Ethanol metabolism in anaerobic digestion systems for better renewable energy recovery from waste feedstocks.

Ethanol plays a key role in advancing the sustainable technology of anaerobic digestion. A comprehensive understanding of ethanol metabolism is essential for optimizing the enhanced recovery of renewable energy from waste feedstocks via anaerobic digestion. This review aims to summarize key findings on ethanol metabolism, focusing on metabolic pathways, kinetics and ecological applications, as well as functional microorganisms and their enrichment strategies. Three ethanol metabolic pathways are discussed: (i) the conventional pathway yielding acetate and hydrogen, (ii) the direct interspecies electron transfer (DIET) pathway, and (iii) the propionate-producing pathway. A comprehensive summary of the kinetics involved in ethanol degradation, microbial growth, and inhibition can enhance reactor design and facilitate the application of the r/K selection theory to enrich functional microorganisms. Operational parameters informed by the r/K selection theory can be used to enrich functional ethanol-oxidizing bacteria, such as Geobacter, thereby influencing ethanol metabolic pathways. These insights offer valuable guidance for further exploration of ethanol degradation toward desired products in the anaerobic treatment of waste feedstocks. Future research should aim to develop a unified ecological framework to explain the coexistence of multiple ethanol metabolic pathways, potentially through concepts such as division of labour or overflow metabolism. Additionally, integrating extracellular electron transfer into thermodynamic models may yield new insights into ethanol metabolism. Finally, DIET-based anaerobic digestion technologies utilizing ethanol should be explored for the degradation of complex substrates.