Design of Outdoor-Gas-Treatment Photobioreactors - American

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Design of outdoor gas treatment photobioreactorsa laboratory simulated approach Di ZHANG, Ka Yip Fung, and Ka Ming Ng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504783s • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 21, 2015

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Industrial & Engineering Chemistry Research

DESIGN OF OUTDOOR GAS TREATMENT PHOTOBIOREACTORS - A LABORATORY SIMULATED APPROACH Di Zhang, Ka Y. Fung, Ka M. Ng*. The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. Email of the corresponding author: [email protected].

In this study, a method for designing the operating parameters (surface light intensity, operating temperature, operating cell concentration, CO2 availability) of an outdoor gas treatment photobioreactor (PBR) and the corresponding control strategies based on laboratory data has been proposed. Moreover, the outdoor performance of the designed gas treatment PBR can be estimated using the novel simulated diurnal cycle experimental setup and procedures developed in this study. Using the design method and the performance estimation experimental procedure, a 10 L outdoor gas treatment photobioreactor has been designed and built in Hong Kong. It is expected to consume 9.97 g CO2 and generate more than 7.25 g O2 during an operation period of 2 weeks. The design method and demonstration reported in this study would set ground for design of large-scale outdoor gas treatment PBRs based on laboratory data.

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1. Introduction Carbon dioxide (CO2) emission has been identified to be responsible for 60% of the global warming.1 The environmental impact has triggered much interest in CO2 sequestration and conversion. Methods proposed include geological storage, ocean storage, chemical mineralization, and biological conversion.2 Among them, photosynthetic conversion of CO2 by microalgae is one of the most attractive methods at present due to its high efficiency of CO2 conversion, mild reaction conditions, co-production of valuable biomass, and co-generation of oxygen (O2).3 The cost of long term CO2 conversion by microalgae is estimated to be 30 USD per ton, which is 10 USD lower than the conventional process through chemical carbon capture and geological storage. Meanwhile, lipids recovered from microalgae are worth 0.37 USD per liter, which makes the process even more economically attractive.4 At the bench scale, cell growth, CO2 consumption, and O2 generation in different cultivation systems have been studied under various illumination,5 temperature,6 pH,7 and nutrient conditions.8,9 Some microalgal species have been proved to be able to tolerate volatile organic carbons, sulfur oxides and nitrogen oxides, and are most suitable for industrial gas treatment.10-11 These studies have provided valuable fundamental knowledge of the microalgal response towards different culture conditions. On the other hand, pilot-scale studies have verified the feasibility and high efficiency of using microalgae for gas treatment.12-13 On-site bioremediation of CO2, NO, and SO2 using a 50 L outdoor photobioreactor fed with flue gas from a steel plant has been reported.14 With a specific Chlorella sp. that could tolerate high temperature and high CO2 concentration, CO2 was fixed at 2.87 g L-1 in a 6-day operation. The removal efficiency of NO and SO2 was about 70% and 50%, respectively.


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Despite all the previous achievements, there is still lack of guidance on designing outdoor gas treatment photobioreactors based on laboratory data. To fill this gap, a method for designing outdoor gas treatment photobioreactors has been developed and demonstrated with Chlorella sp. in this study. Bench-scale experimental setups and procedures facilitating the design have also been established. In addition, the design method allows decision makers to estimate the outdoor performance of a designed photobioreactor under local illumination conditions before field tests, so that feasibility study can be conducted even at the design stage. 2. Methods and Materials 2.1. Preparation of the Chlorella sp. Inoculum The Chlorella sp. used in this study was obtained from the Division of Life Science, HKUST, Hong Kong. The Chlorella sp. was cultured with Bold’s Basal Medium containing (per litre): 250 mg NaNO3, 75 mg K2HPO4, 175 mg KH2PO4, 25 mg CaCl2･2H2O, 25 mg NaCl, 75 mg MgSO4･7H2O, 500 mg EDTA anhydrous, 31 mg KOH, 4.98 mg FeSO4･7H2O, 1 mL H2SO4, 11.42 mg H3BO3, 8.82 mg ZnSO4･7H2O, 1.44 mg MnCl2･4H2O, 1.19 mg NaMoO4, 1.57 mg CuSO4･5H2O, 0.40 mg CoCl2･6H2O. To start with, axenic microalgal culture grown on an agar plate was inoculated into 100 mL autoclaved medium in a 250 mL Erlenmeyer flask (KIMAX, USA). The inoculum was kept by transferring 2 mL culture into 100 mL fresh autoclaved medium every 10 days throughout the study. The inoculum culture was kept on a 150 rpm orbital shaker (OS-20, BOECO, Germany) and was continuously illuminated by two white fluorescent lamps (24 W, 6500 K, Philips, Netherlands). Photosynthetic photon flux density (PPFD) was 170 ± 30 µmol m-2 s-1 at the surface of the flasks, measured with a photometer (LI-250A, LI-COR®, USA). Culture temperature was stabilized at 22 ± 2 oC with air conditioning.


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2.2. Bubble Column Photobioreactor (PBR) and Experimental Setup A bubble column PBR with a working volume of 850 mL was fabricated and its schematic diagram is illustrated in Figure 1. The culture vessel had a water jacket connected to a water bath (Model: 9106 Polyscience®, USA) for temperature control. The culture vessel was sealed with a rubber stopper during the experiments. A gas sparger for feed gas dispersion, a sampling needle, a ventilation needle, and an outlet gas conduit connecting to a gas chromatography (GC) analyzer were imbedded in the rubber stopper. The feed gas was composed of a CO2 stream and a synthetic air (O2/N2 = 21/79, vol./vol.) stream, controlled by separate mass flow controllers (Smart-Trak, SIERRA Instruments, Inc., USA). The two streams were mixed and filtered through a 0.45 µm membrane (MILLIPORE, Ireland) before being dispersed into the culture. Fifty-six light emitting diodes (LEDs), driven by a tunable DC power supply, were grouped onto eight 700 mm-tall aluminum sticks surrounding the bubble column PBR to provide illumination for the culture. 2.3. Effect of Key Operating Parameters on Microalgal Growth To determine the effects of surface PPFD, culture temperature, and CO2 vol.% in the feed gas on microalgae growth, Chlorella sp. was cultivated in the culture vessel under different conditions. Samples of the culture medium were collected every a few hours during the experiment. Cell concentrations were determined by measuring the optical density of the samples at 680 nm (OD680) with a UV/VIS spectrometer (UltrospecTM 4300 Pro, GE, USA) in order to obtain the growth curve. According to the definition in Equation 1, regression was conducted on the growth curve to determine the exponential phase specific growth rate (µexp) of Chlorella sp. under each set of cultivation conditions specified by the surface PPFD, culture temperature, and CO2 vol.% in the feed gas.


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μ =

( )

Eqn. 1

where t is the incubation time. In all experiments, the LEDs were used to provide continuous illumination at different surface PPFDs (0, 105, 450 or 500 µmol·m-2s-1), and the water bath was used to keep the culture temperature at 10 oC, 20 oC, 30 oC, 35 oC or 40 oC. The total feed gas loading rate was 200 mL min-1, in which the two gas streams were adjusted to provide feed gas containing 0.035%, 7.5%, 25% or 100% (vol./vol.) CO2. The dissolved inorganic carbon (DIC) concentrations, a term to quantify the CO2 availability in the medium, were determined to correlate to the CO2 vol.% in the feed gas. Feed gas with 0%, 7.5%, 25%, and 40% CO2 was sparged at a loading rate of 200 mL min-1 into the bubble column PBR with 850 mL autoclaved fresh medium at 35oC for 12 hours to achieve a steady state, respectively. After a steady state was achieved, the DIC concentration in the media was measured, in triplicate, using a carbon dioxide electrode (9502BNWP, Thermo Scientific, USA) connected to a pH/ISE meter (Orion Star A214 Benchtop, Thermo Scientific, USA). 2.4. Gas Treatment Performance under Optimum Growth Conditions The bubble column PBR was operated under the optimum cultivation conditions determined above for the growth of Chlorella sp., and the outlet gas stream was auto-sampled every 13.25 minutes and was analyzed by a GC analyzer (6890N, Agilent, USA). Different types of gas molecules were separated by a CTR1 column (Agilent, USA) and were detected by a thermal conductivity detector kept at 250 oC. The flow rates of CO2 and O2 in the outlet gas were determined from the corresponding calibration curves.


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In order to understand the gas treatment performance of the PBR, the light compensation point of Chlorella sp. was determined and the light attenuation in the culture was analyzed by the following two experiments. 2.4.1. Determination of the Light Compensation Point Light compensation point refers to the light intensity under which the net O2 evolution rate from a photosynthetic cell or culture equals to zero. The net O2 evolution rate from Chlorella sp. was measured at 22 ± 2 oC with the experimental setup shown in Figure 2. A modified dissolved oxygen probe (YSI ODO, USA) was placed in a sealed transparent glass vessel containing 5 mL dilute culture of Chlorella sp. at an OD680 of 0.146. A 6500 K LED driven by a tunable DC power supply was utilized to provide illumination. The PPFD received at the surface of the vessel was controlled by adjusting the power supply for the LED and was measured with the photometer. Since the culture in the glass vessel was dilute and shallow, light attenuation could be neglected. During the experiment, the glass vessel was exposed to a PPFD of 0, 8, 20, 39 and 123 µmol m-2 s-1 for 2 minutes each. The dissolved oxygen (DO) concentration in the sealed culture was auto-logged every 5 seconds to obtain a DO concentration curve, and the net O2 evolution rate under each illumination condition was determined as the slope of the DO concentration – time curve. The light compensation point is then identified by locating the light intensity where the net O2 evolution rate is zero. 2.4.2. Analysis of Light Attenuation Under a fixed surface PPFD, light attenuation through the culture of various cell concentrations (OD680 = 1.5, 2.5, 3.5) was determined using the experimental setup shown in Figure 3. An LED was fixed at one end of a light channel to illuminate the Chlorella sp. culture contained in a glass vessel at the other end. By fixing the power supplied to the LED and the


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position of the culture surface in the light channel, the PPFD on the culture surface was fixed at 108 ± 3 µmol m-2 s-1. Under each cell concentration, the culture depth in the glass vessel was increased from 0 cm to 6 cm, with an increment of 0.5 cm in each measurement. The transmitting PPFD was measured with a photometer at the exit of the light channel for each culture depth and culture concentration. 2.5. Simulated Diurnal Cycle Experiment To estimate the microalgae growth rate and the gas treatment performance under solar illumination, a simulated diurnal cycle experiment was conducted to mimic the light intensity fluctuation received at the surface of an outdoor PBR and determined microalgae growth and the gas treatment performance in the benchtop bubble column PBR accordingly. The light intensity fluctuation during a day was determined in a clear day in Hong Kong as shown in Figure 4 (a). Because the highest light intensity reached 1300 µmol m-2 s-1, which far exceeded the surviving light intensity of Chlorella sp. (Data will be discussed in the next section.), proper shading would be a must for outdoor PBR operated in Hong Kong. Assuming the highest light intensity received at the surface was reduced to 400 µmol m-2 s-1 by shading and the bell-shape of the light intensity curve was kept, the light intensity change in a simulated diurnal cycle experiment could be designed as shown in Figure 4 (b). The cumulative photonic energy supply to Chlorella sp. could be determined accordingly and plotted in Figure 4(b). On top of the light intensity variation, the entire set of operating conditions in a simulated diurnal cycle experiment was designed and summarized in Table 1. Before the simulated diurnal cycle experiment, the bubble column PBR had been operated under the optimal conditions determined from the previous experiments (PPFD: 105 µmol m-2 s-1; Temperature: 35 oC; CO2 vol.% in feed gas: 25%) for 14 h to adapt the culture. After that, the reactor was kept under the


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optimal growth temperature of 35 oC, while the PPFD was controlled according to the light intensity profile listed in Table 1. During the day, feed gas containing 25% CO2 was fed at 200 mL min-1. At night, CO2 was not provided, as photosynthesis was not supposed to happen without light. Nevertheless, the effect of mixing at night was studied. Feed gas was completely stopped in the first night so that the cells would settle under gravity, while synthetic air (air without CO2) was loaded at 200 mL min-1 into the PBR in the second night to provide mixing. The CO2 and O2 flow rates in the outlet stream were analyzed by the same method discussed in Section 2.4. Samples were collected every hour to determine the cell concentrations. The average specific growth rate (µave) within a certain period of time (∆t) was calculated according to Equation 2 μ =

∆( ) ∆

Eqn. 2

3. Results and Discussion 3.1. Effect of Key Operating Parameters on the Growth of Microalgae The growth curves of Chlorella sp. under different operating conditions were obtained, as shown in Figure 5. The exponential phase specific growth rates under different operating conditions were determined and are also listed in Figure 5. Initial OD680 of the culture was 0.150 ± 0.050. Initial pH value of the culture was 6.90 ± 0.01. 3.1.1. Surface Light Intensity Figure 5 (a) shows the growth curves of Chlorella sp. under various surface light intensities when the culture temperature and the CO2 vol.% in the feed gas were fixed at 20 oC and 7.5%, respectively. When the culture was kept at darkness (0 µmol m-2 s-1), slight biomass loss was observed. As the surface light intensity increased to 105 µmol m-2 s-1, the µexp increased to 0.0312 h-1, which further increased to 0.0971 h-1 under a surface light intensity of 450 µmol m-2


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s-1. However, the culture turned from dark green to light green during incubation under a surface light intensity of 450 µmol m-2 s-1. This phenomenon was caused by photooxidation induced by the high level of light intensity which resulted in oxidation and degradation of cell pigments and subsequently turned the culture light green. As the light intensity further increased, the damage could become too severe that cell death would appear.15,16 In fact, it was observed that the cells could not proliferate under a surface light intensity of 500 µmol m-2 s-1 as the cell pigments were damaged, and the culture was completely bleached out. The reason to test and observe the Chlorella sp. growth under different light intensity (0, 105, 450, 500 µmol m-2s-1) in this study was to determine the light intensity that would be too high to cultivate Chlorella sp., instead of selecting the optimum light intensity. This was because although optimum light intensity would be helpful for indoor cultivation with artificial illumination, it would be difficult to achieve during the outdoor cultivation. Thus, the rule of thumb in designing an outdoor gas treatment photobioreactor with affordable operating cost was to only guarantee the survival light requirement of the microalgal species, i.e. to provide shading and prevent lethal photo damage under extraordinarily high light intensity. Through the aforementioned results, it was determined that 450 µmol m-2s-1 would be the upper limit of light intensity for cultivating Chlorella sp. However, in the following experiments, the batch cultivation of Chlorella sp. would be carried out under a relatively low light intensity of 105 µmol m-2 s-1. This was because although Chlorella sp. could survive under 450 µmol m-2s-1, the color of culture changed indicating certain extent of photo-damage, which might not be an ideal status to study the effects of culture temperature and CO2 availability on cell growth. In addition, a light intensity of around 100 µmol m-2s-1 had often been used for microalgae cultivation study.17


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3.1.2. Temperature The exponential phase specific growth rates of Chlorella sp. under different culture temperatures were determined from Figure 5 (b). The surface PPFD value and the CO2 vol.% in the feed gas were fixed at 105 µmol m-2 s-1 and 7.5%, respectively. At 10 oC, no significant growth of Chlorella sp. was observed. At 20 oC, active growth started to appear and the µexp was 0.0312 h-1. As the incubation temperature increased from 20 oC to 30 oC, µexp tripled and was 0.0968 h-1 at 30 oC. At 35 oC, the µexp reached 0.0980 h-1. However, at 40 oC, the specific growth rate of Chlorella sp. dropped to zero. Such a trend could be explained by the change of enzymatic activity and the denature rate of enzymes along with the change of temperature.18 Although a higher temperature led to a higher enzymatic activity which accelerated cell growth, the denature rate of enzymes was also increased and caused opposite effects to cell growth. Above a certain temperature, all enzymes were inactivated by heat so that cells could not grow. In conclusion, the specific growth rate of Chlorella sp. increased with increasing temperature until the optimum temperature range (30-35 oC) and decreased as the temperature was further increased. In the following experiments, 35 oC would be used as the optimum temperature for cell growth. Previous studies suggested that the specific growth rate of microalgae could be doubled with a temperature increase of 10 oC.6 In this study, the growth rate even tripled when the temperature increased from 20 oC to 30 oC. It was also reported that although the temperature range suitable for the growth of a specific microalgal species might be as wide as 15 oC, the specific growth rate would sharply decrease once the temperature went beyond the optimum range, as observed in this study and reported in other studies.19


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3.1.3. CO2 Concentration Figure 5 (c) shows that Chlorella sp. could grow under a wide range of CO2 vol.% in the feed gas, from 0.035 to 100%, at 35 oC under a surface light intensity of 105 µmol m-2 s-1. However, the µexp was low at a CO2 vol.% of 0.035% and 100%. It increased significantly as the CO2 vol.% in the feed gas was adjusted to 7.5% and 25%. This indicated that too low or too high CO2 availability are not preferred by microalgae. Rather, a wide range of moderate CO2 availabilities would favor the microalgae growth. Similar response of microalgae growth to CO2 availability was reported by Widjaja et al.20 Although increasing CO2 supply would promote cell growth, extremely high levels of CO2 might also have adverse effects, probably due to the consequently low pH in the medium. In this case, the optimum CO2 vol.% in the feed gas was from 7.5 to 25%, equivalent to a DIC concentration of 210 to 660 ppm in the equilibrated medium at 35 oC, as determined from Figure 6. This CO2% range was actually very commonly used in studies about CO2 capture by microalgae and coincided with the common CO2 concentration of industrial flue gas, which contained about 8-15% CO2. In the review conducted by Wang et al.21, a table of microalgal strains and their cultivation conditions for CO2 mitigation was presented, where most of the microalgal species were cultivated with a feed gas of 3-40% CO2. 3.2. Gas Treatment Performance under the Optimum Conditions for Growth 3.2.1. CO2 Consumption in the PBR To determine the CO2 consumption rate in the PBR under the optimum cultivation conditions for Chlorella sp. to grow (105 µmol m-2 s-1, 35 oC, 7.5% or 25%), the CO2 flow rate in the outlet was compared to that of the inlet. As CO2 polluted gas being sparged into the medium, CO2 in the gas bubbles diffused into the medium to form DIC species which was utilized by Chlorella


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sp. As shown in Figure 7, CO2 flow rate in the outlet gradually increased in the first 7 h under 7.5% CO2 and in the first 14 h under 25% CO2, which corresponded to the lag phase of cell growth. Although the microalgal cells did consume CO2 to prepare for cell proliferation even at the lag phase, the efficiency of CO2 intake was limited without cell proliferation, and less and less CO2 could dissolve into the medium as it got almost saturated with DIC. Once the cells started to grow, the CO2 flow rate at outlet was observed to slightly decrease due to the increasing consumption of DIC species by Chlorella sp., and thus more CO2 dissolved into the medium. This trend was reversed until severe mutual shading occurred at an overwhelmingly high cell concentration that prevented light penetration into the culture and hampered cell growth, as observed at the tail of the curves for CO2 flow rate in the outlet. Another observation was that the CO2 consumption rate under 25% CO2 was much higher than that under 7.5% CO2 in the period where cells actively proliferated. As shown in Figure 7 (a) and (b), the batch culture reached a maximum CO2 consumption rate of 5 mg min-1 when provided with a feed gas of 7.5% CO2, and 30 mg min-1 with 25% CO2. However, the corresponding growth curves in Figure 5 (c) showed that the cell concentration was actually higher under 7.5% CO2 than that under 25% CO2. This indicated that the average CO2 consumption rate of each cell was higher in a feed gas with 25% CO2 than that with 7.5% CO2. Similar physiological response of microalgae under different CO2 availability conditions has also been observed by other researchers.22, 23 3.2.2. O2 Generation in the PBR Figure 7 (a) and (b) show that the O2 flow rate in the outlet was higher than that in the inlet in the whole experiment. However, this could not be all attributed to the oxygen generation from microalgae. During the first a few hours of experiment where the microalgae was still in the lag


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phase, the higher O2 flow rate in the outlet was probably mainly caused by the feed gas containing low O2 content stripping out dissolved O2 from the medium, although there would certainly be contributions from the photosynthesis of microalgae. As stripping continued, the dissolved O2 concentration in the medium decreased and resulted in a decrease of O2 flow rate in the outlet. An increasing trend on O2 flow rate in the outlet was then observed when microalgae started to actively proliferate and O2 was produced by microalgae through photosynthesis. This increasing trend was continued until at a point where severe mutual shading occurred at a high cell concentration. In that case, the majority of cells were kept in the dark zone to consume the dissolved O2 in the culture without conducting photosynthesis so that the O2 flow rate in the outlet would stop increasing or even start to decrease. This observation agreed with that in a study conducted by Yun et al.24, where the O2% in the outlet gas also reached a maximum value during microalgae cultivation and eventually dropped down. In this experiment, despite the difference of CO2 consumption rate, the maximum O2 generation rates achieved under a feed gas with 7.5% CO2 and 25% CO2 were similar, and were 1.7 mg·min-1 and 1.6 mg·min-1, respectively. Referring to the corresponding growth curves in Figure 5, maximum O2 generation rates for both cases were obtained at a cell concentration of 2.5 in terms of OD680. This was because O2 generation was directly driven by light-induced H2O split in the microalgal cells. Therefore, given the fixed reactor configuration and surface light intensity, the same cell concentration in the PBR would lead to a similar O2 generation rate. Furthermore, the maximum O2 generation rate was repetitively achieved at a cell concentration of 2.5 in terms of OD680 was probably due to the following reason. The cell concentration for achieving maximum O2 evolution rate under a fixed surface light intensity should allow all cells to have positive net O2 evolution rate. This in turn required the light intensity received by all


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cells to be above the light compensation point, which was determined to be 13 µmol m-2 s-1 for Chlorella sp. (Figure 8). According to the Beer-Lambert law, the cells at the central axis of a column PBR would receive the lowest light intensity and would have the lowest net O2 evolution rate. Therefore, the cell concentration for achieving maximum O2 generation rate should allow the light intensity to be no less than 13 µmol m-2 s-1 at the central axis, where the light depth was 2.5 cm. As shown in Figure 9, under a surface PPFD of about 105 µmol m-2 s-1, when the culture concentration increased to 2.5 in terms of OD680, the PPFD at 2.5 cm under the culture surface would decrease to 13 µmol m-2 s-1. Hence, the cell concentration for achieving maximum O2 generation rate should be around 2.5 in terms of OD680. This deductive conclusion showed excellent conformity to the experimental result. Therefore, the cell concentration for optimal O2 generation performance could be selected from bench-scale experiments and this could be used to design the operating cell concentration for the batch operation of an outdoor gas treatment photobioreactor. 3.3. Performance of the PBR under Simulated Diurnal Cycle The simulated diurnal cycle experiment was conducted at 35 oC with a feed gas containing 25% CO2 under the light intensity profile provided in Table 1. The growth curve of Chlorella sp., CO2 consumption rate, and O2 generation rate were determined, as shown in Figure 10. After the adaptation period, the cell concentration, in terms of OD680, was 0.255. OD680 increased from 0.255 to 0.771 during Day I, while it increased from 1.000 to 2.088 during Day II. These corresponded to an average specific growth rate of 0.09 h-1 and 0.06 h-1, respectively, in Day I and Day II.


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The growth curves of Day I and Day II were both ‘S’-shaped, same as that of the cumulative photonic energy supply shown in Figure 4 (b). The specific growth rate (slope of the growth curve) was low in the ‘early morning’ and ‘late afternoon’, but was high during the ‘mid-day’. This suggested that the growth of microalgae under the simulated diurnal cycle might be correlated to the cumulative photonic energy supply. Based on this observation, an empirical model was proposed to estimate the microalgae growth under solar illumination and was expressed in Equation 3. X − = (k + b)E

Eqn. 3

where X is the cell concentration of each collected sample; X0 is the initial cell concentration; E is the photonic energy supply; k and b are the correlation constants. By regressing the microalgae growth to the cumulative photonic energy supply in both Day I and Day II (Figure 11), k and b were determined to be 0.1 m2 mol-1 and 0.0363 m2 mol-1, respectively. This empirical model was proposed by considering the nature of photosynthesis, which is the conversion of photonic energy into chemical energy. The more photonic energy supplies to the system, the more biomass generates. Meanwhile, under a fixed surface light intensity, the higher the cell concentration, the more photonic energy can be converted into chemical energy in biomass, resulting in larger correlation constants. However, this model is valid only before the maximum photonic energy conversion efficiency of the microalgal species is reached and light is the only limiting factor for growth. Despite the limitations of the empirical model, it is much easier to apply in predicting the daily microalgae growth under solar illumination than the kinetic models.25 In Night I and Night II, both illumination and CO2 supply were depleted. Mixing was provided in Night II, but not in Night I. The average growth rate during Night I and Night II were similar


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and were 0.0217 and 0.0295 h-1, respectively. This suggested that mixing could slightly improve cell growth when light was depleted. However, considering the large energy consumption required in mixing, the advantage of mixing at night might be trivial. A surprising observation from this result was that in both nights, the cell density increased, which was different from a previous report where biomass degraded at night.26 In fact, it was also different from the observation in Figure 5 (a), where the amount of biomass decreased when light was depleted during the PBR operation. The continuous growth of Chlorella sp. during the night in this case might be attributed to the excretion or storage of excessive carbohydrates during daytime. Nevertheless, if the microalgae were kept in the darkness for an extended period of time, such as in the case shown in Figure 5 (a), the cells would not have enough metabolites to support cell growth so that biomass degradation would occur. The use of photosynthetic metabolites for cell growth was also observed in other algal species.27 Figure 10 (b) illustrates the flow rate of CO2 and O2 in the inlet and outlet gas streams of the PBR operated under simulated solar illumination. After two diurnal cycles, the CO2 removal rate was 16 mg min-1 and O2 generation rate was 3.45 mg min-1. It was observed that after an intermittent night period, the equilibrium of CO2 and O2 between the gas and liquid phase required 5 h to re-establish, which was much faster than the start-up from fresh medium, as shown in the adaptation period. 4. Design of an Outdoor PBR for Gas Treatment 4.1. Design Methodology of Outdoor Gas Treatment Photobioreactors Outdoor gas treatment PBRs can be designed in a number of repeated units and added up to achieve a target capacity. Theoretically, a uniform configuration with short light path would be preferred. However, in real practice, the configuration and dimensions of an outdoor gas


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treatment PBR are often constrained by constructional and aesthetic considerations, and are designed by architects. Nevertheless, given the PBR configuration, dimensions, and the local climate conditions, the operating parameters (light intensity, temperature, CO2 availability, operating period and initial cell concentration) should be designed based on the requirements of microalgae for growth. Despite the configuration of a PBR, the surface light intensity should not exceed the maximum light intensity (LImax) that a microalgal species can survive and lethal temperatures should be prevented. The LImax and suitable temperature for microalgae growth can be determined using the experimental set-up and procedures described in Sections 2.2 and 2.3. If there is a chance that the solar illumination intensity may exceed the LImax, shading strategies need to be designed for the outdoor PBRs to keep the highest light intensity received on the PBR surface lower than the LImax. Shading can be provided as permanent infrastructures such as roof covers or it can be provided only during the hours with exceedingly high illumination intensities. Another possible measure to prevent photooxidation is to generate a turbulent flow in the culture, which prevents cells from staying at a region of extremely high light intensities for more than seconds. Although this method can create a flashing light effect, which may bring extra benefits for cell growth, much energy would be required.28 On the other hand, temperature control is often required, and if economically feasible, optimum cultivation temperature range is preferred. Waste heat and clear pond can be used for temperature control of outdoor photobioreactors.29, 30 However, in the areas at low latitudes, the gas treatment PBR may not be suitable to operate during the winter time, considering the high energy consumption for heating and low solar illumination intensity. After the unit configuration and dimensions are fixed, the operating cell concentration is determined next. As determined in Section 3.3, cell growth mainly appeared during midday with


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higher surface light intensity. Thus, to assure CO2 consumption and O2 generation during day time, the culture concentration should allow all cells to be fully illuminated during midday. Given the dimensions of a PBR and with the highest surface light intensity fixed by the designed shading strategy, the critical cell concentration that just allows all cells to have positive net O2 evolution rate can be determined using the experimental procedures in Section 2.4.1 and 2.4.2. With reference to the gas treatment performance in the simulated diurnal cycle experiment, it has been noticed that the O2 generation rate and CO2 consumption rate are close to optimal when the cell concentration is around this critical value. Therefore, the initial and final cell concentrations can be selected to be close to the critical cell concentration for having desirable gas treatment performance. The operating period which is selected based on the expected maintenance period for microalgal cells harvest or nutrients replenishment should also be considered when selecting the operating cell concentration. By selecting a shorter operating period, the operating cell concentration range (initial and final cell concentration) can be selected to be closer to the critical cell concentration with optimum gas treatment performance. However, the maintenance cost will be higher. On the contrary, if the operating period is longer, either the operating cell concentration has to be of a wider range or the growth rate should be low, and sometimes both. In such cases, the maintenance cost will be low as well. After determining the operating cell concentration range and the operating period, the required cell growth rate can be calculated. If this desired cell growth rate is lower than the value determined under optimum temperature and CO2 availability conditions in the laboratory simulated diurnal cycle experiments (Section 3.3), the design is feasible. Otherwise, the configuration and the dimensions of the design, or the operating period need to be modified.


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Once the desired average specific growth rate is calculated, the CO2 concentration in the feed gas can be selected. It is hard to give a rigorous correlation between CO2 availability to microalgae and the cell growth rate, especially under changing illumination conditions. One method is to select the desired DIC concentration to achieve the target specific growth rate first through laboratory simulated diurnal cycle experiments (Section 2.5) carried out under different levels of CO2 supply. Then, in the designed outdoor gas treatment PBR, different gas supply strategies (loading rate and concentration) can be used to achieve the desired DIC concentration in the medium. For instance, if gas sparging is adopted to have CO2 transported from the feed gas into the medium, different gas loading rate and CO2 concentration in the feed gas could be applied to adjust the DIC concentration in the medium to the target value selected in the laboratory experiment. Another method is to select the desired CO2 supply strategy directly in a constructed PBR unit. The second method is more straightforward, but the data are completely case-specific. Definitely, the medium should also supply a sufficient amount of other nutrients required by the microalgae. In the design methodology described above, gas treatment is the primary design requirement. However, sometimes, microalgae growth would be more desirable than gas treatment. In such scenarios, an operating period and a target final microalgae concentration should be given. Then, using Equation 3, the minimum cell concentration in each operating period could be estimated if the photonic energy supply was given. 4.2. Case Study The Hong Kong Highway Department intended to build an outdoor PBR with Chlorella sp. at the Hong Kong-Zhuhai-Macao Bridge Information Center. The PBR would use solar illumination to remove CO2 from the ambient air and release O2 into the atmosphere. According


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to the design of the architects, each unit of the PBR should adopt the configuration of a bubble column with a diameter of 10 cm and a height of 150 cm, and contain 10 L culture. The PBR should continuously consume CO2 and generate O2 during an operating period of two weeks. To achieve this target, the operating parameters had been designed as follows. Since the solar illumination intensity in Hong Kong may get to as high as over 1400 µmol m-2 s-1, while the LImax for Chlorella sp. was 450 µmol m-2 s-1, proper shading should be designed to reduce the maximum surface light intensity to no more than 450 µmol m-2 s-1. In this case, roof shading of 20 cm was designed above each unit. Although this measure cannot assure the optimum light intensity as both the solar light intensity and the incident angle changes, the roof shading would at least prevent the microalgae from photo-bleaching to death. The darkness encountered during night and rainy seasons, on the other hand, will not cause lethal damage to the microalgae. According to previous experience, Chlorella sp. remains vital under dim light or complete darkness for more than 4 months, which is enough to resist the natural dark periods. It was determined in the previous sections that the optimum temperature range for Chlorella sp. was 30-35 oC, while the surviving temperature range was 20-35 oC. According to the temperature and illumination conditions in Hong Kong, the outdoor PBR would need cooling most of the year to keep the culture temperature below 35 oC. In winter, if the ambient temperature dropped to below 20 oC, heating devices could be used to keep the microalgal culture grow actively. By checking the light attenuation curve under a surface light intensity of 400 µmol m-2 s-1 (data not shown) using the method discussed in Section 3.2.2, the critical cell concentration allowing all cells in the bubble column to have positive net O2 evolution rate would be about 2.0 in terms of OD680. As shown in Figure 7, the CO2 consumption rate and O2 generation rate could be kept


20

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at a level close to the optimum for a cell concentration around the critical value such that light could penetrate into the entire culture. Thus, the operating cell concentration was selected to be from 1.0 to 3.0 in terms of OD680 during the two-week operating period, which would lead to a biomass concentration from 0.267 to 0.801 g L-1. To achieve the final cell concentration during the two-week operation, the average specific growth rate of Chlorella sp. during the day time should be around 0.003 h-1. It was known that the highest average specific growth rate Chlorella sp. could achieve under solar illumination with optimum temperature and CO2 availability conditions would be 0.09 h-1 (Section 3.3). Therefore, the desired growth rate, lower than the maximum, could be achieved by reducing the CO2 availability to a level lower than the optimum. In this case, ambient air would be used as the carbon source, where the CO2% was fixed. Thus, only the gas loading rate could be adjusted to achieve the CO2 availability that would lead to an average specific growth rate of 0.003 h-1. In this case, the desirable CO2 availability would not be selected through bench-scale experiment. Instead, at the second stage of this design, preliminary experiments will be carried out in a constructed PBR unit to directly select the proper air loading rate. If the determined loading rate is too small to keep the cells suspended in the bubble column, mechanical agitation such as propellers could be used to facilitate cell suspension. For such a PBR unit, to increase the cell concentration from 0.267 g L-1 to 0.801 g L-1 in the 10 L culture, at least 9.97 g CO2 would be consumed, because the dry microalgae biomass contained around 50% carbon by weight and the carbon should unexceptionally come from CO2.31 This equaled to treating 10 m3 polluted air (0.1 vol.% CO2, under which a person’s decision-making performance is impaired) by reducing the CO2% by half during the two-week operating period. Meanwhile, over 7.25 g O2 would be released into the atmosphere during the two weeks as the


21


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molar amount of net O2 generation is approximately equal to the molar amount of CO2 consumed.32 More accurate estimations can be made by conducting the simulated diurnal cycle experiment using a scale-down PBR. 5. Conclusions In this study, a method for designing the operating parameters of an outdoor gas treatment PBR and the corresponding control strategies based on laboratory data, such as the suitable light intensity and temperature range for microalgae growth, the light compensation point, the light attenuation trend, and the growth and gas treatment performance of microalgae in response to CO2 supply, has been proposed. The suitable light intensity range determines whether shading is necessary, while the suitable temperature range affects the temperature control strategies. For a given reactor configuration and dimensions, the desirable specific growth rate of microalgae is determined from the given batch operating period and the operating cell concentration, which is affected by the light attenuation trend in the PBR, the light compensation point of the microalgal species, and the surface light intensity on the PBR. Finally, the CO2 supply strategy should be designed to achieve the desirable specific growth rate of microalgae. In addition to the design methodology, the outdoor performance of the designed gas treatment PBR can be estimated using the novel simulated diurnal cycle experimental setup and procedures developed in this study. Thus, it allows decision makers to have more input right from the designing stage. In the case study, it was determined that the maximum surface light intensity for Chlorella sp. cultivation should not exceed 450 µmol m-2 s-1. The optimum temperature range was 30-35 oC, although the surviving temperature range was 20-35 oC. Hence, for outdoor cultivation of Chlorella sp. in Hong Kong, shading and temperature control would be required, especially during summer. Meanwhile, given a specific PBR design and a batch operating period of two


22

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weeks, the operating cell concentration was selected as 1.0-3.0 in terms of OD680 and the desirable specific growth rate of microalgae would be 0.003 h-1. The CO2 supply strategy will be designed after the outdoor PBR is built such that the desirable specific growth rate can be achieved by varying the air loading rate. Eventually, the estimated CO2 consumption rate and O2 generation rate of such an outdoor gas treatment PBR would be 9.97 g and 7.25 g per batch, respectively.


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Figure 1. Bubble column PBR and experimental setup.


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Figure 2. Experimental set-up for determining light compensation point.


25


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Figure 3. Experimental set-up for characterizing light attenuation.


26

1600

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1400 1200 1000 800 600 400 200 0 0

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8

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t (o' clock) 450 400

9000

(b)

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E (mmol m-2)

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PPFD (µmol m-2 s-1)

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0 0

2

4

6

8

10 12 14 16 18 20 22 24

t (o' clock)

Figure 4. (a) Outdoor PPFD profile of an open area in HKUST on April 16th, 2011 and (b) Simulated solar illumination on the PBR surface for the simulated diurnal cycle experiment.


27


6

(a)

5

PPFD µexp (µmol m-2 s-1) (h-1)

OD680

0 0.0312 0.0971 0

0 105 450 500

4 3 2 1 0 0

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40

60

80

100

120

Incubation Time (h) 6

(b)

0 0.0312 0.0968 0.0980 0

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µexp (h-1)

Temperature (oC)

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3 2 1 0 0

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(c)

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5

Feed Gas CO2 (%)

µexp (h-1)

4

0.035%

0.0189

7.5%

0.0980

25%

0.0932

100%

0.0258

3 2 1 0 0

20

40

60

80

100

120

Incubation Time (h)

Figure 5. Growth curves of Chlorella sp. under different operating conditions.


28

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DIC concentration (ppm)

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1200 1000 800 600 400 200 0 0

10

20

30

40

CO2 in the Feed Gas (%, v/v)

Figure 6. Equilibrium DIC concentration under different CO2% in the feed gas.


29


35

61 CO2 consumption rate = 5 mg min-1

CO2 Flow Rate (mg min-1)

30

60

Inlet CO2 Outlet CO2

25 20

59 58

O2 generation rate = 1.7 mg min-1

15

57 Outlet O2

10 5

56

O2 Flow Rate (mg min-1)

(a)

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Inlet O2

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Inlet CO2 CO2 consumption rate = 30 mg min-1

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45 Inlet O2 44

0 0

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20

25

30

35

40

45

50

Incubation Time (h)

Figure 7. Time course of the inlet and outlet CO2 and O2 flow rate in the PBR under 105 µmol m-2 s-1 at 35 oC with (a) 7.5% CO2 in the 200 mL min-1 feed gas and (b) 25% CO2 in the 200 mL min-1 feed gas.


30

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Net O2 Evolution Rate (µg s-1)

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50 40 30 20 10 0 -10 -20 -30 -40 0

10 20 30 40 50 60 70 80 90 100 110 120 130


Figure 8. O2 evolution rate under different PPFDs.


31


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

OD680 =

1.5

OD680 =

2.5

OD680 =

3.5

60 40 20 0 0

2

4

6

Culture Depth (cm)

Figure 9. Light attenuation in Chlorella sp. culture.


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5.0

(a) 4.5

Adaptation

Day I

Night I

Day II

Night II

4.0

200 mL min-1 200 mL min-1 Feed Gas Feed Gas

OD680

3.5 3.0

25% CO2

25% CO2

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No CO2

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Adaptation

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Night I

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Night II

Inlet CO2

CO2 consumption rate = 16 mg min-1

Outle CO2

80

57 55 53 51

60 49 40

Outlet O2

20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47


45

Inlet O2

0

43 0

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Figure 10. Time course of the (a) microalgae growth and (b) inlet and outlet CO2 and O2 flow rate under simulated diurnal cycles.


33


1.4 Day II, OD0680 = 1.000

1.2 1.0

X-X0 (in OD680)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8 0.6 0.4 0.2

Day I, OD0680 = 0.255

0.0 0

2

4

6

8

10

E (mol m-2) Experiment Day I

Experiment Day II

Model Day I

Model Day II

Figure 11. Correlation of microalgae growth to photonic energy supply during daytime.


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Table 1. Operating conditions during culture adaptation and simulated diurnal cycles PPFD (µmol·m-2·s-1) Adaptation 105, 14 hours Simulated Diurnal Cycles 10, 2 hours 100, 2 hours 220, 2 hours 400, 4 hours 55, 2 hours 0, 12 hours

Temperature (oC) Feed gas 35

200 mL min-1 (25% CO2)

35

Day I & II: 200 mL min-1 (25% CO2) Night I: 0 mL min-1 Night II: 200 mL min-1 (synthetic air, no CO2)

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(29) Wijffels, R. H.; Barbosa, M. J. An Outlook on Microalgal Biofuels. Science. 2010, 329, 706799. (30) Wilde, E. W.; Benemann, J. R.; Weissman, J. C.; etc. Cultivation of algae and nutrient removal in a waste heat utilization process. J. Appl. Phycol. 1991, 3, 159-167. (31) Lardon, L.; Hélias, A.; Sialve, B.; etc. Life-cycle assessment of biodiesel production from microalgae. Environ. Sci. Technol. 2009, 43, 6475-6481. (32) Ke, B. Photosynthesis : Photobiochemistry and Photobiophysics;Springer Netherlands: Dordrecht, 2003; pp 2.


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Design of Outdoor-Gas-Treatment Photobioreactors - American

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