Flocculation and Vacuum Filtration of Algal-Slurry Intermediates to Enable Parallel-Algal Processing

Coproduction of high-value chemicals such as succinic acid is a promising route to enabling economic conversion of algal sugars and lipids to renewable fuels and chemicals. The green alga Scenedesmus acutus was grown at conditions specifically intended to accumulate carbohydrates, in addition to prevalent lipids. After harvesting, the algae biomass was acid pretreated in order to induce cell lysis and hydrolyze the insoluble carbohydrates to soluble sugars. The sugars were subsequently used to produce succinic acid via continuous fermentation by an immobilized culture of Actinobacillus succinogenes, while the lipids were extracted by hexane. The lipids are intended for upgrading to renewable-diesel blendstock, and the succinic acid is a high-value chemical intermediate that may be used to produce renewable chemicals and materials.

While immobilized-culture fermentation offers high specific productivities, it necessitates clarified sugar feedstocks. This presentation will focus on the development and use of a flocculation-enable solid-liquid separation of the pretreated algal slurries. Guided by zeta-potential measurements, trial-and-error testing of several variants of a commercial polyacrylamide flocculent (KemSep-brand, Kemira Chemicals, Helsinki, Finland) identified a suitable flocculent for pretreated algal slurries at the desired pH for fermentation. Additional dosing and mixing studies were performed, enabling vacuum filtration with acceptable permeate flux. The clarified liquor was subsequently used in a 756-hour continuous fermentation, achieving a maximum productivity of 1.2 g/L/h and a process conversion yield of 0.7 g succinic acid/g sugar.