Harvesting Porphyridium purpureum using polyacrylamide polymers and alkaline bases and their impact on biomass quality

This study aims to examine the flocculation efficiency of Porphyridium purpureum (i.e. a red marine microalga with high content of pigments and fatty acids) grown in seawater medium using polyacrylamide polymers and alkaline flocculation. Polymers Flopam™ and FO3801 achieved the highest flocculation efficiency of over 99% at the optimal dose of 21 mg per g of dry biomass through charge neutralisation and bridging mechanism. The addition of sodium hydroxide, potassium hydroxide, and sodium carbonate also achieved flocculation efficiency of 98 and 91%, respectively, but high doses were required (i.e. > 500 mg per g of dry biomass). Calcium hydroxide was not as effective and could only achieve 75% flocculation efficiency. Precipitation of magnesium hydroxide was identified as the major cause of hydroxide-induced flocculation. On the other hand, sodium carbonate addition induced flocculation via both magnesium and calcium carbonate co-precipitation. The large mass of precipitates caused a sweeping effect and enmeshed the microalgal cells to trigger sedimentation. Cell membrane integrity analysis of flocculated P. purpureum indicated that polyacrylamide polymers led to significant compromised cells (i.e. 96%), compared to the alkaline bases (70–96% compromised cells). These results appear to be the first to demonstrate the high efficiency of polyacrylamide polymer and alkaline flocculation of P. purpureum but at the expense of the biomass quality. [Display omitted] •Polyacrylamide polymers achieved 99% flocculation efficiency at low dosage•Addition of NaOH, KOH, Ca(OH)2 or Na2CO3 also induced P. purpureum flocculation•Polymer dose was 40 times lower than that of alkaline flocculants•Charge neutralisation, bridging and co-precipitation are key flocculation mechanisms•Flocculation led to compromised cell membrane and subsequent loss of cell content