CO2 REMOVAL FROM MUNICIPAL WASTE INCINERATOR FLUE GAS AND
BIOMASS PRODUCTION BY MICROALGAE
I. Douskova1, J. Doucha1, P. Novak2, D. Umysova1, M. Vitova1, and V. Zachleder1
1
Department of Autotrophic Microorganisms, Institute of Microbiology, Academy of Sciences
of the Czech Republic, Trebon
2
Termizo Inc., Liberec, Czech Republic
Summary
A municipal waste incinerator flue gas was used as a source of CO2 for cultivation of
the alga Chlorella vulgaris, in order to decrease production costs of the biomass and to
bioremediate CO2 simultaneously. The following results were obtained during the
experiments: The utilization of the flue gas for a photobioreactor agitation and CO2 supply
was proved to be convenient. A growth rate of algal cultures on the flue gas was even higher
when comparing with the control (pure CO2 in air). The toxicological analysis of the produced
biomass showed only a slight excess of mercury while all the others compounds (other heavy
metals, PAHs, PCDD/Fs and PCBs) were below the limits required by foodstuff legislation.
Introduction
Microalgal biomass is a valuable product, which is nowadays widely used in
pharmacy, cosmetics, and as food and feed supplement. Only its relatively high price causes
that it is not used as a source of single cell protein in a large scale. The microalgal biomass
contains all essential amino acids, unsaturated fatty acids, carbohydrates, dietary fibre, and a
whole range of vitamins and other bioactive compounds. In order to achieve high productivity
of microalgal biomass, it is necessary to supply the culture with a sufficient amount of carbon
dioxide. However, when the concentration of dissolved CO2 in culture medium is too high,
inhibition by substrate (CO2) may occur. Generally, the concentration of 2 vol. % of carbon
dioxide in the mixture with air is considered to be optimal (1). Consequently, the CO2
represents a considerable part of the running costs when producing microalgal biomass (2). In
order to reduce the expenditures, a flue gas originating from a municipal waste incinerator
was used as a source of carbon dioxide for the cultivation. From another point of view, the
cultivation of microalgae can be regarded also as an additional step in a flue gas treatment,
which decrease the concentration of CO2 in exhaust flue gas. Similar approaches have already
been applied in case of flue gas from coal power plants (3, 4), industrial heater using kerosene
(5) or simulated flue gas (6). However, these studies suggested only a subsequent burning of
the produced biomass, without considering its value added exploitation. Our aim was to
determine the effect of flue gas from municipal waste incinerator on the growth of microalgae
and also to assess the level of accumulation of possible contaminants in the biomass so as it
can be used for nutritional purposes.
Material and methods
Photobioreactor: A set of glass bubbled columns (inner diameter 36 mm, height 500
mm.) placed in a thermostatic bath and continuously illuminated by fluorescent tube panel
(light intensity 1150 E.m-2.s-1). The columns were “aerated” by cooled flue gas (10 to 13 vol.
% of CO2, 8 to 10 vol. % of O2) or by a mixture of air and pure CO2 as the control cultivation
(2 vol. % of CO2, balance air).
Municipal waste incinerator: The plant burns 90,000 tonnes of waste per year and it is
equipped with up-to-date technology of flue gas treatment. It has been in operation since
1999.
Microorganism: A selected strain P12 of the single-celled fresh water alga Chlorella
vulgaris from the collection of our laboratory was used. This strain has a high specific growth
rate (more than 0.25 h-1) and it is able to grow under a high concentration of CO2.
Culturing medium: (NH2)2CO 550 mg.L-1, KH2PO4 118.5 mg.L-1, MgSO4.7H2O 102
mg.L-1, C10H12O8N2NaFe 19.8 mg.L-1, CaCl2 44 mg.L-1, H3BO3 42 g.L-1, CuSO4.5H2O
47g.L-1, MnCl2.4H2O 164g.L-1, (NH4)6Mo7O24.4H2O 9g.L-1, ZnSO4.7H2O 134g.L-1,
CoSO4.7H2O 30g.L-1, (NH4)VO3 0.7 g.L-1 in distilled water
Culturing conditions: The experiments were carried out in a fed-batch regime, each
experiment in four replicate columns. The concentration of suspended algal biomass was
determined daily both by optical density measurement (750 nm) and dry biomass weight
determination. Dissolved O2 and CO2 in the cultures were measured, too. The pH of cultures
was kept in the range 6.5 - 7.5 by addition of 1 M NaOH. Culturing temperature of 30°C (Fig.
1) was found to be more suitable than 25; 35 or 40°C (data not shown). Distilled water was
added daily in order to eliminate the effect of water evaporation during the cultivation.
Analyses: The biomass was harvested after the cultivation by centrifuging the cell
suspension at 5000 rpm for 5 min and it was freeze-dried. The analyses of possible
contaminants were carried out in an accredited laboratory AXYS VARILAB s.r.o., CZ and
Research. Centre for Environmental Chemistry and Ecotoxicology, Masaryk University, CZ)
Results and discussion
The features of the selected strain of Chlorella vulgaris enable its cultivation on
cooled flue gas containing 10-13 vol.% of CO2 without additional treatment of the gas. It was
proved that the growth rate of this alga, using the flue gas as a source of CO 2, was the same or
even higher in comparison with the growth of a control culture supplied by mixture of air and
pure CO2 (Fig. 1). This fact can be explained by a lower partial pressure of O2 in the flue gas
comparing to air. The levels of toxic compounds present in the flue gas were so low that no
growth inhibition was observed. During the linear growth phase (the interval from the 30th to
the 130th hour of cultivation), which is typical for culturing photosynthetic microorganisms
under constant illumination, the carbon dioxide fixation rate of 4.4 g CO2.L-1.day-1 was
achieved for the flue gas comparing with 2.7 g CO2.L-1.day-1 for the control and 0.65 to 4.0 g
CO2.L-1.day-1 published (7).
16
14
Control 30°C, 2 vol.%
12
-1
Dry weight (g.l )
Fig. 1. Growth curves of the alga
Chlorella vulgaris on the real flue gas
and on the mixture of air and pure CO2.
Culturing temperature 30 °C
Flue gas 30°C, 11 vol.%
10
8
6
4
2
0
0
20
40
60
80
100
120
140
160
Cultivation time (hours)
The analysis of the biomass showed that it complied with the requirements of
European food law (8, 9) in almost all parameters. The only exception was a slight excess of
mercury (Tab. 1), which can be eliminated by installation of an already tested additional step
in the flue gas treatment.
Contaminant
Legal limit
(mg.kg-1)
As
Sn
Al
Cr
Cd
Cu
Ni
Pb
Hg
Zn
PAHs
PCBs
benzo[a]pyren
PCDD/Fs-TEQ
PCDD/F-PCBs-TEQ
0.2
50
10.
4.
0.2
80
6
0.2
0.5 (max. 1)
80
0.01
2
0.001
4 × 10-6
8 × 10-6
Control
biomass
(mg.kg-1)
0.017
0.161
6.76
0.11
0.013
32.5
0.23
0.10
0.074
41.1
<3.4× 10-4
0.001
<2.0 × 10-5
1.00 × 10-7
2.00 × 10-7
Control
biomass
(% of limit)
9
0
68
3
7
41
4
50
15
51
<3
0
<2
3
3
Flue gas
biomass
(mg.kg-1)
0.015
0.155
9.80
0.17
0.011
30.5
0.22
0.12
1.560
35.3
<2.0× 10-4
0.001
<2.0 × 10-5
3.00 × 10-9
1.03 × 10-7
Flue gas
biomass
(% of limit)
8
0
98
4
6
38
4
60
312
44
<2
0
<2
0
1
Tab 1. The amounts of toxicologically relevant compounds in the biomass of Chlorella
vulgaris cultivated on the mixture of air and pure CO2 and on the real flue gas. PAHs =
polyaromatic hydrocarbons, PCBs = polychlorinated biphenyls, PCDD/Fs = polychlorinated
dibenzodioxins and dibenzofurans, PCDD/F-PCBs = polychlorinated biphenyls with dioxin
effect, TEQ = toxic equivalent
Conclusion
It was proved that the application of the municipal waste flue gas as a source of CO2
for microalgal biomass cultivation is feasible. In spite of meeting the foodstuff legal limits, it
is expected that the “waste” origin of the biomass could lead to the rejection of the product by
customers. That’s why the non-nutritional utilization of the biomass should be considered.
There are great possibilities of its exploitation in the field of biofuels, because an important
advantage of the microalgae is the possibility to influence their relative content of lipids,
starch and proteins via different culturing conditions.
References
1.
R. A. Galloway et al., Plant Physiology 39, R8-& (1964).
2.
J. R. Benemann, Energy Conversion and Management 38, S475-S479 (1997).
3.
K. L. Kadam, Energy 27, 905-922 (2002).
4.
K. Maeda et al., Energy Conversion and Management 36, 717-720 (1995).
5.
S. R. Chae et al., Bioresource Technology 97, 322-329 (2006).
6.
L. M. Brown, Energy Conversion and Management 37, 1363-1367 (1996).
7.
N. Kurano et al., Energy Conversion and Management 36, 689-692 (1995).
8.
COMMISSION REGULATION (EC) No 1881/2006 of setting maximum levels for
certain contaminants in foodstuffs (19 December 2006).
9.
VYHLÁŠKA č. 305/2004 Sb., kterou se stanoví druhy kontaminujících a toxikologicky
významných látek a jejich přípustné množství v potravinách (2004).
This work was financially supported by the project EUREKA of Ministry of Education,
Youth and Sports of the Czech Republic (no.OE221), Grant Agency of ASCR (grant no.
A600200701) and by Institutional Research Concept no. AV0Z5020903.