PACl coagulation for the solid-liquid separation of highly concentrated algae suspensions

The ability of algae to absorb CO:, a greenhouse gas, for photosynthesis has received considerable attention in recent years due to the possibility of growing algae to extract into bio-energy. Therefore, technology that rapidly separates and recovers an algal mass from water is needed. In this study, the coagulant polyaluminum chloride (PACl) with high basicity (B) was applied as a chemical coagulant in a Jar Test for a solid liquid separation study using two algal species, Chlorella sp. and Spirulina sp., suspended in both freshwater and seawater. The coagulation process was monitored using an on-line photo-turbidimeter to examine the effect of chemical dosage and operating conditions on solid liquid separation efficiency. Experimental results demonstrate that, for the 200 NTU suspension, Spirulina sp. required a much higher PACl dosage than Chlorella sp. for effective coagulation owing to the higher negative surface charge of the Spirulina sp. Additionally, a wide range of dosages achieved effective coagulation for both Chlorella sp. and Spirulina sp. when suspended in seawater, but not in freshwater. The minimum coagulant dosage for a seawater suspension was the same as that for a freshwater suspension, indicating that high concentrations of ions in seawater do not adversely affect the efficiency of PACl coagulation.