Industrial-scale Microalgae Pond Primary Dewatering Chemistry for Energy-efficient Autoflocculation

Abstract

Industrial-scale microalgae production will likely require large energyintensive technologies for both culture and biomass recovery; energy-efficient and cost-effective microalgae dewatering and water management are major challenges. Primary dewatering is typically achieved through flocculation followed by separation via settling or flotation. Flocculants are relatively expensive, and their presence can limit the reuse of de-oiled flocculated microalgae. Natural flocculation of microalgae-autoflocculation-occurs in response to changes in pH and water hardness and, if controlled, might lead to less-expensive "flocculant-free" dewatering. A better understanding of autoflocculation should also prompt higher yields by preventing unwanted autoflocculation. Autoflocculation is driven by doublelayer coordination between microalgae, Ca +2 and Mg +2 , and/or mineral surface precipitates of calcite, Mg(OH) 2 , and hydroxyapatite that form primarily at pH > 8. Combining surface complexation models that describe the interface of microalgae: water, calcite:water, Mg(OH) 2 :water, and hydroxyapatite:water allows optimal autoflocculation conditions-for example pH, Mg, Ca, and P levels-to be identified for a given culture medium.