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 CO2 delivery with wastewater nutrient recycling. Elucidating the relationships among growth, pH, and inorganic carbon, the precise CO2-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 (NH4+ >70 %, PO43- 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.