Membrane filtration of the cultivation suspension of Scenedesmus o. microalgae: potential and challenges of permeate reuse

In the last couple of decades, interest towards the cultivation and usage of microalgae has grown, thanks to their possible application in the food and the cosmetics industries, in wastewater treatment, in biofuel production, and for CO2 fixation. However, despite their great range of applications and their advantages (like a limited footprint), microalgae cultivation at industrial level is held back by some factors, such as the large water volume required for the biomass growth and the relatively high costs that characterize the process of harvesting and dewatering. The aim of this study is to investigate the dewatering of Scenedesmus Obliquus through 0.14 µm ceramic membrane filtration, analyzing the regulating factors and the possibility of using the obtained permeate as a medium for the growth of a new culture. Filtration tests were carried out to investigate the possible influence that two factors, such as feed biomass quality and hydrodynamic parameters, can have on the filtration process. Filtration tests were performed with healthy and unhealthy biomass. The unhealthy biomass showed a negative effect on the flux (and therefore the productivity), accelerating the fouling of the membrane. Moreover, it had also a negative effect on the permeate quality, causing an increase in the DOC content. The influence of hydrodynamic parameters (crossflow velocity and pressure) was analysed through the use of RSM, associating a value of productivity to every combination of velocity and pressure. Some considerations on energy consumption were made, based on the pressure loss in the filtration module. Finally, concerning microalgae regrowth in permeate water, the results obtained were promising. Specifically, regrowth tests were carried out in a pilot-scale photobioreactor, and the importance of nutrients reintegration (nitrate, calcium, iron, magnesium, manganese) was investigated through two kinds of tests: one with complete reintegration, the other with only nitrate reintegration. While the latter was not successful, the former provided positive results, showing that a more sustainable approach could be possible in microalgae cultivation processes.