(602b) Algal De-Watering Using Temperature Sensitive Hydrogels: A Preliminary Step in the Harvesting Process

Algal cultures from the growth reactors are usually dilute (~0.1-0.5% (w/w)) and direct use of conventional solid-liquid separation methods (e.g. centrifugation, filtration or flocculation) may be cost prohibitive due to large process volumes. In this study, temperature-sensitive gels made of N-isopropylacrylamide (NIPAAm) were investigated as a re-usable absorbent for increasing the concentration of dilute algal suspensions. We observed that when algal cultures were incubated in the presence of NIPAAm gel (at 25°C), cell-free media was absorbed resulting in an increase in cell concentration. The swollen hydrogels were then recovered and heated to 34°C to deswell and release the absorbed media. Since swelling rates were high initially (t<4h) and decreased subsequently (4h < t <15h), cyclic swelling (for 4h) followed by deswelling was applied to more rapidly concentrate the algal cultures. Our experimental results show that the concentration of algal cultures increased from 0.16% to 4.3% (w/w) in 6 cycles. In addition, slurries initially at 3% (w/w) could further be concentrated to 8% (w/w) by incubation with hydrogels without any discernible change in gel swelling/deswelling kinetics. These results suggest that higher solid concentrations could be achieved by applying more swelling/deswelling cycles. In fact, our studies show that removal of nearly all free water is possible to obtain algal pastes with concentrations of nearly 18% (w/w) solids. However, recovery of gels from such concentrated slurries without incurring significant loss of cells is difficult. Overall, our results demonstrate that temperature-sensitive hydrogels can be used to easily dewater dilute algal cultures to  concentrations (8%(w/w) or more) where more traditional solid-liquid separation methods can be easily applied. An economic analysis of the hydrogel based dewatering process will be presented.