Utilization of residues from microalgal industries for agricultural practices: A comprehensive review

Global consumptions of fertilizers and freshwater have been continuously growing due to increased food demand, leading to great concerns on food security. Bio-based resilient nutrient resources such as microalgae-derived waste biomass and wastewater have gained great attention as alternative fertilizer resources because they contain key nutrients (nitrogen and phosphorus), organic compounds (acting as soil conditioner, growth stimulators), and micronutrients. Thus, microalgae-derived solid and liquid fertilizers have great potential in promoting plant growth in soil and/or hydroponic farming, controlling release of nutrients to avoid nutrient leaching and volatilization, and facilitating to achieve circular economy in microalgal industries. However, several challenges, such as imbalanced nutrient element ratios, causes of heavy metal accumulation and increased pH/conductivity, may limit their wide applications. Several recent-published review articles have documented the application of fresh microalgal biomass as fertilizer sources via direct use and conversion methods or recycling cultivation medium for microalgal growth, but no review has been conducted on utilization of microalgal processing wastewater and biomass residues for agriculture practices. Herein, this article provides a comprehensive review on the processes relating to recovery of resources (water, nutrients, valuable plant growth compounds) from microalgae processing wastewater and biomass residues generated in microalgal biorefinery industries, and identifies the key factors that are associated with the resource recovery efficiency and their effects on plant growth.