Setup and testing of a continuous reactor for valorisation of polyhydroxyalkanoates from bacterial sludge

The vast production of conventional plastics and their wide use in commercial applications is critically harmful for the environment. In recent years, bioplastics have become an alternative. Among them, the fully biodegradable polyhydroxyalkanoates (PHA) are polyesters produced by bacteria through aerobic fermentation of various carbon sources. Being PHA more expensive than their fossil counterparts, the aim of this thesis is to valorise bacterial biomass containing PHA by recovering the polymer in a continuous reactor and by converting PHA into propylene through a new hydrothermal protocol. A continuous reactor was set up to extract PHA from bacterial biomass produced via mixed microbial cultures (MMC) by using green solvents. Among them dimethyl carbonate gave the best extraction yields with an overall polymer recovery of 32%. In order to take advantage of the low PHA content biomass produced by MMC, and given the difficulties of extraction, the reactor was further adapted to convert PHA into propylene. Experiments were done setting up a new protocol which used a continuous system for hydrothermal treatment of crotonic acid (the main product of PHB degradation) at different temperatures, flows, reaction times and catalysts. Once determined the yield of propylene from “model” crotonic acid, tests were conducted also using pure PHB and MMC biomass. Crotonic acid was converted into propylene with a yield of 21.1% in the pressurized continuous system at 300°C; the same yield (21.9%) was achieved by treating PHB at 300°C for longer time (> 30 min). Under these conditions MMC biomass gave the highest conversion rate (67.5%), demonstrating the efficiency of the developed system in producing bio-propylene exploiting PHA-containing bacterial biomass.