Study of Polyhydroxyalkanoates production processes

Abstract

The challenge due to the unsustainable situation caused by petroleum-based plastics, requires different alternative process and technologies to obtain biodegradable plastics. Polyhydroxyalkanoates (PHA) are a kind of biodegradable plastic. PHA are polyesters mainly composed by two majority groups: Polyhydroxybutyrate (PHB) and Polyhydroxyvalerate (PHV). Those plastics can be obtained using biological processes which are composed by four phases: an anaerobic acidogenic fermentation, a PHA-producer biomass selection, an accumulation of PHA and the PHA extraction. These processes could use sustainable resources such as wastewaters and Organic Fraction of Municipal Solid Waste (OFMSW) as influent substrate. In this final degree project, the selection and accumulation of bioplastics will be studied. On one hand in lab scale, where the characterization of different selection cycles are carried out based on profiles of Dissolved Oxygen (DO), pH, Reduction-Oxidation Potential (ORP), ammoniacal nitrogen concentration (N-NH4+) and Volatile Fatty Acids (VFAs). And on the other hand, different pilot scale production processes are described and discussed. Regarding to lab experimentation, a synthetic influent, which simulates fermented OFMSW, was used applying an Organic Loading Rate (OLR) of 2.5 g COD L-1 d-1 in a 3.75 L Sequential Batch Reactor (SBR) with temperature control at 35 ºC. This reactor was initially inoculated with sewage sludge from a municipal wastewater treatment plant. The strategy used for the selection of biomass has been an aerobic Feast-Famine with uncoupled nitrogen feeding. Experimental results obtained from the lab study suggest an effective pseudo-stationary selection cycles, where DO profile follows the same trend. Moreover, the biomass adaptation at OLR working value indicates the possibility to increase the OLR to rise the PHA-accumulation and production potential. Accumulation could not have been studied but, based on previous research, PHA compositions are estimated nearby 14% (g PHA) (g VSS)-1 and 44% (g PHA) (g VSS)-1 in the selection and accumulation process, respectively. After the analysis and description of the different pilot plants, it is concluded that different technologies are applicable depending on the main objective of the biorefinery. If nitrogen removal is the principal objective, the use of nitrification and denitrification using part of the accumulated PHA could be used. Furthermore, if PHA production is desired, the use of OFMSW or biological sludges are suitable. Therefore, PHA production is a viable alternative which generates environmental-friendly plastics using wastes as sources