Microalgae biomass concentration and growth solution recovery with ceramic membrane filtration

In recent years, microalgae biomass has been recognized as a useful source for a wide range of applications, such as biofuel production, CO2 fixation, food, and cosmetics. As the demand of microalgae increases, improvements in biomass production and harvesting techniques are required. For example, dewatering processes increase the total production cost, especially when performed in large footprint systems, such as open ponds. The aim of this study is to investigate and optimize membrane filtration as a compact solution for microalgae harvesting with the target to reach the goal of zero liquid discharge: the permeate extracted during the process is reused entirely or partly as a new medium for algae regrowth. Three different types of algae, namely, Spirulina platensis, Scenedesmus obliquus, Chlorella sorokiniana, were tested with four different ceramic membranes having nominal pore sizes of 0.8 μm, 0.14 μm, 300 kDa, and 15 kDa. During flux experiments, the observed flux decline was proportional to the membrane pore size, with negligible differences between the three algae species. Interestingly, similar steady state fluxes were reached using membranes with nominal pore size above 15 kDa, suggesting the existence of a critical flux above which biomass deposition occurs proportionally to the initial flux. On the other hand, cultivation tests showed higher sensibility on the type of biomass rather than on the membrane porosity. Among the three algae species, Scenedesmus obliquus reported the most feasible growth rate. Overall, the 0.14 μm cut-off membrane provided the best combination of productivity and production of permeate stream with suitable quality to sustain the best growth results. This membrane was thus selected for the filtration process optimization in terms of applied pressure and cross-flow velocity. The optimization was carried out by response surface modelling developed through Design Expert software. For any cross-flow velocity, the modeling can suggest the maximum value of applicable pressure, above which foulant deposition counteract the driving force to an extent where overall productivity cannot be further increased. This allows minimization of the applied pressure and of the related energy costs. In summary, this study successfully optimized membrane filtration as an efficient harvesting technique for different algae species and provides insights for possible scale up research necessary to achieve feasible industrial applications.