A techno-economic assessment methodology for the production of polyhydroxyalkanoates and ectoine from biogas in waste treatment plants

Resum

The implementation of the anaerobic digestion technology as a method for reducing the amount of solid waste sent to landfill has been a success with more than 25,000 plants in operation in the world. One of the main factors that has driven this worldwide implementation is the associated production of biogas, further valorized as an energy vector in heat and power co-generation engines (CHP). However, anaerobic digestion plants face nowadays a major dilemma, due to the higher cost of producing energy from biogas in CHP systems, compared to the rapidly declining cost of competing renewable energies such as wind or solar power. These problems have been aggravated by the lack of policy drivers towards the production of renewable energy from biogas and the reduction of feed-in tariffs and fiscal exemptions during the last decade. In fact, the attention of policy makers has shifted to the production of higher added-value products from waste, in the framework of a cleaner, greener and more circular economy and in line with the growingly restrictive environmental policies. Hence, medium and large-scale biogas production plants must reconsider their economic schemes and find innovative sources of revenue for guaranteeing their present and future economic viability. Therefore, there is a growing motivation for transitioning from linear waste treatment plants, where only bioenergy is produced, to more circular urban biorefineries, where all sort of bioproducts can be commercialized. In this transition to urban biorefineries, a better valorization of biogas plays a major role. Its utilization as a source of raw materials (mainly methane (CH4) and carbon dioxide (CO2)) instead of being merely regarded as an energy vector, has gained attention from both the academia and the industry. In this context, the utilization of methanotrophic bacteria, capable of using CH4-biogas as their only source of carbon and energy, has emerged as an opportunity for increasing the current value of biogas. During the last decade, academics have widely demonstrated at laboratory scale the ability of methanotrophic bacteria for manufacturing bioproducts which are ranked higher up in the waste valorization pyramid such as polihydroxyalkanoates (PHA) (chemicals and materials), single cell protein (feed and food) and ectoine (fine chemicals). However, the future technical and economic sustainability of these processes at large scale as well as their robustness in a global economic context in constant change is still unveiled. It is also of paramount importance to evaluate the current biotechnological limitations in bioproducts manufacturing and the potential reduction of production costs derived from future biotechnological advances, thus defining the roadmap to develop cost-competitive biogas biorefineries. This PhD thesis focused on the development and implementation of a methodology for evaluating the technical, economic and environmental feasibility of the bioconversion of biogas into added-value products using methanotrophic bacteria as an alternative to the current utilization of biogas as energy vector in waste treatment plants. This methodology was also designed to identify the biotechnological bottlenecks of these innovative technologies.