The settling and bioflocculation of two strains of algae were investigated in the laboratory to provide insights to help improve algae settling in large-scale, algae-based wastewater treatment systems with simultaneous algae biofuel production. Energy-efficient algae harvesting methods are needed for cost-competitive production of biofuels from algae. The use of bacterially derived exudates (bioflocculation) is promising, but its use in high-rate algae pond (HRAP) wastewater systems has not yielded consistently reliable settling. The settling of the algae species Scenedesmus sp. and Chlorella vulgaris was investigated in the laboratory with and without cultures of the bacterium Burkholderia cepacia to investigate environmental effects on the settling of pure algal cultures and to examine possible bioflocculation by bacteria and bacterial exudates. Scenedesmus sp. and C. vulgaris responded in opposite ways to changes in operating conditions such as culture age and bioflocculation. Scenedesmus sp. settled better in later stationary growth stages, while C. vulgaris settled better in its early exponential growth phase. Scenedesmus sp. settling was improved by bioflocculation during early exponential and late stationary growth stages, but not during early stationary growth. Scenedesmus sp. responded better to B. cepacia cells plus filtrate (filtered bacterial broth) than to filtrate addition alone. In contrast, C. vulgaris settling was not improved via bioflocculation with neither the addition of B. cepacia cells nor filtrate at any growth stage. Longer contact time between the algae cells and bioflocculants improved the settling of Scenedesmus sp., but not the settling of C. vulgaris. Observed differences in settling and bioflocculation of the two algae species were attributed to differences in algal cell size and shape, culture density, and exudate type (i.e., capsular versus dissolved EPS). The widely different settling behavior of the two species of algae used in this research may explain the often varied results observed with mixed cultures in algae wastewater ponds. © 2013 American Institute of Chemical Engineers Environ Prog, 32: 946–954, 2013
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