4. less than a day.(Tredici 2010) Algal harvesting

Potential of microalgae as a feedstock

Characteristics of microalgae for biofuel production                                                                                              

Cultivation and Harvesting of Microalgae


4.   Potential of microalgae as a

As the demand for fuel resources
increases by the growth of population and global economy new alternate
resources are needed.(Razeghifard 2013).The idea about using the microalgae as feedstock for biofuel
production is not new, but now it is discussed seriously because elimination of
fossil fuels and the harm of global warming is associated with fossil fuels.(Chisti 2007).Algae could be prove a viable alternate energy resource as they
use photosynthesis to take CO2 by changing it into carbon source as biofuel.(Razeghifard 2013)t. Now this biofuel is produced by CO2, water, and sunlight which
are all renewable sources.(Razeghifard 2013).Algae have cells that converts CO2 into Biofuel, foods and
bioactive sources.(Chisti 2007) Algae accumulate carbohydrates and lipids that make algae a good
feedstock for biofuels such as bioethanol and biodiesel production. Under
optimum growth conditions algae grow relatively fast than other species and in
few hours, they double its number. (Ahn, Hwangbo et al. 2012) They grow at very high rate 100 time faster than other plants and
their biomass is double in less than a day.(Tredici 2010) Algal harvesting time is very short take less than 10 days and
could be done continuously. They are not like conventional crop which can only
harvested one or two times a year. Most of algae need a tropical environmental
condition however some algal species like chlorella sp. that is isolated form an
arctic sea ice could grow at 4 to 32 C temperature.(Ahn, Hwangbo et al. 2012) Algae and some cyanobacteria like spirulina platensis could grow
in alkaline environments.(Gimmler and Degenhard 2001) Microalgae could provide many types
of biofuels including methane by digestion of algae anaerobically, biodiesel from microalgal oil and gas produced in photosynthesis.(Chisti 2007)

 The potential advantages of microalgae as
feedstock for biofuels are followings:

produced by algae do not have adverse effects on other agriculture because it
does not compete with other food crops for land.

can be grown in different in every environment, like seawater etc.

growth can be helpful in the treatment of waste water as they remove
phosphorous and carbon dioxide.

is helpful as it could be used to produce the broad range of the biofuels such
as hydrogen, syngas and methane also.(Han, Jin et al. 2015)

could be grow round all the year, so production of oil exceeds to the yield of
the oilseed crops.

grow in the aqueous environment but need much less quantity of water than other
crops so they reduce the freshwater source load.

can be harvested in saline water therefore it does not experience land use

have exponential growth rate that they have oil content range from 20-50%
biomass (dry weight) and they double their biomass in very short time as 3.5

  Algal cultivation does not need any
pesticides or herbicides applications.

could also produce many useful co products like residual biomass from oil
extraction and proteins which are used as fertilizers and feed.

The Combination of CO2 fixation,
treatment of wastewater, biohydrogen production and potential of biofuel
production are the potential of microalgae to use as a feedstock. (Brennan and Owende 2010)


Characteristics of microalgae for biofuel production

Microalgae accrue carbohydrate
components and lipid compounds which make them the most favorable feedstock of
biofuels production such as bioethanol and biodiesel. (Razeghifard 2013) They can be grow in every
environment and all year-around which make them suitable for using as
feedstock.(Razeghifard 2013) Microalgae contains fatty acids and
lipids as membrane components, metabolites, source of energy and storage
products that make microalgae a viable possibility as a feedstock for biofuel.(Demirbas and Demirbas 2011) Microalgae use photosynthesis to convert the light into chemical
energy and as they can grow in every environment that is not possible for other
current feedstocks of biodiesel such as rapeseed, soybean, and palm oil.
Microalgal growth rate is higher than agricultural crops, conventional forestry
and other aquatic plants. They need less area than other feedstocks 49 to 132 times
less than soybean crops.(Chisti 2007)  Microalgae can play role as feedstock for many different kinds of
renewable biofuels such as methane, hydrogen, biodiesel and bioethanol.
Microalgal biodiesel does not contain sulfur and performs truly as petroleum
fuel.(Delucchi 2003) The most significant character of
microalgal oil is its biodiesel yield, according to estimation studies the
yield of biodiesel from algae per acre is over 200 time than the yield of other
crops and plants.(Demirbas and Demirbas 2011) If algal biodiesel is produce commercially it would reduce the cost
of per barrel of oil from 100$ to 20$ per barrel only. (Demirbas and Demirbas 2011)

The application microalgae for
production biofuel can also aid other purposes, some of them are the

of CO2 from industrial gases by algal fixation reduce the greenhouse
gasses effect while biodiesel production. (Mata, Martins et al. 2010)

of wastewater by removal of NO3, PO4, NH3 making microalgae to grow by using
these water pollutants as nutrients.(Wang, Li et al. 2008) 

the oil extraction the resultant algal biomass could be treated into methane,
ethanol feedstock used as bio fertilizer because of its high ratio of N:P or
can be burned for energy like heat and electricity.(Wang, Li et al. 2008)

 Microalgae have high value bio derivatives so
microalgae carry the potential to transform huge number of biotechnology fields
such as biofuels, pharmaceuticals, cosmetics, nutrition, food additives and
pollution prevention.(Mata, Martins et al. 2010)


Cultivation and Harvesting of Microalgae


Ø  Cultivation:

Microalgae can adjust or molecular structure such as physiological
and biochemical acclimation so they grow in different enviornments.(Richmond 2004) As they adapt
many modes for their growth so there are severel methods for microalgal
cultivation. For example, photoautotrophic, heterotrophic, photoheterotrophic
and mixotrophic.(Mata, Martins et al. 2010) they can adjust themselves metabolically in response of
environmental change so different species can grow in different environmental
conditions such as

Photoautotrophic cultivation: uses light source
to produce the chemical energy by photosynthesis reaction, In this process, the
light is used as energy source and the inorganic carbon is taken as source of
carbon.(Wang, Yang et al. 2014) Currently, this method is the only process which is
economically and technically suitable for the Viable production of algal
biomass.(Brennan and Owende 2010)  Two methods are developed for the photoautotropically
cultivation of algae, (1) Open pound production. (2) Closed photobioreactors.(Borowitzka 1999) A carbondioxide rich environment can increase the productivity
of biomass to a certain level.(Chiu, Kao et al. 2008) However, the light penetration decreases rapidly by
the increment of broth turbidity this method got difficulty to achieve the very
large-scale productivity of biomass.(Markou and Georgakakis 2011) Lipid concentration have great importance as a
parameter for biodiesel production.(Demirbas and Demirbas 2011)  The lipid concentration (dry weight) varies in microalgae in
ranging 5-68% in photoautotrophic cultivation.(Chen, Yeh et al. 2011)  Large content of lipids could be obtained in the environment
in which nutrient or nitrogen is present in less amount and limited
concentration, however the productivity of biomass that is achieved in this
type of stressed environment if usually much lower than that is obtained in the
normal conditions, which results in even lower productivity of microalgal
lipid.(Wang, Yang et al. 2014) The higher productivity of biomass can be obtained by the
photoautotrophic bioreactors than open pound production of microalgae. (Brennan and Owende 2010)

Heterotrophic Cultivation: In this method
microalgae is grown on organic source. Carbon source is used as both carbon and
energy source.(Brennan and Owende 2010) Heterotrophic method avoids the limitations that were present
in the photoautotrophy, so higher productivity of biomass can be achieved.(Wang, Yang et al. 2014) Lipid concentration in this method is much higher
than photoautotrophic method.(Wang, Yang et al. 2014)  This method is light independent so simpler and smaller
fermenters or reactors are used for cultivation.(Brennan and Owende 2010) These setups produce a higher level of growth and
also decrease the cost of harvesting because of high level of cell densities
obtained.(Chen and Chen 2006) Wide range of organic substrate are utilized in
heterotrophy such as organic carbon, sugar sources are mostly adopted and the
most effective forms.(Liang, Sarkany et al. 2009) however some cheaper sources like glycerol and corn
powder can also provide some good yields.(Wang, Yang et al. 2014)

Large number of organic compounds and fermenters are required for
heterotrophic cultivation so the cost of this method is higher than the photoautotrophy.(Wang, Yang et al. 2014) The other disadvantage of heterotrophy is that susceptibility
to contamination to the other microorganisms which may lower the quantity and
the quality of the products.(Chen, Yeh et al. 2011)

Photoheterotrophic cultivation: this mode is also
knwn as photoassimilation, photometabolism and photoganitrophy, the mode in
which the light is needed to use the organic compounds as the source of carbon.(Mata, Martins et al. 2010)


Mixotrophic Cultivation: Mixotrophic
cultivation is the method in which organism are capable of using both mode
either autotrophic or heterotrophic.(Brennan and Owende 2010)  Microalgae can
utilize both organic carbon and inorganic source.(Wang, Yang et al. 2014) Means that thay can photosynthesis or ingest
organic compounds.(Zhang, Zhang et al. 1999) Inorganic compounds are fixed by photosynthesis and
organic sources are adjusted by aerobic respiration that is affected by the
amount of available organic carbon.(Hu, Min et al. 2012) Main difference among the Mixotrophy and the
photoheterotrophy is that the photoheterotrophy need light as the main energy
source where the mixotrophy could use the organic components to attain that,
photoheterotrophy need light and the organic compounds at the same time. And it
is rarely adopted to produce the microalgal diesel.(Chen, Yeh et al. 2011) Some studies show that the microalgal growth rate
in mixotrophic is almost the total sum of growth under heterotrophic and
photoautotrophic modes.(Marquez, Sasaki et al. 1993)  Meanwhile mixotrophic cultivation utilized the organic
compounds, the growth or algae does not rest on photosynthesis: light is not a
limiting aspect for growth of microalgae, therefore photo-inhibition or
photolimitation can be decrease in the mixotrophic culture when the light levels
are too high or too low.(Wang, Yang et al. 2014). Mixotrophic cultivation can enhance the growth
rate, reduce the growth cycle, decrease the biomass loss when light is not
present due to clean respiration and enhance the productivity of biomass.(Andrade and Costa 2007) Lipid content can be increased as well, that can
lead to higher productivity of lipid and it have much importance for the
production of the microalgal biodiesel.(Wan, Liu et al. 2011) Furthermore the carbondioxide released via aerobic
respiration through microalgae can be reused for the further photosynthesis in
mixotrophic cultivation, which increased the productivity of lipids and biomass.(Mata, Martins et al. 2010) Mixotrophic cultivation of the microalgae produce
higher yields of organic carbon, In view of the required carbon sources donate largely
to the cost/energy inputs of algal cultivation.(Zhang, Yan et al. 2013) Mixotrophic cultivation reduce the cost of cultivation of
microalgae so this mode is much beneficial for the production of microalgal
biodiesel.(Wang, Yang et al. 2014)


Ø  Harvesting:

Microalgal harvesting takes 20-30% cost from the total production of
biomass.(Grima, Belarbi et al. 2003)  Different harvesting methods could be adopt depending on the
density, size and the value of the product.(Olaizola 2003) Generally harvesting methods are consist of two

Bulk harvesting: isolate microalgae from suspension like floatation,
flocculation and gravity sediments.(Lam and Lee 2012)

Thickening: To concentrate the algal slurry after the bulking technique.(Lam and Lee 2012) Techniques that are used in this stage are Centrifugation, ultrasonic aggregation and filtration. So, this is more concentrated
stage then bulking.(Brennan and Owende 2010)

Four key methods are used for harvesting regarding
to biofuel production, belt filtering, sedimentation, flotation and mirceostaning.
The methods depend on the density and size of the microalgae. Microstrainers
are striking method for harvesting because its mechanical simplicity and the large
size unit availability. The new and better accessibility polyester screens have
revitalized their interest for their use in harvesting microalgae. Recent
studies resolve that it would be essential to flocculate the cells before the
microstraining process.(Grima,
Belarbi et al. 2003)

Till now centrifugation and filtration are not feasible mode for microalgal
harvesting at commercial level, they need high energy and their high level of
maintenance cost make then unfavorable for long term use. While on the other
side, flocculation requires less energy for microalgal harvesting. This happen
because the microalgal cells carry -ve charge and due to this they repel each
other and could be suspended in the medium for long period when mixing is not
forced to them. By adding coagulant having +ve charge into the medium, the
charge of microalgal cell would neutralized. Meanwhile flocculant could be
neutralized and form dense flocs which settle under the natural gravity.(Lam and Lee 2012)