Transmission of and fouling by long chain molecules during crossflow microfiltration of algal suspensions: influence of shear

Abstract

Fouling is the main obstacle in membrane filtration, especially in the water industries. Membrane fouling by algae and its secreted extracellular polysaccharide is still an ongoing problem. Flux stepping microfiltration experiments of Chlorella Sorokiniana were carried out using Direct Observation Through Membrane (DOTM) equipment. The trend of extracellular polysaccharide (EPS) transmission during micro-filtration of Chlorella Sorokiniana was found to be dependent on flux as well as crossflow velocity (CFV). Increases in flux during flux stepping experiments generally resulted in an initial increase in transmission of EPS through the membrane. Further increases in flux, however led to unexpected results. At low crossflow velocity, EPS transmission did not vary significantly with flux. At higher crossflow velocity, EPS transmission initially increased as flux increased however the transmission then reduced as flux was increased to higher values. Interestingly, EPS concentration in the permeate was much higher than that of the feed supernatant at higher crossflow velocities. This implies that Cp >Cb , or negative retention, therefore the effect of concentration polarisation alone of solutes in the supernatant cannot be used to explain this phenomena. Furthermore EPS transmission through Anopore membranes was unexpectedly higher for the 0.1 μm membrane than for the 0.2 μm membrane. PVDF, with its broader pore size distribution and interconnected pores, resulted in greater amounts of EPS transmission than Anopore membranes of the same 0.2 μm pore size. These phenomena could result from the changes in the structure and composition of the concentration polarization layer especially near the pore entrances and from shear within the system which may act to remove some of the gelatinous sheath around the cells. The effects due to shear within the system are highlighted here. In order to relate the effect of shear and basic polymeric transport to EPS transmission through membrane pores, a brief review of transport of a polymer through a pore is presented. From this a rationalisation of observations on the deposition and removal mechanisms of algal cells (a physical model of interaction of algae and EPS with the membrane) as seen using DOTM is proposed.