(401a) Development of a Novel Revolving Algal Biofilm Photobioreactor (RABP) for Easy Biomass Harvest

Authors: 
Gross, M. A., Iowa State University
Wen, Z., Iowa State University
Michael, C. I., Iowa State University



Current algal cultivation has been mainly performed in open ponds or closed photobioreactors in which algal cells are suspended in liquid and harvested through sedimentation, filtration, flocculation, and/or centrifugation devices. The objective of this research was to develop a novel attached algal cultivation system to reduce the biomass harvest cost. In the attached growth system, algal cells were attached to a material that was rotating between nutrient-rich liquid phase and carbon dioxide rich gaseous phase for alternative absorption of nutrients and carbon dioxide. The algal cells were harvested by scrapping from the attached surface, and thus, the expensive harvest procedures commonly used in a suspension cultivation system can be avoided.

Compared to the suspended culture systems, the proposed system has several advantages: (i) the biomass can be in-situ harvested DURING the culture process, rather than using an additional sedimentation or centrifugation unit for harvesting (harvested cells have water content similar to that of cells after centrifugation); (ii) the culture enhances the mass transfer by directly contacting algal cells with CO2 molecules in gaseous phase, while traditional suspended culture systems have to rely on the diffusion of CO2 molecules from gaseous phase to the liquid phase which is often limited by low mass transfer rate; (iii) the culture system only needs a small amount of water and land area through utilizing the unique design that only requires the bottom of the reactor system to be bathed in the liquid, this allows for algae cultivation to maximize area by actually growing vertically vs only horizontally.

The attached growth system was optimized for improved biomass productivity.  The major operational parameters optimized include biofilm attachment material, harvest duration, rotational speed, and CO2 concentration.  It was found that the optimal attachment material was duct cotton, harvest duration of 7 days, rotational speed of 4 rpm and atmospheric CO2 concentration. These parameters resulted in a biomass productivity of 10.5 g∙m-2∙day-1.  A compositional analysis (lipid, protein, carbohydrate, ash) was conducted to compare the biofilm based system and standard suspended cultivation system.  The compositional analysis showed that algae grown using the attached system has 5% more protein, and 9% more carbohydrate and 14% less lipid, than suspended culture of the same species of algae. The algae grown in the attached growth system produced a higher content of 5 of the 9 essential amino acids, threonine, valine, isoleucine, leucine, phenylalanine, methionine and histidine.  Two had no significant difference, while lysine and tryptophan had a lower concentration than the suspended systems.  A fatty acid analysis was also conducted but no significant difference was observed.   Overall, the results indicated that the attached growth is a promising algal culture system for an improved biomass productivity with high nutritional value.

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