METHOD AND SYSTEM FOR GROWING MICROALGAE
IN AN EXPANDING PLUG FLOW REACTOR
FIELD OF THE INVENTION
The present invention pertains generally to methods for growing algae.
More particularly, the present invention pertains to the use of an expanding
plug flow reactor to reduce the requirement of using expensive closed system
bioreactors for growing algae. The present invention is particularly, but not
exclusively, useful as a method for growing algae in an open system
comprising an expanding plug flow reactor fed with a medium to maintain a
high concentration of algae cells.
BACKGROUND OF THE INVENTION
As worldwide petroleum deposits decrease, there is rising concern over
shortages and the costs that are associated with the production of
hydrocarbon products. As a result, alternatives to products that are currently
processed from petroleum are being investigated. In this effort, biofuel such
as biodiesel has been identified as a possible alternative to petroleum-based
transportation fuels. In general, a biodiesel is a fuel comprised of mono-alkyl
esters of long chain fatty acids derived from plant oils or animal fats. In
industrial practice, biodiesel is created when plant oils or animal fats are
reacted with an alcohol, such as methanol.
For plant-derived biofuel, solar energy is first transformed into chemical
energy through photosynthesis. The chemical energy is then refined into a
usable fuel. Currently, the process involved in creating biofuel from plant oils
is expensive relative to the process of extracting and refining petroleum. It is
possible, however, that the cost of processing a plant-derived biofuel could be
reduced by maximizing the rate of growth of the plant source. Because algae
is known to be one of the most efficient plants for converting solar energy into
cell growth, it is of particular interest as a biofuel source. Importantly, the use
of algae as a biofuel source presents no exceptional problems, i.e., biofuel
can be processed from oil in algae as easily as from oils in land-based plants.
While algae can efficiently transform solar energy into chemical energy
via a high rate of cell growth, it has been difficult to create environments in
which algae cell growth rates are optimized. Currently, the production of
biofuel from algae is limited by a failure to maximize algae cell growth.
Specifically, the conditions necessary to facilitate a fast growth rate for algae
cells in large-scale operations have been found to be expensive to create.
For instance, while providing high rates of algae cell growth, closed sterile
environments such as inoculant tanks and controlled bioreactors are
expensive to maintain and limited in scale. On the other hand, outdoor largescale
open systems, such as open runways, are plagued by contaminant
organisms which fight the selected algae cells for nutrients and sunlight and
reduce the rate of algae cell growth. Specifically, these contaminants include
non-selected, i.e., "weed", algae, viruses, bacteria, and grazers. Until now, it
has been virtually impossible to prevent contaminant organisms from causing
microbial instability and reducing selected algae cell growth rates in open
systems. In fact, standard open systems typically provide only one to two
days of microbial stability.
In light of the above, it is an object of the present invention to provide a
method for minimizing the need for closed system inoculation of algae cells in
a biofuel production system. Another object of the present invention is to
maximize the cell growth rate of selected algae cells in an open system.
Another object of the present invention is to provide an expanding plug flow
reactor for supporting logarithmic growth of algae cells. Another object of the
present invention is to selectively pump medium into the expanding plug flow
reactor to maintain a high concentration of algae and a selected shallow depth
of medium. Still another object of the present invention is to provide a method
and system for growing selected algae cells in an open system in which
contaminants cannot compete with the selected algae cells. Yet another
object of the present invention is to provide a system and method for growing
selected algae cells that is simple to implement, easy to use, and
comparatively cost effective.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system is provided for
growing selected algae cells in a medium and for preventing the growth of
contaminants in the medium. In this endeavor, the system relies on the initial
use of a closed reactor to grow an inoculum of microalgae. Importantly, the
closed reactor is five times smaller than those used in known algae production
systems. Specifically, the closed reactor comprises 0.4% of the present
system while closed reactors typically comprise about 2% of known systems.
For purposes of the present invention, the closed reactor is a continuous flow
reactor such as a photobioreactor. Further, the closed reactor is designed to
grow the inoculum of microalgae to a full concentration.
After the closed reactor grows microalgae to full concentration, the
inoculum of microalgae is passed in an effluence to an open system.
Specifically, the open system comprises an expanding plug flow reactor and a
standard plug flow reactor. For the present invention, the expanding plug flow
reactor continuously receives the effluence containing the inoculum of algae
cells from the closed reactor. Further, the expanding plug flow reactor
includes a conduit for continuously moving the effluence downstream under
the influence of gravity with little back mixing. Preferably, the expanding plug
flow reactor is an open raceway.
Structurally, the expanding plug flow reactor increases in width from its
first end to its second end. Also, the expanding plug flow reactor is provided
with a plurality of pumps along its length for introducing a growth medium to
the conduit. Initially, the pumps dilute the effluence until the algae reaches a
high concentration. For purposes of the present invention, "high
concentration" is defined as at least about 0.5 grams per liter of fluid.
Thereafter, as fluid evaporates and the algae cells grow, the pumps add
growth medium to maintain the high concentration of algae. Further, the
growth medium includes the nutrients necessary to support the desired
growth of the algae cells.
Importantly, the pumps are controlled in response to the growth rate of
the algae cells. For instance, the algae growth rate may decrease due to a
reduction in the amount of sunlight received and lower air temperatures. As a
result, in order to ensure a high concentration of algae as the expanding plug
flow reactor widens, the pumps will provide less medium. Therefore, the
depth of the medium will decrease slightly, and the flow rate of the algae cells
will decrease due to the viscosity of the algae cells. With the reduced flow
rate, the algae cells are provided with enough time to grow sufficiently to
remain at a high concentration as the expanding plug flow reactor widens.
Because the selected algae is maintained at a high concentration, the
nutrients provided in the growth medium are rapidly consumed by the
selected algae. As a result, the time available for growth of contaminants is
limited.
When the selected algae cells reach the end of the expanding plug flow
reactor, they have reached the desired level of growth. Thereafter, the algae
cells are transferred to a standard plug flow reactor. Typically, the standard
plug flow reactor will have the same width as the downstream end of the
expanding plug flow reactor. Further, a trigger medium may be fed into the
standard plug flow reactor to activate production of oil in the algae cells.
Alternatively, no medium may be fed into the standard plug flow reactor. This
alternative method is effective to trigger oil production because algae cells will
convert stored energy to oil when being starved of certain, or all, nutrients.
Further, as the medium evaporates in the standard plug flow reactor, the
depth of the medium will be reduced until the algae naturally flocculates. In
this manner, the standard plug flow reactor may be designed to self-flocculate
when optimal oil production has been achieved.
For an alternate embodiment of the present invention, a system for
growing algae cells includes a plurality of open ponds. In combination, open
ponds in this plurality are connected for selective fluid communication with
each other, and they are arranged in sequence from a first upstream pond to
a last downstream pond. In a variation from the expanded plug flow reactor
(EPFR) described above, this alternate embodiment of the invention
establishes each downstream pond with an exponentially greater surface area
relative to its adjacent upstream pond.
Structurally, the alternate embodiment of the present invention includes
a first transfer conduit for transferring inoculum from an inoculum source into
the first upstream pond. A culture is thereby created for algae growth in the
first upstream pond. A subsequent transfer of the culture can then be made
from the first upstream pond to successive downstream ponds for further
algae growth. For the present invention, such transfers are periodically
accomplished in a controlled manner, and algae is allowed to grow for a
predetermined time in each of the successive ponds. Eventually, fully grown
algae cells are transferred from the last downstream pond to an oil formation
pond via a last transfer conduit.
Each open pond in the system, regardless of its relative size, will
preferably have a fluid circulating device, such as a paddle wheel or
circulation pump, that can be used to establish liquid flow in the pond.
Preferably, each pond will also have a medium addition conduit for adding
medium into the culture in the pond. Further, as envisioned for the present
invention, the transfer of culture from an upstream pond to its adjacent
downstream pond can be accomplished in either of two ways. For one, each
pond may include a transfer pump for transferring the culture downstream
from the pond to its adjacent downstream pond. For another, the ponds can
be terraced so that a gravity flow can be established from an upstream pond
to a downstream pond.
As implied above, a fixed multiplier is determined to establish a ratio of
the surface areas for adjacent ponds. More specifically, the surface area of
each pond relative to the surface area of an adjacent upstream or
downstream pond will be established by this multiplier. In practice, the value
of the multiplier may vary from system to system. Specifically, in each case
the multiplier will be determined by the growth rate of the algae that is being
used for cultivation in the particular system.
In an operation for the alternate embodiment of the present invention, a
transfer sequence is periodically performed in accordance with a set
procedure. Specifically, the transfer sequence is initiated by first transferring
fully grown algae from the last downstream pond to an oil formation pond.
Once this is done, and the last downstream pond has been emptied, culture
from the adjacent upstream pond is then transferred into the now-empty, last
downstream pond. As the culture is transferred, additional medium can also
be transferred into the last downstream pond for further algae growth in the
last downstream pond. The now-empty, immediately upstream pond can then
receive culture transferred from its respective adjacent upstream pond. This
process of transfer from an upstream pond to an emptied adjacent
downstream pond continues until the first upstream pond has been emptied
and subsequently refilled with inoculum from the source of inoculum. After an
entire transfer sequence has been completed, the cultures in all of the open
ponds are individually circulated to promote algae growth. Once algae growth
in the respective ponds has been completed, the entire transfer sequence can
then be repeated. Preferably, transfer sequences for the alternate
embodiment of the present invention are accomplished during the nighttime.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features of this invention, as well as the invention itself, both
as to its structure and its operation, will be best understood from the
accompanying drawings, taken in conjunction with the accompanying
description, in which similar reference characters refer to similar parts, and in
which:
Fig. 1 is a schematic view of the system of the present invention,
illustrating the flow of algae from the closed reactor, through the expanding
plug flow reactor, and to the standard plug flow reactor in accordance with the
present invention;
Fig. 2 is an overhead view, not to scale, of the expanding plug flow
reactor shown in Fig. 1;
Fig. 3 is a longitudinal cross sectional view of the expanding plug flow
reactor of Fig. 2, showing the depth of the medium in the conduit; and
Fig. 4 is a schematic view for an alternate embodiment of a system in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to Fig. 1, a system for growing selected algae cells is
shown, and is generally designated 10. As shown in Fig. 1, the system 10
includes a closed reactor 12, such as a continuous flow photobioreactor. As
shown in Fig. 1, the closed reactor 12 is fed with an inoculum medium 14 and
continuously grows an inoculum of algae 16. As the inoculum of algae 16
reaches the end 18 of the closed reactor 12, it is at full concentration. Then,
the inoculum of algae 16 passes out of the closed reactor 12 in an effluence
(arrow 20).
As shown in Fig. 1, the effluence 20 containing the inoculum of algae
16 passes from the closed reactor 2 to an open system 22, such as an open
raceway. In Fig. 1, it can be seen that the open system 22 comprises an
expanding plug flow reactor (EPFR) 24 and a standard plug flow reactor
(SPFR) 26. Structurally, the EPFR 24 includes a conduit 28 with a first end
30 for receiving the effluence 20 and a second end 32. Further, the open
system 22 includes a pump 34. As the effluence 20 enters the EPFR 24, the
pump 34 adds a growth medium (arrow 36) to the EPFR 24 to dilute the
concentration of algae 38 within the EPFR 24 to about 0.5 grams per liter of
fluid. Further, the growth medium 36 includes the nutrients necessary to
support the desired growth of the algae 38. As shown in Fig. 1, the open
system 22 may include a plurality of pumps 34 for feeding the growth medium
36 at locations 40 along the length of the EPFR 24.
Referring now to Fig. 2, the structure and operation of the EPFR 24
may be understood. As shown, the first end 30 of the EPFR 24 has a width
and the second end 32 of the EPFR 24 has a width W2 that is substantially
greater than W-|. In Fig. 2, the EPFR 24 is not drawn to scale. In certain
embodiments, W will equal ten feet, while W2 will equal 300 feet. Further, the
EPFR 24 can be seen to include a plurality of sections 42. Further each
section 42 expands in width from its proximal end 44 to its distal end 46. As
shown, the width of each section 42 doubles from its proximal end 44 to its
distal end 46. As a result, the EPFR 24 has a substantially logarithmic
increase in width. While Fig. 2 illustrates an increase in width for each
successive section, it is envisioned that sections 42 having a constant width
could be interspersed among the widening sections 42.
Importantly, the fluid medium 36 and algae 38 flow through the EPFR
24 under the influence of gravity. For purposes of the present invention, this
gravity flow is accomplished using a structured gradient. A preferred
embodiment of a structured gradient for use with the EPFR 24 is shown in
Fig. 3. There it will be seen that the floor 48 of the conduit 28 is formed with a
plurality of steps 50. In detail, the steps 50 are defined by a height "h" of
approximately 3 centimeters, with a distance "s" between the steps 50 being
preferably on the order of approximately 100 meters. Typically, the EPFR 24
may be over 1000 meters long and the algae 38 may have a residence time of
about thirty days in the EPFR 24.
An important aspect of the EPFR 24 for the present invention will be
appreciated with reference to Fig. 3. This aspect is that the depth "d" of the
fluid medium 36 in the conduit 28 needs to be rather shallow (i.e. less than
about 15 cm, and preferably around 7.5 cm). To maintain this depth "d",
however, it is necessary to add the fluid medium 36 along the length of the
EPFR 24 as the EPFR 24 widens. Importantly, the increase in width among
EPFR sections 42 allows for logarithmic growth of the algae 38 while the
concentration of the algae 38 is maintained at the high concentration of at
least 0.5 grams per liter.
In cross-reference to Figs. 1 and 2, as the medium 36 and algae 38
reach the second end 32 of the EPFR 24, they are transferred to the SPFR
26. At this stage, the algae 38 stops growing and, instead, begins to produce
oils to store energy. In order to instigate oil production in the algae 38, a
pump 52 may introduce a trigger medium 54 into the SPFR 26. Specifically,
the trigger medium 54 may lack a desired nutrient, such as nitrogen or
phosphorus, which causes the algae 38 to produce oil. Alternatively, the
SPFR 26 may receive only the medium 36 and algae 38 from the EPFR 24,
without any additional medium 54. In either case, oil production in the algae
38 is triggered by the lack of nutrients to support growth.
In Fig. 4, an alternate embodiment for the present invention is shown
and is generally designated 60. As shown, the system 60 includes an "n"
number of open ponds 62 with the smallest open pond 62(i) being designated
as the "first upstream pond", and the largest open pond 62( being designated
as the "last downstream pond". Intermediate open ponds 62 are arranged in
order, according to size, with an exponentially increasing surface area in a
downstream direction. In this case, the downstream direction extends from
the first upstream pond 62( to the last downstream pond 62(n) . For the
system 60, the ratio between adjacent surface areas of respective open
ponds 62 is established by a fixed multiplier. Importantly, this fixed multiplier
is determined by the growth rate of the particular algae 38 that are to be
cultivated in the system 60.
For the present invention, it is to be appreciated that all of the open
ponds 62 in the system 60 are substantially similar to each other. The
exception here is only in the size of their respective surface areas.
Accordingly, each pond 62 will have a fluid circulating device 64 that is
provided for moving (stirring) algae 38 around in the pond 62. Functionally,
this is done to promote the growth of algae 38 while there is a culture of the
algae 38 in the particular open pond 62. Examples for a suitable fluid
circulating device 6 4 would be a standard circulation pump or a paddle wheel.
Both of these types of devices are well known in the pertinent art.
It will also be seen in Fig. 4 that each open pond 6 2 has a medium
addition conduit (represented by arrow 66) which is provided to add medium
into the respective open pond 62, as needed. Further, the open ponds 6 2 are
connected via respective transfer conduits for selective communication with
each other. For example, the upstream open pond 6 2 (n- ) is connected in fluid
communication via a transfer conduit with its adjacent downstream open pond
62(n). Preferably, the transfer conduits are transfer pumps 68. As shown in
Fig. 4 , the transfer conduit between open pond 6 2 n - ) and open pond 6 2 n ) is a
transfer pump 68( n - i ) . As implied above, however, this particular structure is
only exemplary. As an alternative to using transfer pumps 68, the open ponds
6 2 in system 6 0 can be terraced to provide for a gravity flow of liquid between
the various pairs of upstream and downstream open ponds 62.
In addition to the specific structural components of the system 6 0
described above, inoculum algae 1 6 in an inoculum medium 1 4 can be fed
into the first upstream open pond 62(i) via a first transfer conduit (represented
by the arrow 70). At the downstream end of the system 60, after traversing
the system 60, the now fully grown algae 3 8 can be removed from the last
downstream open pond 6 2 ( n ) via a last transfer conduit (e.g. transfer pump
68(n).
In the operation of the system 60, algae 3 8 are progressively grown as
they are selectively passed from one open pond 6 2 to another. The actual
time spent by the algae 3 8 in each open pond 6 2 in the series will be
substantially the same, and will depend on the type of algae 3 8 that is being
cultivated. As a practical matter, the time spent by algae 3 8 in a particular
open pond 6 2 can be as much as several (e.g. 3 ) days. In the event, the
transfer of algae 3 8 through the system 6 0 is done methodically. And
preferably, the transfer will be accomplished at nighttime when the growth of
algae 3 8 is delayed due to a lack of sun light.
A transfer sequence for moving algae 38 through the system 60 begins
by first emptying the last downstream pond 62(n) . To do this, the fully grown
algae 38 therein are transferred through a transfer conduit (e.g. transfer pump
68(n)) to an oil formation pond (i.e. SPFR 26). Next, the contents of the
adjacent upstream open pond 62 n-i ) are then emptied into the now-empty last
downstream open pond 62(n) . At this time, additional medium can be added to
the last downstream open pond 62(n) via the medium addition conduit 66(n) .
Specifically, this is done to establish proper conditions for further growth of
algae 38 in the open pond 62(n). In turn, the contents of open pond 62(n- 2) (not
shown) are emptied into open pond 62 n-i ), and an appropriate amount of
medium is added. This continues, in sequence, with the contents of each
upstream open pond (e.g. pond 62(2)) being transferred into the just-emptied
adjacent downstream open pond (e.g. pond 62(3)) . The transfer sequence
finally ends when the contents of the first upstream open pond 62( ) have
been emptied into open pond 62(2) and the now-empty upstream open pond
62( ) has been refilled with inoculum of algae 16. The system 60 then
continues to grow algae 38 in respective open ponds 62 until another transfer
sequence is initiated.
While the particular Method and System for Growing Microalgae in an
Expanding Plug Flow Reactor as herein shown and disclosed in detail is fully
capable of obtaining the objects and providing the advantages herein before
stated, it is to be understood that it is merely illustrative of the presently
preferred embodiments of the invention and that no limitations are intended to
the details of construction or design herein shown other than as described in
the appended claims.
What is claimed is:
1. A method for growing algae cells comprising the steps of:
providing an open system comprising an expanding plug flow
reactor (EPFR) for facilitating logarithmic growth of an inoculum of
algae cells, and a standard plug flow reactor (SPFR) for treating the
algae cells to activate oil production therein;
introducing an inoculum of algae cells into a first end of the
EPFR, wherein the EPFR has a second end, and wherein the first end
has a width W and the second end has a width W2, with W 2 ;
adding a growth medium at a plurality of locations dispersed
between the first end and the second end of the EPFR to support
logarithmic growth of the algae cells therein, wherein a selected depth
of medium is maintained in the EPFR;
transferring the algae cells from the second end of the EPFR to
the SPFR; and
triggering the algae cells in the SPFR to activate oil production.
2. A method as recited in claim 1 wherein the concentration of
algae cells in the medium in the EPFR is maintained at approximately 0.5
grams per 1 liter.
3. A method as recited in claim 1 wherein the depth of the medium
in the EPFR is less than approximately fifteen inches.
4. A method as recited in claim 1 wherein the EPFR has a
structured downstream gradient to move the growth medium and algae cells
from the first end to the second end.
5. A method as recited in claim 1 wherein the algae cells have
residence time of about thirty days in the EPFR.
6. A method as recited in claim 1 further comprising the steps of:
determining the amount of nutrients required to support growth
of the algae cells from a first location to a second location in the EPFR,
wherein the width of the EPFR at the second location is greater than
the width of the EPFR at the first location;
periodically determining a growth rate for the algae cells at the
first location in the EPFR;
ascertaining the duration of time needed for the algae cells at
the first location in the EPFR to grow in view of the determined growth
rate;
calculating a volumetric flow rate appropriate for moving the
algae cells at the first location to the second location after the needed
duration of time; and
adding the growth medium between the first location and the
second location to cause the algae cells to move at the calculated
volumetric flow rate, with the growth medium containing the determined
amount of nutrients to support growth of the algae cells from the first
location to the second location.
7. A method for growing selected algae cells in an open system
comprising the steps of:
providing a system comprising a closed reactor for growing an
inoculum of algae cells, an expanding plug flow reactor (EPFR) for
facilitating logarithmic growth of the inoculum of algae cells, and a
standard plug flow reactor (SPFR) for treating the algae cells to
activate oil production therein;
feeding an inoculation medium with a nutrient mix for facilitating
growth of the inoculum of algae cells;
passing an effluence containing the inoculum of algae cells from
the closed reactor to a first end of the EPFR, wherein the EPFR has a
second end, and wherein the first end has a width W-i and the second
end has a width W2, with 2> ;
adding a growth medium at a plurality of locations dispersed
between the first end and the second end of the EPFR to support
logarithmic growth of the algae cells therein, wherein a selected depth
of medium is maintained in the EPFR;
transferring the algae cells from the second end of the EPFR to
the SPFR; and
supplying the SPFR with a trigger medium to activate oil
production in the algae cells therein.
8. A method as recited in claim 7 wherein the closed reactor is a
continuous flow reactor.
9. A method as recited in claim 7 wherein the concentration of
algae cells in the medium in the EPFR is diluted and maintained at
approximately 0.5 grams per 1 liter.
10. A method as recited in claim 7 wherein the depth of the medium
in the EPFR is less than approximately fifteen inches.
11. A method as recited in claim 7 wherein the EPFR has a
structured downstream gradient to move the growth medium and algae cells
from the first end to the second end.
12. A method as recited in claim 7 wherein the algae cells have
residence time of about thirty days in the EPFR.
13. A method as recited in claim 7 further comprising the steps of:
determining the amount of nutrients required to support growth
of the algae cells from a first location to a second location in the EPFR,
wherein the width of the EPFR at the second location is greater than
the width of the EPFR at the first location;
periodically determining a growth rate for the algae cells at the
first location in the EPFR;
ascertaining the duration of time needed for the algae cells at
the first location in the EPFR to grow in view of the determined growth
rate;
calculating a volumetric flow rate appropriate for moving the
algae cells at the first location to the second location after the needed
duration of time; and
adding the growth medium between the first location and the
second location to cause the algae cells to move at the calculated
volumetric flow rate, with the growth medium containing the determined
amount of nutrients to support growth of the algae cells from the first
location to the second location.
14. A system for growing algae cells which comprises:
a source of an inoculum of algae cells;
a plurality of open ponds connected in selective fluid
communication with each other, with the plurality of open ponds
arranged in sequence from a first upstream pond to a last downstream
pond, wherein each downstream pond has an exponentially greater
surface area relative to its adjacent upstream pond;
a first transfer conduit for transferring the inoculum from the
source to the first upstream pond to create a culture therein for algae
growth and to provide for a subsequent transfer of the culture
therefrom to successive downstream ponds for further algae growth;
and
a last transfer conduit for selectively transferring fully grown
algae cells from the last downstream pond to an oil formation pond.
15. A system as recited in claim 14 wherein each open pond
comprises:
a fluid circulating device to establish liquid flow in the pond;
a medium addition conduit for adding medium into the culture in
the pond; and
a transfer pump for transferring the culture downstream from the
pond.
16. A system as recited in claim 14 wherein a fixed multiplier is
determined to establish a ratio of a surface area for each pond relative to a
surface area of an adjacent pond, and wherein the multiplier is determined by
a growth rate of the algae used for cultivation.
17. A system as recited in claim 14 wherein the depth of liquid in
each pond is less than fifteen inches, and the residence time of the culture in
each pond is less than three days.
18. A system as recited in claim 14 wherein a transfer of culture
from a specified pond is completed, and the specified pond is substantially
emptied, before the empty pond receives a transfer of culture from an
adjacent upstream pond.
19. A system as recited in claim 8 wherein the transfer of culture is
accomplished during the nighttime.
20. A system for growing algae cells comprising:
a closed reactor for growing an inoculum of algae cells;
an open system comprising an expanding plug flow reactor
(EPFR) for facilitating logarithmic growth of the inoculum of algae cells,
and a standard plug flow reactor (SPFR) for treating the algae cells to
activate oil production therein;
a means for passing an effluence containing the inoculum of
algae cells from the closed reactor to a first end of the EPFR, wherein
the EPFR has a second end, wherein the first end has a width i and
the second end has a width W2 and wherein W2>W ;
a means for adding a growth medium at a plurality of locations
dispersed between the first end and the second end of the EPFR to
support logarithmic growth of the algae cells therein, wherein the
adding means maintains a selected depth of medium in the EPFR; and
a means for transferring the algae cells from the second end of
the EPFR to the SPFR.