Algal biomass and biodiesel production by utilizing the nutrients dissolved in seawater using semi-permeable membrane photobioreactors

The objective of this study is developing microalgal culture systems to produce biodiesel with low-energy inputs in the ocean. The semi-permeable membrane photobioreactors (SPM-PBRs), which are capable of transferring nutrients dissolved in seawater into the algal broth while containing the cells inside, were operated in a reservoir containing Incheon coastal seawater in the laboratory to observe biomass dynamics, variations in nutrient concentration, pH, and salinity prior to their deployment in the ocean. A green microalga Tetraselmis sp., isolated from Incheon nearshore, was cultivated in a simulated ocean condition under continuous illumination. According to the data obtained from the experiment, microalgal growth was found to be primarily limited by the phosphorus concentration in seawater rather than other major nutrients (carbon and nitrogen) and the pH and salinity of the algal broth, which remained constant. Furthermore, we were able to understand why biomass productivity decreases as the culture progresses. N and P transfer rates through the membranes gradually decreased due to membrane fouling caused by various factors. For an enhancement of nutrient transfer rate and biomass productivity, various SPMs with different nutrient transfer rates and molecular weight cut-offs (MWCOs) were also explored. Biomass productivity increased in proportion to the nutrient transfer rate of the membranes, and the fatty acid content increased from 12 to 30 % at day 0 without a significant change in its composition. With further developments in SPM-PBRs, another technology to simultaneously produce microalgal biomass and reverse eutrophication of the ocean or lakes may be provided.