Characterizing the Growth of Cyanobacteria Using Coiled and Bubble Column Photobioreactors and Its Separation by Flocculation Filtration

This research was conducted with the intention of increasing the cyanobacterial biomass productivity to enhance the biofuel production. In the first part of the research, since the photobioreactors provide ambient growth environment and higher biomass productivity for Anabaena 7120, coiled photobioreactor was developed. The basic growth conditions required were light intensity for photosynthesis, CO2 as carbon source and air bubbling for adequate mixing and to reduce O2 accumulation. The biomass productivity can be enhanced by providing the optimum growth conditions. The parameters varied to obtain the optimum growth conditions were light intensity (10 – 30 μmol/m2/sec), carbon dioxide flow rate (0.5 – 6% of air volume) and air flow rate (2 – 5 L/min). For coiled photobioreactors, the optimum growth conditions were light intensity (20 μmol/m2/sec), carbon dioxide flow rate (6% air flow rate volume) and air flow rate (5 L/min) with maximum biomass concentration of 0.39 g/L. The mere increase in growth parameter did not resulted in the increased biomass productivity and the maximum biomass concentration achieved due to the existence of relative equilibrium in the system. In the second part, due to the fouling of cyanobacteria on the coiled photobioreactor wall and lower biomass productivity, a bubble column reactor was developed. For bubble column photobioreactor, the optimum growth conditions obtained were light intensity of 77 μmol/m2/sec, carbondioxide flow rate of 162.5 mL/min (3.25 % volume of air), and air flow rate of 5 L/min. The maximum biomass productivity achieved through column photobioreactor was 0.70 g/L that was higher than coiled photobioreactor with biomass productivity of 0.39 g/L. The bubble column reactor produced higher biomass than the coiled reactor due to the better mixing of culture which enhances the light exposure and nutrient availability. Also, the height of the water column enhances the CO2 diffusivity and carbon availability. The final part of the research dealt about the separation of the cyanobacterial culture from the BG11 media using dead-end membrane filtration. Three different organic flocculants such as cationic polyelectrolyte, polyquarternary ammonium resin and polyquaternary amine resin at four different concentrations such as 0.01%, 0.07%, 0.13% and 0.19% were investigated for their performance in total filtration time. Cellulose and nitrocellulose membrane were compared for their microfiltration behaviour. Among the three polyelectrolytes, poly amine resin resulted in the shortest filtration time of 1.25 minutes. In comparison to nitrocellulose membrane (8 minutes), the cellulose membrane resulted in the shortest filtration time of 1.25 minutes. The cellulose membrane with poly amine resin at concentration of 0.01% provides the optimum permeate flux with least filtration time. As the optimum growth conditions for wild type anabaena 7120 was identified, it would be interesting to apply those conditions for transgenic species to characterize its biomass productivity. Regarding the downstream processing, continual volatile chemical product separation from growth media produced by transgenic organisms, using adsorption and temperature swing desorption technique would be the future separation strategy could be employed