Dual strategies to mitigate the microalgal harvesting challenges: surface-attached biofilm cultivation and fouling-resistant membrane filtration

Victor O Mkpuma

The interest in microalgae has grown significantly recently due to their numerous economic potentials in diverse fields, such as their role in bioremediation, climate change mitigation, and production of valuable products like foods, feed, biopharmaceuticals, and biofuels. However, the complexity and cost associated with harvesting biomass hinder commercial-scale production of these products. Microalgae are grown in suspension cultures with a biomass concentration of ≤ 0.5 g L-1 dry weight, which must be concentrated to a biomass paste (70-80% moisture or 150-250 g L-1 dry weight) through dewatering. This requires processing large volumes of cultures with accompanying costs estimated to account for about 50% of the production cost. Therefore, developing a cost-effective strategy for generating and recovering microalgal biomass is urgently needed to ensure the economically viable production of microalgal products. I adopted two strategies to address the issue of microalgal biomass generation and harvesting. The first strategy was the attached biofilm growth of microalgae, leveraging the surface attachment potential of microalgae. In a biofilm scenario, the microalgae grow on solid support, separated from the liquid phase, and easily harvested by scraping. The second strategy was to mitigate fouling to improve the membrane filtration process of dewatering suspension cultures. In the first strategy, I utilized a perfused biofilm design, which involves making the algae grow on the semi-permeable surfaces, while the nutrients and moisture flow beneath, reaching the cells on the surface via diffusion. Initially, four filter membranes (Nucleopore filter, Polyvininylidene fluoride (PVDF), Glass microfiber filters (GF/C), and cellulose nitrate (CN) membrane) were screened as potential support materials for growing Chlorella sp. MUR 268. The cellulose nitrate (CN) membrane was a superior material and was selected to grow Chlorella sp. MUR 268 and Scenedesmus sp. MUR 269 and compared the performance with the suspension counterpart. The anaerobic digestate food effluent (ADFE) with a total ammonia nitrogen (TAN) content of 57.8 mg L-1 was used as a nutrient source, thereby incorporating effluent treatment as additional value. Chlorella sp. generated higher biomass in this biofilm (50.38 g m-2), which is 5.4 times better than Scenedesmus sp. Conversely, Scenedesmus sp. was 1.5 times better than Chlorella in suspension cultures. The nutrient removal from ADFE followed the pattern of the biomass yield, with Chlorella sp. achieving higher nutrient uptake in biofilm cultures, while Scenedesmus was better in suspension cultures.

Dual strategies to mitigate the microalgal harvesting challenges: surface-attached biofilm cultivation and fouling-resistant membrane filtration

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