The concerns over food security and protein scarcity, driven by population increase and higher standards of living, have pushed scientists toward finding new protein sources. A considerable proportion of resources and agricultural lands are currently dedicated to proteinaceous feed production to raise livestock and poultry for human consumption. The 1st generation of microbial protein (MP) came into the market as land-independent proteinaceous feed for livestock and aquaculture. However, MP may be a less sustainable alternative to conventional feeds, such as soybean meal and fishmeal, because this technology currently requires natural gas and synthetic chemicals. These challenges have directed researchers toward the production of 2nd generation MP by integrating renewable energies, anaerobic digestion, nutrient recovery, biogas cleaning and upgrading, carbon-capture technologies, and fermentation. The fermentation of methane-oxidizing bacteria (MOB) and hydrogen-oxidizing bacteria (HOB), i.e., two protein rich microorganisms, has shown a great potential, on the one hand, to upcycle effluents from anaerobic digestion into protein rich biomass, and on the other hand, to be coupled to renewable energy systems under the concept of Power-to-X.This work compares various production routes for 2nd generation MP by reviewing the latest studies conducted in this context and introducing the state-of-the-art technologies, hoping that the findings can accelerate and facilitate upscaling of MP production. The results show that 2nd generation MP depends on the expansion of renewable energies. In countries with high penetration of renewable electricity, such as Nordic countries, off-peak surplus electricity can be used within MP-industry by supplying electrolytic H2, which is the driving factor for both MOB and HOB-based MP production. However, nutrient recovery technologies are the heart of the 2nd generation MP industry as they determine the process costs and quality of the final product. Although huge attempts have been made to date in this context, some bottlenecks such as immature nutrient recovery technologies, less efficient fermenters with insufficient gas-to-liquid transfer, and costly electrolytic hydrogen production and storage have hindered the scale up of MP production. Furthermore, further research into techno-economic feasibility and life cycle assessment (LCA) of coupled technologies is still needed to identify key points for improvement and thereby secure a sustainable production system.