Microalgae-Based Wastewater Treatment and Biomass Valorization: Insights, Challenges, and Opportunities from 15 Years of Research

This study highlights 15 years of research conducted at the Federal University of Viçosa, Brazil, on utilizing microalgae for wastewater treatment and biomass valorization within the biorefinery framework. High-rate algal ponds (HRAPs) have emerged as an accessible, efficient, and sustainable technology for decentralized sanitation, particularly in remote areas. The research findings, consistent with global studies, demonstrate that strategies such as ultraviolet (UV) predisinfection and carbon dioxide (CO₂) supplementation significantly enhance the removal of nitrogen (N), phosphorus (P), and organic matter while optimizing biomass productivity. Integrating domestic and industrial wastewater improved nutrient balance, while hybrid systems combining HRAPs with biofilm reactors further increased biomass yields and reduced operational costs. The resulting biomass exhibited high versatility, with proteins (32-42%) as the dominant fraction, followed by carbohydrates (18-23%) and lipids (13-16%), underscoring its potential for bioenergy and agricultural applications. Energy conversion pathways such as anaerobic digestion and hydrothermal liquefaction (HTL) effectively transformed wet biomass into biogas and bio-oil. However, challenges such as high ash and nitrogen content remain to be addressed. In agriculture, microalgae-derived biofertilizers enhanced soil health, reduced nutrient losses, and boosted crop productivity, demonstrating performance comparable to chemical fertilizers. Despite economic and environmental challenges, such as high drying costs and energy demands, the study underscores the significant potential of microalgae-based solutions in advancing the circular bioeconomy. This approach aligns with the Sustainable Development Goals (SDGs) by integrating wastewater treatment, clean energy production, and food security into a cohesive, sustainable framework.