Establishment Of Ph-induced In-situ Flocculation Of Energy Microalgae And Investigation Of The Mechanism

Recent studies have demonstrated that microalga has been widely regarded as one ofthe most promising raw materials of biofuels. However, lack of an economical, efficientand convenient method to harvest microalgae is a bottleneck to boost their full-scaleapplication. Many methods of harvesting microalgae, including mechanical, electrical,biological and chemical based, have been studied to overcome this hurdle.A new flocculation method induced by decreasing pH value of growth medium wasdeveloped for harvesting energy microalgae. The flocculation efficiencies were as high as90%for Chlorococcum nivale, Chlorococcum ellipsoideum and Scenedesmus sp. withhigh biomass concentrations (>1g/L). The optimum flocculation efficiency was achievedat pH4.0. The flocculation mechanism could be that the carboxylate ions of organicmatters adhering on microalgal cells accepted protons when pH decreases and thenegative charges were neutralized, resulting in disruption of the dispersing stability ofcells and subsequent flocculation of cells. A linear correlation between biomassconcentration and acid dosage was observed. Furthermore, viability of flocculated cellswas determined by Evans Blue assay and few cells were found to be damaged with pHdecrease.A new flocculation method was also developed by combination of pH decrease andself-flocculating microalgae for harvesting target microalgae. The flocculation efficienciesof target microalgae were much higher than those with pH decrease only. The mechanismcould be that negatively charged self-flocculating microalgal cells changed into beingpositively charged during pH decrease, resulting in attracting target microalgae cells toform flocs and settling down due to gravity. Additionally, this study revealed thatmicroalgae biomass concentration and released polysaccharide (RPS) from targetmicroalgae could influence flocculation efficiencies, while multivalent metal ions ingrowth medium couldn’t. Furthermore, Spearman’s rank correlation coefficients (Rs)between flocculation efficiency and key factors were also investigated.Recycling of flocculated culture medium for cultivation was studied. After neutralizing pH and adding nutrients to the flocculated medium, it was allowedflocculated medium to be recycled for cultivation.The study provided an economical, efficient and convenient method to harvest freshmicroalgae. Advantages include capability of treating high cell biomass concentrations(>1g/L), excellent flocculation efficiencies, operational simplicity, low cost andrecycling of medium. It has shown the potential to overcome the hurdle of harvestingmicroalgae to promote full-scale application to biofuels from microalgae.