Membrane fouling of forward osmosis in dewatering of soluble algal products: Comparison of TFC and CTA membranes

Forward osmosis (FO) has increasingly penetrated into traditional process, i.e. microalgae dewatering, to become one of the new energy saving technologies. However, FO membranes can be fouled by soluble algal products (SAP) and fouling behavior of the membrane is not clear due to the variety of SAP composition. In this work, two types of membranes including a self-made TFC and a commercial CTA, and three kinds of draw solutions including NaCl, MgCl2 and CaCl2 were adopted to investigate the dewatering of SAP from microalgae of Chlorella vulgaris. The dependence of permeate flux and foulants composition on the membranes, membrane orientation and reverse salt diffusion were then compared for membrane fouling behavior. The results showed that TFC membrane exhibited higher water permeability but more loss of water flux in comparison with CTA. The SAP was inclined to be adsorbed by TFC membrane while adsorbate was easier to be removed by physical cleaning. By contrast, the pollutants accumulated on the membrane surface of CTA were much more irreversible. The interaction between SAP from feed solution and calcium cations from draw solution induced the formation of cake layer on the surface of TFC membrane while irreversible granular deposits of SAP were found accumulated on CTA membrane surface. Possible membrane fouling mechanism was finally discussed for better fabrication of anti-fouling forward osmosis membranes, thus to propel the integration of forward osmosis with traditional field of microalgae dewatering.