Microalgal Photogranules Alchemy: Photobiotransformation of Hydroponic Effluent into Valuable Bioresources

Hydroponics effluent (HE) is characterized by a high nutrient concentration, laden with nitrogen, phosphorus, and root exudates, which leads to eutrophication and deterioration of water quality. Over the years, HE treatment has been a major challenge due to the elevated concentration of the nutrients. Subsequently, conventional treatments such as sewage treatment plants, constructed wetlands, activated carbon adsorption, and advanced oxidation processes are often effective but require high operation and maintenance, energy, and skilled personnel. Hence, low-cost nature-based microalgal wastewater treatment in recent years has gained a lot of attention. Microalgae offer a self-sustaining, sustainable alternative for HE treatment, offering the dual benefits of photomediation and biomass valorization. Therefore, this review highlights the potential of microalgae, particularly microalgal photogranules (MPGs), as an integrated system for HE recycling and high-value bioresource recovery. The review synthesizes key drivers and controlling factors of the microalgal treatment system including the biochemical composition of effluent-grown microalgal biomass. The review highlights that low COD removal efficiency, long retention times, and inadequate focus on circular bioeconomy integration are the major limitations of microalgal systems. Subsequently, MPGs emerge as a next-generation solution integrating microbial synergy, rapid settling, and low energy demand to achieve efficient HE treatment and resource recovery. Overall, the review highlights recent advancements by bridging microalgal biotechnology and environmental sustainability.