Special Issue Summary:

Wastewater treatment is entering a new stage in which pollution control, resource recovery, carbon reduction, and circular bioeconomy must be integrated. Large quantities of biomass waste, including sewage sludge, algal residues, and other organic by-products from wastewater treatment systems, represent both an environmental burden and a valuable resource reservoir. Beyond conventional disposal, advanced thermal conversion, anaerobic and aerobic biotechnologies, green chemistry, and emerging bioelectrochemical and digital-intelligent processes provide new opportunities for recovering energy, nutrients, bio-based materials, and functional products. This Special Issue aims to highlight cutting-edge research on sustainable wastewater treatment driven by circular economy principles, with particular attention to carbon-neutral technologies, low-emission resource recovery, process intensification, life-cycle assessment, and the water–energy–nutrient nexus. We welcome original research articles, reviews, short communications, and perspectives addressing innovative technologies, mechanisms, system integration, and practical applications for transforming wastewater-derived biomass waste into valuable resources.