Pre-concentration of algal cultures using stimuli responsive gels

Algal cultures from growth reactors are usually dilute (typically 0.5 – 2g/L) and direct use of conventional solid-liquid separation techniques (e.g. microfiltration or flocculation) may be cost prohibitive due to large process volumes. In this study, temperature- and pH- sensitive hydrogels were investigated as reusable absorbents for pre-concentrating dilute algal suspensions before subjecting them to conventional harvesting methods.

When algal cultures were incubated in the presence of the temperature-sensitive poly-N-isopropylacrylamide (pNIPAAm) at ambient temperatures (20°C), the aqueous growth medium was absorbed resulting in an increase in cell concentration of the algal culture. The swollen hydrogels were easily recovered using a screen and, when heated to 34°C, readily shrunk back to their original size to released the absorbed medium. The shrunken hydrogels could then be reused to absorb fresh media, and the cyclic swelling and deswelling process could concentrate cultures up to concentrations of 175g-cells/L without any discernible changes in swelling ratios or kinetics.

pH-sensitive poly-acrylic acid hydrogels were able to absorb alkaline growth media (pH >8) and release it when pH was reduced to values <4. pH shifts over this range could be achieved by sparging the growth medium with ambient air (alkaline conditions) or CO₂-enriched air. Initial experiments show that that ~53g of media can be absorbed per gram of hydrogel used and cyclic swelling /deswelling experiments are being performed to evaluated their reusability. Overall, the results from these experiments demonstrate that stimuli responsive gels could be an effective way to concentrate dilute algal cultures for further processing.