Considerations for Photobioreactor Design and
2
Operation for Mass Cultivation of Microalgae
3
Juan Cristóbal García Cañedo and 4
Gema Lorena López Lizárraga 5
Additional information is available at the end of the chapter
6
Abstract
8
Microalgae have great biotechnological potential for production of substances
9
through photosynthesis. Light capture process and electron transportation imply
10
energy losses due to reflection, fluorescence emission, and energy dissipation as heat,
11
giving a maximum theoretical value of ‐ % for microalgae energy capture efficien‐
12
cy and conversion to biomass. For development of full potential of microalgae the
13
knowledge of the light capture process is required. High yields can only be obtained
14
linking photobioreactor design with biological process taking place inside. In massive
15
microalgae cultures, light gradients are generated and this depends on the biomass
16
concentration, cellular types, cells sizes, and pigment content, and also on geome‐
17
try, hydrodynamic, and light conditions inside the photobioreactor. In the present
18
chapter we explain the relationship between light energy capture process and
19
photobioreactor design and operation conditions, like turbulence, gas exchange, and
20
nutrient requirements. Finally, the productivity and costs are discussed, and the
21
parameters that determine the economic viability of any microalgae culture.
22
Keywords: photobioreactor design, photosynthesis efficiency, nutrients, mixing, gas
23
exchange
24
Abbreviations 25
CO carbon dioxide
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
PSI photosystem I PSII photosystem II
N“DPH reduced nicotinamide adenine dinucleotide phosphate
“TP adenosine‐ '‐triphosphate P“R photosynthetic active radiation PCE photon conversion efficiency η energy conversion efficiency
ηsolar solar energy conversion efficiency
LHY light harvesting yield
FEY fractional energy yield of the PSII redox products
QY quantum yield
P * excited PSII reaction center
Q” quinone ”
P”Rs photobioreactors
Io incident light intensity on the photobioreactor surface Is photosystems saturation light intensity
PFDin photon flux density incident light intensity on the photobioreactor surface PFD s photon flux density that saturates the photosystems
achl‐a wavelength dependent Chlorophyll a specific absorption coefficient
[Chl‐a] Chlorophyll a concentration r Photobioreactor radial distance
s Distance between the photobioreactor surface and the hypothetical point where light saturation is reached at a certain PFD
Re Reynolds number
Sn Swirl number
U Mean axial velocity component V Mean circumferential velocity L Photobioreactor length S cross‐sectional surface area
dS differential cross‐sectional surface area
Fv/Fm Maximum photosynthetic efficiency of dark adapted cells Fq'/Fm' Operational photosynthetic efficiency of light adapted cells qe specific light uptake rate
“ photobioreactor superficial area V photobioreactor operational volume C Cellular concentration
Ein input light energy to the photobioreactor Eout outgoing light energy of the photobioreactor PMM“ polymethyl methacrylate
FD daily CO fixation in g CO fixed per g CO injected per day F“t+ accumulation of CO fixed during t + d
F“t accumulation of CO during t d mid mass in grams of CO injected each day g .
FDmax % overall maximum percentage daily CO fixation
1
. Introduction 2
From a biotechnological point of view, the term microalgae refers to unicellular organisms
3
capable of carrying out oxygenic photosynthesis, they contain Chlorophyll a Chl‐a and/or
4
other similar photosynthetic pigments. This definition takes into account a very large and
5
diverse group of photosynthetic prokaryotic and eukaryotic organisms, capable to catalyze
6
the process of carbon dioxide CO fixation and its conversion into organic matter. The number
7
of species of microalgae has been estimated from to million [ , ], these organisms are
8
ubiquitous and have managed to colonize almost every habitat known on the planet, from the
9
tropic to the poles.
10
. . Advantages of culturing microalgae
11
Microalgae possess great biotechnological potential for the production of a wide variety of
12
compounds such as polysaccharides, lipids, proteins, carotenoids, and other pigments,
13
vitamins, steroids, among others. Dozens of algal species are used to produce animal feed,
14
human nutrition, cosmetics, and pharmacy industry components. They also find application
15
as wastewater treatment, CO fixation, and greenhouse gas emissions reduction. Microalgae
16
can be used to produce biofuels, hydrocarbons, and hydrogen [ – ]. They can be a clean and
1
renewable energy source because of their high yield and low spatial requirements, if compared
2
to terrestrial plants. Some authors consider microalgae as biodiesel feedstocks for the future [ ].
3
The only natural process that allows the production of biomass using only sunlight as the
4
energy source and CO is photosynthesis. Unicellular photoautotrophic organisms are capable
5
of using sunlight in a more efficient way than superior plants [ ]. The advantages of microalgae
6
culture compared to superior plants are listed [ , , ]
7
. Microalgae biological systems are considered the most efficient for solar light capture, and
8
the production of compounds through the photosynthetic process.
9
. Whole microalgae biomass can be harvested and used because they lack complex
10
reproductive organs and have no vascular systems.
11
. Many algal species produce and accumulate particular compounds of high commercial
12
value can be induced, for example, proteins, carbohydrates, lipids, and pigments.
13
. The isolation, genetic selection, and strain studies is relatively easy and less time con‐
14
suming because microalgae reproduce themselves by simple cellular division and can
15
fulfill their life cycles in just a few hours or days.
16
. Microalgae can be cultivated with low inorganic nutrients concentration. These make
17
them of particular interest as a protein source, assuring protein availability in regions of
18
low agriculture productivity due to the lack of water and nutrient poor soils.
19
. Systems for biomass production can be adapted or scaled up to different operation levels,
20
allowing later incorporation of these systems to fully automated facilities for large scale
21
production.
22
. . Beginning of mass culture of microalgae
23
Large scale culture of microalgae began in the early s in Japan with the culture of Chlorella
24
[ ], this development was followed by the establishments of other facilities, for example in the
25
s it established the culture of Spirulina in the Texcoco lake in Mexico city by the Sosa
26
Texcoco Co. [ ]. In Dai Nippon Ink and Chemicals Inc. established a commercial Spirulina
27
production facility in Thailand and by there were large scale facilities in “sia that
28
produced more than Kg of microalgae per month, mainly Chlorella [ ] and in near
29
tons of Chlorella were commercialized only in Japan [ ]. The commercial production of
30
Dunaliella salina as a source of ‐carotene was the third microalga of industrial production
31
when Western ”iotechnology Ltd. and ”etatene Ltd. installed a production facility in “ustralia
32
in . These were followed by the installation of other production facilities in Israel and in
33
the United States of “merica [ ].
34
Commercial production success of microalgae in large scale facilities depends on many factors,
35
among which, we can mention the development of large scale culture systems of economic
36
feasibility, and development of these systems has been a gradual process [ ].
37
Productivity of microalgae biomass is affected by factors like photosynthetic pigments
1
efficiency of the in capturing and converting light energy to chemical energy, accumulation of
2
dissolved oxygen produced by photosynthesis, insufficient CO mass transfer rate, depletion
3
of nutrients, and photoinhibition [ , ].
4
. Light capturing process and photosynthetic efficiency 5
“utotrophic growth of microalgae depends on photosynthesis, which involves light electro‐
6
magnetic radiation energy absorption and conversion by photosystems I and II PSI and PSII
7
into electrochemical potential and chemical energy N“DPH and “TP . Energy is later used
8
in the CO fixation process [ ]. Thermodynamic efficiency over the P“R region of systems
9
working with low light regimes – µmole m- s- can be below %, decreasing to %
10
under large solar irradiance > µmole m- s- . In addition, under outdoor conditions around
11
% of the captured total light spectrum energy is converted into heat [ ].
12
In an ideal case, a photobioreactor P”R must capture all light available in the environment
13
and transfer it into the culture to be used for biomass production [ ]. “lthough in normal
14
conditions this does not happen [ ] because photosystems are exposed to an amount of light
15
energy below or greater than the amount that can be transformed into chemical energy. If
16
biomass concentration is too low, some of the light is transmitted through the culture.
17
Conversely, if biomass concentration is too high, a dark zone appears. Maximal productivity
18
will require the exact condition of full absorption of all light received but without a dark zone
19
in the culture volume. This is called luminostat regime. ”ut maintaining luminostat regime
20
over the year in outdoor conditions has no interest in practice as it cannot be applied in actual
21
operating conditions due to the disconnect between the dynamics of irradiation conditions
22
below h and biomass concentration changes days [ ].
23
Under sunlight, biomass growth rate is insufficient to compensate for the rapid changes in
24
sunlight intensity. Consequently, light attenuation conditions that are fixed by biomass
25
concentration are never optimal. This is why determining the maximum photosynthetic
26
efficiency and the upper limits of biomass production through photosynthesis has been a
27
central topic of investigation in mass culture of microalgae [ ] since its beginning.
28
. . Phothosynthetic efficiency
29
Only a fraction of the energy of sunlight can be used to build up biomass and derived products.
30
One form of measuring the overall light usage for biomass production is known as photon
31
conversion efficiency PCE [ ], also called energy conversion efficiency η [ , ]. The
32
conversion of light energy is limited by several factors some due to the physical nature of light
33
itself and others inherent to the photosynthetic process [ ]. First, light must travel from the
34
Sun to the Earth surface, losing one part of the energy just by passing through the atmosphere,
35
for the remaining amount of energy, only the part that has a wavelength between ‐ nm
36
can be used for photosynthesis because this wavelength can be captured by the photosynthetic
37
pigments and is known as photosynthetic active radiation P“R .
38
Solar energy conversion efficiency ηsolar depends on three factors Light‐Harvesting Yield
1
LHY , Fractional Energy Yield FEY of the redox products of PSII, and quantum yield
2
QY , therefore ηsolar=LHY·FEY·QY. LHY value depends on the coordinated functioning of the
3
anthena complex, which absorbs light energy through dozens or even hundreds of protein
4
bound pigments, and the oxygen liberating complex of PSII. This last requires Mn as cofactor
5
to split the water molecules and liberate electrons and generate an excitation of the Chlorophyll
6
P * in the reaction centre. LHY has a maximum value of % under ideal conditions. The
7
FEY depends on the electron transfer chain of the PSII, from P * to quinone ” Q” and also
8
has a maximum value of approximately %, again in ideal conditions. The QY denotes the
9
probability that P * formation results in product formation along the main path of redox
10
chemistry and has an accepted value of approximately . [ ].
11
In the light absorption process and the electronic transport chain FHY and FEY , most of the
12
energy is lost due to reflection, fluorescence emission, and energy dissipation as heat by
13
photosynthetic pigments, this makes that the PCE or η decreases to a value of . %. “lso, the
14
conversion of light energy into biomass diminish even more the energetic yield, making the
15
light energy capture and conversion to biomass efficiency of roughly ‐ % [ , , ].
16
Important is to highlight that this last value is the theoretical maximum accepted for microal‐
17
gae. Maximum light energy conversion efficiency and its conversion to biomass in higher
18
plants have an actual value of just % [ ].
19
. . Light and culture conditions
20
Exposure of photosynthetic cells to an excessive amount of light could lead to photoinhibition
21
and to a decrease in the growth rate. On the other hand, the self‐shading effect between
22
individual cells presented in mass cultures of microalgae causes a productivity decrease, even
23
when the amount of light is sufficient for the population of microalgae in the P”R [ ]. The
24
quality of light, which means the light wavelength that is used in photosynthesis by the
25
microalgae cultures, also affects the culture performance [ , ]. During batch culture, or
26
where light is constant, cells can experience photoinhibition at the beginning of the culture,
27
and the limitation of light when a high cell concentration is reached [ ]. This can be avoided
28
using fed batch cultivation or continuous culture mode. For example, Garcia‐Cañedo [ ] have
29
reported that photosynthetic efficiency is maximum when nutrients like nitrogen are supplied,
30
and fed‐batch culture mode application promotes a high maximum photosynthetic efficiency,
31
close to the reported maximum theoretical value.
32
To develop the full potential of photosynthetic organisms, that can be economically feasible
33
and similar to heterotrophic eukaryotic organisms, like yeasts or filamentous fungi, it is
34
required to identify the bottle neck of the bioprocess. The growth of microalgae requires
35
appropriate light capture and conversion into biomass, therefore, it required novel P”R
36
designs with geometries not commonly used for heterotrophic organisms in different opera‐
37
tion modes that promote higher photosynthetic efficiencies [ , ].
38
. Photobioreactor design 1
The fundamental principle for photobioreactor design is a high surface area to volume ratio
2
in order to use light energy efficiently, and is a requirement to obtain high values of PCE
3
Figure . Higher photosynthetic efficiency can result in higher biomass productivity and
4
concentration, but at much higher cost because of high energy use mixing, cooling, and
5
embodied energy and capital cost [ ]. P”R design must include a short light path, which can
6
be obtained using different geometries and low level of liquid to minimize the energy used for
7
mixing the culture [ ]. “t high liquid level, the water column could generate high hydrostatic
8
pressure and require higher energy input for mixing of the culture inside the P”R with air
9
injection.
10
. . Opens systems: open ponds
11
Normally, microalgae and cyanobacteria large scale mass cultivation is done in shallow open
12
ponds tanks, of circular or raceway type, with solar light. One of the major advantages of using
13
open systems is that they are easy to build, operate, and they have lower costs than closed
14
systems. Even though it has been demonstrated that open pond culture is economically
15
feasible, they still have some disadvantages and limitations, they use light in a very inefficient
16
manner, have evaporation water loses, low CO mass transfer rate from the atmosphere, due
17
to its inefficient mixing mechanisms open ponds also require a large area of land for the culture
18
due to its shallow depth. “dditionally, open systems can be contaminated with predators or
19
fast growing microorganisms like bacteria that can compete with microalgae for nutrients, this
20
is why open systems are only used for organisms that can tolerate extreme conditions [ – ],
21
like high salinity or pH. The scaling up of open ponds culture systems can only be performed
22
by increasing the area, because increasing depth will not increase light penetration leading to
23
lower productivities.
24
. . Closed systems: photobioreactors
25
To overcome the problems detected in open systems it has been proposed the use of closed
26
photobioreactors P”Rs . The former are more appropriate for strains that cannot tolerate
27
extreme environments or when final product is highly susceptible to degradation or contam‐
28
ination. Closed systems also allow the prevention of contamination, allowing the operation in
29
culture modes like photoautotrophic, heterotrophic, or mixotrophic. “lso, closed systems can
30
obtain up to three times more biomass than open systems, thus reducing harvesting costs [ ].
31
Despite the great advances that have been achieved in the construction and operation of P”Rs,
32
its technology is still in development. “round % of current biomass production worldwide
33
is obtained in open systems, despite the fact that P”R technologies offer greater potential in
34
terms of productivity, control of culture conditions, and applicability to cultivate various
35
strains [ ].
36
Several closed P”R designs that are in operation are in laboratory, pilot plant levels, and even
37
some have been successfully scaled up to an industrial level. One of this successful closed P”R
38
design is the tubular type, in this the tubes configuration where the culture is hold is one of
39
the main factors affecting productivity of photosynthetic biomass [ ]. Tubular P”Rs can be
1
built with plastic materials like rigid transparent polyvinyl chloride PVC , polycarbonate or
2
flexible plastic bags, among other materials. They can be arranged in vertical, horizontal,
3
conical, and inclined form, with degasifying units that allow the removal of the O produced
4
during photosynthesis [ ], the tubes can also be arranged in an annular form [ , ], each of
5
these forms affect the productivity expected in this type of systems.
6
“ppropriate design of vertical tubular P”Rs can reduce the culture area and distribute
7
photosynthetic organisms in vertical columns. Vertical reactors can increase the exposure of
8
the organisms to light, and also the contact time between gas and liquid, thus increasing
9
residence time of CO and the efficiency of CO assimilation [ , , ]. Vertical columns can
10
be compact, low cost, easy to operate aseptically [ ], and very promising for large scale
11
culture. It has been reported that vertical P”Rs, vertical columns, and airlift type P”Rs of even
12
. m of diameter can reach a final biomass concentration and a specific growth rate compa‐
13
rable to values reported for P”Rs of smaller diameters [ , ]. The main disadvantages of
14
vertical systems are light reflection and/or incidence of light at the peak hours of the day when
15
the sun is in the summit due to the incidence angle of light on the P”R surface, also vertical
16
form can generate hydrodynamic and shear stress if the height of the P”R is too large.
17
Horizontally displayed tubular P”Rs are considered appropriate for mass cultivation of
18
microalgae because they possess a large illuminated area and have better usage of light at sun
19
summit. Some have been successfully scaled up to a volume of L or more [ , ]. Even
20
though this P”R design generate oxygen accumulation, when used outdoors can present
21
photoinhibition [ ]. When scaling up these systems, it must also be considered that increasing
22
the diameter of the tubes will decrease the area to volume ratio, and the increase of the length
23
of the tubes could generate CO and nutrient gradients and oxygen concentration that could
24
rise up to toxic levels [ , ]. Formerly described designs are combined in inclined tubular
25
P”Rs, which have lower hydrodynamic stress and better illumination because the incidence
26
light angle can be adjusted with the inclination of the P”R, also mixing is better than in
27
horizontal tubular P”Rs [ ].
28
Flat panel P”Rs have also been studied in order to make an efficient use of light for algal
29
biomass production [ , ]. These P”Rs have a large illumination surface and the advantage
30
of high area to volume ratio, and therefore optimum illumination of the cells and low oxygen
31
concentration can be achieved. There are varieties with flat and curved semicircular bottom.
32
In this last form, mixing dead zones is avoided and favour biomass accumulation [ , ].
33
“lthough, it is difficult to achieve efficient biomass productivity per area of land using flat
34
panel P”Rs. Factors affecting biomass productivity in this type of reactors are the angle,
35
direction of flat panels, and the number of panels per land unit [ ], also their scale up requires
36
the addition of compartments and support materials for the P”R [ ]. Generally flat panels are
37
displayed in vertical form but they can also be arranged inclined. Examples of different types
38
of P”Rs can be observed in Figure .
39
1
Figure . Examples of different photobioreactor designs. ”asic designs a flat panel, b vertical tubular, c horizontal
2
tubular [ ], d Flat panel building integrated P”R [ ], e Mass cultivation , Liters P”R in operation in Spain [ ],3
f Floating horizontal P”R [ ].
4
. . Photobioreactor design considerations: power and cost effective designs
5
Unconventional designs have been proposed, which include alveolar type [ ], alpha form
6
Figure a [ ], flat panel and tubular conic form Figure b [ , ], and also spiral tubes [ ].
7
When proposing new design forms it must be considered to combine a high productivity with
8
a low need for auxiliary power [ ]. “ German enterprise called Subitec [ ] has developed a
9
flat panel airlift P”R of L Patents EP ” and EP ” for outdoors culture
10
Figure c, that consists of two fine layers of sealed plastic bags, this design includes baffles
11
to generate vortex that enhance turbulence and mixing of the culture inside the reactor,
12
improving light utilization. This company claims that the energy consumption of their P”R is
13
only W m-, consumption verified in pilot plant scale. Company Proviron [ ], has
14
developed plastic bag P”R in multiple vertical panels of cm thick, this reactor is displayed
15
unrolling a plastic bag film and does not require additional support Proviron claims that in
16
their design, it is possible to achieve a biomass concentration of g L- with a low investment
17
cost of only Euros m-, the most important issue about this design is its low auxiliary power
18
of W m-, which represents approximately half of the maximum value that allows economic
19
feasibility in Central Europe [ ].
20
1
Figure . Examples of unconventional photobioreactor designs. a Conic flat panel b alfa form c Subitec flat panel P”R
2
d Proviron plastic bag P”R. Images taken from [ – ], http //subitec.com/en/flat‐panel‐airlift‐fpa‐photobioreactor and
3
http //www.proviron.com/node/ .4
More recently, buoyant inexpensive plastic film P”Rs has been developed. It consists of two
5
plastic films forming the top and bottom surfaces of the horizontal raceway, sealed to each
6
other and connected to two vertical airlift units. This design combines the advantages of open
7
ponds and closed systems in a cost‐effective way and can be used on both water and ground,
8
depending on the end user's particular needs [ ].
9
There is significant incentive to design and operate algal P”Rs with high biomass productivity
10
and conversion efficiency. “lthough many factors affect performance of P”Rs, such as the type
11
of P”R, culture media, temperature, pH, microorganism used, CO mass transfer, O accumu‐
12
lation, mixing, light intensity, and light/dark cycles. “mong these, the major limiting factors
13
for growth of microalgae are usually light availability and interphase mass transfer [ ].
14
. Turbulence and light capture 15
Nutritional and light requirements of photosynthetic microorganisms can be covered in high
16
light path P”Rs [ ], higher than . m, if the design and operation characteristics are
17
adequate, for example low mixing time and high axial dispersion. “mong the advantages of
18
using high light path P”Rs are the decrease on construction cost and in energy expenditure,
19
also, it can contain more liquid quantity in less land area.
20
In dense microalgae cultures, incident light intensity on the P”R surface Io decreases with
21
culture depth, and in a certain depth it reaches an intensity equal to the saturation of photo‐
22
systems Is , this is why all the light that penetrates further will be used with maximum
23
efficiency [ ]. Grobbelaar [ ] suggested that in a P”R with dense culture of microalgae exists
24
a light gradient and several illuminated regions designated as follows Light limited region,
25
is the deepest part of the P”R, Light saturated region, is the region where all the penetrating
26
light can activate photosynthesis and saturation of photosystems is achieved, Photoinhibi‐
1
tion region, presented in some cases in intense light conditions, and corresponds to the outer
2
part.
3
. . Light gradient determination
4
The exact dimensions of each region depends on the concentration of biomass, geometry,
5
hydrodynamic, and light conditions of the photobioreactor, this will be influenced by cellular
6
size, forms, and pigments content [ , ]. The light penetration depth in a photobioreactor
7
can be calculated using the modified Evers model Figure
8
( )
ππ( )
chl aππ
PFDinθ θ π exp a chl a b dθ
PFD s = . π dθ ..
.
cos éëò cos + éë- - ×éë - ×ùû ùû ùû
ò +
9
( )
2( )
2 2 0.5b= - ×r s cosq+éër - -r s ×senq ùû
10
11
Figure . Light gradient calculus illustration according to the modified Evers Model [ ].
12
where PFDin is the photon flux density of the incident light intensity on the photobioreactor
13
surface, PFD s photon flux density that saturates the photosystems, achl‐a wavelength depend‐
14
ent Chl‐a specific absorption coefficient, Chl‐a concentration, r radial distance, and s distance
15
between the P”R surface and the hypothetical point where light saturation is reached at a
16
certain PFD.
17
From above, it can be deduced that the microalgae inside a P”R will be moving between the
18
three light regions due to the turbulence generated by aeration. Higher aeration rate generates
19
more turbulence, and higher turbulence could generate faster movement of microalgae
20
between the light regions.
21
If the nutritional requirements of a mass culture of microalgae are met, and the culture
1
conditions do not limit growth, then a design aimed to create a turbulent flow will be the most
2
important requisite to obtain higher biomass yields [ ].
3
. . Biomass production enhancing: turbulence and mixing effects
4
Turbulence and mixing of cultures have three main effects prevent microalgae sedimenta‐
5
tion, avoid formation of nutritional CO and O gradients, and moving cells through light
6
gradients, where the quantity and quality of light received by cells vary [ ].
7
The turbulence can be measured in two forms, one is using the Reynolds number Re [ ],
8
and the other form is using the Swirl number Sn [ ]. From the definition of the Swirl number
9
[ ], the average turbulence or liquid movement inside a P”R can be calculated using the
10
following expression
11
0 2 0 L
v L S
S
UVrdSdz Sn =
ò ò ò
U rdSdzò ò ò
12
where U is the mean axial velocity component V mean circumferential velocity component r
13
is the radial distance from z‐coordinate and L is the photobioreactor length [ ].
14
The use of Re is recommendable when the liquid properties and change with time are known,
15
for example viscosity, but it is difficult to determine when the geometry of the photobioreactor
16
is special , this is with less common geometries or with baffles that help to create more
17
turbulence, in those cases the use of the Sn is recommendable.
18
If a mass culture of microalgae has good mixing, then the cells will be exposed to a light
19
gradient in their movement through the P”R [ ]. Near the reactor irradiated surface, algal
20
radiative exposure is usually adequate or in excess, whereas a dark volume with insufficient
21
light for photosynthesis to occur often resides only a few centimeters or less from the irradiated
22
surface, depending on the cell concentration [ ].
23
Normally photosynthetic systems become saturated with light irradiance values of approxi‐
24
mately µmol photon m- s-, this value is equivalent to % of the maximum light irradiance
25
in the summer of approximately µmol photon m- s-. The saturation of light is considered
26
one of the main limitations of using solar light efficiently [ ]. The former has awaken the
27
interest of studying energy light usage with the objective of maximizing photoautotrophic
28
organisms culture productivity [ ], through the design of P”Rs with geometries that can
29
enhance and make better use of turbulence.
30
It has been proposed that the control of light irradiance to the culture can be made by giving
31
the necessary amount of light based in cell concentration, aimed to maintain a luminostatic
32
environment inside the P”R. In this sense specific light uptake rate, qe has been studied
33
( in out)
e E E A
q = VxC-
1
where “ and V are the surface and volume of the column or columns that contain the culture,
2
C is the cell concentration. Ein is the input light energy to the P”R and Eout is the outgoing light
3
energy from the P”R, these are quantified calculating the average value of the light intensity
4
measures on points, every π/ radians, on the inner and outer surface of the P”R [ ].
5
“lthough, luminostatic operation is efficient to generate high cell density cultures, it is evident
6
that an amount of light applied to the exterior of a P”R will only be effective if it is combined
7
with good mixing and turbulence Re ≥ of the culture inside the P”R. For this reason, a
8
key factor in the design of P”Rs is the incorporation of mechanisms to periodically transport
9
or expose cells between light and dark regions of the reactor mixing‐induced light/dark cycles
10
[ ].
11
One alternative to make better use of light is providing internal illumination with light guides,
12
which can increase the illuminated surface in the same volume of P”R [ ] and, therefore,
13
resembles artificial leafs described by Janssen et al. [ ]. Solar light is captured by lenses and
14
then transported to the P”R interior with the use of optic fibers [ ] or with the usage of light
15
guides of Polymethylmethacrylate PMM“ , which is a plastic that has more light transmit‐
16
tance compared with other plastics, this is why it is considered the ideal material for the
17
construction of light guides. PMM“ has a refraction index of . ‐ . for visible light
18
spectrum much greater than the required . , assuring a total reflection of the light inside
19
the guide when it is surrounded by air, while it limits light reflection in the upper part of the
20
guide [ ].
21
Optical fibers have been previously used to provide internal illumination, better light disper‐
22
sion, and increase illuminated surface per volume unit of P”R [ – ]. Unfortunately, large
23
quantities of optical fibers are needed to achieve an increase on the illuminated surface to
24
volume ratio in comparison with externally illuminated P”R. Costs and construction consid‐
25
erations for large scale cultivation systems using optical fiber further limited their application
26
[ ] because P”Rs containing large quantities of optical fibers will not allow achieving good
27
turbulence Re ≥ and mixing at a low cost. Multiple light guides of PMM“ seem to be
28
more promising. One displayed after the other can increase the illuminated surface in large
29
scale P”R because the effect of self shading inside the P”R, common in flat panel and column
30
systems, will be reduced in internally illuminated systems, therefore, the potential of light
31
capture and usage can be increased, achieving optimal conditions with a more uniform
32
illumination.
33
Mixing, which governs the movement of the cells between the illuminated and the dark zones,
34
can considerably enhance the productivity for a wide range of operational conditions, as it can
35
create beneficial light fluctuations onto the cells. Mixing induced light/dark L/D cycles
36
usually occur at frequencies on the order of Hz or less, which is significantly lower than the
37
minimum frequencies required to produce the flashing light effect > Hz . Nevertheless, it
38
has been demonstrated that photosynthesis can be enhanced by low frequency L/D cycles.
39
Mixing time decreases with the increase of the superficial gas velocity. Superficial gas velocity
1
not only enhances the mixing and the light–dark cycle of microalgae, but also increases the
2
rate of shear in the reactor, which is harmful. Mixing and the shear stress should be balanced
3
carefully when a suitable superficial gas velocity is adopted. It has been reported that an
4
optimal superficial gas velocity of . x - m s- for the cultivation of the Chlorella vulgaris.
5
Economic analysis has estimated that mixing accounts for % of the total costs in some types
6
of P”Rs, and one critical challenge to algae biofuel generation was its poor energy balance due
7
to high auxiliary energy requirements for the mixing and the mass transfer [ , ].
8
. Gas exchange, carbon dioxide and oxygen 9
“eration rate is a key parameter to improve the growth of microalgae cell. Gas supplied to the
10
culture increases the mass transfer coefficient, avoiding deficiency of CO , control the toxic
11
level of dissolved O and the inhibitory level of CO , reduce nutrients gradient, avoids cell
12
sedimentation, clumping, fouling, and dead zones [ ], can create an optimized light/dark
13
cycle that can enhance the photosynthesis. However, excessive aeration may produce cell
14
damage due to mechanical shear forces in susceptible microalgae. “lso a high aeration rate
15
will lead to high running costs. “ deep knowledge of the fluid dynamics and the mass transfer
16
is needed for the P”R rational design and optimization. It is necessary to understand the
17
interplay among gas holdup, liquid circulation velocity, mixing, and gas–liquid mass transfer
18
[ , ].
19
. . Carbon dioxide mass transfer in photobioreactors
20
CO consumption is proportional to microalgae growth rate this consumption can be increased
21
by increasing the light irradiation to the culture, but only when light is limiting the photosyn‐
22
thetic process. ”ecause carbon represents approximately half of the dry weight of microalgae
23
biomass, the CO demand for cellular growth will be lower than the maximum demand at low
24
light intensities. For example, the maximum demand of CO in a flat panel P”R considering a
25
maximum radiation of µmol photon m- s-, was a demand of CO for photosynthesis that
26
will require a value of CO specific mass transfer rate KLa CO of only ‐ h- . ‐ .
27
s- , due to the growth rate of microalgae [ ]. Therefore, if the gas contains a low concentration
28
of CO , it will require a high KLa to satisfy the CO demand during microalgae growth [ ].
29
The former KLa values are very low and most of the designed P”Rs under normal operation
30
conditions present a KLa of CO from to times superior to this requirement [ ], because
31
CO can be added to the air supplied to the P”R.
32
Even though from the economic point of view, a high aeration rate will increase costs, it is not
33
recommendable for large scale production P”Rs. It is necessary to establish a minimum
34
aeration rate for each culture conditions [ ]. “n aeration rate of . vvm is appropriate for
35
cell production, and is recommended for an efficient P”R [ ].
36
. . Avoiding O deleterious effects
1
Considering a carbon fraction in the biomass of . and CO as the only carbon source in the
2
medium, it must be provided a minimum of . g of CO to generate g of microalgae biomass
3
[ ]. “dditionally, to this stoichiometric aspect, the competitive inhibition of O and CO for
4
the active site of the Rubisco enzyme Ribulose‐ , ‐diphosphate carboxylase oxygenase must
5
be considered. This is why O removal from the medium is very important. O can accumulate
6
to levels that can be toxic to microalgae. Oxygen concentrations above mg L- are toxic to
7
most of the microalgae species [ ]. In this sense, dissolved O concentration of % is equal
8
to . mg L-, this means that dissolved O concentrations above % of air saturation can be
9
detrimental to algal cells and therefore could reduce productivity [ , ].
10
In closed systems like, for example, horizontal tubular P”Rs of cm of internal diameter with
11
exponential growth of Spirulina, the oxygen concentration can increase up to ‐ mg L-, even
12
when there is ventilation every seconds [ ]. Oxygen build up, generated by photosynthesis,
13
is a particular serious problem in closed P”Rs with high area to volume ratio [ ] and can
14
generate a decrease of the biomass productivity of up to %, this imposes strong limitation
15
to tubes length in tubular P”Rs and scale up of P”Rs [ ].
16
To overcome this problem, some authors have proposed the use of degasifiers [ – ]
17
Figure , even though, to achieve an efficient gas separation from the liquid, the distance
18
between the entrance and exit to the degasifier unit must be of a magnitude that allows the
19
smallest gas bubbles enough time to separate from the liquid [ ]. “nother possible solution
20
is the use of low altitude vertical P”R like for example flat panel P”Rs [ , ] or alveolar
21
systems that allows more contact between the liquid and the air [ ].
22
23
Figure . Degasifiers units coupled to horizontal P”Rs designs [ ].
24
. . CO utilization efficiency
25
“nother aspect that must be considered is CO utilization efficiency. Daily CO fixation as g
26
CO fixated g- of injected CO d- can be calculated with the formula FD = F“t+ – F“t*mid-,
27
where F“t + is the accumulation of CO fixed during t + d , F“t the accumulation of CO
28
during t d and mid g the mass in grams of CO injected each day. It has been demonstrated
29
that is possible to fixate even % of the introduced CO FDmax% in an air current with . %
30
of CO using vertical tubular P”Rs connected in serial three stage manner, and up to % of
31
the introduced CO in an air current with % of CO [ ]. Therefore a high aeration rate results
32
“Q
in low CO utilization efficiency, indicating that it could be highly expensive in an industrial
1
facility to use CO supplementation. “eration conditions must be optimized considering
2
biomass productivity and supplemented CO utilization efficiency.
3
Despite all the presented alternatives are very ingenious, at this moment there is no universal
4
unit to achieve an optimal degasification, the selected choice will depend on the cultured
5
microalga and the preselected objective of this culture for example, biomass, pigments, H
6
production, etc. .
7
. Nutrients 8
Nutrient supply like nitrogen and phosphate is another factor of special interest. The dynamics
9
of these nutrients are strongly coupled to each other, and to the metabolic processes present
10
in the P”Rs. Exploring the fate of nitrogen and other nutrients through the different biological
11
pathways during cultivation in P”Rs is a valuable tool for designing such systems for full scale
12
with an ever growing demand for more efficient nutrient removal systems. For this, knowing
13
the true metabolism of nutrients in P”Rs and its effects on algae growth is vital [ ]. For
14
example, in nitrogen‐deprived culture of Haematococcus pluvialis the photosynthetic electron
15
transport chain is heavily damaged due to the significant reduction of cytochrome b /f
16
complex [ ]. To prevent cells being overreduced by photosynthesis, a correct amount of
17
nitrogen needs to be supplied.
18
. . Culture media development considerations
19
Nutrients are needed to generate biomass and high productivities. Concentration of macro‐
20
nutrients in the medium has a wide range and micronutrients have a narrower range [ ].
21
Minimum requirements for medium composition are obtained from elemental mass balance
22
[ ]. Requirements for the development of culture media are enlisted as follows
23
• The total salt content is determined by the habitat where the microalga was isolated.
24
• First, the composition of the major ionic components must be considered.
25
• Nitrogen sources are mainly nitrate, ammonium, and urea.
26
• The carbon source could be CO , HCO-or organic carbon, like acetate or glucose.
27
• pH is generally required above for maximum specific growth rate.
28
• Trace elements and chelated components, and vitamin requirements are considered last [ ].
29
Medium nutrients can be manipulated in order to obtain a different responses from microalgae,
30
for example, nutritional stress can be a strategy for the production of specific compounds like
31
carotenoids and fatty acids that are produced when there is a nitrogen deficiency in the
32
medium [ , , ], low nitrogen conditions promotes the synthesis of these compounds while
33
the synthesis of proteins and nucleic acids is inhibited.
34
“lso, it must be considered that the fact that some microalgae have the capacity of consuming
1
nutrient in an excessive manner and store them, a phenomena known as luxury uptake [ ],
2
this type of consumption presents when cells have been exposed to a medium with low
3
concentration of certain nutrients starvation or when cells have the capacity of accumu‐
4
lating nutrients, in this last case a previous starvation is not required [ , ]. It is important
5
to highlight that luxury uptake is desirable in a strain used in waste water treatment, because
6
it allows the removal of nutrients or contaminants from waste water without generating large
7
quantities of biomass as a secondary product.
8
. . Importance of nutrient in the photosynthetic process
9
“ll organisms have minimum optimal and maximum nutrient requirements, and the nutrient
10
level in the media affects the growth rate [ ], media usually used on laboratories generally
11
are not suitable to obtain high biomass concentration because high concentration of their
12
components can inhibit growth or precipitate [ ]. Fed‐batch cultivation mode can be used to
13
maintain adequate nutrient concentrations in the media, even though, very few works have
14
been published in this theme [ , – ] and most of these works have been focussed on the
15
fed of organic carbon source [ , ], but heterotrophic cultivation of microalgae using an
16
organic carbon source is not suitable for all species, also some strains often change their
17
chemical composition under heterotrophic culture [ ].
18
It is important to mention that microalgae productivity can be dependent on nutrients like
19
nitrogen, phosphorous, magnesium, iron, and manganese because these are required during
20
the photosynthethic process. Nitrogen is found in the form of proteins that form antenna
21
complexes LHC , reaction centers, and the enzymes that participate in the photosynthesis
22
process. Phosphorous is required in phosphate form to store captured light energy as chemical
23
energy in the form of N“DPH and “TP. The magnesium in the porphyrin ring of the Chlor‐
24
ophyll molecule, iron is part of the ferredoxin molecule, this later is an electron transporter of
25
the PSI, and last manganese is important because it acts as a cofactor in the oxygen liberator
26
complex, this has the function of liberating electrons from the water molecule [ , ].
27
”eside nutrients, there are other factors that can be manipulated in a P”R during its operation
28
like the pH and temperature, but because each of these factors deserves its own review, they
29
are not part of the discussion on the present chapter.
30
. Productivity and costs 31
. . Maximum biomass productivity
32
In P”R design it is important to define the upper limits of light capture efficiency. Maximum
33
biomass productivity has been determined to be . g dry biomass m- d-, considering an
34
average solar irradiation of µmol photons m- s-. ”ut some regions in the world possess
35
a higher solar irradiation, for example in some regions of the United States of “merica and
36
Mexico, maximum solar irradiation ranges between and µmol photon m- s-, this
37
can be equal to productivities as high as . g dry biomass m- d- [ , ]. Main objective in
1
the industrial‐scale deployment of this new technology today is to decrease P”R costs without
2
compromising system performances [ ].
3
. . Biomass productivity in different photobioreactors
4
Reported biomass productivities per unit of land area for different P”R are limited by the sub‐
5
optimal conditions of light inside the P”R, this limits the biological photosynthetic efficiency,
6
or even the design is the limiting factor that affects light usage inside of it. Therefore, high
7
yields can only be obtained linking P”R design with the biological process occurring within.
8
P”R efficiency is determined by factors like capture, transportation, distribution, and use of
9
light energy [ ], and also by the overall use of other main nutrients like carbon, nitrogen,
10
phosphorous, magnesium, and manganese.
11
It has been reported for an outdoor cultivation in a facade P”R run on a whole‐year basis, an
12
expected yearly production of around – tons biomass per ha with Chlorella vulgaris i.e.
13
average daily productivity of . g m- day- , which corresponds to around – tons of
14
CO fixed per year per ha [ ].
15
“ cascade type open culture system with high cellular densities has been developed since
16
and is still in use for Chlorella cultivation in the city of Trebon, Czech Republic. It can reach
17
cellular concentrations as high as g L-, with high growth rates and biomass productivity of
18
even g dry biomass m- d-. Despite its high yields, the implemented system at Trebon is
19
very expensive because of the material glass used to give the P”R slope, but using other
20
materials a similar system can be built at a significant lower cost. “lso, in the geographic
21
location of Trebon, it is only possible to use this P”R during a short period of time during the
22
year because of climate conditions [ ].
23
For economic feasibility of microalgal biorefinery, every cell components of microalgae need
24
to be utilized as much as possible. Continuous or semi‐continuous mode of cultivation for a
25
long period helps to improve microalgae cultivation as commercially successful [ ].
26
“ semicontinuous system was used to produce Chlorella in a lagoon build up with plastic, near
27
the city of Dongara located in Western “ustralia. This system had an average productivity
28
near to g dry biomass m- d-, because of the optimal climate conditions. Unfortunately,
29
technical problems in its scale up led to the closure of this facility [ ].
30
“ narrow light path from . ‐ . cm allows reaching cellular concentrations of up to g
31
L- and a volumetric biomass productivity of . ‐ . g L- d- in outdoors cultures operated
32
in fed batch mode. Ironically, the biomass productivity per area of land unit in a P”R displayed
33
horizontally was . ‐ . g m- d- [ , – ] and was not superior to the reported for an
34
open pond system, this last with a productivity of g m- d- [ ]. The same had been observed
35
in alveolar type vertical panels [ ]. Volumetric productivity in an inclined P”R, with a light
36
path of . ‐ . cm, was only . ‐ . times superior to the one obtained in open cultures of cm
37
of depth [ ].
38
. . Economical experiences in previous designs
1
”ased on cost of materials and manufacturing labour extrapolations it has been estimated the
2
capital cost of wall P”R at full production to be $ , per hectare. Open ponds ranges
3
approximately from about $ , to almost $ , per hectare taking into account the costs
4
of the liner and the paddlewheel [ ].
5
Raceway type open ponds are used in Israel, United States of “merica, China, and other
6
countries. It has been reported that this type of P”R can maintain a cellular concentration of
7
. g L- and a productivity of g m- d- [ ]. Despite their low construction and operation
8
costs, the average cost of these systems is $ ‐ U.S. dollars per Kg of dry biomass [ ]. The
9
system that is more widely used in large scale facilities near L are the ones that use sterile
10
plastic bags near to . m of diameter, adapted with an aeration system. These systems require
11
intensive labour and generally present poor mixing. This makes very expensive to produce
12
microalgae biomass. The costs are nearly $ U.S. dollars per Kg of dry microalgae biomass,
13
for smaller cultures the costs can raise up to $ and even to $ U.S. dollars per Kg. Costs
14
are very high and superior to the estimated for the production of Chlorella, Spirulina, and
15
Dunaliella, which are between $ and $ dollars per Kg [ ].
16
Productivities and cost are important questions for an algal industry whose economic survival
17
depends on production, and vary accordingly to cultivation methodology. From the perspec‐
18
tive of scaling between laboratory to large‐sized outdoor facilities, differences might arise in
19
products or co‐products expected if cultivation methods are not the same [ , ].
20
. Conclusions 21
Microalgae autotrophic growth is first limited by the photosynthethic process itself, and by
22
the process of light energy captures and CO conversion into biomass. This is why P”R design
23
must consider this biological process and focus on providing light, CO, and other nutrients at
24
a low cost.
25
Microalgae mass cultures can only be achieved with P”Rs designs aimed to improve photo‐
26
synthetic efficiency in light capture, maintaining at the same time adequate turbulence
27
conditions that can promote cells movement through the different illuminated regions, a high
28
mass transfer rate and high usage efficiency of supplied CO , allowing efficient O removal
29
produced by photosynthesis, and avoiding the generation of nutrients gradients besides
30
supply adequate nutrient quantities in the moment they are required, in order to improve their
31
use by the culture. This can be done using P”Rs with a long light path operating in fed‐batch
32
cultivation mode, if the design and operation characteristics are adequate.
33
Therefore, current designs of P”R still can be improved with the objective of lowering costs,
34
increase efficiencies, and maintain high productivities. New P”R materials and different
35
culture modes need to be investigated and evaluated because responses of specific strains
36
cannot be inferred from other P”Rs or culture conditions. Future investigations must consider
37
microalgae as systems and aim to evaluate interactions between photosynthetic efficiency,
1
CO and nutrient assimilation under different culture modes and operation conditions.
2
Acknowledgements 3
To CINVEST“V and to CON“CYT for the scholarship granted for Juan Cristóbal García
4
Cañedo grant number .
5
Author details 6
Juan Cristóbal García Cañedo* and Gema Lorena López Lizárraga
7
*“ddress all correspondence to [email protected]
8
Department of ”iotechnology and ”ioengineering, CINVEST“V‐IPN, “v. Instituto
9
Politécnico Nacional , Col. San Pedro Zacatenco, México, D.F., C.P. , México.
10
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