Harvesting microalgae using ceramic and polymeric crossflow membrane systems

Abstract

Harvesting microalgae from its growth media remains a challenge for the industrial production of microalgal biomass and associated products. Harvesting microalgae, specifically Scenedesmus sp., using crossflow ceramic and hollow fiber membrane techniques is studied in this thesis. A bench-scale membrane harvesting system was designed and operated with 0.2 μm, 0.45 μm, 0.8 μm and 1.4 μm ceramic membranes; and 0.1 μm, 0.2 μm and 0.45 μm hollow fiber membranes. Notably, all of the tested pore sizes achieved 100% microalgae biomass recovery for both types of membrane. The highest filtration flux achieved was 184 L/m2/h, when using the 1.4 μm ceramic membrane at a crossflow velocity of 0.5 m/s. For both membrane types, critical transmembrane pressure points existed, beyond which further increasing the transmembrane pressure (TMP) did not result in higher filtration flux. Since all the membrane pore sizes achieved the desired microalgae biomass rejection rate, the importance of membrane operation needs to be taken into consideration. Among various methods used to maintain operational efficiency, the use of backwash stands out due to its advantages, including no required chemical addition, energy saving and ease of operational design. Backwash effectiveness was affected by various conditions, including backwash duration, flux and intervals. The most efficient backwash was obtained with the 1.4 μm ceramic membrane, when the backwash flux was 2.0 times the filtration flux, with a 30 seconds duration and a 480 seconds (8 minutes) backwash interval. This increased the filtration flux by 70%. Additionally, the 0.2 μm hollow fiber membrane demonstrated significantly better filtration flux maintain ability compared to the 1.4 μm ceramic membrane, an average of 2.5 times higher flux was observed from the filtration cycles.