Examination of microalgal biofouling in a constant-flux crossflow filtration system comparing polymeric and carbon-nanotube membranes

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2020-08

Authors

Gol, Reuben Deane

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Abstract

The commercial microalgal industry consists of three major stages in the development of bioproducts, biomaterials, and biofuels: the growth of microalgae in open ponds or closed photobioreactors, the various harvesting and dewatering processes, and the final product preparation, extraction and/or conversion. Harvesting and dewatering of microalgae accounts for ~30% of the production costs in the industry and there is no economy-of-scale, limiting the impact of this industry to high-value, low-volume products. There are several methods utilized across the industry for the harvesting and dewatering of microalgae, with no universal method identified for all microalgal strains and products. This research focused on crossflow filtration, a relatively new harvesting method with the potential to reduce costs and energy inputs at scales suitable for high-volume products, e.g., animal feed, human food, biofuels, etc. The major fouling effects algae have on membranes are the primary limitations of this method, incurring high maintenance and operating costs. Polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes were compared with novel, non-woven carbon nanotube (CNT) membranes to assess the anti-biofouling effects of these materials. Membranes were characterized through contact-angle measurements and permeability assessments in a novel constant-flux crossflow filtration system. The fouling effects on the membranes were assessed using cultures of Chlorella vulgaris and a wild strain of Nannochloropsis through threshold flux (TF) calculations and scanning electron microscopy. In all experiments, CNT membranes had less pore blocking and were more resistant to adhesion by bacteria, microalgae, extracellular organic material, salts, and minerals with less cake layering, suggesting improved fouling resistance when compared to PVDF and PES membranes. TF and permeability experiments demonstrated that CNT membranes exhibited improvements in fouling resistance by 1.6-fold and 1.9-fold for Chlorella and Nannochloropsis cultures, respectively, compared to PVDF membranes. CNT membranes also resulted in 8.7-fold and 7.9-fold improvements in fouling resistance compared to PES membranes for the Chlorella and Nannochloropsis cultures, respectively. Lastly, a techno-economic analysis (TEA) was performed to estimate the production costs for harvesting microalgal biomass using an industrial spiral-wound filtration system. Using this model, TEA evaluations supported that CNT membranes with increased permeability have the potential to reduce production costs.

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