Dual flocculation strategy with pH adjustment for cost-effective algae harvesting

Flocculation-sedimentation is an efficient approach for algae harvesting, but flocculant usage inevitably results in additional expenses. Therefore, it is essential to lower the flocculant dose for cost-efficient algae harvesting. The use of dual flocculants enhances algae harvesting, but the effect of pH has not been investigated. In this study, pH regulation was applied with dual flocculants of ferric sulfate and cationic polyacrylamide to improve the harvesting efficiency and reduce the flocculant dosage. Zeta potential measurements and floc characterization were performed to establish the mechanism. The results showed that the optimal flocculating strategy included initial addition of 75 mg/L ferric sulfate followed by addition of 10 mg/L cationic polyacrylamide at pH = 4. With pH regulation, the flocculant dosages were significantly reduced without compromising the harvesting effectiveness. The floc size was modestly increased by charge neutralization with a low ferric sulfate dosage at pH = 4, and subsequent addition of cationic polyacrylamide produced large flocs by bridging effect without cell lysis. Additionally, the dissolved extracellular organic matter was successfully eliminated. Based on an economic analysis at the industrial scale, the total cost of optimized dual flocculation with pH regulation was estimated to be 26.88 USD ton -1 algae, which was significantly lower than those of other flocculating strategies and hence proved the economic viability of this approach.