Comparative Evaluation of Batch, Semi-Batch, and Continuous Shikakai-Generated Colloidal Gas Aphrons Flotation for Microalgae Removal

Abstract

Microalgae can impair water quality and complicate downstream treatment. This study evaluated Shikakai (Acacia concinna), a natural saponin-rich surfactant, for colloidal gas aphron (CGA)-assisted microalgae removal using batch, semi-batch and continuous flotation. Previous studies show that pH can alter microalgal surface charge and flotation behaviour and that saponin surfactants can promote bubble–algae attachment. A microalgal suspension with an initial turbidity of 1940 NTU and an estimated concentration of 0.5 wt.% was treated with Shikakai-generated CGAs. The initial suspension pH was 7.4 and was not adjusted during flotation. Samples were collected at predetermined intervals, and residual concentration was estimated from turbidity using a calibration relationship. Removal percentage was calculated relative to the initial concentration. Batch flotation gave the highest and most sustained removal, increasing from 48.72% immediately after agitation stopped to 70.50% after 60 min, with turbidity decreasing to 407 NTU. Semi-batch removal increased to 57.72% at 60 min, corresponding to 620 NTU. Continuous flotation achieved a maximum removal of 55.92% at 10 min but fluctuated thereafter. Because pH was not systematically varied, its independent effect on removal could not be quantified. Shikakai-generated CGAs effectively removed microalgae, with batch flotation showing the best overall performance. Future optimisation should systematically evaluate pH, CGA dosage, flow conditions and residence time.