Sustainable production of microalgae biomass for biofuel and chemicals through recycling of water and nutrient within the biorefinery context: A review

Abstract Commercial‐scale production of microalgae biomass for biofuel and biochemicals requires a substantial amount of water and nutrients. Many studies are conducted to explore the potential of various aqueous streams originating from harvesting stage and different energy recovery steps as an alternative for water and nutrient supply. Presence of toxic organic compounds, unassimilated ions, particulate matter, and high alkalinity in post‐harvest water limit its recyclability. Nutrient recovery from biomass via anaerobic digestion (AD) and various hydrothermal processes is being explored. So, there is a need to understand the impact of harvesting methods, nature and impact of organic compounds, buildup of algogenic organic matter (OM), amount of unused nutrients, and salinity on water recycling. Optimum conditions for maximum nutrient recovery from AD and hydrothermal processes are discussed for effective nutrient recycling. This review is an attempt to understand the challenges associated with the recycling of aqueous streams for water and nutrient requirement for sustainable microalgae cultivation. The effectiveness of a recycling stream is defined here as “biomass ratio.” Possible growth inhibiting factors are identified, and their solutions are suggested along with potential directions for future research. Large‐scale sustainable cultivation of microalgae through recycling of different streams depends on better understanding of biological activity of algal OM through detailed characterization and in‐depth understanding of physiochemical properties.