Continuous harvesting of microalgae biomass using foam flotation

Abstract

Biomass harvesting and dewatering are major operational costs that constrain the development and expansion of the industrial use of microalgae; particularly for low value biofuels. In this study we demonstrate, for the first time, a surfactant-aided foam flotation column that is designed and optimised for the continuous harvest of microalgae not only to improve the productivity of microalgae biomass but also as an attempt to overcome the trade-off between recovery efficiency and enrichment in batch and semi-batch foam flotation. The following operational parameters were optimised; surfactant concentration, air flow rate, feed flow rate, column height, liquid pool depth, and sparger type. Additionally, the effects of cell surface characteristics (hydrophobicity, zeta potential, and contact angle) were investigated on Chlorella vulgaris flotation performance. Hydrophobicity was enhanced using three surfactants; the cationic cetyltrimethylammonium bromide (CTAB), the anionic sodium dodecyl sulphate (SDS), and the non-ionic TWEEN®20; with CTAB producing the greatest enhancement. Surfactant concentration, column height, and air flow rate had the greatest effect on the algae concentration factor (CF) and recovery efficiency (RE). The optimised design (CTAB = 35 mg L−1, air flow rate = 1 L min−1, feed flow rate = 0.1 L min−1, column height = 146 cm, liquid pool depth = 25 cm, with a fine porous sparger) yielded RE of 95, 93, and 89% with 173, 271, and 143-fold biomass enrichments for freshwater C. vulgaris and marine Isochrysis galbana and Tetraselmis suecica microalgae respectively. Achieving high RE for freshwater and in the case of marine microalgae (irrespective of ionic strength) at moderate surfactant dosages, gives foam flotation the advantage of being a growth media independent harvesting process. The process had a very low power consumption (0.052 kWh m−3 of algae culture). Our findings demonstrate the potential for continuous, low cost, scalable flotation harvesting with particular relevance for the biofuels, water and wastewater treatment industries.