Resource recovery from domestic waste sources using purple phototrophic bacteria in photo-biorefineries

Abstract

This doctoral thesis was carried out at the Department of Chemical and Environmental Technology at Rey Juan Carlos University, as part of the Biomass and Bioenergy research line, in the Area of Design of biorefineries based on the production and integral valorization of microorganisms. It was funded by the European project DEEP PURPLE (Bio-based Industries Joint Technology Initiative (BBI-JTI) under the European Union's Horizon 2020 research and innovation program under grant agreement No. 837998). A three-month predoctoral stay was also conducted at the Nova University of Lisbon within the Biochemical Engineering research group. The growing scarcity of freshwater is forcing us to seek alternative sources, such as recycled water, obtained from wastewater treatment. When properly treated, this water allows nutrients (such as nitrogen and phosphorus) to be recovered and can be reused in agricultural irrigation, reducing the use of non-organic fertilizers. Therefore, it is essential to develop technologies capable of ensuring the recovery and reuse of wastewater to comply with environmental standards. In general, wastewater treatment plants (WWTPs) are divided into three phases: primary, secondary, and tertiary. In the latter phase, aerobic and anaerobic biological treatments predominate, especially the activated sludge system and anaerobic digestion, respectively. In both cases, they are limited by high sludge production, the energy cost of the processes,and, specifically in anaerobic digestion, long retention times. Despite being implemented at an industrial level, and with research continuing to improve their energy and environmental efficiency, the aim is to promote new technologies that do not have such high economic or energy requirements. This has led to a search for the integration of urban waste treatment within the framework of the circular economy. This need responds to the urgency of replacing the linear model of production and consumption based on the extraction, use, and disposal of resources. This change introduces the concept of the circular economy, proposing a structural shift towards a system where materials are kept within the production cycle, minimizing waste and reducing polluting emissions. Biorefineries are emerging as key tools in this process, transforming waste into value-added products such as energy, materials, or nutrients, promoting energy efficiency, and reducing dependence on non-renewable resources. Within this context, WWTPs are evolving into resource recovery plants, where clean water, energy, and nutrients are recovered, thus integrating the principles of sustainability and circularity. Improving the energy efficiency of WWTPs is one of the main challenges today, as these facilities consume a significant portion of the energy used in the urban water cycle, almost 44%. Reducing this consumption is essential to decreasing greenhouse gas emissions and moving toward increasingly self-sufficient systems. Among the most promising innovations in biological wastewater treatment are purple phototrophic bacteria (PPB). These bacteria are facultative photosynthetic organisms that use light and have remarkable metabolic versatility. They are capable of simultaneously assimilating carbon, nitrogen, and phosphorus, generating biomass rich in high-value-added compounds. Among these compounds, their ability to store carbon in the form of polyhydroxyalkanoates (PHA) or glycogen, and phosphorus as polyphosphate, stands out, making them potential sources of fertilizers. In addition, their ability to fix nitrogen and synthesize high-quality proteins positions them as a viable single-cell protein (SCP) alternative for animal or human consumption. The metabolism of PPB varies according to environmental conditions, and they can develop in photoheterotrophic, heterotrophic, or chemoautotrophic environments. This metabolic flexibility of PPB allows them to adapt to different types of wastewater and recovery objectives. The balance between carbon and nitrogen in the culture medium is decisive: high ratios favor the accumulation of bioplastics, while more balanced proportions optimize nutrient removal. Factors such as light and dark cycles, dissolved oxygen concentration, and light intensity also influence this process. Proper control of redox potential and aeration is crucial to prevent oxidation of photosynthetic complexes. On the other hand, feeding strategies and lighting conditions can improve the production of polymers and pigments, which would help reduce the energy costs of the process. Furthermore, unlike other photosynthetic microorganisms such as microalgae, PPB use wavelengths in the near-infrared range (between 800-1050 nm), which reduces microbial competition and favors their potential for implementation in open systems. In this case, scaling up PPB-based photobioreactor systems is one of the main technological challenges. Operation in open systems, such as raceway channels, is more economical, although it poses challenges in terms of oxygen control and microbial competition. Closed reactors, such as flat plate reactors, allow for greater control but are more expensive. In any case, optimizing the design and light transfer is essential to maintain the productivity and stability of bacterial communities. To this end, computational modeling tools are a great ally in optimizing the hydraulics and light distribution in these systems effectively in different environments. The implementation of large-scale PPB photobiorefineries capable of integrating wastewater treatment with the recovery of value-added products represents a decisive step towards sustainability. European projects such as DEEP PURPLE have made progress in this direction, combining the management of municipal organic waste, sludge, and domestic wastewater using photobioreactor systems that harness the ability of PPB to generate bioplastics, fertilizers, and other valuable compounds. This approach demonstrates that it is possible to conceive of treatment plants not only as purification infrastructures but also as productive biofactories that close the resource cycle, reduce emissions, and contribute to the development of a truly circular economy. PPB-based wastewater treatment is positioned as one of the most promising alternatives for simultaneously addressing the water crisis, pollution, and dependence on fossil resources. Its combination of efficiency in removing pollutants, low energy consumption, and ability to generate high economic value products makes it a strategic tool for moving towards a sustainable resource management model. The future challenge lies in consolidating its industrial scalability and holistically integrating the environmental, economic, and social aspects that will enable it to realize its potential as a driver of the circular bioeconomy. Within this framework, and to contribute to the advancement of PPB photobiorefineries, this doctoral thesis addresses the need to change the paradigm of urban waste management (wastewater and the organic fraction of municipal solid waste) from treatment and disposal to recovery and nutrient recovery, under the auspices of the circular economy. One of the main challenges in the treatment of domestic wastewater (DWW) using PPB lies in the imbalance between the chemical oxygen demand (COD) and the nutrients present in the effluent, particularly nitrogen (N). This disproportion makes it challenging to remove carbon, nitrogen, and phosphorus simultaneously without an external carbon source, which is necessary to achieve the optimal COD:N ratio required by PPB.