Valorization of microalgae fermentation waste for a close-loop process

Heterotrophic microalgae culture has given new insights to the commercial biomass and biofuel production. Ultrahigh density microalgae cultures have been reported by many studies in this decade, while the waste management has not been comprehensively addressed in this regime and now a critical environmental concern. For this purpose, current review has attempted to design a close-loop fermentation process by recycling of water and nutrients from waste stream (spent culture media and lipid extracted microalgae biomass slurry) to the secondary microalgae fermentation. The idea of close-loop process is novelty of this study, which has been addressed by qualitative analysis of the existing studies in the fields of photoautotrophic and heterotrophic microalgae cultures along with bacterial and yeast fermentation. Secondary fermentation has challenges of heat based sterilization, excessive salts, organic growth inhibitors, carbon to nitrogen ratio, and turbidity. These problems are resolvable by effective pretreatments (membrane separation, activated carbon and enzymatic hydrolysis), nutrient alternatives and non-heat based sterilization processes. At the end, application of close-loop microalgae fermentation is concluded to reduce greenhouse gas emissions, eutrophication, raw material consumption and biomass production cost of the scale-up heterotrophic microalgae fermentation. The insights of the current review are helpful for the future studies a line with the united nations sustainable development goals SDG 7 and 8.