Comprehensive insights into conversion of microalgae to feed, food, and biofuels: Current status and key challenges towards implementation of sustainable biorefineries

Microalgae have been promoted as important feedstocks for producing biofuels and bioproducts. However, their production on a large scale would require a large amount of water and nutrients. Considering the importance of food security and the high economic value of microalgal biomass, food and feed applications should be prioritized if microalgae are grown using conventional water resources. Nevertheless, microalgae can also be grown in nutrient-rich wastewater streams while producing valuable biomass and treating harmful effluents. Because of health issues, food and feed applications of microalgal biomass grown using waste resources are not allowed; hence, it can be used as a feedstock for biofuel production. In line with that, this review first briefly discusses the applications of microalgae in food, feed, and wastewater treatment and then summarizes the efforts put into utilizing microalgal biomass to produce biofuels through different pathways. These technologies, including thermochemical (200–1300 °C), biochemical (25–55 °C), and chemical (25–200 °C) conversion methods, are critically discussed. The pros and cons of using different microalgal conversion technologies are outlined to identify possible future directions for the field. Different microalgal biomass harvesting methods are also reviewed, and the process conditions are summarized. Parameters affecting technology selection, such as energy costs, reaction time, and technological sophistication, are also explored and presented. In general, microalgal biofuels still face many challenges in replacing traditional fossil fuels. Future work should focus on maximizing the yield and quality of microalgal biofuels while enhancing their economic viability.