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Sustainable biofuel production from microalgae cultivated in domestic wastewater: Process development and environmental assessment
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A crescente busca por sistemas energéticos sustentáveis e por tecnologias eficientes para o tratamento de águas residuais promove o desenvolvimento de soluções integradas que abordem simultaneamente a recuperação de recursos e a produção de combustíveis renováveis. Neste contexto, a presente tese de doutoramento investiga a produção de biocombustíveis a partir de biomassa de microalgas cultivadas em esgoto doméstico, bem como a subsequente avaliação do desempenho ambiental, no âmbito de um enquadra mento de bioeconomia circular. Três culturas de microalgas (Chlorella vulgaris, Tetradesmus obliquus e uma cultura mista de microalgas e bactérias - consórcio) foram avaliadas em relação à sua capacidade de tratar esgoto doméstico e gerar biomassa adequada para a produção de biocombustíveis. Todos os sistemas demonstraram remoção eficaz de azoto e fósforo, contribuindo para a melhoria da qualidade da água. A cultura mista apresentou a maior remoção de matéria orgânica, evidenciando interações sinérgicas entre algas e bactérias presentes na amostra. A caracterização da biomassa revelou diferenças significativas no teor de azoto entre as culturas, um parâmetro relevante que influencia a qualidade do biocombustível nas etapas subsequentes do processo. A biomassa colhida foi convertida em bio-crude através de liquefação hidrotérmica (HTL), um processo termoquímico particularmente adequado para matérias-primas húmidas. O estudo analisou a transferência de azoto da biomassa para o bio-óleo, considerando que teores elevados de azoto comprometem a estabilidade do combustível e promovem emissões de NOx durante a combustão. A microalga Tetradesmus obliquus apresentou os menores teores de proteína e de azoto celular, bem como a menor concentração de azoto no bio-óleo final. Estes resultados demonstram que a seleção da cultura constitui uma estratégia relevante para melhorar a qualidade do bio-óleo e minimizar impactos ambientais. Com o objetivo de melhorar adicionalmente as propriedades do combustível, procedeu-se ao hidrotratamento catalítico dos bio-óleos obtidos por HTL, utilizando os catalisadores CoMo/Al2O3, CoMo/Al2O3 sulfurado (sCoMo) e Pt/Al2O3. O upgrading catalítico permitiu aprimorar as propriedades físico-químicas do bio-óleo, incluindo o teor de carbono e hidrogénio e o poder calorífico superior (HHV), promovendo simultaneamente desoxigenação e desnitrificação. O catalisador sCoMo apresentou melhor desempenho no aumento dos teores de C e H, na melhoria do HHV e na remoção de azoto e oxigénio. Contudo, considerando propriedades comerciais, o catalisador Pt alcançou a maior fração de compostos na gama de ebulição de combustíveis de aviação, demonstrando que o óleo tratado apresenta elevado potencial como intermediário para a produção de combustível sustentável para a aviação (SAF). Por fim, foi realizada uma análise de ciclo de vida para avaliar os impactos ambientais associados à produção do bio-óleo. Os resultados indicam que os principais hotspots estão associados ao processo de sulfuração do catalisador. A avaliação ambiental fornece ainda dados primários que suportam o escalonamento industrial do processo de produção de biocombustível via HTL. Neste sentido, esta tese demonstra que a integração do tratamento de esgoto doméstico com microalgas selecionadas, liquefação hidrotérmica e upgrading catalítico constitui uma via relevante para a produção de biocombustíveis sustentáveis. Ao alinhar a valorização de resíduos com a geração de energia renovável, este trabalho contribui para os princípios da economia circular e apoia a transição energética sustentável.
The increasing demand for sustainable energy systems and effective technologies for wastewater treatment promotes the development of integrated solutions that simultaneously address resource recovery and renewable fuel production. In this context, this PhD thesis investigates the production of biofuels from microalgal biomass cultivated in domestic wastewater and further environmental performance assessment within a circular bioeconomy framework. Three microalgal cultures (Chlorella vulgaris, Tetradesmus obliquus and a mixed culture of microalgae and bacteria (consortium)) were evaluated regarding their capacity to treat domestic wastewater and generate biomass suitable for biofuel production. All systems demonstrated effective nitrogen and phosphorus removal, contributing to improved water quality. The mixed consortium achieved the highest organic matter removal, highlighting synergistic interactions between algae and bacteria. Biomass characterisation revealed significant differences in nitrogen content among strains, an important parameter that influences downstream biofuel quality. The harvested biomass was converted into bio-crude through hydrothermal liquefaction (HTL), a thermochemical process particularly suitable for wet feedstocks. The study examined nitrogen transfer from biomass to bio-oil, given that excessive nitrogen content compromises fuel stability and promotes NOx emissions during combustion. Tetradesmus obliquus, exhibited the lowest protein and cellular nitrogen contents and also the lowest nitrogen concentration in the final bio-oil. These findings demonstrate that strain selection is an important up- stream strategy for improving bio-oil quality and minimising environmental impacts. To further enhance fuel properties, catalytic hydrotreatment of the HTL-derived bio-oils was performed using CoMo/Al2O3, sulphided CoMo/Al2O3 and Pt/Al2O3 catalysts. The catalytic upgrading improved physicochemical properties of the crude oil, including carbon and hydrogen content and higher heating value (HHV), while promoting deoxygenation and denitrogenation. Sulphided CoMo (sCoMo) presented better performance in increasing C, H and HHV contents and in promoting nitrogen and oxygen removal. However, when considering commercial properties, Pt catalyst achieved the highest fraction of com- pounds within the jet fuel boiling range, demonstrating that the treated oil has strong potential as an intermediate for sustainable aviation fuel (SAF) production. Finally, a life cycle assessment (LCA) was conducted to evaluate the environmental impacts associated with the bio-oil production. The results indicate that the main hotspots are associated with the catalyst sulphiding process. The environmental assessment also provides primary data to support the industrial scale-up of the HTL-based biofuel production process. In this sense, this thesis demonstrates that the integration of domestic wastewater treatment using selected microalgae, hydrothermal liquefaction and catalytic upgrading constitute an important pathway toward the production of sustainable biofuels. By aligning waste valorisation with renewable energy generation, this work contributes to circular economy principles and supports the sustainable energy transition.
The increasing demand for sustainable energy systems and effective technologies for wastewater treatment promotes the development of integrated solutions that simultaneously address resource recovery and renewable fuel production. In this context, this PhD thesis investigates the production of biofuels from microalgal biomass cultivated in domestic wastewater and further environmental performance assessment within a circular bioeconomy framework. Three microalgal cultures (Chlorella vulgaris, Tetradesmus obliquus and a mixed culture of microalgae and bacteria (consortium)) were evaluated regarding their capacity to treat domestic wastewater and generate biomass suitable for biofuel production. All systems demonstrated effective nitrogen and phosphorus removal, contributing to improved water quality. The mixed consortium achieved the highest organic matter removal, highlighting synergistic interactions between algae and bacteria. Biomass characterisation revealed significant differences in nitrogen content among strains, an important parameter that influences downstream biofuel quality. The harvested biomass was converted into bio-crude through hydrothermal liquefaction (HTL), a thermochemical process particularly suitable for wet feedstocks. The study examined nitrogen transfer from biomass to bio-oil, given that excessive nitrogen content compromises fuel stability and promotes NOx emissions during combustion. Tetradesmus obliquus, exhibited the lowest protein and cellular nitrogen contents and also the lowest nitrogen concentration in the final bio-oil. These findings demonstrate that strain selection is an important up- stream strategy for improving bio-oil quality and minimising environmental impacts. To further enhance fuel properties, catalytic hydrotreatment of the HTL-derived bio-oils was performed using CoMo/Al2O3, sulphided CoMo/Al2O3 and Pt/Al2O3 catalysts. The catalytic upgrading improved physicochemical properties of the crude oil, including carbon and hydrogen content and higher heating value (HHV), while promoting deoxygenation and denitrogenation. Sulphided CoMo (sCoMo) presented better performance in increasing C, H and HHV contents and in promoting nitrogen and oxygen removal. However, when considering commercial properties, Pt catalyst achieved the highest fraction of com- pounds within the jet fuel boiling range, demonstrating that the treated oil has strong potential as an intermediate for sustainable aviation fuel (SAF) production. Finally, a life cycle assessment (LCA) was conducted to evaluate the environmental impacts associated with the bio-oil production. The results indicate that the main hotspots are associated with the catalyst sulphiding process. The environmental assessment also provides primary data to support the industrial scale-up of the HTL-based biofuel production process. In this sense, this thesis demonstrates that the integration of domestic wastewater treatment using selected microalgae, hydrothermal liquefaction and catalytic upgrading constitute an important pathway toward the production of sustainable biofuels. By aligning waste valorisation with renewable energy generation, this work contributes to circular economy principles and supports the sustainable energy transition.
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Palavras-chave
Microalgae wastewater treatment hydrothermal liquefaction catalytic up-grading life cycle assessment