Microalgae-mediated treatment of steel industry reverse osmosis concentrate integrated with flue gas carbon fixation: From laboratory selection to pilot-scale verification.

To address the dual environmental challenges of CO2 emissions and reverse osmosis (RO) concentrate in the steel industry, this study developed an integrated microalgae-based treatment strategy. The strategy consisted of two sequential phases, including laboratory-scale screening of microalgal strains and subsequent pilot-scale validation under real industrial conditions. Eight microalgal strains were initially evaluated using RO concentrate and simulated flue gas (20 % CO2), among which Chlorella LJ34 exhibited superior performance in biomass production (1.26 g/L dry weight), nutrient removal (87.9 % total nitrogen (TN), 37.4 % total phosphorus (TP)), and carbon fixation (0.19 g/L/day). Pilot-scale cultivation in a 6000 L tubular photobioreactor using actual flue gas (13 % CO2) and RO concentrate confirmed LJ34's adaptability, with 85.3 % TN and 76.7 % TP removal and simultaneous reductions in TOC, hardness, and iron, indicating comprehensive water quality improvement. Notably, LJ34 increased fatty acid accumulation in parallel with carbon fixation, particularly enriching polyunsaturated fatty acids (PUFAs), highlighting its potential for biodiesel production. The system also demonstrated potential scalability, with each cubic meter of reactor annually removing approximately 228.11 m3 of flue gas CO2 and 36.4 m3 of RO concentrate, offering practical benchmarks for industrial-scale implementation. Overall, the results demonstrate the potential of microalgae-based systems for pollutant removal, CO2 mitigation, and biomass valorization in industrial settings. These findings provide a strong foundation for system optimization and offer a sustainable path toward industrial decarbonization and resource recycling.