Biotechnological approaches to advance organic waste-to-energy: Anaerobic Digestion of end-of-life bioplastics and selection of superior H2-producing microbes

The climate and energy crises that society is facing nowadays require a coordinated effort from governments and industries to find sustainable alternatives to fossil fuels. Among the various renewable energy sources, the recovery of energy from organic wastes is a relatively unexplored and yet promising strategy that may help tackle the dual challenge of reducing our dependency on fossil fuels and effectively generating value from proper organic waste management options. Different routes have been explored in recent years to recover energy from organic wastes, but the microbial-driven process of Anaerobic Digestion (AD) is the more mature to convert organic wastes into biofuels. The present thesis aimed to identify superior hydrogen-producing microbes that can represent valid alternatives to improve the OFMSW-to-Hydrogen conversion and focused also on developing biotechnological solutions to advance AD treatment of bioplastic items that are disposed of in the Organic Fraction of Municipal Solid Waste (OFMSW) but are not effectively treated in AD plants. Large-scale hydrogen production from organic waste is still unachievable as economic and technical challenges persist, with low hydrogen yields and the absence of robust microbial strains suitable for industrial processes being the major ones. To address these limitations, H2-producing microbes, adapted to complex substrates, have been previously isolated in the Waste to Bioproducts-Lab from an anaerobic digestor. Ten morphologically distinct bacterial species were selected and genetically identified by 16S rDNA sequencing. Among these species, the Clostridium sartagoforme SA1 strain exhibited high hydrogen yields from glucose, soluble starch and carboxymethylcellulose (CMC). The strain was then tested on the OFMSW, which is particularly rich in starch and cellulose, with outstanding productions, up to 39 mL H2 gVS-1. Additionally, C. sartagoforme SA1 confirmed its ability to produce H2 even in the presence of OFMSW’s indigenous microflora, increasing the H2 yield by 36%. This finding is of great importance since the strain proved to be efficient as H2 producer even in a highly competitive environment. These findings suggest that the identified C. sartagoforme SA1 holds promise as a robust microbe to be efficiently used for the H2 production from OFMSW. Bioplastics are sustainable alternatives to conventional fossil plastics and their fast-growing production requires new strategies for their end-of-life effective management. This thesis aimed to evaluate the ability of recombinant enzymes, produced in S. cerevisiae, to hydrolyse Mater-Bi® shoppers, a common starch-based bioplastic item. Several hydrolysis experiments were carried out to identify the best-performing enzyme combinations. Then, the selected enzymes were tested also in simulated AD settings. Moreover, the BMP (Biochemical Methane Potential) of OFSMW supplemented with Mater-Bi® bags, with or without the recombinant enzymes, was also assessed and proved that the biotechnological solution so far developed has great potential towards industrial application in AD plants.