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International Journal of Sustainable and Green Energy
2015; 4(1-1): 20-32
Published online December 31, 2014 (http://www.sciencepublishinggroup.com/j/ijrse)
doi: 10.11648/j.ijrse.s.2015040101.14
Biological purification processes for biogas using algae
cultures: A review
Rameshprabu Ramaraj, Natthawud Dussadee
School of Renewable Energy, Maejo University, Sansai, Chiang Mai-50290, Thailand
Email address:
rrameshprabu@gmail.com , rameshprabu@mju.ac.th (Ramaraj R.), natthawu@yahoo.com, natthawu@mju.ac.th (Dussadee N.)
To cite this article:
Rameshprabu Ramaraj, Natthawud Dussadee. Biological Purification Processes for Biogas Using Algae Cultures: A Review. International
Journal of Sustainable and Green Energy. Special Issue: Renewable Energy Applications in the Agricultural Field and Natural Resource
Technology. Vol. 4, No. 1-1, 2015, pp. 20-32. doi: 10.11648/j.ijrse.s.2015040101.14
Abstract:
Bioenergy is a type of renewable energy made from biological sources including algae, trees, or waste from
agriculture, wood processing, food materials, and municipalities. Currently, the uses of renewable fuels (bioethanol, biodiesel,
biogas and hydrogen) are increased in the transport sector worldwide. From an environmental and resource-efficiency
perspective biogas has several advantages in comparison to other biofuels. The main components of biogas are methane (CH
4
)
and carbon dioxide (CO
2
), but usually biogas also contains hydrogen sulphide (H
2
S) and other sulphur compounds, water, other
trace gas compounds and other impurities. Purification and upgrading of the gas is necessary because purified biogas provides
reductions in green house gas emissions as well as several other environmental benefits when used as a vehicle fuel. Reducing
CO
2
and H
2
S content will significantly improve the quality of biogas. Various technologies have been developed and available
for biogas impurity removal; these include absorption by chemical solvents, physical absorption, cryogenic separation,
membrane separation and biological or chemical methods. Since physiochemical methods of removal are expensive and
environmentally hazardous, and biological processes are environmentally friendly and feasible. Furthermore, algae are abundant
and omnipresent. Biogas purification using algae involved the use of algae’s photosynthetic ability in the removal of the
impurities present in biogas. This review is aimed at presenting the algal characteristics, scientific approach, gather and clearly
explain the main methods used to clean and purify biogas, increasing the calorific value of biogas and making this gas with
characteristics closest as possible to natural gas through algae biological purification processes.
Keywords:
Algae, Biogas, Biological Purification, Renewable Energy
1. Introduction
Bioenergy should play an essential part in reaching targets
to replace petroleum-based transportation fuels with a viable
alternative, and in reducing long-term CO
2
emissions, if
environmental and economic sustainability are considered
carefully. The world continues to increase its energy use,
brought about by an expanding population and a desire for a
greater standard of living. This energy use coupled with the
realization of the impact of CO
2
on the climate, has led us to
reanalyze the potential of plant-based biofuels [1]. The term
biofuel is referred to as liquid or gaseous fuels for the transport
sector that are predominantly produced from biomass. A
variety of fuels can be produced from biomass resources
including liquid fuels, such as ethanol, methanol, biodiesel,
Fischer-Tropsch diesel, and gaseous fuels, such as
biohydrogen and biogas.
The process of biogas production from algal biomass is an
alternative technology that has larger potential energy output
compared to green diesel, biodiesel, bioethanol, and hydrogen
production processes. Moreover, anaerobic digestion can be
integrated into other conversion processes. The organic
fraction of almost any form of biomass (from plants, algae and
other microorganisms), including sewage sludge, animal
wastes and industrial effluents, can be broken down through
anaerobic digestion (AD) into CH
4
and CO
2
mixture called as
‘‘biogas”. The first methane digester plant was built at
Bombay, India in 1859 [2, 3]. AD approaches steadily
growing role in the renewable energy mix in many countries.
AD is the process by which organic materials are biologically
treated in the absence of oxygen by naturally occurring
bacteria to produce ‘biogas’ which is a mixture of CH
4
(40-70%) and CO
2
(30-60%) with traces of other gases such as
hydrogen, hydrogen sulphide and ammonia [4]; the biogas
process also produces potentially useful by-products in the
form of a liquid or solid ‘digestate’ [5].
International Journal of Sustainable and Green Energy 2015; 4(1-1): 20-32 21
Normally, biogas is comprised of CH
4
, CO
2
, and other trace
gas compounds gases such as water vapour, H
2
S, halogenated
hydrocarbons, siloxanes, ammonia, nitrogen, and oxygen [4].
Biogas is a valuable fuel which is produced in digesters filled
with the feedstock like dung or sewage. All types of biomass
can be used as substrates for biogas production as long as they
contain carbohydrates, proteins, fats, cellulose, and
hemicelluloses as main components. The composition of
biogas and the methane yield depends on the feedstock type,
the digestion system, and the retention time. In general, the
use of plant biomass for energy generation today is
problematic because of the competition with food or feed
production. This is because most of the plants used for energy
generation today (crop plants, sugar cane, sugar beets, canola,
etc.) have to be grown on arable land. Low demand
alternatives like switchgrass are only beginning to emerge.
Algae have got a number of potential advantages compared to
higher plants because of faster growth rates and the possibility
of cultivation on non-arable land areas or in lakes or the ocean,
therefore attenuating food and feed competition [6,7]. Of the
potential sources of biogas the most efficient producers of
biomass are the photosynthetic algae (micro and macroalgae).
Photosynthetic pigments, including chlorophyll, have an
important role since it provides the oxygen and the source of
energy for all living things. Plant and algae growth is affected
by the photosynthesis speed which depends on the availability
of CO
2
. Biological CO
2
fixation by algae is another such form;
i.e. sunlight being used to reduce CO
2
to carbon. Capturing
CO
2
from flue gases is the precautionary principle which
needs preventive action, at both national and international
levels to minimize this potential action [8]. A promising
approach therefore seems to be the use of fast-growing algae
species for anaerobic fermentation to produce biogas, which
then can substitute natural gas resources.
To utilize biogas as a transport fuel, CO
2
and H
2
S must be
removed from the concentration to leave biomethane. Biogas
purification is the process where any impurities are removed
such as sulphides and ammonia. Biogas upgrading on the
other hand is the process which removes CO
2
and the end
product is bio-methane. The bio-methane which has been
upgraded is suitable for injection into the national gas grid or
vehicle fuel [4]. Biogas needs cleaning for two main reasons;
the first is to improve the calorific value of the product gas and
the second is to reduce the chance of damaging downstream
equipment which is due to the formation of harmful
compounds [9]. Thus, biogas has a wide availability and
renewable nature due to the organic materials and
microorganisms required for biogas synthesis. Biogas
purification methods can be divided into two generic
categories:
1 Those involving physicochemical phenomena (reactive
or non-reactive absorption; reactive or non-reactive
adsorption).
Those involving biological processes (contaminant
consumption by living organisms and conversion to less
harmful forms). Biological processes are widely employed for
CO
2
and H
2
S removal, especially in biogas applications.
For CO
2
capture from biogas, physical and chemical
absorption methods are generally applied with fewer
complications; however, these methods are needed to post
treat the waste materials for regeneration of cycling utilization.
The biological methods of CO
2
capture from biogas are
potentially useful [10]. Biological processes are widely
employed for H
2
S removal, especially in biogas applications
[11]. Furthermore, biogas is an environment friendly, clean,
cheap and versatile fuel. Consequently, the purpose of the
current paper is to present an integrated review of the biogas
production methodologies and purification process, algal
characteristics, approaches and clearly explain the main
methods used to clean and purify biogas, increasing the
calorific value of biogas and making this gas with
characteristics closest as possible to natural gas through algae
biological purification processes
2. Growth Characteristics of Algae and
Importance
Algae are the most important primary producer in aquatic
ecosystem [12]. Many species of algae are present such as;
green, red and brown algae which belong to the group of
Chlorophyta, Rhodophyta and Phaeophyta, respectively.
Algal growth is found in a wide range of habitats, like fresh
water, marine water, in deep oceans, in rocky shores, the
plank-tonic and benthic algae can become important
constituents of soil flora and can exist even in such extreme
conditions as in snow, sands/desert or in hot springs, open and
closed ponds, photo bioreactors, sewage and wastewater,
desert as well as CO
2
emitting industries etc [13]. Generally
they are found in damp places or water bodies and are
common in terrestrial as well as aquatic environments. Algae,
a broad category encompassing eukaryotic microalgae,
cyanobacteria and macroalgae, can be cultivated to produce
biomass for a wide range of applications [14].
Algae are a very diverse group of predominantly aquatic
photosynthetic organisms of tremendous ecological
importance, because they were the beginning of the food chain
for other animals. Algae played an important role in
self-purification of contaminated natural waters and offered an
alternative for advance nutrition removal in water or
wastewater [15, 16]. The idea to incorporate microalgae as an
agent of bioremediation was firstly proposed by Oswald and
Gotaas in 1957 [17]; the biomass recovered was converted to
methane, which was a major source of energy [18]. Hence,
algae provided the basis of the aquatic food chain and they
were fundamental to keep CO
2
of carbon cycle via
photosynthesis as a substantial role in biogeochemical cycles
[12]. Most algae were photoautotrophic, converting solar
energy into chemical forms through photosynthesis.
The mechanisms of algal photosynthesis were very similar
to photosynthesis in higher plants and their products are
molecularly equivalent to conventional agricultural crops [19].
The main advantages of culturing algae as a source of biomass
were as follows: (1) high photosynthetic yields (up to a
22 Rameshprabu Ramaraj and Natthawud Dussadee: Biological Purification Processes for Biogas Using Algae Cultures: A Review
maximum of 5-6% conversion of light c.f. 1-2% for the
majority of terrestrial plants); (2) the ability to grow in fresh,
salt and wastewater; (3) high oil content; (4) the ability to
produce non-toxic and biodegradable biofuels; (5) many
species of algae can be induced to produce particularly high
concentrations of chosen compounds–proteins, carbohydrates,
lipids and pigments - that are of commercial value; (6) the
ability to be used in conjunction with wastewater treatment
[13,17–20]. Since algae was a key primary producer
global-wide, algae biomass was essential biological natural
resources which played an important role in nutrient, food,
fertilizer, pharmaceutics and biofuel.
In addition, algae application is widely accepted in practice
as one of the best strategies in bioengineering. There are
several reasons for this approach: (1) the best growth rate
among the plants, (2) low impacts on world’s food supply, (3)
specificity for CO
2
sequestration without gas separation to
save over 70% of total cost, (4) excellent treatment for
combustion gas exhausted with NOx and SOx, (5) high value
of algae biomass including of feed, food, nutrition,
pharmaceutical chemicals, fertilizer, aquaculture, biofuel, etc
[13, 20]. Algae an important application for the cultivation of
algae is the production of biomass for energy purposes. Due to
the energy crisis, renewable energy becomes a popular issue in
this world today and there are several alternatives such as
bioenergy, solar, wind, tide, geothermal, etc. For bioenergy,
algae are the third generation biofuel [20]. For the reasons of
the best energy conversion efficiency of sunlight [15] and the
highest growth rate [18], algae have the best potential among
all the energy crops. Because of the fast growth, many high
valuable products are generated, e.g. food, biofuel, etc [Figure
1].
Figure 1. Potential products from algae
Algae produce biomass, which can be converted into energy
or an energy carrier through a number of energy conversion
processes. They include thermochemical conversion
(gasification, direct combustion and pyrolysis), biochemical
conversion (anaerobic fermentation, anaerobic digestion and
photobiological hydrogen production) and esterification of
fatty acids to produce biodiesel [13,18,20]. A lot of studies
was indicated the importance of algae in carbon dioxide
fixation [12–16,18,20]. Driver et al. [21] stated that algae are
an attractive feedstock for the production of liquid and
gaseous biofuels that do not need to directly compete with
food production. Figure 2 illustrated the detailed information,
process including algal stain selection, water type, cultivation
methods, growth mode and harvesting methods. Furthermore,
the various scenarios for biofuel development from algae are
represented. Many options are available with regard to algae
type and strain choice, including both eukaryotic algae and
prokaryotic cyanobacteria, the source of water for cultivation,
cultivation method and mode of growth, the method of algae
harvesting and the biofuel conversion process. The
understanding of biological phenomena, algal genetics,
carbon storage metabolism, photosynthesis and algal
physiology, have the potential for significant advances in algal
biofuel feasibility [21]. This is being driven by advances in
genomic technologies to provide the potential for genetic and
metabolic engineering, plus the development of
high-throughput techniques for the screening of natural strains
for suitable biofuel characteristics.
Figure 2. Algae production system
3. Algae Biogas Production Process and
Technology
Anaerobic digestion (AD) is a common process for the
treatment of a variety of organic materials and biogas
production. Macroalgal and microalgal biomass can be AD to
produce methane. Recently, microalgae have also become a
topic of interest in the production of biogas through anaerobic
fermentation [22].The AD of algae is a prospective
environmentally feasible option for creating a renewable
source of energy for industrial and domestic needs. Algal AD
International Journal of Sustainable and Green Energy 2015; 4(1-1): 20-32 23
on is a key unit process that integrates efficiency and
beneficially into the production of algal derived biofuels. Both,
macro- and microalgae are suitable renewable substrates for
the anaerobic digestion process. The process of biogas
production from algal biomass is an alternative technology
that has larger potential energy output compared to green
diesel, biodiesel, bioethanol, and hydrogen production
processes [4]. Moreover, anaerobic digestion can be integrated
into other conversion processes and, as a result, improve their
sustainability and energy balance. Opposite to biohydrogen,
bioethanol or biodiesel that only uses determined
macromolecules (carbohydrates and lipids), biogas is
produced by biological means under anaerobic conditions that
converts all algae macromolecules into methane [5, 8].
Figure 3. Stages of Anaerobic Digestion (methane fermentation process)
AD is an application of biological methanogenesis which is
an anaerobic process responsible for degradation of much of
the carbonaceous matter in natural environments where
organic accumulation results in depletion of oxygen for
aerobic metabolism. Since AD is a process by which almost
any organic waste can be biologically converted in the absence
of oxygen. This process, which is carried out by a consortium
of several different microorganisms, is found in numerous
environments, including sediments, flooded soils, animal
intestines, and landfills. Accordingly, this is a complex
process, which requires specific environmental conditions and
different bacterial populations. Mixed bacterial populations
degrade organic compounds, thus producing, as end-product;
a valuable high energy mixture of gases (mainly CH
4
and CO
2
)
termed biogas [9]. Methane fermentation is a complex process,
which can be divided up into four phases: hydrolysis,
acidogenesis, acetogenesis/dehydrogenation, and methanation
(Figure 3). These four stages are involved in the breakdown of
organic matter on the path to methane production; stages
include hydrolysis, fermentation (or acidogenesis),
acetogenesis and eventual methanogensis (1). Hydrolysis
involves the conversion of complex molecules and
compounds–carbohydrates, lipids and proteins – found in
organic matter into simple sugars, long chain fatty acids and
amino acids, respectively. Acidogenesis in turn converts these
into volatile fatty acids, acetic acid, CO
2
and H
2
. Acetogenesis
converts the volatile fatty acids into more acetic acid, carbon
dioxide and hydrogen gas. Methanogens have the ability to
produce methane by using the carbon dioxide and hydrogen
gas or the acetic acid produced from both the acetogenic or
acidogenic phases [10,11].
3.1. Anaerobic Digestion of Macroalgae Biomass
Macroalgae is one such source of aquatic biomass and
potentially represents a significant source of renewable energy.
The average photosynthetic efficiency of aquatic biomass is
6–8%, which is much higher than that of terrestrial biomass
(1.8–2.2%). Macroalgae are fast growing marine and
freshwater plants that can grow to considerable size (up to 60
m in length). Annual primary production rates (grams
C m
−2
yr
−1
) are higher for the major marine macroalgae than
for most terrestrial biomass [23]. Macroalgae can be
subdivided into the blue algae (Cyanophyta), green algae
(Chlorophyta), brown algae (Phaeophyta) and the red algae
(Rhodophyta). Either Freshwater macroalgae or marine
macroalgae (kelp or seaweed) could be used for solar energy
conversion and biofuel production [23]. Macroalgae received
a large amount of attention as a biofuel feedstock due to its
prolific growth in natural habitat of freshwater system,
eutrophic coastal water fouling beaches and coastal
waterways.
Macroalgae can be converted to biogas by process of AD to
biogas (~ 60% CH
4
) [24]. Research conducted in the 1980’s
on macroalgae (giant brown kelp (Macrocystis)) [25] still
provides a bench mark for biogas yields for a number of
macroalgal species, but since this time there have been
developments in AD technology and an enormous increase in
its use. In comparison to terrestrial biomass crops, macroalgae
contain little cellulose and no lignin and therefore undergo a
more complete hydrolysis. AD has been used to dispose and
process this material for the production of biogas; the AD of
macroalgae biomass could meet two currently important
needs, the mitigation of the eutrophication effects and the
production of renewable energy. Because of the abundance of
seaweed/ freshwater macroalgae biomass its conversion can
be highly desirable and convenient, mostly for countries with
long coastlines or eutrophic environments [26].
Investigations on the use of macroalgae of the brown algae
division in processes of methane fermentation were conducted
by Vergara-Fernàndez [27]. He was examining the possibility
of applying to this end the biomass of Macrocystis pyrifera
and Durvillea antarctica macroalgae and a substrate based on
the mixture of these species. His study proved that for all
substrates tested the yield of biogas production was
comparable and reached 180.4±1.5 dm
3
/kg d.m.d. Singh and
Gu [28] and Parmar et al. [29] were also analyzing the yield of
biogas production with the use of microphytobenthos plants as
an organic substrate. They achieved the highest technological
effects during fermentation of Laminaria digitata brown algae
belonging to the order Laminariales. In that case, methane
production was high and reached 500 dm
3
CH
4
/kg o.d.m. The
use of Macrocystis sp. enabled achieving
24 Rameshprabu Ramaraj and Natthawud Dussadee: Biological Purification Processes for Biogas Using Algae Cultures: A Review
390–410 dm
3
CH
4
/kg o.d.m., whereas upon the use of
Gracilaria sp. and Laminaria sp. methane production
accounted for 280–400 dm
3
CH
4
/kg o.d.m. and
260–280 dm
3
CH
4
/kg o.d.m., respectively [30].
The feasibility of biogas production from macroalgae
collected from the Orbetello lagoon. Maroalgae biomass
collected from the same lagoon was used for biogas
production in batch reactors. He demonstrated that it is
possible to produce CH
4
directly from macroalgae, preserving
the spontaneous epiphytic microorganisms, as microbial
starter of the digestion process. Moreover, it is possible to
foster CH
4
yield by using anoxic sediments collected from the
same lagoon as a further microbial inoculum. In fact, the
addition of sediment improved the degradation activity,
accelerating the removal of volatile fatty acids (VFA) from the
medium and their conversion into methane, reducing the
digestion time and increasing CH
4
yield [31]. The promising
results obtained despite the harsh conditions (high salts,
sulphur and heavy metals concentration) have been favoured,
in our opinion, thanks to a pre-existing adaptation and mutual
interactions within the native microorganisms. The bacterial
pool was highly adapted both to biotic and abiotic factors, that
is to macroalgal tissue composition and to the salts and toxic
components present in water and sediments. Furthermore, this
approach solely based on the exploitation of the intrinsic
degradation potential of the reference ecosystem, proved to be
suitable for a selective and non-intensive anaerobic digestion
of macroalgae. In the review by Dębowski et al. [30] presented
the effectiveness of biogas production with the use of
macroalgae as a substrate in methane fermentation processes
(Table 1). Huesemann et al. [32] stated that AD of macroalgae
was technically feasible at scale and it has been suggested that
it could be a cost-competitive with anaerobic digestion of
terrestrial biomass and municipal solid waste.
Table 1. Effectiveness of biogas production with the use of macroalgae as a
substrate in methane fermentation processes.
Macroalgae taxon Quantity of biogas/methane
Durvillea antarctica 179.3±80.2 dm
3
CH
4
/kg d.m. d
Gracilaria sp. 280–400 dm
3
/kg o.d.m.
Laminaria sp. 260–280 dm
3
/kg o.d.m.
Laminaria digitata 500 dm
3
/kg o.d.m.
Macrocystis 390–410 dm
3
/kg o.d.m.
Macrocystis sp. 189.9 dm
3
CH
4
/kg o.d.m.
Macrocystis pyrifera 181.4±52.3 dm
3
CH
4
/kg d.m. d
M. pyrifera+Durvillea antarctica 164.2±54.9 dm
3
CH
4
/kg d.m. d
Pilayella+Ectocarpus+Enteromarpha 40.0–54.0 dm
3
/kg
29.2–39.4 dm
3
CH
4
/kg
Ulva sp. 200 dm
3
/kg o.d.m.
Ulva lactuca 157–271 dm
3
CH
4
/kg o.d.m.
3.2. Anaerobic Digestion of Microalgae Biomass
Microalgae are highly productive and are able to produce
large quantities of biomass more efficiently [13,14,16].
Generally, the composition of microalgae is
CO
0.48
H
1.83
N
0.11
P
0.01
[13], and microalgae have been found to
have several constituents, mainly including lipids (7–23%),
carbohydrates (5–23%), and proteins (6–52%). The chemical
compositions of microalgae are mainly dependent on the
species and culture conditions. Microalgae AD is a key unit
process that integrates efficiency and beneficially into the
production of microalgae derived biofuels. The first authors to
report on the anaerobic digestion of microalgae biomass were
Golueke et al. [33]. They investigated the anaerobic digestion
of Chlorella vulgaris and Scenedesmus, microalgae species
grown as part of a wastewater treatment process.
The technical feasibility data on the anaerobic digestion of
algal biomass have been reported for many species of algae.
Among the microscopic algae, the following cultures have
been successfully used for the production of methane: the
mixed culture of Scenedesmus sp. and Chlorella sp., the mixed
culture of Scenedesmus sp., Chlorella sp., Euglena sp.,
Oscillatoria sp., and Synechocystir sp., the culture of
Scenedesmus sp. alone, and together with either Spirulina sp.,
Euglena sp., Micractinium sp., Melosira sp., or Oscillatoria SP.
The production of biogas through AD offers significant
advantages over other forms of bioenergy production. Since
AD consists of organic carbon degradation into organic acids
and biogas. Biogas mainly consists of methane (around 65%),
which is carbon most reduced state, and carbon dioxide
(around 35%), which is its most oxidized state. Other gases
(normally less than 1%), such as nitrogen, nitrogen oxides,
hydrogen, ammonia and hydrogen sulphide are also formed
[34, 35].
Table 2. Effectiveness of biogas production with the use of microalgae as a
substrate in methane fermentation processes.
Macroalgae taxon Quantity of biogas/methane
Arthrospira platensis 481±13.8 dm
3
/kg o.d.m.
Chlamydomonas reinhardtii 587±8.8 dm
3
/kg o.d.m.
Chlorella kessleri 335±7.8 dm
3
/kg o.d.m.
Chlorella vulgaris 150 dm
3
CH
4
/kg o.d.m.
240 dm
3
CH
4
/kg o.d.m.
Dunaliella salina 505±24.8 dm
3
/kg o.d.m.
Euglena gracilis 485±3.0 dm
3
/kg o.d.m.
Phaeodactylum tricornutum 350±3.0 dm
3
CH
4
/kg o.d.m.
Scenedesmus obliquus 210±3.0 dm
3
CH
4
/kg o.d.m.
S. obliquus 287±10.1 dm
3
/kg o.d.m.
Scenedesmus sp.+Chlorella sp. 986 dm
3
/kg o.d.m.
Scenedesmus sp+Chlorella sp.
180±8 dm
3
/dm
3
d
573±28 cm
3
/dm
3
d
818±96 cm
3
/dm
3
d
Spirulina maxima 240 dm
3
CH
4
/kg o.d.m.
Spirulina platensis 280±0.8 dm
3
CH
4
/kg o.d.m.
Sialve et al. [35] stated that an organic matter composition
can be converted stoichiometrically into methane for
calculating the theoretical methane yield. Thus, lipids
(1.014 L/g VS), followed by proteins (0.851 L/g VS) and
carbohydrates (0.415 L/g VS) have the highest theoretical
methane yield. Indeed, inducing a particular macromolecule
accumulation in microalgae cells has proven to successfully
increase the methane yield. Research conducted with
carbohydrate-enriched cyanobacteria Arthrospira platensis by
phosphorus limitation attained a methane yield of 0.203 L/g
COD when biomass had 60% of carbohydrates in respect to
0.123 L/g COD when the carbohydrate content was 20% [36].
In the review by Dębowski et al. [30] presented the
effectiveness of biogas production with the use of macroalgae
International Journal of Sustainable and Green Energy 2015; 4(1-1): 20-32 25
as a substrate in methane fermentation processes (Table 2).
The biogas yield of plants is generally limited by the greater
or lesser proportion of lignocellulose, which is difficult to
recycle. Efficiency of biogas production is related to the
species-dependent, efficiency of cell degradation and presence
or absence of molecules. However, the use of microalgae with
a low lignocellulose content, for example Chlorella vulgaris,
Phaeodactylum tricornutum and Spirulina platensis, permits
an almost complete utilization of the organic substance.
Golueke et al. [33] demonstrated the ability of microalgae to
pass through an anaerobic digester intact and remain
undigested. The authors noted that microalgal cells are known
to be able to effectively resist bacterial attack and found intact
microalgae cells in digestate leaving a digester after a 30-day
hydraulic retention time. The composition of the biogas and
the yield could be varied depending on the cell contents, the
cell wall components and the stability of the cell wall. In
particular the protein content of the cell plays a decisive role.
Depending on the type of algae, the biogas yield was between
280 and 400 L/kg total volatile solids. Generally the
variability is related to two main aspects: (i) the
macromolecular composition, and (ii) the cell wall
characteristics of each microalgae species. The difference in
anaerobic biodegradability due to the macromolecular
composition lies on the methane potential of different organic
compounds in microalgae cells. Consequently, pretreatment
techniques have been used to solubilize particulate biomass
and improve the anaerobic digestion rate and extent.
4. Pretreatment Methods for Increased
Biogas Production from Algae
Algae anaerobic biodegradability is limited by their
complex cell wall structure. Thus, pretreatment techniques are
being investigated to improve algal methane yield. Various
pretreatment technologies have been developed in recent years.
These pretreatment technologies aim to make AD faster,
potentially increase biogas yield, and make use of new and/or
locally available substrates, and prevent processing problems
such as high electricity requirements for mixing or the
formation of floating layers. Pretreatment methods can be
divided into four categories: thermal, mechanical, chemical
and biological processes (Figure 4).
Pretreatment methods have been studied in order to
disintegrate microalgae cells, solubilise the organic content,
and increase the anaerobic digestion rate and extent. Thermal
pretreatments have been the most widely investigated already
in continuous reactors and leading to net energy production
[36, 37]. Mechanical pretreatments have mostly been
investigated in batch assays using algae cultures [38]. Thermal
pretreatments have been the most widely studied already in
continuous reactors and leading to net energy production [39].
Mechanical pretreatments were less dependent on algae
species, but required a higher energy input if compared with
chemical, thermal and biological methods [38]. Chemical
pretreatments have been proved successful, particularly when
combined with heat [39]. Enzymatic pretreatment seem to
improve microalgae hydrolysis [40], which is promising due
to its low energy input.
Figure 4. Pretreatments for improving algae biogas production
4.1. Pretreatment Methods for Increased Biogas Production
from Macroalgae
Pretreatment of the algae is thus needed to aid both
mechanical transport (pumping) as well as microbiological
AD. Biogas can be derived via anaerobic fermentation of any
organic matter, including the cellulose and hemicellulose
within macroalgae, although the biomass must be subjected to
pretreatment processes in order to liberate the sugars needed
for fermentation. The effect of the pretreatment technologies,
thermal treatment, thermochemical treatment, mechanical
treatment, wet oxidation, hydrothermal pretreatment, steam
explosion, plasma-assisted pretreatment and ball milling. One
option is mechanical pretreatment of the algae; however a
method which can handle the long fibrous material in
macroalgae species is needed. Another method, which is
relatively untested but promising, is enzymatic pretreatment
which during recent years has been tested on many substrates
to investigate effect on biogas potential [41].
The mechanical pretreatment effectively broke up the
structure of all macroalgae into homogenous slurry.
Mechanical pretreatment could increase the soluble
COD-concentration of the tested algae by 1.5 to 3 times
compared to raw algae. Enzymatic treatment increased it by
1.3 to 1.7 times. The best results were achieved by combining
mechanical and enzymatic treatment where the concentration
could was increased 3.5 times compared to raw algae [42]. A
mechanical pretreatment phase is usually the first step not only
for methane [43]. Nielsen and Heiske [44] was discussed the
effect on methane yield of U. lactuca by various pretreatments
including mechanical maceration and autoclavation. Sodium
hydroxide soaking at room temperature prior to AD led to a
18% increase in methane potential in macroalgae as (Palmaria
palmata), possess a high methane potential (308 ± 9 mL g
VS−1
)
[45]. Nielsen and Heiske [44] studied four macroalgae
species-harvested in Denmark-for their suitability of
bioconversion to methane. In batch experiments (53 °C)
26 Rameshprabu Ramaraj and Natthawud Dussadee: Biological Purification Processes for Biogas Using Algae Cultures: A Review
methane yields varied from 132 ml g volatile solids(-1) (VS)
for Gracillaria vermiculophylla, 152 ml gVS(-1) for Ulva
lactuca, 166 ml g VS(-1) for Chaetomorpha linum and 340 ml
g VS(-1) for Saccharina latissima following 34 days of
incubation. With an organic content of 21.1% (1.5-2.8 times
higher than the other algae) S. latissima seems very suitable
for anaerobic digestion. However, the methane yields of U.
lactuca, G. vermiculophylla and C. linum could be increased
with 68%, 11% and 17%, respectively, by pretreatment with
maceration. Nielsen and Heiske [44] data of methane
potentials in different macroalgae with pretreatments were
presented in Table 3.
Table 3. Methane potentials of different macroalgae with pretreatments.
Macroalgae taxon Pretreatment Methane yield (ml g VS
-
1
) Methane production (ml g algae
-
1
')
Batch screening of methane potentials of different macroaigae
a
Chaetomorpha linum Washed, chopped 166 ± 43.5 11.4 ±2.98
Chaetomorpha linum Washed, macerated 195 ± 8.7 13.4 ±1.46
Saccharina latissima Washed, chopped 340 ± 48.0 68.2 ± 9.63
Saccharina latissima Washed, macerated 333 ± 64.1 66.8 ± 12.87
Gracillaria vermiculophylla Washed, chopped 132 ± 60.0 17.3 ±4.88
Gracillaria vermiculophylla Washed, macerated 147± 56.3 19.3 ± 7.39
Ulva lactuca Washed, chopped 152 ± 18.7 9.9 ± 1.21
Ulva lactuca Washed, macerated 255 ±47.7 16.5 ± 3.08
Pretreatments of U. lactuca
b
Ulva lactuca Unwashed, chopped 174± 23.3 12.8 ± 3.33
Ulva lactuca Unwashed, macerated 271 ± 16.2 17.6 ±1.12
Ulva lactuca Washed, chopped 171 ±22.3 12.2 ± 1.06
Ulva lactuca Washed, macerated 200 ±11.0 14.3 ± 1.53
Ulva lactuca Washed, 110 C/20 min 157 ± 13.4 11.3 ±0.96
Ulva lactuca Washed, 130 C/20 min 187 ± 23.2 13.4 ± 1.72
Ulva lactuca Dried, grounded 176 ±17.3 95.6 ± 9.42
Note:
a
34 days of incubation;
b
42 days of incubation (source: Nielsen and Heiske, 2011)
4.2. Pretreatment Methods for Increased Biogas Production
from Microalgae
The digestibility of microalgal biomass varies significantly
even between closely related species [46]. CH
4
yields from
microalgae vary due to variation in cellular protein,
carbohydrate and lipid content, cell wall structure, and process
parameters such as the bioreactor type and the digestion
temperature. Regarding the cell wall characteristics, it is
mostly composed of organic compounds with low
biodegradability and/or bioavailability, such as cellulose and
hemicellulose. This tough cell wall hinders the methane
production, since organic matter retained in the cytoplasm is
not easily accessible to anaerobic bacteria [47]. AD is carried
out by heterogeneous microbial populations involving
multiple biological and substrate interactions. Anaerobic
biodegradation can be divided into four main phases:
hydrolysis, acidogenesis, acetogenesis and methanogenesis
(before mentioned). AD (sometimes also called methanogenic
fermentation) is widely applied in digestion of manure,
sewage sludge and organic fraction of municipal solid wastes
in industrial and agrarian societies. Anaerobic digestion of
microalgal biomass has been studied from many freshwater
and marine microalgae in various combinations. Rigid
eukaryotic cell walls of microalgae can limit the anaerobic
digestion of the biomass [33,47]. Pretreatment techniques
were pointed out as a necessary step for microalgae cell
disruption and biogas production by Chen and Oswald [47].
The effectiveness of pretreatment methods on biogas
production depends on the characteristics of microalgae, i.e.,
the toughness and structure of the cell wall, and the
macromolecular composition of cells. For instance,
Scenedesmus sp. has one of the most resistant cell walls, since
it is composed by multilayers of cellulose and hemicellulose
on the inside, and sporopollenin and politerpene on the outside
[48].
Microalgae complex cell wall structure confers a resistance
to biological attack. In fact, species without cell wall (e.g.
Dunaliella sp. and Pavlova_cf sp.) or containing a
glycoprotein cell wall (e.g. Chlamydomonas sp., Euglena sp.
and Tetraselmis sp.) showed higher methane yields than those
with a more complex cell wall, containing recalcitrant
compounds (e.g. Scenedesmus sp. and Chlorella sp.) [49].
Rates and yields of CH
4
formation from microalgal biomass
often increase with digestion temperature. For example, [33]
reported 5–10% increase in digestibility of microalgal
biomass, when the digestion temperature was increased from
35 to 50 °C. Chen and Oswald [47] increased the CH
4
yield by
33% by heat pretreating microalgal biomass at 100 °C for 8 h.
In both examples, however, the amount of energy consumed in
the heating and pretreatment was higher than the
corresponding energy gain from increased CH
4
production
[50].
Retention times required to obtain high CH
4
yields from
untreated microalgal biomass are relatively long, 20–30 days
[51,52]. AD of microalgal biomass has been investigated in
batch and fed-batch systems as well as in continuously stirred
tank reactors [50]. Zamalloa et al. [52] suggested that
anaerobic sludge blanket reactors, anaerobic filter reactors and
anaerobic membrane bioreactors should be tested due to their
high volumetric conversion rates. In the review by Passos et al.
[30] presented the effectiveness of biogas production with the
main pros and cons of microalgae pretreatment methods
(Table 4). As can be seen, thermal pretreatment seems
International Journal of Sustainable and Green Energy 2015; 4(1-1): 20-32 27
effective at increasing biogas production, while energy
demand is low compared to mechanical ones. Nevertheless,
biomass thickening or dewatering is crucial. Scalability may
be a handicap for microwave pretreatment. Regarding
thermo-chemical pretreatment, studies have shown positive
results on microalgae biodegradability increase; however
further studies should evaluate the risk of contamination in
continuous bench and pilot-scale reactors.
Table 4. Comparison of pretreatment methods for increasing microalgae anaerobic biodegradability.
Pretreatment Control parameters Biomass
solubilization
Methane yield
increase Pros Cons
Thermal (<100 °C) Temperature; exposure
time √√√ √√
Low energy
demand;
Scalability
High exposure time
Hydrothermal
(>100 °C)
Temperature; exposure
time √√√ √√ Scalability
High heat demand; thickened or dewatered
biomass; risk of formation of refractory
compounds
Thermal with steam
explosion (>100 °C)
Temperature; exposure
time; pressure √√√ √√√ Scalability High electricity demand; scalability; biomass
dewatering
Microwave Power; exposure time √√ √√ –
Ultrasound Power; exposure time √√ √√ Scalability High electricity demand; biomass dewatering
Chemical Chemical dose; exposure
time √ √ Low energy
demand
Chemical contamination; risk of formation of
inhibitors; Cost
Thermo-chemical Chemical dose; exposure
time; temperature √√√ √√ Low energy
demand
Chemical contamination; risk of formation of
inhibitors; Cost
Enzymatic Enzyme dose; exposure
time; pH, temperature √ √ Low energy
demand Cost, sterile conditions
5. Algae Biogas Impurity Removal and
Upgrade Technology
Biogas produced in AD plants or landfill sites is primarily
composed of CH
4
and CO
2
with smaller amounts of H
2
S, NH
3
and N
2
. Trace amounts of H
2
, VOCs and O
2
may be also
present in biogas and landfill gas. Usually, the gas is saturated
with water vapor and may contain dust particles. Additionally,
organic silicon compounds are usually present in particular
with reference to landfill gas, however their presence was
highlighted also in AD biogas. The heating value of biogas is
determined mainly by the methane content of the gas [53].
The main impurities are CO
2
which lowers the calorific
value of the gas and sulfuric acid (H
2
S) which could cause
several problem on the plants and for human health, in fact on
the plants it causes corrosion (compressors, gas storage tank
and engines), while it’s toxic after its inhalation. Although
CO
2
is a major problem in the biogas as its removal is useful to
adjust the calorific value and the relative density, and the
removal of H
2
S can be of crucial point to the technological and
economic feasibility of upgrading process of the gas [54].
Biogas production is growing and there is an increasing
demand for upgraded biogas, to be used as vehicle fuel or
injected to the natural gas grid. To enable the efficient use of
biogas in these applications the gas must be upgraded. Since
separation of CO
2
and N
2
from CH
4
is significantly important
in natural gas upgrading, and capture/removal of CO
2
, CH
4
from air (N
2
) is essential to greenhouse gas emission control.
Removal of CO
2
is done in order to reach the required Wobbe
index of gas. As methane has a 23-fold stronger greenhouse
gas effect than CO
2
, it is important to keep methane losses low,
for both economic and environmental reasons.
In general, in the standards requirements on Wobbe index
values and limits on the concentration of certain components
such as sulfur, oxygen, dust and the water dew point, as well as
a minimum methane volumetric concentration of 96% are
defined. There are several different commercial methods for
reducing the CO
2
content of biogas. Two common methods of
removing carbon dioxide from biogas are absorption (water
scrubbing, organic solvent scrubbing) and adsorption
(pressure swing adsorption, PSA). Less frequently used are
membrane separation, cryogenic separation and process
internal upgrading, which are a relatively new method,
currently under development. The upgraded biogas is often
named biomethane. Various technologies can be applied for
removal of contaminants.
When CO
2
and other impurities are removed during the
upgrading process, the methane concentration increases and
thus the resulting biomethane can be utilized as an alternative
to natural gas. Starr et al. [55] articulated on the carbon
capture technologies that upgrade biogas by removing its CO
2
content. There are quite a few different technologies on the
market today. The main unit operations used are absorption,
adsorption, membrane separation and cryogenic separation;
further information about these unit operations and their
associated technologies shown in Table 5. A common factor of
all of these techniques is that the removed CO
2
is normally
released back into the atmosphere. In some cases, if its quality
is high enough, it can be used for industrial purposes such as
increasing the CO
2
concentration for photosynthesis in
greenhouses or for carbonation in food production.
Strevett et al. [56] investigated the mechanism and kinetics
of chemo-autotrophic biogas upgrading. In this experiment,
different methanogens using only CO
2
as a carbon source and
H
2
as an energy source were examined. The selection between
mesophilic and thermophilic operation temperatures is
typically based on whether the completion of reaction or the
rate of reaction is of primary concern. Thermophilic
28 Rameshprabu Ramaraj and Natthawud Dussadee: Biological Purification Processes for Biogas Using Algae Cultures: A Review
methanogens exhibit rapid methanogenesis, while mesophilic
bacteria give more complete conversion of the available CO
2
[56]. They selected Methanobacterium thermoautotrophicum.
The organism works optimally at temperatures of 65–70 °C
and has a specific requirement for H
2
S, so both unwanted
components are removed. A synthetic biogas of 50–60% CH
4
,
30–40% CO
2
and 1–2% H
2
S was mixed with H
2
to a final
mole fraction of H
2
: CO
2
equaling 0.79:0.21. The gas mixture
was fed to the hollow fibers packed with organisms. This
biological system can effectively remove CO
2
and H
2
S, while
approximately doubling the original CH
4
mass. Alternative
physicochemical treatment methods only remove the
contaminating gas components, without changing CH
4
mass.
Furthermore, physicochemical treatment generates additional
waste and unwanted end products. The purified biogas
contains about 96% CH
4
and 4% CO
2
, while H
2
and H
2
S were
not detected [56].
Table 5. Current biogas upgrading technologies (adopted from Starr et al. [55]).
Unit operation
Technology Acronym Description of process
Absorption High pressure water scrubbing HPWS Water absorbs CO
2
under high pressure conditions. Regenerated by depressurizing
Chemical scrubbing AS Amine solution absorbs CO
2
. The amine solution is regenerated by heating
Organic physical scrubbing OPS Polyethylene glycol absorbs CO2. It is regenerated by heating or depressurizing
Adsorption Pressure swing adsorption PSA Highly pressurized gas is passed through a medium such as activated carbon. Once the
pressure is reduced the CO
2
is released from the carbon, regenerating it
Membrane Membrane separation MS Pressurized biogas is passed through a membrane which is selective for CO
2
Cryogenic Cryogenic separation Cry Biogas is cooled until the CO
2
changes to a liquid or solid phase while the methane
remains a gas. This allows for easy separation
Table 6. Comparison table of CO
2
fixation in the uptake rate and consumption efficiency.
Culture Species CO
2
source biomass Uptake rate
(mg/L/day)
consumption
efficiency (w/w) Reference
Pure Chlorophyta sp. 10% CO
2
8.2 g/m
2
/d 4 (10% CO
2
) 9% Hase et al. [57]
Chlorella sp. 13.2 g/m
2
/d 6 (10% CO
2
)
Chlorella sp. 1, 5, 10% CO
2
2.25 g/L 83 (10% CO
2
)
4% Ramanan et al. [58]
Spirulina platensis 2.91g/L 70 (10% CO
2
)
2% a
Mixed - Diffusion from air 22 t as C 19 (50cm) 44% Green et al. [59]
- Injected CO
2
11.5 kg/day as C
10.2 kg/day as C 187 148% Weissman and Tillett
[60]
Dominant species
a
Diffusion from air 0.126 g/L (TSS
c
) 162 123% Tsai [20]
Dominant species
b
Diffusion from air 0.136 g/L (TSS
c
) 175 131% Ramaraj [13]
Note:
a
the genera Chlorella, Oscillatoria, Oedogonium, Anabaena, Microspora and Lyngbya
b
the genera Anabaena, Chlorella, Oedogonium and Oscillatoria;
c
total suspended solids (as a biomass)
6. Biogas Purification Using Algae
Biological Biogas Purification Methods
and Techniques
Microalgae are a group of unicellular or simple
multicellular photosynthetic microorganisms that can fix CO
2
efficiently from different sources [12–16], including the
atmosphere, industrial exhaust gases, and soluble carbonate
salts. Furthermore, combination of CO
2
fixation, biofuel
production, and wastewater treatment may provide a very
promising alternative to current CO
2
mitigation strategies.
Presence of chlorophyll and other pigments help in carrying
out photosynthesis. The true roots, stems or leaves are absent.
Mostly they are photoautotrophic and carry on photosynthesis,
some of these are chemo heterotrophic and obtain energy from
chemical reactions as well as nutrients from preformed
organic matter. Beside the plants, since algae had high
potential CO
2
fixation in the current knowledge.
Microalgae can fix CO
2
using solar energy with efficiency
ten times greater than terrestrial plants [13, 16]. The issue of
greenhouse gas attracts an enormous attention worldwide
recently. When atmospheric CO
2
concentration increased, it
would gradually disturb the balance of global climate to cause
unusual and astounding phenomena on earth. Therefore, we
require the rapid development of bio-carbon-fixation
technology to eliminate the adverse effects of CO
2
, to transfer
atmospheric CO
2
through the carbon cycle and to promote
carbon balancing ecologically. Currently, many innovative
alternatives of physical, chemical and biological technologies
of CO
2
mitigation are rapidly developed.
At present, algae application of CO
2
sequestration has
developed as a popular topic and the current interests are
including: species, power plant flue gas utilization, reactor
design, growth condition, growth kinetics and modeling. The
most studies in the literature concerned the maximum CO
2
uptake rate by the artificial photo-bioreactors [12, 13, 20].
Among those techs, bio-eco-technology is the most natural
and ecological way to accomplish the designed targets by the
utilization of “self-designed” bio-functions of nature [12, 13,
15, 16]. The different sources and approaches of algal CO
2
uptake rate and consumption efficiency was presented in
Table 6. Accordingly, algae production has a great potential
for CO
2
bio-fixation process and deserves a close look.
Biogas purification/scrubbing using algae involved the use
of algae’s photosynthetic ability in the removal of the
impurities (mainly CO
2
and H
2
S) present in biogas, leaving a
purified biogas containing almost pure methane, which could
International Journal of Sustainable and Green Energy 2015; 4(1-1): 20-32 29
be used for energy generation. Biological purification
technology is worth examining because has double impact.
The method about removing CO
2
from biogas by microalgal
culturing using the biogas effluent as nutrient medium and
effectively upgrade biogas also simultaneously reduce the
biogas effluent nutrient [61]. Using biogas as a source of
carbon dioxide has two main advantages: the biomass
production costs are reduced and the produced biomass does
not contain harmful compounds, which can occur in flue gases.
Hendroko et al. [62] verified xhibit that microalgae
(Scenedesmus sp.) in laboratory experiments using biogas
slurry as growing medium and biogas are given periodically
generate 21% of CO
2
compared with 24% of controls. They
summarized: digestion slurry with seed cake JatroMas cultivar
as raw material is able to increase growth of microalgae
Scenedesmus sp. higher than standard media; microalgae
Scenedesmus sp. is able to capture CO
2
gas in bio-methane;
with integration of slurry and bio-methane intake, there is
tendency Scenedesmus sp. growth is more increasing;
Mutualism symbiosis among slurry, bio-methane and
microalgae Scenedesmus sp. will give impact to increasing of
CH
4
content in bio-methane. In other word, microalgae can be
work as purification biologic from bio-methane [62].
There are several authors [10, 62, 63] reported that
Arthrospira sp, Chololera vulgaris SAG 211-11b, Chlorella
sp. MM-2, Chlorella sp. MB-9, Chlorella vulgaris ARC1,
Chlamydomonas sp. dan Scenedesmus sp. was a positive
synergy with biogas. The productivity of the system with
Zarrouk media and biogas almost 5 times higher than that for
the same media without biogas when piggery waste was used,
the utilization of biogas brings a productivity gain of about
2–5 times higher [63].
Kao et al [64] demonstrates that the microalga Chlorella sp.
MB-9 was a potential strain which was able to utilize CO
2
for
growth when aerated with desulfurized biogas (H
2
S < 50 ppm)
produced from the anaerobic digestion of swine wastewater.
The demonstrated system can be continuously used to upgrade
biogas by utilizing a double set of photobioreactor systems
and a gas cycle-switching operation. Furthermore, they
demonstrated that the efficiency of CO
2
capture from biogas
could be maintained at 50% on average, and the CH
4
concentration in the effluent load could be maintained at 80%
on average, i.e., upgrading was accomplished by increasing
the CH
4
concentration in the biogas produced from the
anaerobic digestion of swine wastewater by 10%.
Some literatures mentioned about the cultivation
microalgae using biogas as CO
2
provider. Kao et al. [64] used
biogas that contained 20±2% CO
2
for Chlorella sp. culture
with variation of light intensity which was at cloudy and at
sunny day. Kao et al. [10] used biogas that contained 20±1%
CO
2
for Chlorella sp. culture with variation flow rate of biogas
which was 0.05; 0.1; 0.2; 0.3 vvm. Douškova et al. [65]
investigated the potential of biogas as CO
2
provider for
Chlorella vulgaris; and optimization of biogas production
from distillery stillage is described. The growth kinetics of
microalgae Chlorella sp. consuming biogas or mixture of air
and CO
2
in the concentration range of 2–20% (v/v)
(simulating a flue gas from biogas incineration) in
laboratory-scale photo-bioreactors. It was proven that the raw
biogas (even without the removal of H
2
S) could be used as a
source of CO
2
for growth of microalgae. The growth rate of
microalgae consuming biogas was the same as the growth rate
of the culture grown on a mixture of air and food-grade CO
2
.
Several species of algae can metabolize H
2
S [66]. Using a
biological system to remove H
2
S has similar benefits to using
one to remove CO
2
: lower upkeep costs, more
environmentally sustainable and non-hazardous waste.
Furthermore, Tongprawhan et al. [67] used oleaginous
microalgae to capture CO
2
from biogas for improving
methane content and simultaneously producing lipid. They
screened several microalgae for identify their ability to grow
and produce lipid using CO
2
in biogas. Finally, they reported a
marine Chlorella sp. was the most suitable strain for capturing
CO
2
and producing lipid using biogas (50% v/v CO
2
in
methane) as well as using 50% v/v CO
2
in air. Sumardiono et
al. [68] established to evaluate the design of the
photobioreactor system for purifying biogas through the
culturing of microalgae. This system represented a simple
promising way for the current forthcoming technologies of
biogas purification. It helps to decrease the concentration of
CO
2
in biogas concomitantly producing microalgae biomass.
The microalgae Nannochloropsis is able to use CO
2
from
biogas produced from the anaerobic digestion of tannery
sludge. The results show that cultivation of microalgae under
the biogas to scrub out CO
2
and promote enrichment of
methane in the biogas in this work and obtained scrubbing of
27% from 30%.
The biocapture of CO
2
by microalgae can be applied to
improve the quality of biogas by reducing the CO
2
content as
this would lead to an increase in the methane content [69]. The
microalgae Chlorella sp.was analysed in terms of conditioning
biogas. As a result the biogas components CO
2
and H
2
S could
be reduced up to 97.07% and100%, respectively. Also an
increase of microalgae cell count could be documented, which
provides interesting alternatives for the production of algae
ingredients. Consequently, the algae biological purification is
an alternative to other biogas purification methods.
7. Conclusion
Biogas is a promising and valuable renewable energy
source. Biogas can be utilized in several ways; either raw or
upgraded. As a minimum, biogas has to be cooled, drained and
dried immediately after production, and almost always it has
to be cleaned for the content of CO
2
, H
2
S and other impurities.
Using the photosynthesis of algae to remove the CO
2
from
biogas is an alternative method that solves the problems of the
common non-biological methods. Algae are self-sustaining
with the addition of minimal nutrients and light. Algae were
used as a biological method to remove CO
2
through
photosynthesis. Algae has several advantages over
conventional chemical CO
2
removal methods because algae is
inexpensive to obtain, requires only light and minimal
nutrients in addition to the CO
2
for growth, and the waste can
30 Rameshprabu Ramaraj and Natthawud Dussadee: Biological Purification Processes for Biogas Using Algae Cultures: A Review
be harvested for biofuels. Several species of algae can
metabolize H
2
S. The H
2
S content in biogas, at levels higher
than 300–500 ppm, damages the energy conversion technique.
Today biological cleaning reduces the content of hydrogen
sulphide to a level below 100 ppm. Using a biological system
to remove H
2
S has similar benefits to using one to remove
CO
2
: lower upkeep costs, more environmentally sustainable
and non-hazardous waste. Maintaining a pure culture would
increase the efficiency of the algae in processing CO
2
. Using
biological metabolism to purify biogas is a promising means
of biofuel production. The incorporation of algae in
photobioreactors to purify biogas has several advantages over
conventional chemical methods of CO
2
removal. Obtaining
algae is relatively inexpensive because culturing algae
requires minimal nutrients for their growth. Growth of the
algae requires a light source as well, which does not
necessarily have to be expensive if illumination is provided by
natural sunlight, which is not limited in supply.
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