1. Introduction
Climate changes triggered by excessive emissions of greenhouse gases (GHG) pose
the major environmental hazard today [1]. The
unceasing and successive increase in concentrations of atmospheric carbon dioxide
(CO), methane (CH), and nitrous oxide (NO), observed for many
years, has significantly accelerated adverse climate changes [2]. The phenomena
induced by the greenhouse effect directly affect the ecological homeostasis,
destroy natural ecosystems, and adversely affect both the health status of
populations and the economy [3].
CO is estimated to account for almost 65% of the total global emissions
of greenhouse gases (GHG) [4]. The concentration of atmospheric CO has
increased from approximately 310 ppm in the 1960s to over 410 ppm today [5]. This
increase is mainly due to anthropogenic activities; with the exploitation of
fossil fuels being the greatest contributor in this respect [6]. Therefore, there
is a justified need to search for possibilities to minimize CO emissions
and methods to reduce its concentration in the atmosphere. This effect can be
achieved by harnessing primary methods based on renewable energy sources or other
low-emission or zero-emission technologies for the production and use of fuels
[7]. Another way is to develop effective CO fixation methods, involving all
activities that lead to its capture and subsequent long-term storage and
deposition [8]. The most frequently described methods of CO sequestration
include mineral carbonation, CO deposition in geological structures, and
biological methods [9].
Considering economic and ecological concerns, the CO fixation methods
engaging photosynthesizing microorganisms seem to be a viable approach compared
to the physical and chemical techniques [10]. Due to the phytoplankton inhabiting
natural marine ecosystems, this process plays a key role in maintaining CO
balance in the atmosphere [11]. Marine phytoplankton accounts for half of the
global primary productivity, fixing approximately 50 gigatons CO annually
[12]. Ample studies have proved the higher efficiency of CO fixation and
biomass productivity by microalgae compared to vascular plants [13].
Thus far investigations have provided evidence for the feasibility of using
controlled systems from microalgae biomass proliferation in the processes of
wastewater and effluent treatment, waste and sewage sludge management, CO
bio-sequestration, bio-gas enrichment or exhaust gas purification [14, 15]. The
produced microalgae biomass is deemed to be a valuable raw material for producing
energy carriers and a source of many economically valuable compounds and
chemicals, which makes this technology economically and environmentally viable
[16, 17].
The dynamic development of bioenergy systems based on the use of methane
fermentation processes often poses difficulties with post-fermentation sludge
management [18]. After dehydration, the solid phase is either used as a
fertilizer or dried and used in co-incineration processes. On the other hand, the
liquid phase is difficult to neutralize due to its large volume and high
concentration of pollutants [19]. Many studies have described the possibility of
using post-fermentation leachate in microalgae biomass proliferation as a source
of biogenic compounds and microelements in the culture medium [20, 21]. The high
concentration of CO in the leachate has been proved to intensify the growth
rate of microalgae, which has a direct impact on the efficiency of pollutant
degradation [22]. So far, there have been few reports only describing the
possibility of using exhaust gases from biogas combustion in the production of
microalgae [23]. To date, this source of CO has been seen as a promising
element of photobioreactors, but these assumptions have not been supported by the
results of experimental works [24]. Therefore, there is a justified need to
assess the possibility of using waste gases generated during biogas combustion
for intensive production of microalgae biomass, and to simultaneously verify the
effectiveness of biological CO fixation. An important step of this
assessment is the selection of microalgae species that can be cultivated in a
medium containing leachate and used for the assimilation of CO from biogas
combustion. It is necessary to take into account their growth rate, resistance to
specific pollutants present in the leachate and waste gases, eurybiontic nature,
high adaptability to changing environmental conditions, and pollutant removal
efficiency [24]. Studies have shown that multicellular algae with a low growth
rate cannot be used for this purpose due to the difficulties in maintaining their
constant growth and efficiency of the purification process [25]. Instead,
microalgae are preferred, including mainly the fast-growing strains of
Chlorella sp., Scenedesmus sp., and Chlamydomonas sp.
[26].
The aim of this study was to determine the possibility of using flue gases from
biogas combustion in the production of Chlorella vulgaris biomass by
assessing the impact of this technological treatment on the efficiency of
CO removal, growth efficiency and composition of microalgal biomass, as
well as changes in chlorophyll a concentration and the effectiveness of
nutrient removal from the culture medium.
2. Materials and Methods
2.1 Experimental Design
The experiment was conducted under laboratory conditions, in two series
differing in the source of CO fed to photobioreactors (PBR). In series 1
(SA), it was atmospheric air, whereas in series 2 (SE), these were exhaust
emissions (flue gases) from biogas combustion. In both experimental series,
microalgae were cultured for 19 days.
2.2 Microalgal Biomass and Culture Medium
The experiment was carried out with the Chlorella vulgaris UTEX 2714
culture obtained from the Culture Collection of Algae (University of Texas,
Austin, USA). This taxon features a huge potential for utilizing pollutants,
including waste gases. The advantages of this species include its eurybiontic
nature, high adaptability to varying environmental conditions, resistance to
pollution, and a fast growth rate.
The cultivation bold balsam medium 3N-BBM was used in microalgae culture (Table 1). At the beginning of culture, the 3N-BBM medium and microalgae were fed to PBR
in the amount ensuring the initial concentration of Chlorella vulgaris
biomass at approximately 40 mg/dm.
Table 1.Composition of the 3N-BBM medium.
Specification |
R-r initial, g/dm |
Dose, cm |
Microelements |
R-r initial, mg/dm |
NaNO |
75.0 |
10 |
FeCl·3HO |
97 |
CaCl·2HO |
2.5 |
10 |
MnCl·4HO |
41 |
MgSO·7HO |
7.5 |
10 |
ZnCl |
5 |
KHPO·3HO |
7.5 |
10 |
CoCl·3HO |
2 |
KHPO |
17.5 |
10 |
NaMoO·2HO |
4 |
NaCl |
2.5 |
10 |
|
|
Microelements |
|
6 |
|
|
2.3 Sources of CO
Flue gases were derived from biogas combustion in a Bunsen burner
(Sigma-Aldrich, Darmstadt, Germany), with atmospheric air as the source of
oxygen. Biogas was obtained from a fermentation tank operating under mesophilic
conditions [27]. The flue gases were accumulated in a metal dome fixed above the
burner. Then, they were discharged through a snorkel to a 2.5-meter aluminum pipe
(20 cm in diameter) to get cooled, and finally were stored in tedlar bags. The
mean CO concentration in the flue gases was 13% 0.5%. In both
experimental series, the mass flux of CO to PBR was ensured at 0.054
mgCO/min. In series 1 (SA), atmospheric air was fed to PBR with the yield
of 100 cm/min (Mistral 200, Aqua Medic). In series 2 (SE), exhaust gases
were fed to the PBR using a peristaltic pump (FASTLoad Programmable control
peristaltic pump, VWR Germany) with the yield of 0.3 cm/min. In SA series,
their air which had flown through the culture medium was discharged from the PBR.
In SE series, the exhaust gases were recirculated with the peristaltic pump (VWR
Germany) with the yield of 100 cm/min, owing to which the gas volume flux
was analogous in both reactors. In SE series, the peristaltic pump was also used
to discharge the gas outside the reactor, with the yield of 0.3 cm/min.
2.4 Experimental Station
The experiment was performed in glass PBR, in which the culture medium occupied
the volume of 1.0 dm, and the gaseous phase occupied 0.3 dm. The
inlet of gases with CO to PBR was in the culture medium directly above the
bottom, whereas the gases were discharged in the upper section of PBR (Fig. 1).
Track of peristaltic pumps provided protection against gas backflow. The tube for
collection of medium and microalgal biomass samples was equipped with a valve.
The reactors had probes for pH measurements (pH meter 340/ION-Set WTW,
Oberbayern, Germany). pH was measured continuously, once a day, and the results
were sorted, averaged and recorded in a pH-meter memory. The reactors were
continuously illuminated by fluorescent lamps (T8 Luxine Plus 15W Sylvania United
Kingdom, color temperature 6500K), with the illuminance on reactor’s surface from
the light side at 2 klux. The temperature of flue gases and air fed to reactors,
and culture temperature was 20 ℃ 2 ℃.
Fig. 1.
Experimental station scheme. SA: (1) glass photobioreactor; (2)
valve supplying compressed air to the culture medium; (3) collection of
microalgal biomass samples; (4) pH measurement; (5) air feeding pump; (6) air
discharge. SE: (1) glass photobioreactor; (2) valve supplying flue gases to the
culture medium; (3) collection of microalgal biomass samples; (4) pH measurement;
(5) flue gas feeding pump; (6) flue gas discharging pump; (7) gas recirculating
pump.
2.5 Analytical Methods
The culture medium (20 cm) was collected from the PBR once a day and
determined for organic dry matter content with the gravimetric method. The
filtrated samples were analyzed for total nitrogen content (LCK Hach-Lange USA).
Chlorophyll a content was determined with the fluorescent method using
an algae online analyzer (AlgaeOnlineAnalyser – bbe Moldaenke GmbH, Germany).
Algae of a given taxonomic class possess a similar composition of photosynthetic
pigments and thus have a typical in vivo fluorescence-excitation
spectrum, whereby the emission wavelengths of the measured fluorescent light are
between 680 and 700 nm. It is thus possible to allocate an algal species to a
spectral algal class based on its fluorescence spectrum. In order to obtain a
meaningful fluorescence excitation spectrum, six LEDs were used at frequencies of
370 nm, 430 nm, 470 nm, 525 nm, 590 nm, and 610 nm, respectively. The excitation
wavelengths of the LEDs were adapted to the absorption wavelengths of the
light-harvesting pigments of different algal classes: phycocyanin, phycoerythrin,
fucoxanthin, peridinin, and chlorophyll a. The excitation of the algal
pigments was performed after dark adaptation by switching on the LEDs one after
the other at a high frequency. The fluorescence emission of the chlorophyll
a resulting from the excitation was measured in the phases between these
pulses. Spectra of different algal classes of an algal sample consisting of
cyanobacteria, chlorophytes, diatoms, dinoflagellates, and cryptophytes were
recorded. A mean excitation spectrum normalized by chlorophyll a content
(fingerprint) of an algal class was determined. Using these “fingerprints” and
a mathematical operation (best-fit procedure) enabled calculating the chlorophyll
a concentration from a complex mixture and the distribution of up to 4
different algal classes in a water sample. The fifth pre-installed class was
reserved for the detection of fluorescent yellow substances (humic substances)
and used for chlorophyll a correction. The chlorophyll determination
(calibration) was quantitatively based on an established HPLC separation method
of algal pigments [28].
Once a day, samples of gases (flue gases and air) were collected at the inlet to
and the outlet from PBR (CO, CO, NO, SO, O, N),
whereas samples of crude biogas were collected before combustion (CH,
HS, H, N, O). The quality of gases was measured using an
Agilent Technologies gas chromatograph (GC) with a TC detector (Model 7890A with
columns 6Ft 1/8 2 mm MolSieve 5A 60/80 Ultimetal, 9Ft 1/8 2 mm Porapak Q 80/100
Ultimetal) under the following conditions: detector temperature 250 °C; oven
temperature 40 °C; carriers: He 10 mL/min and N 10 mL/min; and a portable
flue gas analyzer Testo 340 (Testo Ltd., Poland) certified for compliance with
the EN 50379 standard. Inside the reactors, pH electrodes were tightly installed
for on-line measurements (WTW 340/ION-Set WTW, Oberbayern, Germany). pH was
measured continuously, once a day, and the results were averaged and saved in
analyzer memory.
At the end of the culture, microalgal biomass was subjected to quality analysis.
Contents of organic dry matter and mineral dry matter in the biomass were
determined with the gravimetric method. Biomass samples dried at 105 °C were
determined for contents of total carbon (TC), total organic carbon (TOC), and
total nitrogen (N). The above analyses were performed using a Flesh 2000
Organic Elementary Analyzer (Thermo Scientifics, USA). The content of total
phosphorus (P) was determined with the colorimetric method with ammonium
metavanadate (V) and ammonium molybdate after prior mineralization of the sample
in a mixture of sulfuric (VI) and chloric (VII) acids at a wavelength of 390 nm
using a DR 2800 spectrophotometer (HACH Lange). The content of total protein was
determined with the Kjeldahl method. The samples were mineralized in sulfuric
acid (VI) in the presence of catalysts. Protein nitrogen is converted under these
conditions to the ammonium ion which, after alkalization, is distilled as
ammonia. The ammonia content was determined by acid-base titration. The
conversion factor of 6.25 was used to convert nitrogen to protein. The content of
reducing sugars was determined with the colorimetric method with an anthrone
reagent, at the wavelength of 600 nm, using a DR 2800 spectrophotometer (HACH
Lange). Lipid concentration was determined with the Soxhlet method using an
extraction apparatus (Buchi).
2.6 Statistical Analysis
Analyses were conducted in five replications for both experimental series. The
results were subjected to one-way analysis of variance at the assumed
significance level (p 0.05). Differences between mean values were
determined with the Tukey test for honestly significant differences (HSD).
3. Results and Discussion
3.1 Changes in the Composition of Gases
The qualitative composition of crude biogas produced in the fermentation tank
and used for combustion was as follows: CH – 64.2 1.9%; CO – 35.4 2.4%; HO – 3.1 0.4%; N – 1.4
0.2%; O – 0.3 0.1%; H – 9800 1300 ppm; H –
700 140 ppm, and HS – 1500 560 ppm. Table 2 presents the
composition of flue gases supplied to the PBR in series SE. It needs to be
emphasized that CO concentration in the flue gases outflowing from the
photobioreactor in series SE was stable throughout the 19-day culture period and
reached 1.0% 0.5%. The gas assimilation rate was not correlated with
the observed growth phases of C. vulgaris biomass in PBR. Considering
that CO concentration in the exhaust gases was 13% 1.0%, the
efficiency of CO fixation in the technological system reached 0.05
mgCO/min. This technological effect was influenced by two factors, namely:
CO assimilation by the growing microalgal biomass and its dissolution in
the culture medium solution [29]. Given the high hardness of water used to
prepare the culture medium, i.e., 490 20 mg CaCO/dm, it had
significant buffering properties and a significant capability for CO
fixation by calcium or magnesium ions. This phenomenon has been earlier described
by Liu et al. (2022) [30], who optimized the growth rate of
Prymnesium parvum. In addition, PBR supply with exhaust gases in SE
series enabled the complete removal of NO and SO, whose
concentrations in waste gases were at 150 20 ppm and 1200 70 ppm,
respectively, and allowed increasing oxygen content to 21.4 0.1%.
Analyses conducted in SA series demonstrated a decrease in CO concentration
from 390 10 ppm to 310 10 ppm, an increase in oxygen content to
21.2 0.1%, and the removal of nitrogen and sulfur oxides from the air.
The effectiveness of CO removal from exhaust emissions was also described
by Jiang et al. (2013) [31] who demonstrated that the effectiveness of
CO utilization by Scenedesmus dimorphus might reach even up to
75.61%. In addition, they proved that S. dimorphus may tolerate high
concentrations of CO and NO, and that CaCO addition mitigated the
inhibiting effect of flue gases on microalgae [31].
Table 2.Composition of dried gases supplied to and discharged from PBR
in particular experimental series.
Component |
Unit |
Series |
SE |
SA |
Inflow to PBR |
Outflow from PBR |
Inflow to PBR |
Outflow from PBR |
CO |
% |
13.0 1.0 |
1 0.5 |
0.039 0.001 |
0.031 0.001 |
N |
% |
76.2 0.4 |
77.9 0.2 |
78.1 0.1 |
78.1 0.1 |
O |
% |
9.2 0.3 |
21.4 0.1 |
20.9 0.1 |
21.2 0.1 |
CO |
ppm |
1.4 0.2 |
0.0 0.0 |
0.0 0.0 |
0.0 0.0 |
NO |
ppm |
150 20 |
0.0 0.0 |
42.0 3 |
0.0 0.0 |
SO |
ppm |
1200 70 |
0.0 0.0 |
19.0 2 |
0.0 0.0 |
Other researchers [32] suggested flue gases to be a fine source of CO and
concluded that their use allowed reducing costs of microalgal culture
supplementation with other CO sources. This technological treatment may
reduce both this gas emissions to the atmosphere and the costs of chemical and
physical purification of flue gases [33, 34]. Limitations in deploying crude flue
gases are driven by their high temperature and potentially toxic pollutants they
contain [35]. The studies conducted so far have proved that only a few species of
microalgae tolerate high concentrations of SO and NO. For this
reason, the choice of species is essential to ensure high effectiveness of
CO fixation from flue gases [31, 36]. An eurybiontic and resistant to harsh
environmental conditions genus Chlorella sp. is claimed to be promising
in this respect as it ensures CO fixation rates from 0.73 to 1.79
g/dm/d [37].
A complete assessment of the effectiveness of net CO fixation by
microalgae can be made taking into account the amount of energy introduced into
the cultivation system (lighting, mixing, gas injection, separation and drainage,
nutrient dosing, etc.). This can only be reliably done in installations operated
on a technical or pilot scale, where the operating conditions are similar to
full-scale systems. This is an important aspect that determines the application
potential of each technology. Research of this kind in the novel photobioreactor
with a total volume of 30 m which required merely 100 m of land
footprint was carried out by Chen et al. (2012) [38]. These researchers
determined the potential of CO fixation in the culture of Spirulina
platensis and proved that the total capture of CO in a photoautotrophic
culture was 2234 kg CO year. However, after taking into account the annual
energy consumption of 1494 kg CO, the found that the net amount of fixed
CO in the biomass was only 740 kg CO/year. Ultimatelty, upon
deducting the energy consumption of bioreactor unit operation, the estimated
amount of CO to be fixed by a scaled-up reactor would be 74
tons/hayear [38].
3.2 Changes in Microalgal Biomass Concentration and Characteristics
The experimental series differed significantly in the rate and amount of
microalgal biomass produced. In SE series, within the first 9 days of culture, in
the logarithmic growth phase, the biomass growth rate was 77.8 3.1
mgVS/dmd, and biomass concentration reached 745 42
mgVS/dm. In the subsequent 4 days of C. vulgaris population
development, no significant changes were observed in biomass concentration, and
the culture entered into the stationary phase of growth. On day 13 of the
culture, biomass concentration peaked to 754 45 mgVS/dm and then
successively decreased in the consecutive days of culture (the decay phase). At
the end of culture in SE series, i.e., after 19 days, the concentration of
C. vulgaris reached 365 62 mgVS/dm (Fig. 2). The above
profile of C. vulgaris population development is consistent with
observations made by Lee et al. (2000) [39], who proved that the growth
rate of microalgae can be affected by tolerance of their species to the
concentrations of major inhibitory compounds (NO and SO) in flue gas.
Other authors [40] have emphasized that microalgae cannot be used for direct
CO fixation from exhaust emissions because industrial flue gases contain
approximately 100–300 ppm SO [40]. In the present study, the SO
concentration determined in SE series was substantially higher and reached 1200
70 ppm. In turn, other authors [41] have concluded that the NO
compounds have no direct impact on microalgae growth at concentrations below 300
ppm NO. The growth of microalgae may be inhibited by the excess of acidic
gases, part of which cannot be effectively consumed by algae nor dissolved in
water, producing multiple ionized H. This may lead to culture medium
acidification and, consequently, to the inhibition of microalgal population
development. Huang et al. (2016) [42] have emphasized that certain
methods are effective in mitigating the toxic effects of SO and NO on
microalgal biomass. Lee et al. (2000) [39] have reached this goal in the
case of Chlorella sp. KR-1 strain by maintaining optimal pH values and
using high concentrations of inoculating cells.
Fig. 2.
Changes in the concentration of C. vulgaris biomass in
particular experimental series.
In SA series, the rate of microalgal biomass growth determined within the first
9 days was significantly lower and reached 56.1 2.7
mgVS/dmd. In contrast to SE series, the concentration of
C. vulgaris biomass in PBR was also observed to increase within the 6
subsequent days, peaking to 735 64 mgVS/dm. But still, this value
was lower than the value determined in SE series, where it reached 780 58
mgVS/dm on day 12 of culture. In SA series, the lag phase and stable
biomass concentration in the culture medium, approximating 700 mgVS/dm,
were observed till the end of culture (Fig. 2). Sydney et al. (2010)
[43] achieved similar results in their study, where the C. vulgaris
LEB-104 biomass concentration showed exponential growth from the 96th to the
168th hour of the experiment. The maximal cell concentration, reaching 1.94
g/dm, was achieved in the last day of culture (15th day), whereas the
maximal microalgal biomass productivity reached 0.31 g/dmd [43].
The analysis of the present study results allows concluding that the C.
vulgaris population grew faster and achieved a higher final biomass
concentration in SE series. In addition, it earlier entered into the lag phase,
followed by decay phase as early as on day 14 of culture. Although the rate of
microalgal biomass production turned out to be lower in SA series, this culture
featured greater stability and significantly lesser fluctuations in biomass
concentration in PBR.
The correlations noted in biomass growth were confirmed by the observed changes
in chlorophyll a concentration in PBR. The course of these changes in
time was alike, though not identical compared to the changes in C.
vulgaris biomass concentration. No statistically significant differences were
observed in chlorophyll a concentration until day 13 of culture. The
rate of its increase and its final concentration were analogous in both
experimental series, i.e., 7056 785 g/dm in SE and
6790 258 g/dm in SA (Fig. 3). In the subsequent days
of culture, a significant and rapid decrease in chlorophyll a
concentration was observed in SE, reaching 4650 521
g/dm at the end of the culture (Fig. 3). In SA series, its
concentration remained stable, ranging from 6820 478
g/dm on day 14 to 6180 480 g/dm
on day 19 of culture (Fig. 3). The analysis of chlorophyll a
concentration in the culture medium confirmed that the short-time supply of flue
gases was a viable technological solution, while the long-term feeding of this
CO source had an adverse effect on C. vulgaris population and
contributed to a rapid decrease in microalgae count after 14 days of culture.
Also Yang and Gao (2003) [44] investigated the impact of supplying microalgal
cultures with flue gases and their effect on changes in chlorophyll a
concentration. They observed that high concentrations of bisulfites, reaching 2
mmol/dm, caused damage to chlorophyll a in B. braunii and
ascribed this toxic effect to the generation of active oxygen radicals
contributing to chlorophyll a whitening and peroxidation of membrane
lipids [44]. In turn, Vuppaladadiyam et al. (2018) [45] have emphasized
that acidic conditions may enhance the toxic effects of bisulfites, which is
related to the tolerance to SO.
Fig. 3.
Changes in the concentration of C. vulgaris biomass in
particular experimental series.
The source of CO had no significant effect on the composition and
characteristics of C. vulgaris microalgae biomass. The contents of basic
parameters characterizing the biomass were similar in both experimental series.
The content of volatile solids oscillated around 91%, that of protein
approximated 30%, whereas contents of lipids and sugars were at 19% and 37%,
respectively. Table 3 presents detailed biomass characteristics. A similar lipid
concentration in C. vulgaris culture, reaching 17.23%, was achieved by
Álvarez-Díaz et al. (2017) [46]. In turn, Yeh et al.
(2010) [47], who used a dissolved inorganic carbon source (sodium bicarbonate)
and a fluorescent light source (TL5), produced the biomass of C.
vulgaris microalgae with the following composition: 25–30% of proteins,
6–10% of carbohydrates, and 30–40% of lipids.
Table 3.Composition of C. vulgaris biomass in particular
experimental series.
Parameter |
Unit |
Series |
SE |
SA |
Volatile solids |
% dry matter |
91.1 1.2 |
90.6 2.0 |
Mineral solids |
% dry matter |
8.9 1.2 |
9.4 0.6 |
N |
mg/g dry matter |
49.4 3.1 |
50.7 3.5 |
P |
mg/g dry matter |
18.7 1.7 |
19.9 2.3 |
TC |
mg/g dry matter |
511.2 39.2 |
504.8 52.4 |
TOC |
mg/g dry matter |
469.5 11.9 |
455.2 19.6 |
Proteins |
% dry matter |
30.6 1.4 |
29.1 1.0 |
Lipids |
% dry matter |
19.4 1.7 |
18.9 0.9 |
Sugars |
% dry matter |
37.2 2.6 |
36.6 3.3 |
CO capture and utilization (CCU) is defined as the conversion of this gas
into valuable products with lower or no emissions such as fuels, chemicals,
carbon fibers, biomass, and building materials [48]. CCU should contribute even
to negative net emissions [49]. CCU with the use of microalgae is a biological
process in which CO is assimilated in the photosynthesis process [50], and
the produced biomass replaces non-renewable resources in the production of
chemicals, fuels, plastics, building materials, dyes, dietary supplements,
cosmetics, pharmaceuticals, feed, and fertilizers [51]. An example is their use
in the production of cement [52] or biochar, which, when introduced into the
soil, allows for long-term storage of CO and promotes sustainable
agriculture [53]. Another direction of deploying microalgae biomass is the
production of bioplastics [54]. These types of plastics are environmentally
friendly because they do not increase the CO pool and are more easily
biodegradable [55]. Microalgae biomass can be an alternative to other bioplastics
and replace traditional plastics or biodegradable plastics such as polylactic
acid and polyhydroxyalkanoates [56].
3.3 Changes in Nitrogen Concentration and pH Value
The more rapid development of the microalgal culture observed at the initial
period of culture in SE series was also confirmed by the analysis of changes in
the concentration of nitrogen compounds in the culture medium. A significantly
more effective consumption of this medium component was observed between day 4
and day 6 of culture. The nitrogen concentration reached 26 4
mgN/dm in SE and 38 4 mgN/dm in SA (Fig. 4). An interesting
phenomenon was observed in SE series, namely the increase in nitrogen
concentration from 6.8 2.6 mgN/dm on day 15 to 22.2 4.1
mgN/dm on day 19 (Fig. 4). This increase was correlated with the decay
process of microalgal biomass, mineralization of organic matter, and nitrogen
release to the dissolved phase. For comparison, PBR supplied with atmospheric air
allowed reaching a stable nitrogen concentration in the culture medium, fitting
within a narrow range from 5.5 2.6 mgN/dm to 5.7 3.2
mgN/dm in the analogous days of culture (Fig. 4).
Fig. 4.
Changes in total nitrogen concentration in the culture medium in
particular experimental series.
In SA series, the pH value increased significantly since the onset till day 10
of culture (Fig. 5). This increase was strongly correlated with the growth
dynamics of the C. vulgaris population and an increased microalgal
biomass concentration in PBR. This phenomenon is typical of periodical cultures,
where the increasing concentration of exometabolites produced during
photosynthesis leads to culture medium pH increase. Once the threshold
concentrations are achieved and pH value increases substantially, the growth of
the microalgal population is firstly diminished and then ultimately inhibited. In
SA series, the intensive production of the microalgal biomass was inhibited
around day 11 of culture, as manifested by the recorded concentrations of biomass
and chlorophyll a. Afterward, the pH value reached 8.81 0.4
(Fig. 5). In the subsequent days of culture, the concentration of C.
vulgaris biomass remained stable, thereby limiting dynamic pH changes. At the
end of SA culture (day 19), the pH value reached 9.15 0.2 (Fig. 5).
Fig. 5.
Changes in culture medium pH in particular experimental series.
The pH values recorded in SE series were lower in the entire culture period.
Analogous pH changes to SA series were observed at the microalgal population
development and growth phases. The pH value increased from 7.08 0.2 at
the beginning of the experiment to 8.12 0.4 after 14 days of PBR
operation (Fig. 5). In the subsequent days, the SE series culture was observed to
decay, which was reflected in reduced production of exometabolites by microalgae
and pH decrease to 7.9 0.2 (Fig. 5). The lower pH values recorded in SE
series were also affected by CO source. Even though the total CO
concentration was the same in both experimental series, the concentration of
carbon dioxide was higher in exhaust emissions. A greater difference of
concentrations contributes to faster diffusion, penetration, and dissolution of
CO in the culture medium, which caused pH to decrease when the buffering
capability had been depleted. The pH of the culture is an important effector of
the microalgal CO-concentrating mechanism [57]. Valdes et al.
(2012) [58] have demonstrated that the pH profile provides information about the
behavior of the microalgae/photobioreactor system in terms of CO
consumption effectiveness.
3.4 Algal CO Fixation—Limitations and Challenges
Effective CO biosequestration in the systems for intensive
microalgae biomass production raises multiple controversies due to the lack of
carbon neutrality and, in many cases, even a positive carbon footprint of this
type of technology, as earlier proven [59]. As autotrophic organisms, microalgae
fix CO through photosynthesis, which should directly affect the low or none
carbon footprint and high handprints. It takes place in natural ecosystems, where
the growth of phytoplankton population occurs without additional elements
intensifying the rate of biomass production. However, many technological
solutions implemented for the industrial processing of microalgae are based on
specialized installations, like photobioreactors; heating and lighting systems;
devices for mixing and for dosing a carbon dioxide source and nutrients; as well
as solutions for biomass thickening and dehydration [60]. These solutions require
energy consumption both for the production of components and for the operation
and service of the technology. What is more, their carbon footprint can be high.
This is especially important in climatic zones with low temperatures and poor
insolation. Therefore, it is necessary to strive for implementing simple,
energy-saving installations, and for identifying and optimizing solutions
operating similarly to natural ecosystems.
Ekendahl et al. (2018) [61] have demonstrated that the year-round
cultivation of microalgae under natural conditions is possible in the far north,
like Borås in Sweden, thanks to the supply of heat from waste heat and carbon
from the flue gases of pulp and paper mills. Due to the low photosynthesis
efficiency of only 1.1%, the biomass was collected only once a year. Despite the
relatively low photosynthesis efficiency, research works have documented
applicable practices for carbon biosequestration in microalgae cultivation, even
at higher latitudes. The trophic relationships (autotrophy, heterotrophy and
mixotrophy) occurring in local microalgae consortia were determined and
technological recommendations for CO capture, reduction of energy
consumptionm and minimization of carbon footprint were developed [61].
Other studies have proven the possibility of energy-efficient cultivation of
microalgae in northern latitudes. The production system achieved a photosynthetic
efficiency of 1.1% net, an energy index (NER) of 0.25, and the predicted annual
energy biomass yield from the area was 5.2 times higher than the respective
oilseed rape production [62]. Energy-efficient production of microalgae in a cold
continental climate has been proven as well. The recovered biomass had high
calorific values of 20–23 MJ kg and contained 14–19% of oil with a
predominance of C16 and then C18 fatty acids. The presented technological
solution was found utile for carbon sequestration and energy storage in biomass
[62]. In turn, Deprá et al. (2019) [63] investigated new bioreactor
designs to maximize carbon mass transfer from the culture medium to the biomass
or microalgae metabolites. A hybrid photobioreactor containing a bubble column
and an ‘illumination platform’ was designed, as inspired by recent advances in
biofilm culture strategies. The configuration of the model resulted in an average
CO conversion rate of 45.32 kg CO/m/d, but only 1.28% of
CO was incorporated into the biomass. Most of the converted CO, i.e.,
82.75% carbon mass transfer, was consumed for the synthesis of volatile organic
compounds [63].
The carbon footprint of each subsequent stage of microalgae processing
(harvesting, dehydration, upgrading) needs to be balanced so that the end
products are emission-negative, i.e., have a positive net carbon footprint. The
Life Cycle Analysis (LCA) is the tool to balance CO emissions and energy of
any biologically-mediated carbon capture and utilization (bio-CCU) system. In
this way, it is possible to verify the carbon footprint. The development and
implementation of sustainable practices and energy-saving technologies is
essential for the development of CO-neutral microalgae biorefineries [60].
Undoubtedly, the use of cheap sources of nutrients and CO is an important
element allowing to increase energy and economic efficiency. Therefore, it seems
advisable to conduct research on the possibility of using waste CO,
including that from biogas combustion installations, as a source of this
microalgae biomass production-limiting chemical compound.
4. Conclusions
Exhaust emissions from biogas combustion may be deployed to intensify the
culture of C. vulgaris species microalgae. The use of this CO
source (series SE) caused a higher rate of biomass growth in the acceleration and
logarithmic growth phases, reaching 77.8 3.1 mgVS/dmd. In
addition, it enabled producing a higher concentration of microalgae, i.e., 780
58 mgVS/dm.
Nevertheless, it needs to be emphasized that the C. vulgaris culture
supplied with flue gases turned out to be very sensitive and after a few days of
the stationary phase rapidly entered into the decay phase. This phenomenon
enforces strict control and monitoring of microalgal biomass production as well
as rapid responses in flue gas-fed systems. This is an important hint for
potential operators of such technological systems on the large scale. In turn,
the culture fed with atmospheric air as a CO source was far more stable and
featured a long phase of stationary growth.
The course of the C. vulgaris culture and the observed changes in
biomass concentration were correlated with changes in chlorophyll a
concentration, culture medium pH, and effectiveness of nitrogen consumption by
the microalgae. PBR feeding with flue gases had no significant effect on biomass
characteristics in terms of contents of organic substances, including lipids,
proteins, and sugars.
C. vulgaris biomass effectively assimilated CO from the
emissions, and its concentration recorded before the decay phase decreased from
13% in crude flue gases to 1% in the effluent from the photobioreactor. The
concentration of CO in gases fed to PBR approximated 1% throughout the
culture period.
It should be understood that the complete assessment of the effectiveness of net
CO fixation by microalgae can be made taking into account the amount of
energy introduced into the cultivation system. By converting the energy consumed
into the CO produced, it is possible to determine the net reduction of this
gas. This can only be reliably done in installations operated on a technical or
pilot scale, where the operating conditions are similar to full-scale systems.
Determining the size of the carbon footprint by means of a properly conducted LCA
is a prerequisite for recognizing the technologies based on the production of
microalgae biomass as those enabling the real CO sequestration. The
possibility of long-term storage of carbon in the microalgae biomass by
developing products that can be used in products or building materials is
important as well. Regardless of the possibility of deploying microalgae to fix
and utilize CO, a justified avenue of research is to look for cheap sources
of CO-rich gases.