The transition away from chemical flocculants: Commercially viable harvesting of Phaeodactylum tricornutum

Harvesting can contribute up to 15% of the total production cost for microalgal biomanufacturing. Using flocculants is potentially a cost-effective approach but there has been considerable debate on the efficacy, cost, toxicity to the cell and environment, and the effect on the biomass further downstream. In this study, a range of biobased flocculants (chitosan from crab shells, Moringa oleifera seed extract, pectin from bananas, tannic acid-based derivatives from Acacia tree bark and egg shell powder) were compared with traditional chemical flocculants (aluminium sulphate, iron chloride and sodium hydroxide) for harvesting the diatom Phaeodactylum tricornutum. It was concluded that Tanac’s tannin based Tanfloc 8025 (SL range) was the most promising, requiring a low concentration (10.4 kg ton−1 biomass), at low cost ($27.04 ton−1 microalgal biomass). The flocculant was effective over a wide pH (7.5–10.0) and temperature (15–28 °C) range and harvesting (>85% efficiency) occurred in 10 mins, which resulted in a biomass concentration factor of ≥ 5.69.