Integrated two-stage continuous system with pH-triggered CO2 delivery for sustainable algal wastewater-to-bioenergy production

Microalgae offer high lipid content and significant carbon–neutral potential, yet inefficient bioprocess and carbon source management fundamentally hinder their scalability. Addressing these challenges, this study developed an innovative integrated two-stage continuous cultivation system for Synechococcus sp., combining pH-triggered CO 2 delivery with wastewater nutrient recycling. Elucidating the relationships among growth, pH, and inorganic carbon, the precise CO 2 -based pH strategy eliminated salt stress, enhancing biomass productivity by 25 % over conventional chemical methods. The optimized nitrogen-deficiency two-stage mode achieved a 2.3-fold increase in lipid productivity and enabled high-efficiency nutrient removal (NH 4 + >70 %, PO 4 3− near-complete). A comprehensive techno-economic analysis confirmed superior cost-effectiveness, demonstrating a ∼45 % reduction in minimum biodiesel production cost compared to single-stage operations. These findings establish a highly feasible, scalable, and sustainable wastewater-to-bioenergy solution.