Fouling phenomena during membrane filtration of microalgae

Harvesting of secreted fuel precursors is an alternative approach to sustainable biofuel production from algae, and relies on selective separation of growth media from algal suspensions. Membrane fouling and species fractionation for selected ultrafiltration (UF) and microfiltration (MF) membranes were investigated for this purpose. Suspensions were filtered using total recycle, flux-stepping protocols to measure fouling rates and subsequent fouling reversibility. Severe fouling was observed at high flux values for 50 kg/mol UF and 0.22 μm MF membranes, with much of the fouling irreversible. In contrast, a 5 μm MF membrane showed essentially no increase in resistance, even though the whole algal cells were rejected to form a cake on the membrane surface. Analysis of the permeate from the 5 μm MF filtration indicated the presence of “protein-like”, “humic-like”, and high-molecular-mass polymeric materials, as well as submicron particles. However, model solutions of dissolved organic material showed very low fouling compared to the as-grown algal suspensions. Instead, submicron particles present in the algal suspensions and in the 5 μm MF permeate are thought to be the primary foulants by forming high-resistance cakes on the UF and 0.22 μm MF membranes and also by plugging the pores of 0.22 μm MF membranes. Thus, the flux decline and lack of “easy“ reversibility for our algal suspensions, filtered on specific membranes, appears to be primarily associated with deposition of smaller colloidal species within the membrane and/or near its surface, rather than with macromolecules or a cake of whole algal cells. Furthermore, membrane characteristics similar to those of the 5 μm MF membrane employed may offer a unique strategy for separation of these undesirable particulate materials from healthy cultures of algae with large-enough size.