Flocculation with intermittent dosing for enhanced microalgae harvesting

Abstract

One of the bottlenecks to the industrialization of algal biofuel is the high cost of harvesting. Flocculation is an efficient technique for algae harvesting, but flocculant consumption results in additional expenses. To lower the overall cost of algae harvesting, it is essential to explore approaches to reduce flocculant usage. In this study, a novel flocculant dosing strategy of intermittent dosing was investigated for enhanced algae harvesting efficiency, with comparisons to traditional one-time dosing, continuous dosing and dual dosing. The impacts of critical operational parameters were examined, and the mechanism was investigated based on floc analysis. The results demonstrated that intermittent dosing can significantly enhance harvesting efficiency. At the optimal operational conditions of 5 s dosing interval, 10 mg/L min−1 dosing speed and 5 min stirring time, the algae harvesting efficiency increased from 71.1 % to 91.3 % compared to one-time dosing when 15 mg/L CPAM was used as a flocculant. According to floc analysis, intermittent dosing tended to form larger and loosely connected flocs, while traditional one-time dosing tended to produce smaller and denser flocs. The different floc growth mechanisms can help to explain how intermittent dosing improved algae flocculation. In addition, intermittent dosing resulted in a higher removal efficiency of dissolved extracellular organic matter and improved algae recultivation. Finally, the cost estimation was performed, and the operational cost was lowered from 32.65 USD ton−1 algae for one-time dosing to 20.64 USD ton−1 algae for intermittent dosing. Through the investigation of this study, the technical, mechanical and economic feasibility of intermittent dosing for enhanced algae harvesting were established.