Titolo della tesi: Conversion of plastic waste into bioplastics
The rise of plastic waste, particularly those that are difficult to recycle, such as “Plasmix”, poses an increasingly severe issue to worldwide waste management. Plasmix is a heterogeneous mixture of plastics and other materials (such as paper, glass, metal, and organic matter) generated through recycling waste plastic (usually from municipal separate plastics collection) via standard mechanical processes. Because of its heterogeneity, it cannot be recycled using traditional procedures that require homogeneous plastics to produce high-quality materials, leaving it as a non-recyclable byproduct that cannot be further processed. In many situations, such litter is burned or disposed of in landfills, intensifying the environmental problem and hampering the plastic sector transition to a circular economy. Achieving a virtuous management of all resources, minimizing the waste of potentially recyclable materials that could be reintroduced into other production cycles is, in this context, one of the main challenges of our times. Hydrothermal liquefaction (HTL) may, in fact, be an interesting solution for converting such waste into higher-value chemicals. HTL is a thermochemical process that turns diverse organic materials (including algal biomass, forest wastes, and even urban waste) into biofuels and other products such as gas, solid char, and aqueous residue (HTL-AP) using water at medium temperature and high pressure. In addition, this method has been used to decompose different polyolefins. In light of this, there exists the possibility of employing the HTL process to facilitate the removal of plastic waste such as Plasmix, for which it could serve as an effective thermochemical pre-treatment that, when combined with other processes, could help transform Plasmix into an energy resource, reducing reliance on fossil fuels while likewise decreasing the volume of waste destined for unsustainable treatments such as landfill or incineration. The aqueous phase produced by HTL is a residue rich in organic compounds that is generally regarded as a negative by-product of the process due to the presence of toxic compounds, but which, as a matter of fact, offers an opportunity for recovery through biological processes such as anaerobic acidogenic fermentation. This metabolism can produce carboxylic acids (e.g., acetic, propionic, butyric, valeric, and hexanoic acids). The latter are compounds with commercial applications in a variety of industrial sectors, including chemical, farming, agricultural, food, as well as in the research field. Furthermore, these acids serve as useful substrates for the production of biopolymers such as polyhydroxyalkanoates (PHAs), a family of biodegradable bioplastics that, due to their properties, which can be modulated based on the monomeric units that comprise the polymer, are emerging as a bio sustainable alternative to traditional fossil plastics. Nevertheless, when compared to the latter, their manufacturing is still costly, owing to the sterility required to maintain pure microbial strains and the high cost of ad hoc required feedstocks. To lower these expenses, researchers are looking on mixed microbial cultures (MMC), which can also exploit low/no cost substrates like organic waste streams (e.g., agro-industrial wastewaters). As a result, assessing the feasibility of generating PHA using plastic waste treated by hydrothermal pre-treatment would represent the dual advantage of making PHA production more cost-effective while simultaneously facilitating the management of non-recyclable plastic waste.
The research theme investigated in this PhD thesis is therefore focused on the exploration of innovative strategies for the recovery of plastic waste that are difficult to process, such as Plasmix, exploiting its potential for conversion into value-added compounds, with the secondary aim of reducing the cost of producing bioplastics such as PHA, exploiting the synergy between different technological approaches (thermochemical, mechanical, and biological), promoting a circular economy and reducing the environmental impact of fossil-based plastics.
In particular, the thesis has been divided into three major blocks.
The first block discussed the idea of converting the aqueous byproduct of the hydrothermal liquefaction (HTL) of plastic waste, such as Plasmix and polystyrene, into carboxylic acids using acidogenic fermentation approaches. The primary goal of the study was to assess the effectiveness of the fermentation process on thermochemically pre-treated plastic waste, comparing it to model substrates, with a focus on determining the most suitable HTL conditions and the impact that chemical species generated by the liquefaction of a Plasmix mixture could have on the process yield.
In the second block, in collaboration with the Instituto de Agroquímica y Tecnología de Alimentos (IATA) in Valencia (Spain), the problem of upgrading Plasmix was approached in a more straightforward manner, evaluating how to incorporate it as a filler in a poly(hydroxybutyrate-hydroxyvalerate), PHBV, based materials, reducing the material's production costs while also providing an innovative solution for reintroducing non-recyclable plastic waste into commercial products. The major goal was to determine the effect of adding Plasmix at various concentrations and pulverization degree on the most essential PHBV properties while retaining the material's biodegradability.
Finally, in the third block, with the partnership of the company NextChem s.p.a. the economic aspect of the treatment was investigated, preliminarily assessing the technical and economic feasibility of an industrial plant for the production of PHAs from mixed microbial cultures from plastic waste such as HTL-pretreated Plasmix, estimating in particular the main investment CAPEX) and operational (OPEX) costs.