The efficiency of a novel microalgal culture system (an airlift loop bioreactor [ALB] engaged with a fluidic oscillator to produce microbubbles) is compared with both a conventional ALB (producing fine bubbles without the fluidic oscillator) and non-aerated flask culture. The impact of CO2 mass transfer on Dunaliella salina growth is assessed, through varying the gas (5% CO2, 95% N2) dosing flow rate. The results showed that approximately 6 - 8 times higher chlorophyll content was achieved in the aerated ALB cultures than in the non-aerated flasks, and there was a 20% - 40% increase in specific growth rate of D. salina in the novel ALB with microbubbles when compared with the conventional ALB cultures. The increase in chlorophyll content was found to be proportional to the total amount of CO2 mass transfer. For the same dosing time and flow rate, higher CO2 mass transfer rate (microbubble dosing) resulted in a greater growth rate.
Microalgae have been considered for CO2 capture from flue gas by many industries recently, due to their high CO2 uptake efficiencies which are one order of magnitude (10 to 50 times) higher than those of terrestrial plants [
Many studies have demonstrated the correlation between light intensity and algal productivity based on the assumption of unlimited CO2 supply, however, in practice CO2 mass transfer was always limited due to conventional bubble dosing. In order to achieve sufficient CO2 dissolution, higher dosing rate and longer dosing time (e.g. 24 hrs/d) were employed to compensate for the lower mass transfer. Nevertheless, by doing so, under higher aerating flow rate, most of the gas was wasted due to low mass transfer, and the intensive agitation could cause damage to the algal cells. Besides, longer dosing time means more energy consumption which would result in a low yield/power ratio. Therefore, design of a CO2 dosing system with a relatively high gas mass transfer and low energy cost tends to be a major consideration for cost-competitive microalgae culture. Since an energy efficient microbubble dosing system has been developed [
Zimmerman et al. [
The experimental setup for lab bench ALB cultures is shown in
D. salina was pre-cultured in a growth room (25˚C ± 2˚C, light intensity 50 µmol·m−2·s−1) in a similar culture medium, but with added HEPES (20 mM) as a buffer (pH 7.5). At the beginning of the main experiments, 50 ml of pre-cultured D. salina was added to 2.5 L of fresh culture medium for each culture. Each ALB culture was
dosed with CO2 enriched gas (5% CO2, 95% N2) for 30 minutes per day. 50 ml algal samples were taken after gas dosing or mixing (for flask cultures), followed by topping up the culture with 50 ml of fresh medium. pH and DO levels in each of the bioreactors were measured daily before and after gas dosing using a SevenGo Duo Pro pH/DO meter.
The chlorophyll content of the samples of D. salina culture taken each day was determined by measuring the optical density at wavelengths of 645 nm and 663 nm using the method described by Zimmerman et al. [
The CO2 concentration dissolved in the medium was calculated from the current pH using Equation (2), of which the detailed derivation is shown in Appendix A.
Each day, the difference between the concentration of dissolved CO2 before and after dosing, calculated based on the pH, indicates the amount of CO2 that has been transferred into the medium (dosed CO2). The reading taken the following day before dosing indicates the decrease in the dissolved CO2 and gives the amount of CO2 uptake by D. salina.
than in the flask cultures for the same culture period. It is easily understood that the ALB engaged with microbubble/fine-bubble dosing enables a high mass transfer of CO2 dissolution and O2 removal, which makes the culture both CO2 sufficient and O2 stripped, therefore, algae grew better in such “well served” circumstances. Zimmerman et al. [
Apart from the relatively higher CO2 mass transfer and an appreciable O2 stripping by “micro/fine-bubbling”, a better pH control is also one of the reasons that explain why ALB cultures exceeded the flask cultures in productivity. Commonly, pH in the culture medium increases as the algae grows, and when the pH increases beyond the optimum range, the culture may be adversely affected. As algae grow, the photosynthetic uptake of CO2 leads to the increase in pH, but as a consequence of increasing pH, increases while and CO2 decrease, which inhibits the photosynthetic reaction and improves the rate of algal respiration [
growth occurs. In terms of ALB cultures (FO engaged or not), chlorophyll increased dramatically (from around 0.05 - 0.15 mg·L−1 to 26.43 - 32.65 mg·L−1) until the growth entered steady phase (the last 3 days). Correspondingly, pH was supposed to rise even faster than control culture, however, due to daily micro-bubble (300 μm) or fine-bubble (600 μm) dosing, the culture pH was maintained in a suitable range of 6.5 - 8.5 (
One thing needs to be clarified that for all ALB cultures in this study, the pH value seems to be similar despite the different dosing flow rates or dosing methods. But theoretically, for various dosing conditions with different mass transfer capabilities, the dissolved CO2 in the culture medium differs, correspondingly, the pH value indicating the amount of dissolved CO2 differs as well. Such a ‘contradiction’ can be explained by
culture. Generally, for each dosing flow rate D. salina grew better in FO engaged ALBs (microbubble dosing) than in normal ALBs (fine-bubble dosing). The peak chlorophyll content reached 27.03 - 32.65 mg·L−1 when FO was applied, while only 23.13 - 26.47 mg·L−1 was achieved without FO. To quantify the comparison of D. salina growth under different dosing conditions, the overall specific growth rate was estimated from the slope of a semilog plot of ln(Ct/C0) versus time. Hence the specific growth rate under each ALB dosing condition was obtained, which was plotted in
Generally, the specific growth rate (μ) was found to increase along with dosing flow rate, either with or without FO engaged. The maximum μ of 0.13 d−1 and 0.17 d−1 was achieved at flow rate of 0.9 L·min−1 (without FO) and at 1.1 L·min−1 (with FO), respectively. This overall trend was found similar to gas-liquid mass transfer study [
(
For the ALB cultures with fine-bubble dosing (NoFO), within the flow rate range of 0.3 - 1.1 L·min−1, mass transfer coefficient Kla (either for CO2 dissolution or for O2 removal) increased with flow rate, and consequently CO2 dissolution and O2 stripping efficiency were enhanced. The culture therefore had more dissolved CO2 available for algal uptake and less O2 inhibition. Thus, specific growth rate increased as the flow rate went up. The same scenario was observed for the novel ALB cultures (microbubble dosing) under the flow rate of 0.3 - 0.7 L·min−1. However, the specific growth rate did not significantly increase by further increasing the flow rate when it exceeded 0.7 L·min−1. This can be explained by assuming that for 0.3 - 1.1 L·min−1 of dosing (ALB cultures, NoFO) and 0.3 - 0.7 L·min−1 of dosing (ALB cultures, FO), the daily total amount of CO2 mass transfer (average CO2 mass transfer rate × dosing time) did not reach or exceed the saturation concentration, therefore higher mass transfer led to a greater amount of available CO2, which consequently resulted in a higher growth rate. For the flow rate of 0.9 - 1.1 L·min−1 with mcirobubble dosing, the total CO2 mass transfer is likely to be excessive (average CO2 mass transfer rate × dosing time > CO2 saturation). The extra CO2 was therefore released to the atmosphere and did not contribute to the algal growth. Thus increasing the flow rate over a valid range may not effectively improve the growth. Based on the above discussion, 30 min·d−1 of dosing under 0.7 L·min−1, close enough to reach CO2 saturation, turns out to be the optimal dosing condition for the 3L-ALB culture (with microbubble dosing).
For the ALB cultures under each condition, the amount of total CO2 uptake and the increase in the chlorophyll content were calculated for certain culture periods, which are shown in
Therefore, an assumption can be made that in the same time period, the CO2 uptake rate should be proportional to the instant algal concentration (chlorophyll content), which is shown as follows:
where μ is the overall specific growth rate (constant for a certain culture condition); VChl and represent chlorophyll growth rate and CO2 uptake rate, respectively; [Chl] and [CO2] mean the chlorophyll content and CO2 concentration, separately.
Indeed, the experimental data, shown in
In order to correlate the algal growth to CO2 mass transfer, the correlations between the amount of CO2 uptake and the CO2 transferred to the liquid still needs to be understood, which is presented in
where represents CO2 average mass transfer rate; tdosing means the dosing time.
By combining Equation (3) and Equation (5), it gives
From Equation (6), the chlorophyll content increase has been shown to be in direct proportion to the mass transfer rate for the ALB cultures in this study, which again explains why the ALB cultures with microbubble dosing have higher growth rates than the ones with finebubble dosing.
An about 6 to 8 times enhancement of D. salina growth was found in ALB cultures, compared with the flask incubation. Instead of buffer solution (e.g. HEPES), daily 30 minutes of 5% CO2 gas dosing maintained pH at a suitable level (6.5 - 8.5). Besides approximately 20% -
40% increase in specific growth rate was found in the FO engaged ALB cultures, over a wide range of gas dosing flow rate. Furthermore, the chlorophyll content (growth) was found to be directly proportional to the mass transfer rate for D. salina ALB cultures. Further modelling of these observations is being carried out.
WZ would like to thank the Royal Society for a Brain Mercer Innovation Award. We acknowledge support for microbubble dynamics from EPSRC. DJG would like to acknowledge support from Carbon Trust.
The dissolved CO2 is in equilibrium with and, which can be described by the following chemical reactions [16,17].
where the relevant equilibrium constants are:
.
The system must satisfy the electro-neutrality constraint, therefore
Assuming constant concentrations of other cations and anions, it gives
By solving above equations, it gives
However Equation (1) can only be used to calculate the concentration of CO2 in water and when the pH is less than 7. For the [CO2] estimation in the medium containing NaHCO3 modification needs to be made. The system would still need to satisfy the electro-neutrality constraint. But since NaHCO3 is added into medium, other cations and anions are not equal.
Therefore, the additional amount of Na+ needs to be taken into consideration:
Finally, Equation (1) is modified as:
(a)
(b)