This project aims at developing a multi-stage process for polyhydroxyalkanoates (PHA) production by mixed microbial cultures (MMC) whereby the combination of physical-chemical and biotechnological steps offers an innovative route to connect the upcycle of conventional plastic wastes through their conversion into new bio-based and biodegradable polymers, so fullfilling the principles of circular economy.
Although the use of MMC for PHA production from fermentable organic waste is being studied for several years at laboratory scale and recently also at pilot scale, plastic waste is still an unexplored feedstock. An initial physical-chemical pretreatment stage is required to make this waste a suitable substrate for its biological conversion and the obtained composition of the pretreated carbon source primarily affects the polymer composition, which also depends on the performance of the following biological stages.In particular, a pivotal role is played by the microbial selection stage since the better its operation the higher the PHA production in the accumulation reactor which, in turn, affects the following extraction step.
Indeed, downstream steps are also quite important, which include not only PHA extraction from biomass but also its further processing into bioplastics with desired properties. In particular, functionalisation to confere anti-microbial activity and 3D-printing will be investigated.
To achieve these targets , the project is organized into 3 Workpackages, each one containing significant steps forward with respect to the state of art:
WP1: Physical-chemical pretreatments of plastic wastes, including a combination a thermal treatment and chemical oxidation , either in sequence or simultaneously
WP2: Set-up of a biological system for PHA production with MMC, with a brand new approach of a sequence of continuous reactors
WP3: Downstream processing, including supercritical fluid PHA extraction, antimicrobial functionalization and 3-D printing
This project aims at developing a multi-stage process for polyhydroxyalkanoates (PHA) production by mixed microbial cultures (MMC) whereby the combination of physical-chemical and biotechnological steps offers an innovative route to connect the disposal of conventional plastic wastes to circular economy. The latter enhances the recycling and reuse of plastic materials and their conversion into PHA, which are both biobased and biodegradable polymers, poses a concrete and attracting solution to environmental issues. Although the use of MMC for PHA production from waste materials is being studied for several years at laboratory scale and recently also at pilot scale, plastic waste is still an unexplored feedstock. An initial physical-chemical pretreatment stage is required to make this waste a suitable substrate for its biological conversion and the obtained composition of the carbon source primarily affects the polymer composition, which also depends on the performance of the following biological stages. However, the optimal operating conditions of the overall process will be assessed also thanks to the expertise acquired by the participation of this research group to various European Horizon 2020 Projects. In particular, a pivotal role is played by the microbial selection stage since the better its operation the higher the PHA production in the accumulation reactor which, in turn, affects the following extraction step. The microbial selection is typically operated in an SBR which allows to obtain the feast and famine (FF) conditions required to establish the selective advantage towards PHA-storing microorganisms. Here, the intent is to test a novel FF system consisting of two sequentially operated reactors (instead of a single SBR) where the feast and famine phases are physically and not temporally separated. In this configuration, the hydraulic retention time of the first and the second reactor correspond to the feast and the famine phase of the SBR cycle, respectively. As a consequence, the feast reactor can be continuously fed with the organic substrate and this brings an advantage over the SBR that is periodically fed. Indeed, from an operational point of view, the continuous operation is simpler than the intermittent one since it can be used without a computerized system of control to run the pumps of the reactors (as required for the SBR system). However, in order to determine the impact of the operation mode of the FF system on the efficiency of microbial selection, the results obtained with the proposed new two-reactor configuration will be compared to those acquired in previous studies operated with the SBR configuration. Also, another idea to be developed in the project relies on the possibility to infer specific characteristics to the produced polymer for ad hoc applications, with main reference to food packaging. PHA functionalization follows its extraction from microbial cells, that will be performed with a recently developed method based on the use of supercritical CO2 coupled to an enzymatic pre- or post-treatment to increase the polymer purity. Clearly, the optimal conditions to which operate the extraction step need to be identified since PHA composition and purity are strictly related to the nature of the feedstock that, in this project, is complex and heterogeneous. This makes particularly interesting the opportunity to establish an adequate procedure by which creating three-dimensional devices made of PHA starting from plastic wastes. The integration of the emerging 3-D printing technology with the proposed process represents a further and substantial advancement of the project that permits to provide a new life to conventional plastic waste.
Moreover, we will use antimicrobial molecules to surface-functionalize PHA films for industrial packaging. In fact, the quest for new antimicrobial materials, such as hydrogels of Fmoc-protected peptides and amino acids has gained momentum due to their ease of synthesis and cost effectiveness. Fmoc protected peptides have been shown to inhibit the activity of many Gram-positive (e.g. Staphylococcus aureus, Staphylococcus epidermis) and Gram-negative (e.g. Escherichia coli, Pseudomonas aeruginosa) bacteria (Ryan et al., 2011, Langmuir 27:4029-39; Toke 2005, Biopolymers 80:717-735; Gahane et al., 2020, Biomater Sci 8:1996-2006).