6.1 Introduction
Global warming which results from increasing concentration of CO2 has become an important issue of environmental concerns. There are many attempts for CO2 recovery
including physical, chemical, and biological methods (Abu-Khader, 2006; Lee and Lee, 2003).
Among these attempts, the biological method using microalgal photosynthesis is believable as an effective approach for biological CO2 fixation (Yanayi et al, 1995; Wang et al., 2008). By the biological approaches, CO2 can be fixed into microalgal biomass by photosynthesis. The photosynthetic organisms can produce proteins, fatty acids and pigments as dietary
supplements for humans and animals (Ono and Cuello, 2004). Furthermore, lipids from microalgae are chemically similar to common vegetable oils and are high potential sources for biodiesel production (Chisti, 2007). The microalgal-based biodiesel compared with fossil fuels, that is renewable, biodegradable, and low pollutant produced (Chisti, 2008; Vicente et al,, 2004). Thus, reducing atmospheric CO2 by microalgal photosynthesis is considered safe and reliable for living environment (Lee and Lee 2003; Cheng et al., 2006).
Using outdoor microalgal cultures, such as open pond, has been proposed to reduce CO2 emission (Jeong et al., 2003; Ono and Cuello, 2004). However, outdoor culture system is limited to microalgal growth, not easy to control the environmental parameter and shows low productivity as a result of variable environmental temperatures, system circulation and light utilization (Carvalho et al., 2006). In comparison with open culture system, closed
photobioreactor is easy to control environmental parameters (Molina Grima et al., 1999) and can achieve high growth rate (Pulz, 2001; Sierra, 2008). A closed photobioreactor can be a bioscrubber for waste gas treatment and the microalgal cells cultured in the photobioreactor to convert CO2 from the waste gas into biomass is an energy-efficient and economical approach (Chiu et al., 2008; Suh and Lee, 2003). Several studies have proved that using microalgal cells cultivated in photobioreactors is useful and practical method for CO2 removal (Chiu et al., 2008; Ono and Cuello, 2004; Keffer and Kleinheinz, 2002). There were several types of photobioreactors, such as tubular, flat and column photobioreactors, reported. Vertical tubular type photobioreactors, such as bubble and air-lift photobioreactors, were often thought to be the most efficient mixing and the best volumetric gas transfer (Eriksen, 2008). Besides, the
traditional fermentation bioreactor, bubble column equipped with perforated draft tube has been shown significant improvements over traditional air-lift reactors in mixing and mass transfer performance (Bando et al., 1992a, b); however, the bioreactor equipped with perforated draft tube is never used in photosynthetic organism. Criteria for high-density culture in photobioreactor are good mixing, mass transfer and light utilization. In addition, in a high-density culture of microalgae, light utilization could be improved if good mixing providing the flash light effect of microalgal photosynthesis (Barbosa et al., 2002). As mentioned above, we tested whether a photobioreactor with porous centric tube is potential for the microalgal culture with high cell density.
In the present study, we designed a photobioreactor which is an air-lift type photobioreactor with porous centric tube for high-density microalgal culture and the performance evaluation was compared to the other two designs, bubble column and centric tube photobioreactors. The microalgal species, Chlorella sp. NCTU-2, was Taiwan native microalgal species and was screened as a potential candidate for growth and biomass production in this study. For determining the capacity of daily biomass production of the microalga, a semicontinuous culture operation was performed. Moreover, the CO2 removal efficiency was evaluated under different aeration rates and microalgal densities (i.e., biomass concentration) of the culture.
6.2 Results and discussion
6.2.1 Growth of Chlorella sp. NCTU-2 in the photobioreactors
The comparison of the growth of Chlorella sp. NCTU-2 cultivated in the photobioreactors without inner column, with centric-tube column and with porous
centric-tube column was performed as a batch culture in an incubator at 26±1°C, with a light intensity of approximately 300 µmol m-2 s-1 at the surface of the photobioreactor provided by a continuous, cool white, fluorescent light source. The cultures were provided with 5% CO2
gas. The cultured samples were collected for density measurements at 12-h intervals. Figure 10 shows the growth curves of the three cultures, and the results indicate that the microalgae cultured in the porous centric-tube photobioreactor performed at the highest growth rate.
The maximum biomass concentrations and specific growth rates of the cultures in the photobioreactors without inner column, with centric-tube column and with porous
centric-tube column in batch culture mode were 2.369, 2.534 and 3.461 g L-1, and 0.180,
0.226 and 0.252 d-1, respectively (Table 9). This result indicates that the maximum biomass concentration in the porous centric-tube photobioreactor could be enhanced by 46% and 37%
compared to those in the bubble column photobioreactor and in the centric-tube
photobioreactor, respectively. The culture in the porous centric-tube photobioreactor showed not only an improved maximum biomass concentration but also a better specific growth rate.
Recently, Ranjbar et al. (2008) reported that the maximum cell density of Haematococcus pluvialis cultured in an air-lift photobioreactor was 18% higher than that in a bubble column photobioreactor. Oncel and Sukan (2008) demonstrated that an air-lift photobioreactor culture yielded a maximum biomass concentration value of 2.21 g L-1 whereas a bubble column photobioreactor culture yielded only a maximum biomass concentration value of 1.87 g L-1. The air rising randomly through the photobioreactor was the only driving force for culture mixing in the bubble column photobioreactor (Oncel and Sukan, 2008; Ranjbar et al., 2008).
The air-lift-type photobioreactor with the centric tube could provide a regular circulation of the culture in that the air rising from the inner column made the circulating liquid flow out of the inner column whereupon it was gravitationally forced downward (as shown in Figure 3) (Chisti, 1989; Chisti, 1998). The regular circulation of the culture resulted in a more effective mixing for growth (Oncel and Sukan, 2008). Ranjbar et al. (2008) also reported that the light regime inside a photobioreactor could be improved and a high-density growth was attainable by using an air-lift-type photobioreactor. In our designed photobioreactor, there are
perforations of 5 mm diameter regularly distributed along the porous centric tube. A similar concept has also been reported in which the liquid flowing through the rising zone could horizontally flow through the perforations (Xu et al., 2008). The result indicated that the growth of Chlorella sp. NCTU-2 cultured in a photobioreactor with a porous centric tube was even 37% higher than when cultured in a photobioreactor with a centric tube. The perforations along the centric tube in the porous centric-tube photobioreactor could provide a shorter mixing time; therefore, this photobioreactor possesses a better mixing efficiency (Fu et al., 2003). In a high-density culture, the main limitation is the light penetration, which will decrease due to the self-shading effect of the microalgal cells. However, a photobioreactor providing a light/dark zone could minimize the self-shading effect (Chae et al., 2006). The perforations along the centric tube could increase the frequency with which the microalgal cells are exposed to light/dark cycles. The more frequent light/dark cycles affect the productivity and the yield of biomass and have been reported to lead to higher growth and photosynthesis rates (Hu and Richmond, 1996; Nedbal et al., 1996).
6.2.2 CO2 removal of Chlorella sp. NCTU-2 in the photobioreactors
The CO2 removal efficiency of Chlorella sp. NCTU-2 cultivated in the three types of photobioreactor was evaluated and compared. The microalgal cells were cultured in batch cultures. When the biomass concentration reached 3 g L-1, the microalgal cells were centrifuged and resuspended in fresh medium. Then, the microalgal cells were divided into equal parts and cultured in the photobioreactors at a biomass concentration of approximately 2 g L-1 and with 5% CO2 aeration at 0.25 vvm. The CO2 removal efficiency was determined by measuring the influent load and the effluent load. Table 9 shows a comparison of the CO2
removal efficiencies of Chlorella sp. NCTU-2 cultivated in photobioreactors without inner column, with centric-tube and with porous centric-tube column. The results show that the CO2
removal efficiency in the porous centric-tube photobioreactor is 45 and 52% higher than those in the bubble column and centric-tube photobioreactors, respectively. The CO2 removal efficiency in the porous centric-tube photobioreactor showed the highest efficiency of CO2 removal, which is probably due to the higher mixing efficiency and the higher photosynthetic rate. A similar result was also reported by Xu et al. (2008) that the quality of mixing is critical for the performance of a bioreactor, and a shorter mixing time was obtained with an air-lift reactor with a net draft tube, which also provided a horizontal flow, in comparison with the mixing time in a bubble column reactor and an air-lift reactor without a net draft tube. The result has also been confirmed by Grobbelaar (1994) who indicated that higher mixing resulting in more frequent light/dark cycles would enhance the photosynthetic efficiency.
6.2.3 Biomass productivity of Chlorella sp. NCTU-2 in semicontinuous cultivation
Before the operation of semicontinuous cultivation, the microalgal cells were cultivated in a porous centric-tube photobioreactor. When the cell density reached approximately 3 g L-1 (beginning of the early stationary phase), the mode was changed to fed-batch cultivation in that the feed medium was only supplied with 750 mg NaNO3 and 44.11 mg NaH2PO4·H2O per liter every 2 days (as shown in Figure 10). After 6 days of cultivation, the biomass concentration reached approximately 5 g L-1. This almost corresponds to the maximum biomass concentration of a Chlorella sp. NCTU-2 culture grown in the porous centric-tube photobioreactor at a high growth rate. The growth potential would be decreased due to the decreasing light utilization at the higher biomass concentration of the culture. For the
maintenance of the high-density culture and of biomass production, the semicontinuous culture mode was then applied. For the semicontinuous culture mode, the growth profiles of Chlorella sp. NCTU-2 cultured in the centric-tube photobioreactor aerated with 5% CO2 and operated with 1/4 (i.e. one fourth of the volume of the culture broth was replaced by fresh medium at intervals of 2 d), 1/3 (one third of the broth replaced at 3-day intervals) or 1/2 (one half of the broth replaced at 8-day intervals) replacement are shown in Figure 11. A
steady-state growth profile was seen with each broth replacement during semicontinuous culture. The steady-state growth profile indicated that continuous growth of the microalgae cultivated in the porous centric-tube photobioreactor could be sustained in a high-density culture.
Table 10 shows the performance of the broth replacement strategies in the
semicontinuous culture mode. At a high culture density (biomass concentration from 2.47 to 4.94 g L-1), the specific growth rates and biomass productivities in the 1/4, 1/3 and 1/2
replacement were 0.106, 0.118 and 0.132 day-1, and 0.61, 0.53 and 0.51 g L-1 d-1, respectively.
Compared with our previous study (Chiu et al., 2008), the biomass productivity in the
maintained semicontinuous culture mode was 0.458 g L-1 d-1. The results indicate that, for the porous centric-tube photobioreactor in semicontinuous culture mode, the maximum biomass productivity was 0.61 g L-1 when one fourth of the culture broth was recovered from the culture every 2 d.
6.2.4 CO2 removal efficiency at a variety of culture densities at different aeration rates For the study of CO2 removal, microalgal cells were collected and cultured in a biomass concentration range from 1.03 to 5.15 g L-1. Different culture densities were obtained by condensing the microalgal cells by centrifugation. The microalgal cells were resuspended in fresh medium for further experiments. All the cultures at a variety of microalgal densities were provided with premixed 10% CO2 gas at different aeration rates. The CO2 removal efficiency was evaluated by measuring the influent load and effluent load airstreams at different aeration rates and cell densities of the microalgae. Figure 12 shows the correlation between CO2 removal efficiency, culture aeration rate and biomass concentration. The regression lines and the equations were as follows: y1=0.1204x+0.0295 for 0.125 vvm, y2=0.108x-0.0308 for 0.25 vvm, and y3=0.0483x-0.0223 for 0.5 vvm. Here, the value of y1 is the CO2 removal efficiency (%) and the value x is the biomass concentration (g L-1).
The result indicates that an increasing CO2 removal efficiency could be achieved by a lower aeration rate. Also, the CO2 removal efficiency could be increased by cultivation at a high density. Mandeno et al. (2005) have revealed that the CO2 removal efficiency decreased with increasing gas flow. This may have resulted from bubble coalescence. The amount of coalescence would have increased with the increased gas flow rate. As the bubbles coalesce, the bubble surface area per unit gas volume would decrease and the larger bubbles would rise faster than the smaller ones. Moreover, CO2 absorption from the bubbling gas would also decrease with decreasing surface area per unit gas volume of the bubbles. In order to obtain a higher CO2 removal efficiency, high-density cultivation was performed because, under these conditions, more CO2 was consumed by the microalga Chlorella sp. NCTU-2. Furthermore, an increase in CO2 removal efficiency during high-density cultivation may also result from the high-density culture broth causing a higher viscosity, which would in turn increase the gas retention time for CO2 absorption. The optimal conditions for CO2 removal in this study were culturing Chlorella sp. NCTU-2 at a high biomass concentration of 5.15 g L-1 and aeration of the culture at 0.125 vvm. The maximum efficiency of CO2 removal was 63% (with 10% CO2 in the aeration gas). Keffer and Kleinheinz (2002) have demonstrated that the maximum CO2 reduction was 74% with Chlorella vulgaris cultured in a bubble column photobioreactor, but the CO2 concentration for aeration was 100-fold lower than that in this study. In this study, the CO2 removal efficiency could still reach 63% when using the porous centric-tube
photobioreactor with a high-density culture, although the CO2 concentration for aeration was 100-fold higher than that reported by Keffer and Kleinheinz (2002). Compared with our previous study (Chiu et al., 2008), due to the achievement of a high-density culture in the porous centric-tube photobioreactor, the CO2 removal efficiency was increased from 20 to 53% at the same aeration rate and CO2 concentration. By using the porous centric-tube photobioreactor with semicontinuous cultivation strategy, a high culture density could be maintained, up to approximately 5 g L-1; besides, a high efficiency of CO2 removal could be achieved.
7. Part IV:Microalgal biomass production and on-site bioremediation of