From waste to biopolymers: integrated process development for polyhydroxyalkanoate (PHA) production from wastewater treatment sludge. Bridging laboratory innovation and industrial application

Wastewater treatment sludge represents both an environmental challenge and a resource within the circular bioeconomy. This thesis investigates the valorization of sewage sludge through the production of polyhydroxyalkanoates (PHAs) using mixed microbial cultures (MMCs), focusing on process efficiency, operational stability, and industrial scalability. Reactor configurations were systematically explored, from batch to semi-continuous and continuous operation. In sequencing batch reactors, decoupling carbon and nitrogen feeding and withdrawing biomass immediately after the feast phase enabled selective enrichment of PHA-storing microorganisms while maintaining high intracellular polymer content. Operational parameters, including organic loading rate and carbon-to-nitrogen ratio, were critical in balancing growth and storage metabolism. The same strategy of carbon and nitrogen decoupling, combined with biomass withdrawal immediately after the feast phase, was implemented in a continuous-feeding configuration designed to simplify reactor operation. This configuration maintained the selective enrichment of PHA-storing microorganisms while providing more stable substrate availability and reducing the complexity of reactor operation, effectively combining the benefits of feast–famine dynamics with enhanced operational practicality. Finally, a fully integrated continuous process was developed in collaboration with B-Plas sbrl, combining hydrothermal carbonization (HTC), anaerobic fermentation and microaerophilic-aerobic PHA production. HTC solubilized sludge organic matter, anaerobic fermentation converted it into volatile fatty acids, and the continuous reactor enriched PHA-storing microbes under microaerophilic-aerobic conditions, achieving high polymer yields without complex cyclic feeding or nutrient limitation. These results directly supported the design of an industrial plant at in a wastewater treatment facility, capable of treating approximately 3500 t·y⁻¹ of sludge. This work demonstrates that wastewater sludge can be efficiently converted into high-value PHAs, highlighting both opportunities and challenges, including polymer extraction, feedstock variability, and long-term operational stability. By integrating microbial ecology, process optimization, and engineering design, the research provides a pathway for sustainable, circular, and energy-positive wastewater treatment systems.