Utilizing the nucleation activity of filamentous cyanobacteria to construct photogranules: treatment performance, formation process and recycling phycobiliprotein.

This study characterizes the dynamic development process of photogranules, explores the role of related substances in their formation, and verifies the resource recovery potential in nitrogen and phosphorus removal from wastewater. The characteristics of photogranules were observed, and the nitrogen and phosphorus removal efficiency of the system was measured. The content of protein and polysaccharide in extracellular polymeric substances was determined over time. Excitation-emission matrix fluorescence was used to analyse the types and changes of substances in the system. Resource recovery assessments were conducted by adopting the method of freeze-thaw combined with trituration to extract phycocyanin and determine the content of phycobiliprotein. Photogranule development went through three sequential phases: initially, photogranules were under 1 mm in diameter, with peripheral filaments proliferating to create an irregular, meteorite-like surface; densification: loosely woven filaments contracted, increasing density and reducing surface pits, forming more compact structures; at maturation, granules reached approximately 1.3 mm and became spherical. The system achieved approximately 98% nitrogen and 56% phosphorus removal. Protein and polysaccharide content in extracellular polymeric substances increased slightly over time. Excitation-emission matrix fluorescence showed abundant humic-like substances in the early phase, while microbial byproducts and aromatic proteins remained stable later. The freeze-thaw combined with trituration yielded the highest concentration of phycocyanin (0.31 mg L-1), with phycobiliprotein content stable at about 50 mg L-1. This study describes photogranule growth characteristics, confirms the structural role of EPS, and demonstrates the dual functionality of photogranules in nutrient removal and resource recovery.