Microalgal membrane bioreactors for wastewater treatment: Valorisation pathways, micropollutant removal, and scale-up applications

Global water scarcity and strict discharge regulations demand wastewater treatment systems to simultaneously achieve high treatment efficiency and resource recovery. Conventional activated sludge-based technologies remain energy intensive and are not particularly effective for nutrient recovery and emerging micropollutant removal. Microalgal membrane bioreactors (MBRs) have therefore gained attention as integrated treatment platforms that combine biological assimilation with membrane-based solid-liquid separation. This review critically examines recent progress in algal MBR for wastewater treatment, with a focus on valorisation pathways, micropollutant removal, and scale-up applications. Notably, key operating parameters are analysed, including algal-bacterial symbiosis, membrane-assisted biomass retention, and the coexistence of multiple removal mechanisms governing nutrient assimilation, biosorption, biodegradation, and photodegradation. This review further assesses major valorisation pathways for wastewater-grown algal biomass, including anaerobic digestion, integrated biorefineries, and nutrient recycling, highlighting their potential to improve the overall techno-economic and environmental sustainability of treatment systems. Significant technological challenges remain, particularly membrane fouling associated with algal extracellular products, process optimisation under variable wastewater and climatic conditions, and uncertainties surrounding long-term and large-scale operation. Finally, critical knowledge gaps are identified, including the need for standardized performance metrics, fouling-resilient bioreactor and membrane designs, integrated life cycle assessments, and robust pilot- and full-scale demonstrations. These gaps should be addressed to advance algal MBR from lab-scale systems to practical wastewater resource recovery technologies.