Life Cycle Assessment of Microalgae Production in a Sanitary
Effluent Medium Supplemented with Glycerol
Eduarda Torres Amaral
a
, Tiele Medianeira Rizzetti
b
, Michele Hoeltz
c
,
Gleison de Souza Celente
d
, Lisianne Brittes Benitez
e
and Rosana de Cassia de Souza Schneider
f
1
Environmental Technology Post-graduate Program, Universidade de Santa Cruz do Sul,
Av. Independência, 2293, Santa Cruz do Sul, Brazil
lisianne@unisc.br, rosana@unisc.br
Keywords: Microalgae, Wastewater Treatment, Glycerol, LCA, Environmental Impact, Chlorella sp.
Abstract: Microalgae are considered renewable and sustainable raw materials. They can be cultivated in wastewater,
enabling its treatment for disposal into water bodies due to the sequestration of residual nutrients such as
nitrogen and phosphorus. The biomass obtained can be used to develop biofuels and biosupplies. Recognizing
the production potential of Chlorella sp. in an alternative culture media, this work aimed to evaluate the major
environmental impacts on biomass production cultivated in three scenarios with NPK solution, effluent, and
effluent with glycerol supplementation. Life Cycle Analysis was performed using data from a 20 L production
scale. The most impacted categories in the process were the production of carcinogenic and non-carcinogenic
pollutants, inorganic respiratory pollutants, ecotoxicity, land acidification, land occupation, global warming,
and non-renewable energy use. Given the best environmental results, cultivation in effluent with glycerol
supplementation led to fewer environmental impacts on Chlorella sp. cultivation since it showed higher
biomass yield than the other two scenarios.
1 INTRODUCTION
Microalgae have been widely studied as a great
potential raw material for the production of biofuels
and biosupplies (Cheng et al. 2019, Do et al. 2022),
e.g., biodiesel, dietary supplements, nutraceuticals,
cosmetics, animal feeds, and pharmaceuticals
(Molazadeh et al. 2019, Fawcett et al. 2022).
Microalgae also can grow in wastewater and
produce low-cost biomass while removing or
consuming organic and inorganic nutrients from
wastewater, making them a sustainable alternative
(Singh et al. 2020).
These organisms offer economic
and environmental advantages, including
photosynthetic efficiency, high growth rate, and CO
2
sequestration (Muhammad et al. 2021, Li et al. 2022).
a
https://orcid.org/ 0000-0002-1140-9173
b
https://orcid.org/0000-0001-6539-1397
c
https://orcid.org/ 0000-0002-6156-2600
d
https://orcid.org/ 0000-0002-9314-2782
e
https://orcid.org/ 0000-0002-0604-1407
f
https://orcid.org/ 0000-0003-1400-8401
In addition, wastewater from various sources such
as agriculture, households, and industries can contain
anthropogenic pollutants that microalgae can
remediate, which otherwise would pose risks to
human health and the environment (Mofijur et al.
2021, Ahmed et al. 2022).
In this sense, microalgae’s benefits to the
environment are unquestionable since they can bring
benefits during growth and biomass use. On the other
hand, biomass production involves inputs and outputs
that potentially impact each life cycle, related to the
need for energy, inputs, equipment sanitization,
separation, and drying. Thus, considering the entire
process, there is an impact on the life cycle of
industrial production of microalgae, whether in a
conventional environment or with effluents. Life
Cycle Assessment (LCA) is a valuable tool to
Amaral, E., Rizzetti, T., Hoeltz, M., Celente, G., Benitez, L. and Schneider, R.
Life Cycle Assessment of Microalgae Production in a Sanitary Effluent Medium Supplemented with Glycerol.
DOI: 10.5220/0011905800003536
In Proceedings of the 3rd International Symposium on Water, Ecology and Environment (ISWEE 2022), pages 77-82
ISBN: 978-989-758-639-2; ISSN: 2975-9439
Copyright
c
2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)
77
determine the environmental impacts associated with
the production of microalgae, evaluating emissions,
energy, and resource consumption (Sun et al. 2019,
Wu et al. 2019), highlighting critical points in the
production process and comparing scenarios (Herrera
et al. 2021).
In the context of the production of the microalgae
Chlorella sp. in an effluent medium with and without
glycerol (organic carbon source) supplementation,
the effluent treatment was investigated, showing the
benefit for the water treatment by this biological
method and the importance of this biomass as a
source of lipid (Amaral et al. 2022). This research
seeks to recognize, from the experimental research
already conducted, the potential environmental
impacts of the production process in comparison with
the use of NPK solution, an agricultural input widely
used for microalgae biomass production.
2 SCOPE AND DELIMITATION
OF THE SYSTEM
This LCA study aimed to quantify the potential
environmental impacts of microalgae cultivation in
sanitary effluent with and without glycerol
supplementation, promoting the bioremediation of
the effluent and enhancing the production of
microalgal biomass compared to the production in
NPK-rich medium. Thus, three scenarios were
compared in this LCA research for Chlorella sp.
production:
- Scenario 1: Microalgae cultivation in 3 g L
-1
NPK
solution according to Pacheco et al. (2019);
- Scenario 2: Microalgae cultivation in sewage
effluent;
- Scenario 3: Microalgae cultivation in sanitary
effluent with glycerol supplementation;
In all three scenarios, the steps were the same, using
the same equipment for cultivation, harvesting, and
drying. What differs one from the other is the culture
medium and biomass yield for the same production
duration.
The LCA of microalgal biomass production
grown in sanitary effluent with and without glycerol
supplementation was performed at bench-
photobioreactor experiments (Figure 1) with sanitary
effluent from the Wastewater Treatment Plant
(WWTP/UNISC) obtained after anaerobic digestion.
The NPK (Yara, 12:11:18) used was previously
dissolved in water and filtered.
Figure 1. Photobioreactor used in microalgae cultivation.
When the microalgae reached a cell density of 4.75 x
10
5
cel mL
-1
, they were cultivated in a 20 L
photobioreactor with constant aeration and lighting.
The experiment was terminated on day 10 (240 h).
After cultivation, each sample was centrifuged
individually in a benchtop centrifuge for 15 min at
2500 rpm, followed by drying for 24 h at 50 °C. The
input and output data from each step, energy
consumption, yield, inputs, and waste, were used in
SimaPro version 8.5 software to conduct the Life
Cycle Impacts Analysis (LCIA). The equipment used
is shown in Table 1, and the process inputs and
outputs are shown in Table 2.
The potential environmental impacts arising from
the LCA for producing 1 kg of biomass considering
cultivation, separation, and drying were evaluated in
all three scenarios. All inputs and outputs were found
in the Ecoinvent 3.6 database.
Table 1: Equipment used in the cultivation of microalgae in
tubular photobioreactors.
Steps Equipment Specifications
Cultivation
Lamps
LED T8 9W 100-
240V, ~ 50/60HZ
Pumps
Aeration pump
Vigo Ar/ 60Plus.
AC 220V – 60Hz,
2.5 W, 90 L h
-
1
,120 mbar.
Separation Centrifuge
Centrifuge Sigma/
6-16KS, Ano
2015, ~V/Hz 220-
2 40/60, 2300 W.
Drying Greenhouse
Greenhouse
Tecnal/ TE-394,
+7°C-70°C, 1.1
kW.
ISWEE 2022 - International Symposium on Water, Ecology and Environment
78
Table 2: Inputs and outputs of the three scenarios evaluated.
Scenarios
Item NPK Wastewater Wastewater +
g
l
y
cerol
In
p
ut
N fertilize
r
(
k
g)
1.62 - -
P fertilize
r
(kg) 1.48 - -
K
fertilize
r
(
k
g)
2.43 - -
Wastewate
r
(L) 20 20 20
Gl
y
cerol
(g)
- - 12.5
Wate
r
(L) 20 20 20
Clean Product
(g)
2 2 2
Output
Treated Effluent (L) 20 20 20
Comments: total values are shown in the table
3 LIFE CYCLE IMPACT
ANALYSIS
In the study of environmental impacts in the three
scenarios evaluated to produce microalgal biomass, it
was observed that of the 15 categories presented by
the Impact 2002+ method, the categories that are most
significant in the process are related to the production
of carcinogenic and non-carcinogenic pollutants,
inorganic respiratory pollutants, ecotoxicity, and
terrestrial acidification, land occupation, global
warming, and non-renewable energy use. The
impacts are associated with necessary inputs from the
technosphere (water, detergent, glycerol, and NPK)
and energy needs.
In the cultivation stage, there is a great influence
of the use of electricity to operate the pumps to
transfer CO
2
to the medium and to produce light
energy, essential for photosynthesis and the
conversion of inorganic carbon into biomass. In
separating biomass by centrifugation and in oven
drying, there was a greater contribution of impacts
associated with electricity use.
By adding glycerol, the effect of electricity
consumption was minimized due to the increased
production of microalgal biomass, enabling
production in a mixotrophic metabolic mode.
Producing more biomass at the same energy
consumption is a premise for achieving a lower
environmental impact. Using inputs more efficiently
leads to a higher biomass yield can reduce
production’s environmental impacts. Residual inputs
can determine better yield without adding more
economic and environmental impacts (Kabir et al.
2022).
Figure 2. Impact categories obtained in the biomass production with different mediums (wastewater, wastewater
supplemented with glycerol, and NPK solution) using Simapro 8.5 with Impact 2002+ method and Ecoinvent database.
It was observed that the glycerol supplementation
scenario generated the least environmental impacts
(Figure 2), even though purified glycerol was used as
an input, which added environmental impacts to the
system but led to a greater gain in biomass. Residual
glycerol, obtained after methanol recovery, should
have its pH analyzed and adjusted and, therefore, may
be responsible for an even better environmental
Life Cycle Assessment of Microalgae Production in a Sanitary Effluent Medium Supplemented with Glycerol
79
performance of microalgal biomass production.
Crude glycerol is already recognized as a suitable
carbon source in mixotrophic microalgae production
(Xu et al. 2019, Gougoulias et al. 2022). In previous
research conducted by this group, it was observed that
adding glycerol may be a factor in improving the C/N
ratio and using microalgae to reduce residual nitrogen
after conventional urban wastewater treatment (not
published). Figure 3 shows the main normalized
environmental damages associated with microalgal
biomass production from the scenarios under study.
The use of NPK in microalgae production adds the
most impact in all categories, being the greatest for
human health. Li et al. (2022), when studying the life
cycle of microalgae production in wastewater using
the ReCiPe method, which presents the impacts on
mid and endpoints, also observed the benefits of using
wastewater in microalgae production, showing a
reduction in long-term impacts to human health and
the ecosystem. They also noted the energy input
requirement and no nutrient recovery effect as factors
responsible for lower environmental performance.
In the uncertainty analysis for the three scenarios
concerning the main impact categories, the data in
Table 3 were obtained. The greatest uncertainties in
the impact categories in both methods are related to
the scenario in which microalgae was cultivated in a
medium with NPK.
Figure 3. Environmental damage categories obtained in the biomass production with different mediums (wastewater,
wastewater supplemented with glycerol, and NPK solution) using Simapro 8.5 with Impact 2002+ method and Ecoinvent
database.
The uncertainties of these data for the main
categories of impacts analyzed correspond to the
division of the standard deviation by the mean in each
category. The largest uncertainties in the impact
categories in both methods are in the categories of
land cover in the wastewater cultivation scenario and
carcinogenic and non-carcinogenic pollutants for
cultivation in NPK solution; however, all
uncertainties were considered low since they are less
than 0.3 (Pearson and Casarim 2018).
4 FUTURE PERSPECTIVES
For the production of Chlorella sp., besides the
benefits associated with the fact that microalgae are
photosynthesizers, responsible for carbon capture
from the atmosphere, and are suitable for capturing
nutrients from the water, it is observed by the Life
Cycle Inventory (LCI) that they can also be promising
as a cleaner production process if we use effluent for
cultivation. Supplementing glycerol, especially crude
glycerol, also adds a better environmental
performance associated with a higher biomass yield.
To improve the process, it is still possible to
reduce impacts associated with biomass drying by
using a solar dryer with heated air through the
biomass, as presented by Silva et al. (2021) for
Spirulina platensis, which reached 11% of biomass
moisture in less than 3.5 h.
Notably, the reduction of the impacts of the
process by cultivating in an effluent medium with
ISWEE 2022 - International Symposium on Water, Ecology and Environment
80
residual glycerol can lead to the loss of some
properties and the risk of contamination, which
reduces the possibilities of biomass use. This process
can be environmentally cleaner, with several benefits.
However, the use of the biomass obtained should be
considered. Nevertheless, the biomass can be used as
biofertilizer (Vishwakarma et al. 2022), and biofuels
(de Souza Celente et al. 2019, de Souza et al. 2021)
are more likely products to be developed with the
biomass.
Another aspect of microalgae production's
environmental viability is the cultivation location (de
Souza et al. 2022). In this LCA, the need for effluent
and glycerol transportation was not considered, and if
there is transportation, the impacts increase due to
fuel consumption. For many microalgae, the use of
effluent in cultivation can be promising; however, if
there are large distances between the generation of
effluent and the production of microalgae, the
alternative is no longer viable.
Table 3: Uncertainty analysis by Monte Carlo simulation (1000 interactions and 95% confidence) of the impact results of the
categories highlighted in the LCIA with the Impact 2002+ method.
Categories Unit
NPK Wastewater Wastewater + glycerol
Average CV Average CV Average CV
Carcino
g
ens k
g
C
2
H
3
Cl e
q
3.73E+01 0.183 3.70E+00 0.004 2.14 7.15E-03
Global warmin
g
k
g
CO
2
e
q
3.11E+03 0.036 3.15E+03 0.000 1791.28 1.67E-04
Land occupation m
2
or
g
.arable 5.48E+02 0.033 5.22E-02 0.132 5.04 9.15E-02
N
on-carcino
g
ens k
g
C
2
H
3
Cl e
q
1.57E+02 0.120 9.72E+01 0.000 55.35 8.40E-04
N
on-renewable ener
gy
MJ primar
y
3.65E+04 0.036 4.21E+04 0.000 23906.24 1.46E-04
Respirator
y
inor
g
anics k
g
PM
2.5
e
q
3.25E+00 0.047 2.33E+00 0.000 1.33 2.75E-04
Terrestrial acid/nutri k
g
SO
2
e
q
9.33E+01 0.042 4.80E+01 0.000 27.35 3.81E-04
Terrestrial ecotoxicit
y
k
g
TEG soil 1.50E+05 0.046 4.86E+03 0.002 3132.06 2.21E-02
5 CONCLUSIONS
The environmental performance of microalgal
biomass production was obtained by LCA comparing
three cultivation scenarios (NPK solution, effluent,
and effluent with glycerol supplementation). When
comparing the scenarios, it was observed that using
sanitary effluent with glycerol supplementation led to
less environmental impacts in the cultivation process
of Chlorella sp. Thus, glycerol can reduce microalgal
biomass production impacts when supplemented with
effluent. However, this will depend on the biomass’s
applicability, production logistics, and transport
impacts.
ACKNOWLEDGEMENTS
This work was supported by Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior –
Brasil (CAPES) – Finance Code 001 and Conselho
Nacional de Desenvolvimento Científico e
Tecnológico - CNPq – protocol 310228/2019-0 and
400862/2019-0.
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