Enhanced Microalgal Harvesting Using Microalgae-Derived Extracellular Polymeric Substance as Flocculation Aid

Abstract

Coagulation-based harvesting has been widely used in microalgal biomass harvesting. However, the coagulant contamination in the harvested biomass may negatively affect the applications in feedstock processing for food, feed, and fuel. In this study, extracellular polymeric substances (EPSs) were derived from microalgae, Scenedesmus acuminatus, and then used as a bioflocculant to aid the flocculation of the same algae. The results show that the alum coagulant (Al3+) usage was significantly reduced from 77.6 to 4.5 mg g–1 when adding this EPS bioflocculant at a dose of 3.2 mg g–1, which potentially reduces the chemical cost from $282 per metric ton to $71 per metric ton dry biomass that is harvested. To analyze the compositions of this bioflocculant, molecular fractionation was performed. The functional fractions such as protein-like and humic-like organic substances were characterized by fluorescence excitation–emission, followed by polysaccharide analysis. Low-MW (<3 kDa) EPS contributed to the flocculation process more than the large-MW fractions. Low-MW EPS contained higher contents of glucose and mannose in the polysaccharide that influence the interactions of the algae and the alum coagulant. Microalgal-derived bioflocculants may open up new avenue toward the low-cost and sustainable bioflocculation processes for algal and other biomass separation.

Sustainable microalgal biomass production in food industry wastewater for low-cost biorefinery products: a review

“…EPS produced by Scenedesmus acuminatus are used for the flocculation of the same microalgae and the harvesting efficiency > 80%. Low molecular weight EPS composed of mannose and glucose influences the microalgal biomass harvesting process (Yang et al 2020 ). Detailed bio-flocculation techniques and their flocculation mechanisms are discussed in a review reported by Ummalyma et al ( 2017 ).…”

Recent Advances of Microalgae Exopolysaccharides for Application as Bioflocculants

“…This process was demonstrated by Guo et al [7], using supernatant and cell suspension from the autoflocculating Scenedesmus obliquus AS-6-1 culture for the recovery of non-flocculating microalgae biomass. Furthermore, to increase the efficiency of bioflocculation processes, other flocculants such as Al 3+ and Fe 3+ can be added together with extracellular polysaccharides [7,10,76]. According to Yang et al [76], the extracellular polymeric substance co-extracted in the Al 3+ recovery process after the primary flocculation step contributed to the clotting process of Scenedesmus acuminatus.…”

“…In this sense, Yang et al [10] reported bioflocculant activity of EPS from Scenedesmus acuminatus in the recovery of the biomass of this same microalga. The results showed that the addition of microalgae EPS (3.2 mg g −1 ) significantly reduced the use of aluminum coagulant (Al 3+ ) from 77.6 to 4.5 mg g −1 .…”

Treatment of Wastewaters by Microalgae and the Potential Applications of the Produced Biomass—A Review

“…Several microalgae could release various polysaccharides, also known as exopolysaccharides (EPS), into the growth media [59]. Negatively charged groups within EPS molecules are also capable of absorbing metals [60]. Both living and dead cells of microalgal are capable of metal removal, although the metal removal efficiencies for living cells are higher compared to dead cells [61].…”