerobic digestion systems (mesophilic and thermophilic) and the microbial dynamics in such systems. Bench-top batch reactors were used in this study, during which samples were taken at selective times for mesophilic and thermophilic conditions. In order to determine the gas yield and methane production, the BIOGAS5000 analyser was used. Samples collected from these systems were then subjected to physico-chemical and molecular analyses. Microbial communities during selected times were determined using the MiSeq next generation sequencing (NGS) platform. The data obtained from NGS were analysed through Qiime2® and used to determine the microbial community and metabolic pathways. The major phyla observed in mesophilic conditions included Firmicutes, Chloroflexi, Bacteroidetes, Actinobacteria, Euryarchaeota, Synergistetes and Proteobacteria. While thermophilic conditions were almost similar but dominated by Firmicutes, Proteobacteria, Actinobacteria, Chloroflexi, Synergistetes, Thermotogae, and Bacteroidetes. Biogas production in mesophilic conditions with higher yields than that of thermophilic conditions was observed; this could have been due to the nutrient imbalance (ammonium inhibition). Methane yields were observed in both conditions and shown a lower methane yield compared to other studies, due to the inhibition of methanogens; although methane production was low in both temperature conditions mesophilic conditions compared to thermophilic conditions still produced more Biogas- and methane yields during anaerobic digestion. Thermophilic conditions increase free ammonia in an anaerobic system, which is toxic to methanogens, and therefore, are more likely to inhibit methanogenesis. The predicted metabolic pathway in these systems was unknown and ammonia oxidation and dehalogenase were identified as the leading predicted pathways. Pre-treatment of Sludge to remove high ammonium in wastewater would result in better gas production.