Foam fractionator as a tool to remove dissolved organic matter and improve the flocculation of the marine microalga Nannochloropsis oceanica

Abstract

Microalgae have great biotechnological potential, yet they are difficult to harvest. Flocculation is a promising technology, but the dissolved organic matter (DOM) released by microalgae during growth interacts with the flocculant, hindering the process. This DOM is composed mostly of proteins and carbohydrates which may present a polarity ranging from hydrophobic to hydrophilic. In aquaculture, foam fractionators are routinely employed to remove DOM from cultures, improving the water quality. However, this equipment has never been employed in microalgae cultures. Therefore, the present study aimed to evaluate the use of the foam fractionator to remove DOM from a Nannochloropsis oceanica culture to improve the flocculation process. First, DOM accumulation was monitored for 33 days in a 330 L outdoor culture to characterize the dissolved organic carbon (DOC) and protein and carbohydrate contents. Samples from logarithmic (day 13) and stationary (day 29) growth phases were fractionated by polarity (hydrophobic, transphilic, and hydrophilic), the DOC and protein and carbohydrate contents of each fraction were measured, and flocculation was performed. Then the culture was treated with a foam fractionator at bench (11 L) and pilot scales (1600 L), and the composition and flocculation were compared with the non-treated culture. Proteins strongly hindered flocculation, and the foam fractionator was effective in removing this element and significantly improved the flocculation efficiency. Furthermore, N. oceanica cell integrity was not affected by the foam fractionator. These results indicate that DOM can be efficiently removed by foam fractionation, improving the microalgae harvesting process.