JOURNAL OF FERMENTATION AND BIOENGINEERING Vol. 81, No. 5, 470-472. 1996

Carbon

TECHNICAL NOTE

Dioxide Fixation in Batch Culture of ChZoreZla sp. Using a Photobioreactor with a Sunlight-Collection Device

SATOSHI HIRATA,‘*v2 MASAO HAYASHITANI,‘*2 MASAHITO TAYA,3 AND SETSUJI TONE3

Akashi Technical Institute, Kawasaki Heavy Industries Ltd., Akashi, Hyogo 673,’ Research Institute of Innovative Technology for the Earth, Kizu, Kyoto 619-02,2 and Department of Chemical Engineering, Faculty of

Engineering Science, Osaka University, Toyonaka, Osaka 560,j Japan

Received 16 August 1995/Accepted 5 February 1996

In a batch culture of Chlorella sp. using sunlight as a light source, the cell concentration reached a maximum of 150 mg dry cells dme3 at 200 h. The mean rate of CO2 fixation during the culture was 31.8 mg CO2 dmm3 d-l. The efficiency of conversion of energy to biomass was estimated as 4.3%.

[Key words: photobioreactor, CO2 fixation, sunlight utilization, Chlorella sp.]

In recent years, many attempts have been made to reduce the quantity of CO2 in the atmosphere. Studies

lenses was transmitted via 9 quartz optical fibers (0.8 mm in diameter and 3 m in length), a connecting joint and

on photosynthesis, the biological method for CO* fixa- 133 plastic optical fibers (1.5 mm in diameter and 1.5 m tion, and utilization of microalgal biomass have been carried out (l-3). At present, open-pond type bioreac-

in length) to an illumination plate. The configuration of

tors utilizing sunlight as a light source are being used the illumination plate is shown in detail in Fig. 2. The plate was made of Pyrex glass, the surface of which was

for the production of fodder and foodstuff from cultured microalgae (4). However, problems involving the control

frosted to minimize the dispersion of light in the fermen- tor. The irradiance area of this plate was 0.12m2. The

of culturing conditions, the evaporation of medium and fermentor which was made of Plexiglas was 320mm the low availability of CO2 can be encountered when using these bioreactors. In addition, the light intensity

high, 240mm wide, 46 mm deep and had a working volume of 3 dm3. All surfaces of the fermentor were cov-

decreases with increasing medium depth in these bioreac- tars, since sunlight shines only on the reactor surface.

ered with aluminum foil to prevent light from entering the fermentor.

In the present study, cultivation of the microalga, Chlorella sp., was carried out using a photobioreactor in which the collected sunlight was channelled to the algal cells via a specially designed illumination plate. The aims of this study were to examine the efficiency of light collection, the efficiency of light transmission to the algal cells, the rate of CO* fixation by the alga, and the effi- ciency of photosynthetic conversion using the illumina- tion plate in the photobioreactor.

To supply the carbon source and to agitate the medi- um, a gaseous mixture, CO2 : O2 : N2= 10 : 3 : 87 (v/v), was sparged from a glass tube (1.5 mm bore) at a con- stant flow rate of 0.5 dm3 min I. The tube was inserted vertically into the fermentor along one corner with the tube outlet located 10mm from the bottom of the fer-

Chlorella sp. UKOOl, a unicellular, green freshwater microalga with a mean diameter of 4 pm, was used in this study. The microalga could grow in an atmosphere containing 0.03 to 40% CO1 and the optimum pH and temperature for growth were pH 5.5 to 6.0 and 3O”C, respectively (Hirata, S. et al., Abstr. Autumn Meeting Sot. Chem. Engrs. Japan, vol. 1, p. 334, 1995). The microalgal strain was provided by the Research Institute of Innovative Technology for the Earth, Kyoto. All cultures were carried out using C medium (5) at 30°C without sterilization. Cultures did not become highly contaminated during experiments.

collection device

4---) To illumination plate

I

<OF Outdoors Indoors

A schematic of the experimental apparatus is shown in Fig. 1. A sunlight-collection device (Himawari XD-50, La Foret Engineering Co., Tokyo), which was placed on a stand located outdoors, automatically tracked the sun using a sensor and collected the sunlight. The device was equipped with 18 Fresnel lenses which were divided into 2 sets of 9 lenses. The light collected by the 9 Fresnel

* Corresponding author.

FIG. 1. Schematic of the photobioreactor system. 1, Sunlight- collection device with 18 Fresnel lenses; 2, 9 quartz optical fibers; 3, plastic joint; 4, 133 plastic optical fibers; 5, illumination plate; 6, fermentor for cultivation; 7, fermentor for measurement of light intensity; 8, quantum sensor; 9, thermostatically controlled water bath; 10, glass tube for aeration; 11, gas mixer; 12, pH electrode; 13, recorder; 14, plastic tube; 15, stainless steel tube.

470

VOL. 81, 1996 TECHNICAL NOTES 471

10 mm

t

2

200 mm E

.I.. L200 mmG lmm

Cross-sectional view Front view Side view

FIG. 2. Configuration of the illumination plate. 1, Illumination plate made of Pyrex glass; 2, 133 plastic optical fibers covered with stainless steel plates; 3, 133 plastic optical fibers covered with a stain- less steel tube; 4, 133 plastic optical fibers; 5, stainless steel plates; 6, stainless steel tube.

mentor. The composition of the gaseous mixture was determined based on the composition of exhaust gas from boilers in a power plant.

Cultivation of Chlorella sp. was started by the addi- tion of 0.1 dm3 of preculture to 3 dm3 of C medium and was carried out from September 13 to 21, 1994 in Akashi, Japan.

The microalga was also cultivated at 30°C in a 1 dm3 Roux flask (medium: 0.2 dm3) shaken at 50rpm on a rotary shaker (R-20, TAITEC Corp., Saitama). A gaseous mixture (CO2 : 02 : N2= 10 : 3 : 87 (v/v)) was supplied to the medium in the Roux flask at a constant flow rate of 0.05 dm3 min *. The flask was continuously illuminated by 5 white fluorescent lamps (FL-15, Matsushita Electric Industrial Co., Osaka) or by a xenon lamp (UXL-SOOD- 0, Ushio Inc., Tokyo).

The concentration of algal cells was measured in terms of optical density at 680nm and the cell concentration was converted into dry weight per dm3 of culture by a correlation factor.

The sunlight intensity was measured using an LI-

Outdoor light intensity (W m-2)

FIG. 3. Correlation between the outdoor light intensity and the light intensity inside the fermentor.

160

0 0 50 100 150 200

Cultivation time (h)

FIG. 4. Time course of Chlorda sp. UK001 culture in the photobioreactor. Symbols: l , cell concentration; 0 , outdoor light intensity; A, light intensity measured inside the fermentor.

190SB quantum sensor (LI-COR Inc., Lincoln, USA) placed horizontally on a stand. The light intensity inside the fermentor was measured using a QSL-100 quantum sensor (Biospherical Instruments Inc., San Diego, USA) placed 1Omm from the center of the illumination plate in an empty fermentor which was of the same composi- tion and dimensions as the fermentor used to culture the algal cells to avoid the effect of light attenuation by algal cells. The dispersion of light in the fermentor was less than 10%. In both the above measurements (detected wavelength range: 400 to 700 nm), the light intensity was expressed as the average value determined for 2 h.

After cultivation, the algal cells were centrifuged at 12,OOOxg at 4°C for 15 min. The harvested cells were dried under reduced pressure, and were then subjected to elemental analysis. Carbon, hydrogen and nitrogen contents were measured using an MT-3 Ultramycro Elemental Organic Analyzer (Yanagimoto Mfg. Co., Kyoto). Phosphorous and sulfur contents were measured using an AP2 automatic sulfur determination apparatus (Yoshida Kagaku Kikai Co., Osaka), and ash content was measured after burning the cells at 815°C. The calorific value of the biomass was measured with an automatic bomb calorimeter (CA-3, Shimadzu, Kyoto).

Light is the most important factor in the process of CO* fixation by microalgae. The efficiency of light collec- tion and transmission in the photobioreactor system was first examined. The correlation between the outdoor light intensity and the light intensity inside the fermentor is shown in Fig. 3. The efficiency of light collection and transmission to the algal cells in this apparatus was approximately 8% from the slope of the correlation line. At lower light intensities, the efficiency was reduced to some extent, which may have arisen from the en- trance of light rays normal to the lens surfaces of the sunlight-collection device.

TABLE 1. Elemental analysis of Chlorella sp. UK001 cells

Element (wt. %) C H 0 N s P Ash

54.0 8.6 31.0 3.3 0.36 0.95 2.1

472 HIRATA ET AL. J. FERMENT. BIOENG.,

TABLE 2. Comparison of cultures of ChloreNa sp. UK001 in photobioreactor and Roux flasks illuminated with a fluorescent or xenon lamp

Light intensity Irradiance Culture Mean rate of Mean rate of Maximum cell Reactor Light source area time cell growth COZ fixation concentration

(W m-r) (m2) (h) (mg dry cells dn-’ d-r) (mg COr drn-j d-r) (mg dry cells dm-r)

Photobioreactor Sunlight O-15.7” 0.12 200 16.1 31.8 150 Roux flask Xenon lamp 59.9b 0.018 45 368 728 694 Roux flask Fluorescent lama 71.4b 0.018 46 437 865 842

a Value inside the fermentor. b Value at the surface of Roux flask.

The average light intensity inside the fermentor was 9.3 W me2 on a day when sunlight intensity was the highest during the culture period. A light intensity of 10.8 W me2 in a fermentor with internal illumination of optical fibers using a metal-halide lamp as the light source was reported by Takano et al. (6), which is only slightly higher than the light intensity in our fermentor.

The results of cultivation and elemental analysis of the alga are shown in Fig. 4 and Table 1, respectively. This alga was highly dispersed in the culture medium, and did not adhere to the wall of the fermentor and the surface of the illumination plate during cultivation. The initial concentration of the algal cells was 16 mg dry cells dm-3 and the final cell concentration was 150 mg dry cells dmP3 after 200 h cultivation. The mean growth rate was 16.1 mg dry cells dmP3 d-l. Since COz was the sole car- bon source and the carbon content of the biomass was 54%, the average rate of COz fixation was calculated as 31.8 mg CO2 dmP3 d-l.

Table 2 compares the culture conditions, cell growth rate, CO2 fixation rate and maximum cell concentration of Chlorella sp. UK001 in the photobioreactor and in Roux flasks illuminated by a fluorescent or xenon lamp. The mean rates of cell growth and CO2 fixation obtained from the photobioreactor culture were lower than those from the Roux flask cultures. These data indicate that the main limiting factor for algal growth was the light energy in the culture with the photobioreactor. A new type of apparatus is now under investigation to improve the efficiency of light collection and light transmission to the algal cells.

The elemental composition and calorific value of the alga were analyzed after cultivation. The contents of hydrogen, oxygen and nitrogen were 8.6, 31 and 3.3%, respectively. The calorific value of the alga, Eb (J per g dry cells), was 2.40~ 104 J g-l, which was larger than that of a land plant (1.78 x lo4 J gg’) (7).

In the present study, the efficiency of photosynthetic conversion on a radiated light basis, E, (%), was defined by the following equation, assuming that the light is completely absorbed by the culture.

E = 100Eb.AX c E (1)

where AX (g)= total amount of biomass produced and E (J)=total light energy channelled via the illumination plate to the culture. The value of E was calculated from the following equation.

E=A (Ildt (2)

where A(m2) =irradiance area, Z(W md2) = light intensity inside the fermentor and t(s)=culture time. In the cul- ture shown in Fig. 4, 4.3% of E, was obtained from Eqs. 1 and 2 using Et,=2.40x 104 J g-l, AX=O,402g, A=0.12m2 and t=7.2x lo5 s. Considering that E, of crops such as corn, rice and soybeans is 5.0 to 9.2% during active propagation (7), the value estimated in the algal culture is thought to be reasonably high.

In conclusion, a microalga, Chlorella sp. UKOOl, was cultivated in a photobioreactor using sunlight as a light source. The average rate of COz fixation was 31.8 mg CO2 dm-3 d-l and E, was estimated as 4.3%. Methods to improve the CO2 fixation rate and the efficiency of energy utilization in a photobioreactor system need to be further investigated.

This work was supported by a grant from the New Energy and Industrial Technology Development Organi- zation (NEDO), Tokyo.

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