Microbial valorization of agro-industrial wastes through chain elongation, pha production, and biofuel generation: a study on grape pomace, sewage scum, sewage sludge, vegetable waste, olive mill pomace, and wetland biomasses

The increasing generation of agro-industrial waste necessitates sustainable valorization strategies that mitigate pollution while recovering valuable bioproducts. This thesis thus explores the biotechnological conversion of five industrial byproducts—grape pomace, the cellulose-hemicellulose-rich fraction of sewage scum, sewage sludge and vegetable waste, olive mill pomace, and wetland biomasses—into high-value chemicals and biofuels. Microbial processes investigated include chain elongation for hexanoic acid production, polyhydroxyalkanoate synthesis, and anaerobic digestion, complemented by extraction studies for process optimization. For grape pomace valorization, ethanol leaching generated a liquid stream rich in ethanol and nutrients, enabling efficient hexanoic acid production through microbial chain elongation. The process was adaptable to different grape pomace sources and resulted in high hexanoic acid productivity. Hexanoic acid was recovered at 89% purity through acidification and phase separation, while the solid residue remained available for further valorization, including polyphenol or tartaric acid extraction and anaerobic digestion. Enzymatic saccharification of the cellulose-hemicellulose-rich fraction from sewage scum yielded a glucose-rich liquor, supporting hexanoic acid production in semi-continuous setups with yields comparable to similar studies. Pertraction was evaluated for hexanoic acid extraction, revealing bicarbonate as a safer alternative to boric acid in the stripping phase, improving process safety and product purity. Polyhydroxyalkanoate production was achieved using anaerobically co-digested sewage sludge and vegetable waste pretreated with hydrodynamic cavitation, yielding a high-purity polyhydroxybutyrate. The use of filtered but non-sterilized substrate with a pure strain improved process feasibility, lowering costs associated with sterilization. Biochemical methane potential tests assessed the anaerobic digestion potential of olive mill pomace and wetland biomasses, demonstrating methane yields comparable to literature values despite challenges such as high organic loading and polyphenol content. This thesis advances the sustainable valorization of agro-industrial waste by integrating microbial conversion and extraction technologies, contributing to circular economy principles and enabling the production of bio-based chemicals and fuels from diverse waste streams.