In this study, we establish an outdoor photobioreactor system for microalgal biomass production. The study is divided into four parts: 1. The strategy of increasing CO2 tolerance (> 15% CO2) and cell density in the microalgal cultures was performed in this study. 2. The semicontinuous operation strategy enhanced the microalgal biomass production. 3. We also demonstrated the correlation between CO2 removal efficiency, culture aeration rate and culture density by using porous centric tube photobioreactor. 4. We demonstrated an outdoor photobioreactor aerated with flue gas for on-site bioremediation. The conclusions are showed in below, respectively.
8.1 Part I. Reduction of CO2 by a high-density culture of Chlorella sp. in a semicontinuous photobioreactor
1. In previous studies, researchers focused on the effects on high concentration of CO2
aeration for microalgae growth but low concentration of CO2 aeration (< 2% CO2) for CO2 removal efficiency. In our study, CO2 removal efficiency in the microalgal cultures aerated with higher CO2 concentration was evaluated. Our data show that the CO2 removal
efficiency in the developing microalgal photobioreactor at 2%, 5%, 10% and 15% CO2
aeration could reach to 58%, 27%, 20% and 16%, respectively.
2. The strategy of increasing CO2 tolerance and cell density in the microalgal cultures was performed in this study. At the initiating stage of culture, the microalgal cells were grown and adapted to an enriched- CO2 (2%) environment. When a high-density microalgal culture reached, the culture could be executed under 10% or 15% CO2 aeration without decreasing performance of cell maintenance and growth. It is valuable information on the use of CO2 reduction system of microalgal photobioreactor.
3. It is also confirmed that CO2 reduction system could be extended to multiple units in parallel photobioreactor for discharging waste gas in a large scale. The amount of lipid and biomass production and CO2 removal efficiency in the individual parallel photobioreactor and in the single photobioreactor also were similar.
8.2 Part II. Lipid accumulation and CO2 utilization of Nannochloropsis oculata in response to CO2 aeration
1. The lipid accumulation from microalgae with different concentration of CO2 aeration
would be evaluated for the efficiency of biodiesel production. In our study, CO2 utilization for microalgal lipid accumulation in the cultures aerated with different concentration of CO2 was evaluated. Our data show that the CO2 utilization for microalgal lipid
accumulation at low concentration of CO2 aeration would be more potential and more efficient.
2. The CO2-tolerance for microalgae is an important issue in the use of waste flue gas as a carbon source. N. oculata grew well at 2% CO2 aeration, but completely inhibited at 5, 10, and 15% CO2 aeration. After a pre-adapting culture and applying to semicontinuous system, N. oculata could grow well at high CO2 aeration. The growth profile is similar and could be applied to CO2 removal system.
3. We demonstrate the strategy of increasing total lipid production for biodiesel production.
The comparison of productive efficiencies in semicontinuous system was performed with half (for one-day) or three fifth (for three-day) replacement culture approaches. Our data show the culture broth being daily replacement could be more efficient not only for the biomass production but also for lipid production. It is valuable information that the
microalga cultured in the semicontinuous operation system could efficiently introduced 2%
CO2 aeration for biomass and lipid production in the long-term operation.
8.3 Part III. The air-lift photobioreactors with flow patterning for a high-density culture of microalgae and carbon dioxide removal
1. In our study, a porous centric tube photobioreactor was designed for microalgal culture.
The comparison of the growth of Chlorella sp. NCTU-2 cultivated in the photobioreactors without inner column, with centric tube and with porous centric tube column was evaluated.
Our data show that the maximum optical density and specific growth rate of the cultures in photobioreactors without inner column, with centric tube and with porous centric tube column in a batch culture were 11.5, 12.3 and 16.8 (at A682), and 0.18, 0.226 and 0.252 d-1, respectively. Furthermore, the efficiency of CO2 removal in the porous centric tube
photobioreactor is 45% and 52% higher than those in the bubble column and centric tube photobioreactors, respectively. It is valuable information on the design of microalgal photobioreactor.
2. We demonstrate a strategy of a long-term maintained high-density culture in our designed porous centric tube photobioreactor. Chlorella sp. NCTU-2 cultured in the centric tube
photobioreactor and operated by 1/4 (i.e., one-fourth volume of cultured broth was replaced by fresh medium at an interval of 2 days) and 1/3 (one-third broth replaced at 3 days interval) and 1/2 (one-second broth replaced at 8 days interval) replacement. A steady state of growth profile was seen in each broth replacement of semicontinuous culture. The maximum biomass productivity could achieve to 0.61 g L-1 in 1/4 of the culture broth recovered from the culture every 2 days.
3. We also demonstrated the correlation between CO2 removal efficiency, culture aeration rate and culture density by using porous centric tube photobioreactor. The result indicates that increasing of CO2 removal efficiency could be accomplished by lower aeration rate. The optimal condition for CO2 removal could achieved in this study was that Chlorella sp.
NCTU-2 was cultured at high optical density of 25 (A682) and the culture was aerated at 0.125 vvm. The maximum efficiency of CO2 removal was 63% (the aerated gas with 10%
CO2). According to our best knowledge, this is the first report addressed the efficacy on CO2 removal with different culture density and aeration rate in a closed microalgal photobioreactor aerated with high (10%) CO2 concentration.
8.4 Part IV. Microalgal biomass production and on-site bioremediation of carbon dioxide, nitrogen oxide and sulfur dioxide from flue gas using Chlorella sp. cultures
1. In this study, a microalga, Chlorella sp. MTF-7, was isolated by chemical mutagenesis. The microalgal strain Chlorella sp. MTF-7 grew in the presence of flue gas, and the flue gas temperature was 40-45oC when it was introduced into the microalgal cultures. It is a potential microalgal strain for bioremediation of flue gas.
2. We demonstrated an outdoor photobioreactor aerated with intermittent flue gas for on-site bioremediation. On-site bioremediation of CO2, NO and SO2 in flue gas by the Chlorella sp. MTF-7 cultures could reach approximately 60%, 70% and 50%, respectively.
According to our best knowledge, this is the first report addressed and evaluated on-site bioremediation of CO2, NO and SO2 in flue gas.
3. We also demonstrated a double-set of photobioreactor systems alternately aerated with flue gas for continuous flue gas bioremediation. An on-site outdoor photobioreactor system using a gas-switching cycle operation could remove a high percentage of the CO2, NO and SO2 from flue gas generated from a coke oven. We believe that it is valuable information for the use of flue gas bioremediation system of microalgal cultures.