Humanity is facing an energy crisis due to depleting fossil resources, industrialization, and a growing world population, which has forced us to focus on finding alternative energy sources. Moreover, increasing carbon dioxide concentration in the atmosphere has led to climate change with severe consequences such as changing weather patterns and disruption of the balance of nature. In this context, microalgae have attracted global attention considering a potential solution for both sustainable energy and CO2 sequestration. Microalgae biomass with its excellent qualities such as rapid growth rates, high carbon dioxide absorption capacity, resistance to harsh conditions, and the potential for continuous production throughout the year are superior to other traditional feedstocks considered for bioenergy. Despite the huge potential of microalgae biomass, its current use is limited to a few products and applications due to the low productivity and high production cost of biomass. To offer a feasible solution, the current PhD thesis focuses on harnessing microalgae biomass as a sustainable resource for biogas production with potential applications in food, feed, nutraceuticals, cosmetics, and medicine industries when integrating a biorefinery concept. The PhD thesis aims to develop a novel improved microalgae biomass production system for biogas plants integrating biogas waste streams, namely digestate and flue gases as low-cost nutrients. The produced microalgal biomass is returned as anaerobic digestion feedstock closing the loop of nutrient circulation. This innovative approach seeks to integrate a circular economy model by utilizing waste streams from biogas plants, thereby transforming by-products and emissions into valuable resources for energy generation. During the doctoral studies, an innovative technology for microalgae cultivation effective in colder climates was designed as a pilot, constructed and patented. This technology overcomes the limitations of existing cultivation methods by improving light access for the microalgae, minimizing land use with a stacked modular system, and integrating artificial LED lighting. Furthermore, potential microalgae species for low-temperature climates were selected based on the literature review and tested. Altogether three microalgae strains were selected for suitability for biomass production using digestate. The PhD thesis consists of three main chapters: Literature review, Research methodology and Results and Discussion. In the literature review chapter, the factors affecting microalgae growth and biomass production are discussed, as well as state-of-the-art technologies for microalgae cultivation, potential biorefinery routes of microalgal biomass, digestate management and biomass harvesting techniques are reviewed. Moreover, the role of microalgae in the global bioeconomy is debated. In the Research methodology chapter methods applied are described. The obtained results are described and discussed in the Results and Discussion chapter. The PhD thesis is based on seven scientific publications and presented in five international scientific conferences.