Accelerating microalgal growth with CO2 transformation

Abstract

The maximising carbon dioxide capture in the water as inorganic carbon source for accelerating microalgal growth was carried out where the effects of pH, gas-liquid contacting area, height, gas flowrate and salinity on dissolved Total Inorganic Carbon (TIC) concentration were observed. The experiments were conducted in 1-3 m high bubble column and packed column. The pH of the demineralised water was adjusted as required using NaOH 0.5 M and HCI 0.5 M. % Efficiency of CO₂ dissolution (Dissolved CO₂(g)/ Input CO₂(g)) was then computed. From the findings, an increase in pH could lead to a greater dissolution of CO₂ resulting in a greater effluent TIC concentration and high %CO₂ dissolution efficiency. Adding gas-liquid contact area and gas hold up (ℇg) caused a slightly greater dissolution of CO₂. A triple increase in height from 1 m to 3 m in packed column gave the better overall % efficiency but not in a direct proportional to the height. Furthermore, the gas phase CO₂ decreased along The axial position resulting in a lesser quantity of CO₂ at higher position in the column. At high CO₂ flowrate, CO₂ did not have enough contact time with the solution and wastefully released to the atmosphere causing low %efficiency. The design of the –“Circulating Counterflow Contactor –C.C.C.” system was employed where the liquid and gas were counter-flowed to enhance the contact time and better mixing. The C.C.C. offered the steadily high %efficiency not only in early stage but also later stage in the range of 34-56% at the liquid flowrate of 2 LPM was employed. Higher degree of salinity was found to lower the dissolution of TIC in the solution. Additionally, the effect of bicarbonate on microalgal growth was examined by initially manipulating pH in range 6-9. NaHCO₃ at 30 ppm was fed into the cultivation system where pH 6 and 7 exhibited most suitable initial pH the in which maximum cell concentration and specific growth rate were slightly different. An increase in NaHCO₃ concentration from 30 to 80 and 200 ppm did not seem to have significant effect on the growth. CO₂ dissolution from C.C.C. system was also employed to cultivate C. Vulgaris and obviously the results illustrated a higher growth where the maximum cell concentration and specific growth rate were higher than using dissolved NaHCO₃.