Optimization of microalgae harvesting by ultra-fine bubble flotation

Abstract This study investigates the optimization of microalgae harvesting through an ultra-fine bubble (UFB) flotation system. Microalgae possess high potential for carbon capture and biofuel production, but the harvesting stage remains a critical bottleneck in large-scale commercialization due to high costs and inefficiencies in traditional techniques. This research develops and evaluates a UFB-assisted flotation method aimed at improving microalgae recovery efficiency. The novelty of this study lies in the integration of nanometric ultra-fine bubbles (10–250 nm), systematic chemical screening, and response surface methodology (RSM)-based optimization for microalgae harvesting. Unlike conventional flotation systems that typically employ microbubbles in the micrometer range, the developed UFB system generates highly stable nanobubbles, thereby enhancing bubble-microalgae interactions and harvesting performance. A custom-designed UFB generator was used to produce nanobubbles with an average diameter of 30–70 nm, demonstrating strong stability and uniform distribution. The freshwater microalga Chlorella sp . was cultivated and subjected to flotation tests under varying conditions of pH and activator (polyaluminum chloride, PACl) dosage. Key operational parameters were screened through jar tests and further optimized using a central composite design integrated with RSM. Results indicated that PACl outperformed alum in enhancing microalgal recovery, regardless of whether collectors (sodium oleate (NaOl) or cetyltrimethylammonium bromide) was co-applied. Regression analyses yielded second-order polynomial models that accurately predicted the recovery efficiency for OD₆₈₀, turbidity, chlorophyll-a, and biomass, with R² values exceeding 0.80. Optimal recovery was achieved at 23 mg L -1 PACl and pH 5.3, with observed efficiencies reaching up to ≈ 100%. Validation tests confirmed model reliability, with predicted values within 10% of experimental results. The study concludes that UFB-assisted flotation is a promising, cost-effective approach for efficient microalgae harvesting and can facilitate the development of microalgae-based carbon capture and biofuel production systems.