The effects of trace elements on the microbial communities of thermophilic biogas production
Abstract: The increasing need for alternatives to fossil fuels and traditional waste management systems can be simultaneously addressed by the production of biogas using organic waste as input material. The biogas produced can be used directly for energy production or upgraded to increase methane purity for use in the engines of motor vehicles. Furthermore, the volume of waste sent to landfills greatly reduced in the process and, when a hygienization step is included, the nutrient-rich output digestate can be used as agricultural fertilizer. Not only does the use of digestate as fertilizer reduce emissions of greenhouse gases by avoiding landfilling, it also decreases the use of fossil fuels associated with the production of chemical fertilizer. Several hindrances to biogas production exist, however, and at different levels. Economic interests demand as high efficiency as possible which includes operating a system close to its limits regarding organic loading rate (OLR), fatty acid content, ammonia content, sulfide content and trace element content. Such operation presents challenges on a technical level. The presence of trace elements may counteract certain forms of inhibition that arise but are an expense biogas plants would gladly do without. Meanwhile political will may lie with other energy sources or waste management systems. The mass balance of trace elements on agricultural land from biogas derived fertilizer must also be considered. In this study, the effects of trace elements in thermophilic biogas production (52°C) were investigated through the operation of two 5-litre CSTR reactors. Surveillance parameters were tracked over the 184 day experiment period with microbiological analysis performed on 9 digester liquid samples taken during three key periods: startup, 3 hydraulic retention times after startup and 3 hydraulic retention times after an increase in OLR. Terminal Restriction Fragment Length Polymorphism (TRFLP) analysis was used to analyze the dynamics of microbial community members carrying the functional gene, fhs, encoding the enzyme formyltetrahydrofolate synthetase (FTHFS) active in the Wood-Ljungdahl pathway and present in syntrophic acetate oxidizing bacteria. These acetogens are especially important given their role in helping to define the balance between the two possible pathways of methanogenesis, acetoclastic and hydrogenotrophic. A clone library of variants of the fhs gene was constructed to identify dominant members of this population. A phylogenetic analysis was also carried out to place gene variants found in this analysis in the greater context of variation of the fhs gene from previous sequencing of other biogas reactors. This study revealed a lack of differences between biogas production with or without trace element additions which was in contrast to previous studies. While gaps in knowledge in thermophilic systems limited more extensive analysis and thus more definitive conclusions, the variation in composition of food waste must be considered a main variable when considering differences between seemingly similar systems.
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