Harvesting Nannochloropsis oculata by Chitosan and AlCl3-Induced Flocculation: Effects of Microalgal Condition on Flocculation Performance

Flocculation showed the potential for harvesting microalgae at a low cost. The inorganic flocculant (AlCl 3 ) and organic flocculant (chitosan) are two representative and common flocculants, which were suited for different conditions (pH, salinity, and cell density). To investigate the effects of the three factors on the flocculation performance of the two flocculants, especially for the interaction effects among these factors, single-factor experiments and response surface methodology (RSM) were conducted. The results indicated that AlCl 3 performed better under alkaline conditions, whereas chitosan showed better performance at acidic pH. Salinity had little influence on the flocculation efficiency (FE) of AlCl 3 . By contrast, increasing salinity reduced the FE of chitosan, especially above 8.4 g/L. As cell density increased, the optimal dosage of AlCl 3 increased to a constant ~ 26 g/L. In contrast, the optimal dosage of chitosan continuously increased within the studied range. Also, RSM revealed significant interaction effects of [salinity][cell density] for AlCl 3 , due to which AlCl 3 showed a good flocculation performance under either high cell density or high salinity conditions but poor with both high together. Significant interaction effects of [salinity][cell density] for chitosan was observed, and one of the two factors hence alleviated the negative influence from the other. Considering their performance under the different condition and the interaction effects, dual flocculation with AlCl 3 + chitosan on multiplex conditions was evaluated, and the results indicated that higher FE with wider applicability could be achieved with lower dosages of the dual flocculant compared with utilization of the two flocculants individually under most of the conditions.