Valorization of waste activated sludge for microalgae cultivation: Differential roles and mechanisms of extracellular and intracellular substances.

The increasing volume of waste activated sludge (WAS) poses environmental risks, yet its rich organic matter and nutrients offer potential for resource recovery. This study systematically compares the roles of completely crushed sludge extract (CCS), extracellular substances (ESS), and intracellular substances (ISS) from municipal WAS in the cultivation, biomass production, and harvesting of Chlorella pyrenoidosa. Results showed that ISS primarily served as a nutrient source, achieving the highest biomass (0.37 g L-1). ESS, rich in extracellular polymers (polysaccharides 45.4 mg L-1, proteins 38.3 mg L-1), significantly enhanced microalgal flocculation, attaining the best settling performance. CCS yielded the highest productivities of valuable metabolites: lipids (25.2%, 8.4 mg L-1 d-1), proteins (5.7%, 1.9 mg L-1 d-1), and polysaccharides (3.4%, 1.1 mg L-1 d-1). Transcriptomic analysis revealed that WAS extracts upregulated key genes in glycolysis, TCA cycle, and lipid synthesis pathways, enhancing energy metabolism and carbon flux for growth and metabolite accumulation. This study provides a theoretical basis and technical approach for sustainable microalgae cultivation and harvesting through WAS resource utilization.