Mass algal culture system

An apparatus and process for the culture of algae in a liquid medium is disclosed. The medium circulates through an open trough and is exposed to an atmosphere which is temperature regulated. The nutrient content of the liquid medium is regulated to control the chemical composition growth and reproduction characteristics of the cultured algae. Before it is allowed to strike the medium, sunlight is passed through a filter to remove wavelengths which are not photosynthetically active. Heat energy can be recovered from the filter.

BACKGROUND OF THE INVENTION 
This invention relates to a method for growing photosynthetic 
microorganisms in liquid suspension, and specifically to the mass culture 
of unicellular algae. 
In recent years cultured algae has been recognized as a promising source of 
food and even of chemical feedstocks. As a result, a variety of 
apparatuses and methods have been devised to facilitate the growth of 
algae. High yields have been obtained in some tightly controlled 
laboratory experiments, but heretofore, efforts at mass algal culture have 
been disappointing in that they were inefficient and uneconomical. 
Prior mass algal culture systems have yet to prove economical because (1) 
they require relatively deep containment (20-100 cm) in order to provide 
for temperature control; (2) they produce comparatively dilute cultures; 
(3) they make inefficient use of carbon dioxide and little use of 
infra-red radiations from sunlight; (4) they require substantial energy 
inputs to provide mixing to avoid thermal stratification; (5) they must 
process larger volumes of water to obtain the same harvest yields of algal 
matter that might be collectible from shallower systems; and (6) they 
permit little or no control and/or regulation of those environmental 
elements which control and regulate the performance characteristics of the 
cultured cells. 
Further, no prior mass algal culture system has been equipped to induce and 
regulate the flashing-light effect efficiently. Prior apparatuses have 
suffered from rapid contamination by unwanted organisms and have required 
extensive sterilization treatments for both equipment and media. Nutrient 
and temperature management has not been conducted with precision in such 
mass culture system. 
Harvesting is usually the single largest deterrent to realizing practical 
and economical unicellular algal production. The usual methods employed 
include settling, perhaps enhanced by floculation, centrifugation, and bed 
evaporation. All such processes require too much time, space, and/or 
energy to permit reasonable commercial utility. 
Even the best of existing apparatus have been operated at least than peak 
efficiency because currently known methods of operation are not regulated 
to maximize the production of cell matter. 
SUMMARY OF THE INVENTION 
It has now been discovered that the deficiencies of existing systems can be 
overcome by the use of novel apparatus and processes which permit a 
substantial gain in the net energy (outputs vs. inputs) obtained from the 
system, without being substantially more complex to operate than systems 
heretofore used. The mass culture apparatus disclosed can precisely 
regulate many variables so that the cells harvested can be controlled to 
be of chosen chemical compositions and produced at rates representing high 
and nearly constant conversion efficiencies of sunlight into stored 
chemical-free energy. In this way, the algal product can be chosen to meet 
a variety of needs. 
As in some prior systems, algae is cultured in a liquid medium which flows 
through a shallow trough. In the present system, the trough is positioned 
beneath a filtering means which absorbs the infrared and ultraviolet 
wavelengths of sunlight passing therethrough. Algal cells in the liquid 
medium thus receive light of the photosynthetically active wavelengths 
which stimulate growth and reproduction, but are not exposed to 
substantial amounts of light of wavelengths which retard or are not used 
in those life functions. The captured wavelengths heat the filtering means 
which therefore functions as a solar collector. The heat energy developed 
can be used to control the temperature of the algal culture medium or can 
be converted into electrical energy for driving pump motors and other 
essential system components. 
The algae-containing liquid medium moves through a channel or trough by 
gravity flow. A pump removes medium from a discharge end of the trough and 
redeposits it in an inlet end of the trough. A gas lift pump is uniquely 
advantageous for this purpose because such a pump not only circulates the 
liquid medium but also can be used to separate organic wastes from the 
medium. A stream of minute gas bubbles can be injected into the liquid 
medium 12 as it passes through the gas lift pump. The bubbles possess a 
static charge so that organic wastes in the liquid medium become attached 
to oppositely charged bubbles. The bubbles rise to the surface of the 
liquid medium carrying the electrostatically absorbed organic substances 
with them. When at the surface, the bubbles form a froth. This froth and 
the undesirable organic substances is contains, may then be easily 
separated from the liquid medium. 
Channels of the present invention can be arranged in a serpentine pattern. 
Liquid medium containing growing algae is circulated through the 
serpentine channels at a rate sufficient to cause mixing or turbulence 
therein, thereby to achieve a desired periodicity of the fluid element 
such that its components are alternatively given access to a surface layer 
of the fluid and deeper layers therein. Individual algal cells, following 
the flow pattern of the liquid medium, are transported continuously 
between surface locations and regions deeper within the channels. Because 
the overlying algal cells are continuously and cyclically exchanged with 
those deeper within the medium, and because they extinguish light by 
absorption and scattering in direct proportion to their concentration in 
the medium, they progressively shade the cells to the point of virtual 
darkness in the deeper zones within the channels. Thus, individual cells 
growing within the shallow (2-5 cm deep) fluid element are exposed to 
alternating periods of light and darkness and exhibit the desirable growth 
characteristics associated with the phenomenon commonly known as the 
"flashing light effect" throughout most of the channel system. 
Near its discharge end, the channel deepens and widens so that the 
cross-sectional flow area is increased. As a result, laminar flow is 
established within the fluid element as it approaches the discharge end. 
Flow is regulated so that near stagnant conditions are produced in the 
surface waters at the discharge end whereby algal cells within the fluid 
element tend to rise to the surface, forming a thick surface film. This 
algal film is readily harvested either by regulating its flow over a wier 
or by other skim harvesting techniques. The efficiency of these techniques 
is improved by processes which increase the flotation of the algal cells. 
Such processes include (1) the electrostatic attachment of algal cells to 
gas bubbles within the fluid element due to the production of minute 
bubbles in the gas-lift pump and/or the natural formation of oxygen 
bubbles by the cells during photosynthesis, (2) the modification of the 
chemical composition of the algal cells by judicious selection and 
regulation of those environmental factors which direct the biosynthesis of 
chemical compounds less dense than the growth medium, and (3) the addition 
of surfactants which are less dense than water and which absorb to and 
increase the flotation of the algal cells. 
Prior to innoculation with an algal culture, the apparatus is sterilized by 
preparing a liquid medium precursor containing all the acidic components 
of the desired liquid medium. The trough is filled with this precursor to 
kill any potentially contaminating organism. An alkaline substance is then 
added to the precursor to complete the liquid medium and raise its pH to 
within a range suitable for the growth of algae. The medium is innoculated 
with the desired algae and maintained in an environment conducive to 
growth and reproduction. 
During operation, contamination is reduced because the channels are 
contained within an enclosure, in which a somewhat elevated pressure is 
maintained. The system is provided with means to carefully regulate 
nutrient and carbon dioxide content of the liquid medium and to maintain 
the medium within preferred temperature and pH ranges. In this way the 
system affords an output of algal products having more uniform and 
controllable chemistry, adjustable over a greater range of desirable 
compositions, than have heretofore been obtained in mass culture systems. 
By appropriate selection of nutrient schedules it is possible to maximize 
cell reproduction, cell enlargement and/or the concentration of certain 
chemical compounds, such as lipid constituents, in the algal product. 
Continuous operation can be achieved by continuously or periodically 
adding nutrients to the medium to make up for those consumed by the 
growing algae and by recycling liquid medium until such time as the liquid 
becomes contaminated with undesirable organisms or with a toxic level of 
some algal secretion which can not be removed satisfactorily. 
It is therefore an object of this invention to provide an algal culture 
system wherein algal cells are exposed to light which is chiefly comprised 
of photosynthetically active wavelengths and wherein algal cells are grown 
in shallow media (2-5 cm) and sheltered from light of wavelengths which 
inhibit growth and reproduction. 
A further object is to provide such an algal culture system wherein 
filtering means absorb essentially all radiation below 350 nanometers and 
above 700 nanometers from incident sunlight. 
It is a further object to provide a system wherein the temperature of the 
algal culture is regulated to provide an ideal growth environment. 
An additional object is to use the nonphotosynthetic wavelengths of light 
to provide energy for nonphotosynthetic operations of the system. 
Another object is to provide means of bringing potentially toxic dissolved 
organic materials into contact with gas bubbles so that they will float to 
the surface of a liquid culture medium where they can be conveniently 
removed. 
An additional object is to provide a gas lift pump mechanism which both 
moves liquid culture medium through a trough and brings algal cells into 
contact with gas bubbles. 
Another object is to provide a gas lift pump as aforesaid to regulate and 
control the build up of potentially toxic dissolved organic materials. 
An object is also to provide an algal culture system wherein algal cells 
can be continuously grown and harvested. 
Yet another object is to provide an algal culture apparatus wherein algae 
growing in a liquid medium are alternately exposed to periods of light and 
darkness to obtain the favorable growth characteristics induced by the 
"flashing light effect." 
In addition, it is an object to provide an algal culture system as 
aforesaid which is of simple construction and which stores sunlight as 
chemical energy in excess of the amounts required for system operation. 
Another object is to provide a mass culture system wherein an algal 
nutrient medium can be regulated to control uniformly the chemical makeup 
of the harvested algal product and to achieve routinely a greater 
productivity than has heretofore been practical. 
An object is to provide a mass algal culture process capable of regulating 
the physiological characteristics of algae grown and reproduced therein. 
It is also an object to provide a method of liquid culture medium 
formulation wherein a liquid precursor of the culture medium acts as a 
sterilizing agent for the culture growth environment. 
A further object is to provide an efficient outdoor system for storing 
solar energy in chemical form represented by the mass culture of marine 
algae in a seawater based medium. 
These and other objects and features of the present invention will be 
apparent from the drawings and description of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred apparatus for the production of algae according to the 
present invention is shown in FIGS. 1, 2 and 7. A trough 13 is provided to 
contain a liquid algae culture medium 14 in a shallow layer most 
preferably two to five centimeters deep. The trough has an inlet end 15 
where the liquid medium is introduced and a discharge end 16 where liquid 
medium is withdrawn. The trough may be level or slightly inclined 
downwardly, e.g. at about one percent, toward the discharge end 16 to 
facilitate the flow of liquid medium therethrough by gravity. 
Legs 18 support chamber walls 20, a floor 24 and a substantially optically 
transparent ceiling member 26 which together comprise an enclosure 28 for 
the trough 13. The enclosure defines an interior gas space 29 wherein an 
appropriate gaseous atmosphere may be maintained at a slightly elevated 
pressure so that airborne contaminants can not enter the enclosure. The 
walls 20 and floor 24 are preferably opaque so that incident light can 
enter the gas space 29 only through the ceiling member 26. With this 
arrangement, light and heat are the only external elements which effect 
the environment inside the growth apparatus. 
Although nonrecirculating trough systems may be used as described below, 
the embodiment shown in FIGS. 1 to 7 is a recirculation system wherein the 
liquid medium 14 makes multiple trips through the trough 13. When the 
liquid medium 14 reaches the discharge end 16 of the trough, it is 
recirculated to the inlet end 15 by means of a suitable pump. Preferred is 
a gas lift pump mechanism 30 shown in FIG. 3. This pump includes a 
substantially vertical stand pipe 32 having a medium inlet 34 and a medium 
outlet 35 located above the inlet. 
A stream of gas is injected into the pipe 32 through an inlet opening 36 
shown in FIG. 3 as being located below the medium outlet 35. As the gas 
bubbles rise in the stand pipe 32, they carry liquid medium in the pipe 
upwardly and into the trough inlet end 15. In the illustrated embodiment, 
gas from a source such as an air compressor 37 or a compressed air 
cylinder (not shown), is delivered to the inlet 36 via a gas delivery tube 
38. 
The inlet 34 of the gas lift pump is connected by a recirculation pipe 39 
to the discharge end 16 of the trough; and the outlet 35 of the gas lift 
pump is positioned to deposit liquid medium from the pump into the inlet 
end 14 of the trough. With this arrangement the pump 30 can be used to 
continuously recirculate algal culture medium through the trough 13, by 
withdrawing liquid from the trough's discharge end 16 and replacing it in 
the inlet end 15. Recirculation through line 39 is accomplished at a 
steady rate controlled by the rate of air addition to the pipe 32 from air 
compressor 37. A check valve 40 prevents the circulating culture medium 
from backing into the air delivery tube 38 during operation and pump 
downtimes. 
The gas lift pump mechanism shown in FIG. 3 includes several additional 
features. The pump mechanism includes means for regulating liquid medium 
temperature throughout a wide range of temperatures between freezing and 
boiling. This feature is included because each species of algae develops 
most efficiently in a narrow range of temperatures. To maintain optimum 
growth temperatures the gas lift pump is equipped with jacket 41 which 
surrounds a portion of the piping through which the medium flows. FIG. 3 
shows such a jacket surrounding the pipe 32. When the liquid culture 
medium 14 drops in temperature to below a desired range, a hot liquid may 
be circulated through the jacket 41 to elevate the temperature of the 
medium. Likewise, an overheated medium will give up heat to a relatively 
cold liquid circulating through the jacket 41. Thermostatic controls (not 
shown) may be provided to automatically operate this heat exchange 
mechanism for maximum efficiency. Heat exchange means for regulating 
medium temperature could, of course, be provided at other locations along 
the liquid medium's path as will more fully be described herebelow. 
Resistance heaters could also be located along the trough 13 to heat the 
medium. 
The illustrated gas lift pump is further preferred because it includes 
integral means for injecting special purpose gases into the liquid medium. 
Although other means could be provided for this purpose, the structure of 
the illustrated pump mechanism 30 is unique and especially advantageous. 
Gas cylinders 43 or other gas sources are each connected by a valved feeder 
line 44 to a special gas supply line 45. The line 45 connects to the pipe 
32 at a location beneath the medium inlet 34. A porous element 47 shown in 
FIG. 3 is located inside the pipe 32 between the medium inlet 34 and the 
gas supply line 45 to disperse gas admitted through the supply line into 
bubbles. The illustrated element comprises upper and lower perforated 
plates 48, 49, each having uniform hole patterns identical to one another. 
Bubble size is selected by regulating the degree of pore overlap between 
the two adjacent plates. Pore overlap is varied by twisting a ring 
structure 50 to which the lower plate 49 is attached with respect to the 
upper plate 48 which is fixedly mounted in the pipe 32. Other contructions 
such as a block of microporous material, could also be used for the 
element 47. 
Because the porous element 47 disperses gas into a stream of bubbles in the 
liquid medium, carbon dioxide advantageously can be added to the medium 
through the element. Carbon dioxide is an essential compound consumed by 
algae during photosynthesis; and in a mass algal culture system, it is 
rapidly consumed from liquid culture medium and must be replaced. 
Preferably, at least 235 grams of carbon dioxide should be replaced per 
each hundred liters of medium per day. Carbon dioxide from stack gasses, 
process gas or a gas production plant may conveniently be used. When the 
carbon dioxide is bubbled into liquid medium 14 circulating through the 
pipe 32, the high surface area of the bubbles and the long period of 
liquid contact facilitates efficient dissolution of the carbon dioxide. 
Carbon dioxide can thus be added to the medium at rates required to 
maintain a desired medium alkalinity and nitrient carbon content. Because 
carbon dioxide is admitted to the gas lift pump through an inlet separate 
from the circulating gas inlet 36, the rate of carbon dioxide addition is 
easily controlled independently of the liquid medium circulation rate and 
can be established to minimize the escape of heat-retaining carbon dioxide 
gas into the gas space 29. 
To retain sufficient amounts of carbon dioxide in a liquid culture medium, 
the pH of the liquid should be maintained within an appropriate range. For 
fresh water media wherein carbon dioxide is retained as a dissolved gas, a 
pH of between 6.0 and 7.5 is preferred. A range of 7.5 to 9.5 is best for 
salt water media wherein carbon dioxide is present as dissolved 
bicarbonate ions. 
Substantially water insoluble gasses such as oxygen, hydrogen and ozone may 
also be delivered from one of the tanks 43 into the pipe 32. Bubbles of 
such gasses, produced by the porous element 47, carry a static charge 
which is determined by the chemical natures of the gas added through the 
line 45 and of the liquid medium. The static charge makes the gas bubbles 
attractive to oppositely charged substances in the liquid medium and the 
two tend to adhere to one another due to electrostatic adsorption. After 
the liquid medium 14 is carried from the pipe 32 into the inlet end 15 of 
the trough, the entrained gas bubbles tend to rise to the surface carrying 
with them the adhering substances. 
Small, lightweight particles, such as organic waste substances tend to 
adhere to oppositely charged gas bubbles and to be carried to the medium 
surface in this manner. When bubbles carrying the organisms reach the 
surface, a froth tends to form on the surface of the medium 14 near the 
inlet end 15. By providing an appropriate foam separation device near the 
inlet end, the froth containing undesirable organic substances can be 
separated from the liquid medium. 
One suitable foam separation device 52 is shown in FIGS. 1 and 5. In this 
device the stand pipe 32 of the gas lift pump extends upwardly through the 
floor of trough 13. Inside the trough, a drum 53 surrounds the pipe 32 and 
is provided with a liquid discharge slit orifice 54 located below the 
uppermost end of the pipe 32. A gate 55 is provided to regulate the fluid 
head inside the drum 53, by adjusting the size of the orifice 54. A 
collector tray 56 surrounds the top of the drum 53; and a channel flume 57 
extends downwardly from the tray. 
The previously described froth, which contains organic materials, collects 
on the surface of liquid medium inside the drum 53 as the liquid medium is 
continuously discharged through the orifice 54 into the trough 13. As the 
froth collects, it rises and eventually flows over the top of the drum 
into the tray 56 from which it is discharged through the flume 57. 
Other devices, such as the skimming device hereinafter described, could be 
used for removing froth from the surface of liquid medium 14 in the trough 
13. 
Certain water insoluble gasses injected through the porous element 47 may 
also have an affinity for algal cells. If the rate of charged gas 
injection is sufficiently fast, bubbles of gas may adhere to cells passing 
through the pipe 32. Once carried into the drum 53, the algal cells tend 
to rise to the surface. Due to their relatively large size, the cells do 
not rise as rapidly as the bubbles bearing organic substances, but instead 
are caught up in the subsurface flow of liquid medium and carried through 
the orifice 54 into the trough 13. 
The relative attraction of algal cells and organic substances to the gas 
bubbles can be regulated by selection of the types and amounts of gas 
injected as minute bubbles, by regulating the size of the bubbles and by 
adjusting the flow rate of liquid medium and the injection rate of gasses. 
To prevent algal cells from being removed in the foam separation device 
52, the above factors may be selected to minimize algal cell flotation and 
maximize organic material flotation inside the drum 53. 
A skimming device 59 is located in the trough 13 for continuously 
harvesting algal cells. One skimmer means suitable for this purpose is 
shown in FIGS. 1 and 6. This device comprises a porcelain drum 60 which 
may be lowered so that the lowermost portion of the drum extends just 
beneath the surface of the liquid medium 14. The drum thus provides a 
surface barrier without substantially impeding subsurface flow of the 
liquid medium. As liquid medium moves downstream in the direction of 
arrows 61, it passes beneath the drum 60. The drum 60 is rotated 
(counterclockwise in FIG. 6) so that the rising face of the drum faces 
upstream. As the drum rotates, floating algal cells adhere to the rising 
face of the drum 60 and are carried upwardly over the drum. A doctor blade 
or squeegee 62 is positioned against the drum 60 to scrape the algal cells 
from the drum surface and into a discharge chute 63. The skimming device 
60 may conveniently be located in the trough 13 near the discharge end 16. 
It could, however, be located at any position along the trough 13. 
It is advantageous to bring as many algal cells as possible to the surface 
of the liquid medium so that they are accessible for skim harvesting. 
Flotation of algal cells can be enhanced by any means capable of slowing 
the flow rate and reducing the mixing of liquid medium ahead of the 
harvester so that the pull of the liquid stream is overcome and the 
naturally buoyant algal cells rise to the surface. In the illustrated 
embodiment, flow is reduced at the discharge end 16 of the trough by 
regulating the rate at which liquid is removed through the pipe 39. The 
gas lift pump 30 is operated at a constant rate such that there is always 
a semistagnant buildup of medium at the discharge end 16. 
The flotation of algae is further enhanced because the channel both widens 
and deepens as it approaches the trough's discharge end 16. This causes a 
laminar flow pattern to be established in the liquid medium commencing 
somewhat upstream of the skimmer 60. FIG. 4 shows flow vectors 
(represented by arrows) observed at the discharge end of the preferred 
channel. It can be seen that increased cross-sectional flow area causes 
transition from transitional and/or turbulent flow to laminar flow. 
The drum 60 of the skimmer 59, acts as a gate which reduces the flow of 
surface liquid further so that a near-stagnant surface layer of medium is 
maintained upstream of the skimmer 59 as well as between the skimmer and 
the discharge end 16. Adjustment of the drum's skimming action regulates 
the discharge of surface water in the region of near stagnation. 
Upstream of the discharge end 16, the liquid medium is subjected to 
continuous mixing as will be described hereinafter. This mixing tends to 
maintain algal cells in suspension. When the algal cells reach the region 
of laminar flow, cells are freed from the mixing action and those having 
densities lower than the medium tend to rise to the surface where they 
form a thick film in the near-stagnant surface layer. Flotation of algal 
cells can further be enhanced by regulating the growth environment in the 
trough 13 to maximize the cell's biosynthesis of chemical compounds less 
dense than the liquid medium 14. Surfactants less dense than the medium 14 
can be added to the medium upstream of the discharge end 16. The 
surfactants adsorb to and increase the flotation of the algal cells. 
Once on the surface, algae may be skimmed off readily by the porcelain drum 
60 and constitute the end product of the system. 
As an alternative to skim harvesting, surface algal cells at the discharge 
end 16 can be harvested from the liquid medium by a conducting regulated 
flow of liquid medium over an end channel weir (not shown). Control of 
flow over such a weir regulates the discharge of surface waters within the 
region of near-stagnation. 
Harvesting may be accomplished most conveniently during daylight hours 
because algal cells emit oxygen during photosynthesis. The oxygen collects 
in small bubbles which cling to the algal cell wall and thus increase the 
flotation of the cells. Flotation is also enhanced by the minute bubbles 
which are formed in the airlift pump mechanism 30 and which may remain 
attached to the cells up to the time of harvesting. 
Other devices for continuously harvesting algal cells, such as rotary 
screens or hydroclones, may be used. A skimmer is preferred, however, 
because such a device is highly efficient when used as described above. 
To avoid contamination of the liquid culture medium 14 by undesirable 
airborne species, it is advisable that the gas space 29 be sealed from the 
surrounding atmosphere or be maintained at a slight positive pressure so 
that gas will tend to flow out of the space 29 to the surrounding 
atmosphere. One suitable way to avoid contamination is by continuously 
pumping a stream of filtered air into the space 29 and allowing any excess 
gas to flow out of the chamber. Some gas is, or course, added to the space 
from the supply lines 38, 45 via the standpipe 32. In addition, it may be 
helpful to inject cool, filtered air taken from the surrounding air to 
reduce the temperature of gas contained in the space 29. This prevents gas 
in the space 29 from becoming overheated which could adversely affect 
algal production. 
To further enhance the growth of algae, this apparatus is provided with 
means for filtering radiation of undesired wavelengths from incident 
sunlight. One suitable filtering means is a shallow container of light 
filtering liquid 64. Such a container, as shown in FIGS. 1 and 6, may be 
of "sandwich" construction, including the ceiling member 26 which serves 
as a lower panel of the container, a spaced upper transparent panel 65 and 
walls 66 joining the perimeters of the two panels to define a watertight 
compartment. The compartment contains a layer of liquid 64 which is 
selected for its ability to filter undesired wavelengths from the sunlight 
before it enters the gas space 29. If algae is to grow properly in the 
system, it is necessary that the panels 26, 65 be made of a material which 
is substantially transparent to those wavelengths of sunlight which are 
photosynthetically active. Preferably the panel 65 will have a flat face 
inclined toward the sun at an angle of up to about 60.degree. from 
horizontal to avoid reflection loss of photosynthetically active 
wavelengths. 
A variety of different liquids might be suitable for use in the container 
depending upon their light absorption spectra. One especially suitable 
liquid is an aqueous solution of CuSO.sub.4. An effective solution will 
contain about five to ten weight percent CuSO.sub.4.5H.sub.2 O divided by 
the length, in centimeters of the light's path inside the solution. Such a 
solution layer will trap the ultraviolet and infrared wavelengths which 
inhibit algal growth and/or normally would be unutilized. 
A radiation absorbing gas or gel could be used in the container 62 in place 
of the liquid solution. A gas suitable for this purpose is a mixture of 
ammonia and sodium thiocyanate. Alternately, filtering can be achieved 
using a solid filter plate. If, for example, one of the container panels 
26, 65 is impregnated with copper sulfate salt, a transparent liquid or 
gas could be circulated between the panels to receive heat energy absorbed 
from solar radiation by the impregnated panel. As a filter, the 
impregnated panel will suffice alone if recovery of heat energy is not 
desired. 
Regardless of filter type, the filter should be capable of transmitting 
photosynthetically active wavelengths and at the same time absorbing or 
reflecting at least ninety-nine percent of all incident radiation below 
350 nanometers and above 700 nonometers. 
Because the air in the gas space 29 will begin to receive excessive heat 
during daylight hours from the filter if the filter's temperature 
increases to an undesirably high level, a pane of transparent material 
(not shown) can be placed a small distance beneath the ceiling member 26 
to form a compartment. A layer of air trapped in such a compartment would 
thermally insulate the air space 29 from the filter. Cool air could be 
circulated through such a compartment to further prevent the transfer of 
heat from the filter into the air space 29. 
When using a liquid solution as a filtering medium, heat can be removed 
directly from the liquid 64 to prevent overheating inside the air space 
29. FIG. 8 shows schematically a heat exchange system both for controlling 
the temperature of liquid 64 and for utilizing heat energy collected in 
the liquid. A supply of the liquid 64 is maintained in a reservoir 72. A 
stream of the relatively cool liquid in the reservoir 72 is pumped by pump 
74 through a distribution tube 76 and into the compartment between the 
panels 26, 65 of a "sandwich-type" solar collector 77 as described above. 
Preferably, the tube 76 connects at the lowermost part of the compartment 
so that liquid 64 is injected along the lowest sidewall of the container, 
but other flow patterns could be used. 
As the pump 74 continues to operate, the injected liquid flows upwardly 
between the panels 26, 65 collecting heat. The heated liquid flows out of 
the upper end of the compartment through a tube 78 which is connected to a 
heat exchange unit 82. Inside the unit 82, excess heat is removed from the 
stream of filtering liquid by indirect heat exchange with a cooling fluid, 
preferably water, or some other non-toxic fluid having a high heat 
capacity. The fluid is circulated through a heat exchange coil 84 whereby 
it receives heat energy from the liquid 64. The temperature or the cooling 
liquid is preferably thermostatically controlled to maintain the filtering 
liquid at a decreased temperature. After it is cooled in the heat exchange 
unit 82, the filtering liquid 64 is returned to the reservoir 72 through a 
return tube 85. If heating of the liquid 64 is required at any time, a 
heated fluid can be circulated through the coil 84. 
Heat energy recovered in the exchange unit 82 can be used in other process 
applications, stored for heating use during cold periods or, if of 
sufficient quantity and quality, converted into electrical energy. The 
heat cooling fluid can be pumped from the heat exchange unit 82 into a 
distribution line 86 by a pump 87. The distribution line connects to an 
inlet pipe 88 of the previously described jacket 41. An outlet pipe 89 
returns cooled fluid to the heat exchange unit via a collection line 90. 
Another device which can utilize the heat in the cooling fluid is a trough 
heating system 92. The system includes temperature control lines 94 which 
are connected between an input manifold 96 and an output manifold 97. As 
best shown in FIGS. 1 and 7, the lines 94 are embedded in a layer of sand 
or similar material 98 in the bottom of the trough 13. A layer of flexible 
sheeting 100, e.g. of polyvinyl chloride or butyl rubber, covers the sand 
98 and comprises the floor of the channel defined by the trough 13. When 
the temperature of liquid medium 14 in the trough 13 descends below a 
desired level, fluid heated in the heat exchange unit 82 can be diverted 
through the input manifold 96 into the temperature control lines 94. Heat 
from the fluid thus defuses into the sand 98 to warm liquid medium 14 in 
the trough 13. The fluid is returned to the collection line 90 via the 
output manifold 97. 
Heated fluid can be diverted to other process apparatus, represented 
schematically in FIG. 8 as a box 102. Such apparatus might include means 
for converting heat energy into electricity for powering pumps and other 
equipment. 
The apparatus of the present invention further includes a device to induce 
mixing of the liquid medium 14 as it passes through the trough 13. The 
illustrated embodiment includes a serpentine trough having a series of 
curves positioned at intervals. Liquid medium is circulated through the 
trough at a rate sufficient to induce controlled mixing therein. As a 
result vertical eddies are induced and the liquid medium follows a 
vortical path. In this mixing pattern algal cells periodically move 
between surface layers of the medium and layers near the bottom thereof. 
Periodic mixing in turn reduces settling and early floatation of algal 
cells. 
Periodic or continuous medium mixing is further useful because plants grow 
most effeciently when subject to alternating periods of light and relative 
darkness. Because algal cells extinguish light by absorption and 
scattering in direct proportion to their concentration in the medium, 
relatively little sunlight penetrates to the deepest portion of the liquid 
medium 14 as compared to the amount of light available just below the 
surface. The above described periodic mixing thus causes algal cells to 
move alternately between well lighted positions (adjacent to the surface) 
and shaded areas (distant from the surface). This in turn causes the cells 
to exhibit the desirable growth characteristics associated with the 
phenomenon commonly known as the "flashing light effect." Preferably, the 
trough 13 is designed to produce periodic culture mixing such that 
individual algal cells follow a substantially sinusoidal path, as shown by 
the arrows in FIG. 9, during their travel from the inlet end 15 to the 
discharge end 16. 
To obtain the most efficient use of available sunlight, the trough system 
should contain the liquid medium in a relatively thin layer, between about 
0.5 and 5.0 centimeters in depth and more preferably between 2.0 and 5.0 
centimeters. Maintaining such a shallow layer facilitates culture mixing 
for the purpose of inducing the "flashing light effect" phenomenon. 
When operating a recirculating system with liquid medium in a 0.5 to 5.0 
centimeter depth range, the "flashing light effect" is maximized if the 
algal culture density is maintained so that at all depths, light intensity 
is extinguished between one hundred and one thousand fold multiplied by 
the liquid medium depth in centimeters. In other words, the culture 
density will be in the most proficient range if light intensity at a given 
depth multipled by the depth in centimeters gives a figure between about 
0.1 and 1.0 percent of the light intensity at the surface of the liquid 
medium. 
If a portion of the algal culture is not recirculated with the liquid 
medium, culture density at the inlet end of the trough may be at a density 
below the above specified amount. The proscribed culture density should, 
however, be reached at some point along the trough so that algal growth 
thereafter will be progressively enhanced by optimum "flashing light" 
conditions. 
While the serpentine trough of the illustrated device is quite suitable for 
moving cells in a vortical path, other options are available. It might, 
for example, prove desirable to culture algae in a long straight trough. 
Appropriate mixing will automatically result if a suitable liquid flow 
rate is maintained. Mixing can further be created by positioning baffles 
or other mixing devices at spaced intervals in a trough. Such devices can 
provide the turbulence necessary to obtain a mixing pattern of the type 
illustrated by arrows in FIG. 9. Other techniques well known in the art 
can be applied to generate the desired turbulences. 
One especially suitable mixing device is a cylindrical riffle 104 as 
illustrated in FIGS. 10 and 11. Such riffles may be located at intervals 
along the trough's channel bottom to create turbulence and thus culture 
mixing in the flowing liquid culture medium 14. The greatest amount of 
turbulence occurs just downstream of a riffle and recedes thereafter. 
Riffle spacing should thus be set so that a riffle is located at each 
point where turbulence induced by the immediately proceeding riffle has 
died down. 
To make up for nutrients which are consumed by the algae during their 
growth and to regulate the pH of the nutrient medium, the apparatus of the 
present invention includes lines 106 shown in FIG. 1 as extending from a 
plurality of tanks 108 containing solutions of make up nutrients and/or 
liquids for adjusting the pH of the liquid medium 14. Make up nutrients 
from the tanks are pumped into the trough 13 via the lines 106 to 
replenish the liquid medium 14. A plurality of such nutrient make up 
and/or pH adjustment lines may be positioned at intervals along the trough 
to add nutrients or liquids to adjust pH wherever needed. The lines can 
also by used to add water to the trough 13 to make up for losses due to 
evaporation and to add carbon dioxide to the liquid medium. Periodic 
sampling and testing of the liquid medium can be used to determine the 
amount of nutrients, pH and salinity adjusting materials, carbon dioxide 
or water to be added through each nutrient make up line 106. A variety of 
alternative means for monitoring and replenishing the nutrient medium 14 
will be apparent to one skilled in the art. 
By careful regulation of the nutrient medium, it is possible to uniformly 
control the quality and chemical composition of the ultimate algal 
product. This is because algal cells of a single species are found to have 
physiological characteristics which differ depending upon the environment 
in which they are cultured. Factors affecting the algae include light 
wavelength and exposure timing, chemical composition of the nutrient, 
concentration of cells in the culture, and temperature. Each such factor 
is carefully controlled by the above described system. 
It has been found, for example, that maximum lipid production can be 
achieved if the nitrogen concentration of a culture medium is reduced when 
the culture approaches its maximum cell population. Thus, by regulating 
the nutrient medium it is possible to control the lipid content of the 
product algae. 
OPERATION 
The basic operation of the present invention will be apparent from the 
forgoing description of the apparatus. The trough 13 is filled with liquid 
culture medium and innoculated with algae. The airlift pump 30 is 
activated by introducing gas through the line 40. This causes the liquid 
medium, containing rapidly growing algal cells to circulate through the 
channels as a shallow layer overlying the flexible sheeting 100. The flow 
rate is selected to induce the desired mixing of liquid as it flows 
through the channels. 
While the system operates, the thermostatically controlled heat exchanger 
82 maintains the temperature of the filtering liquid 64 at a desired 
level. Also, a fluid at an appropriate temperature is circulated through 
the jacket 41 and/or temperature control lines 94 to maintain the liquid 
medium within an efficient operating temperature range. Carbon dioxide is 
continuously fed to the liquid medium 14 through the line 45 of the gas 
lift pump mechanism and/or the lines 106. After an initial operating 
period, samples of the nutrient material are taken and, if the samples 
indicate that the nutrient level has dropped below a desired minimum, 
makeup nutrients are added through line 82 to promote further algal 
reproduction and/or cell growth. Once the algae has reproduced to a 
desirable concentration and cell size, the harvesting mechanism 59 is 
lowered into the liquid 14 and the drum 60 activated to commence 
harvesting. 
The continuous growth of high-lipid algal cells in recirculating liquid 
medium can be accomplished by establishing coordinated harvesting and 
nutrient addition schedules as follows. The culture is established in a 
full strength liquid culture medium. As the culture grows, nutrients are 
added periodically to maintain their concentrations within their desirable 
range. When the rate of cell reproduction reduces as the cell 
concentration approaches its limit, as determined by an electronic 
particulate cell counter or similar device, nitrogen compounds are 
eliminated from the makeup nutrients until the nitrogen content of the 
liquid medium drops to about fifty percent of the amount initially present 
in the complete medium. Subsequent nutrient additions are adjusted to 
maintain nitrogen at the fifty percent level while other nutrients are at 
full strength. 
After a period at these conditions, total cell mass of the culture will 
have increased, especially the lipid content thereof. Harvesting is then 
commenced until about fifty percent of the cells are separated from the 
liquid medium. Once this has been done harvesting is stopped, the nitrogen 
content returned to full strength and the entire process repeated. 
It is desirable that both the trough 13 and the liquid medium 14 be 
sterilized before use. This is especially true where natural seawater is a 
component of the liquid medium. If such a preliminary step is not taken, 
the liquid medium may be contaminated with undesirable organisms. The 
present system provides a unique method of self-sterilization as will be 
apparent from the following example. 
EXAMPLE 
A culture of Phaeodactylum tricornutum was grown in an apparatus of the 
type previously described. This particular marine algae was selected 
because of its unique growth characteristics and usefulness as an end 
product. It is further advantageous for cultivation because it has been 
observed to secrete substances having antibiotic activity. Such secretions 
may inhibit the growth of bacteria in the liquid nutrient medium. Because 
Phaeodactylum tricornutum does not require external supplies of silica or 
vitamins, these nutrients can be excluded from the liquid medium to 
inhibit the growth of contaminating micro-organisms requiring these 
nutrients. 
A variety of liquid mediums might be used successfully, for the culture of 
Phaeodactylum tricornutum, but exceptional results are achieved using a 
medium which includes sea water in which is dissolved a mixture of 
nutrient salts which, in grams per 100 liters of sea water, comprises 
38.0--63.4 g. HNO.sub.3, 5.9--9.8 g. H.sub.3 PO.sub.4, 3.2-5.4 g. KCl, 
1.40-2.33 g. Na.sub.2 EDTA (ethylene diaminetetraacetic acid disodium 
salt), 0.015-0.025 g. FeCl.sub.3.6H.sub.2 O, 0.00275-0.00460 g. 
(NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, 0.0008-0.0013 g. H.sub.3 
BO.sub.3, 0.00030-0.00050 g. ZuCl.sub.2, and lesser amounts of both 
CuCl.sub.2.2H.sub.2 O and MnCl.sub.2.4H.sub.2 O, and an amount of KOH 
sufficient to raise the pH of said solution to within the range of 
7.6-7.8. 
An experimental liquid nutrient medium was accordingly prepared from 100 
liters of sea water and 50.7 g. HNO.sub.3, 7.8 g. H.sub.3 PO.sub.4, 4.3 g. 
KCl, 1.86 g. Na.sub.2 EDTA, 0.020 g. FeCl.sub.3.6H.sub.2 O, 0.00368 g. 
(NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, 0.00105 g. H.sub.3 
BO.sub.3, 0.00040 g. ZnCl.sub.2, lesser amounts of both 
CuCl.sub.2.2H.sub.2 O and MnCl.sub.2.4H.sub.2 O, and an amount of KOH 
sufficient to raise the pH of the nutrient medium to within the range of 
7.6-7.8. All of the above ingredients except the KOH were combined to form 
a liquid nutrient medium precursor which was introduced into the trough 13 
and pipes 32, 39 of the apparatus. Due to its nutrient content, the 
precursor was substantially acidic (pH 2.2) so that potentially 
contaminating organisms in the sea water, trough and pipes were killed by 
a period of exposure to the acidity of the precursor. 
At the completion of this sterilization step, the KOH was added to the 
precursor to bring the precursor to a pH suitable for growth of the 
Phaeodactylum tricornutum, to complete the nutrient content of the liquid 
nutrient medium and to bring the level of liquid medium in the trough up 
to an average running depth of 2.2 centimeters. The pH range of 7.6-7.8 
was well within the pH range of 7.5 to 9.5 preferred for maintaining a 
substantial bicarbonate concentration in the sea water based medium. The 
medium was circulated by the gas lift pump for a period sufficient to 
completely mix the ingredients. Thereafter the completed medium 14 was 
innoculated with a culture of the algae. Artificially formulated sea water 
could have been used in place of natural sea water to reduce the need for 
sterilization steps, but it is almost impossible to formulate sea water 
accurately. 
The trough was located beneath a transparently bottomed tray containing a 
three centimeter layer of an aqueous solution containing three weight 
percent copper sulfate. Using this filter nearly ninety percent of 
infrared radiation was removed from incident sunlight. 
The innoculated liquid medium was continuously circulated at a mean flow 
rate of one foot per second. The algae were allowed to grow and reproduce 
and did so rapidly. Nutrients were replenished as needed. After a period 
of rapid reproduction, cell division rate decreased, indicating that a 
near maximum cell concentration in the light had been reached. Thereafter, 
the liquid medium and entrained algae were further circulated and 
nutrients added as needed to maintain cell metabolism, but no HNO.sub.3 
was added with the makeup nutrients. The nitrogen content of the medium 
was thus allowed to become substantially depleted. In this nitrogen-lean 
medium, algal cells continued to grow in size and weight, but cell 
division rate was greatly reduced. The amount of cell matter continued to 
increase and the lipid content of the cells increased markedly. Most 
efficient lipid production resulted when the liquid medium was maintained 
in the temperature range between 22.degree. and 34.degree. C. 
When the concentration of algae reached approximately four grams per liter 
of liquid medium, about half the algae was harvested leaving about two 
grams of algae per liter of medium. Makeup nutrients and water were added 
to the remaining liquid medium to bring its volume and nutrient content, 
including nitrogen, back up to the full strength. Cell division, growth 
and harvesting proceeded continuously according to the steps previously 
described. 
During the entire growth period, a temperature condition was maintained to 
enhance the growth and reproduction of algal cells. Temperature in the 
airspace 29 was maintained at a maximum of 30.degree. C. by circulating 
cooled, filtered air through the space on warm days. The temperature of 
the liquid medium 14 was maintained between 18.degree. and 28.degree. C. 
with the highest temperatures occurring during daylight hours and the 
lowest temperatures at night. Average medium temperature during the 
experimental period was 24.degree. C. 
Operating in this manner a Phaeodactylum tricornutum production rate 
significantly higher than has previously been reported for mass culture 
systems was achieved. The presence of KOH in the liquid medium appeared to 
cause a change in the algal cell morphalogy and to cause an increase in 
the cells' nutrient intake. Nitrogen intake by the cells was extremely 
rapid considering that the medium contained nitrogen as nitrate ions. 
The source culture used to innoculate the liquid medium 14 was typical of 
commonly described Phaeodactylum tricornutum cultures and contained the 
typical ovate forms. However, following exposure to the nutrient medium 
described, the ovate forms disappeared, leaving only the fusiform type. In 
this medium, the fusiform cells enlarged greatly, attaining forty microns 
in cases, and the entire cell volume filled to the extent that extension 
arms (normally present) were absent. Cells grown under these conditions 
contained nearly four times the cytoplasm mass of normal fusiform cells 
and thus were producing nearly four times the weight for each parent cell 
division. 
Furthermore, the cells showed an entirely new mode of cell division 
possibly due to the high concentration of potassium ions in the liquid 
medium. Large fusiform cells were observed having one or more buds 
gradually protruding near the apex. These buds continued to expand 
outwardly until an equivalent number of legs would form. Indentation of 
the cell would continue until there were two or more cells, still 
connected apically. It was not uncommon to see three and four cell 
clusters radiating outward from the center where they would remain 
connected. It thus appears that Phaeodactylum tricornutum may have the 
capacity to produce three to four cells per cell division in a properly 
selected liquid medium. 
It was further observed that the normal sequence for cell enlargement 
during night hours and cell division during daylight hours was reversed 
during this experiment. 
From the harvest results obtained in this experiment, it can be 
extrapolated that a yield of 38.06 ash free dry tons of algae per 
acre-year would be obtained when using the system described. Assuming 
further optimization, yields of seventy to ninety tons seem within reach 
for certain locations and climatic conditions. The experimental results 
are far superior to those previously reported and indicate a 
photosynthetically active radiation utilization efficiency rate far above 
the values achieved by previous mass algal culture systems. 
By the method of operation described in the above example, a liquid medium 
can be continuously recycled and reused until such time as it becomes 
contaminated with undesired organisms or until waste substances which 
cannot be removed from the liquid medium accumulate to a toxic level. 
The apparatus described can be used to culture both fresh water and marine 
algae. Although Phaeodactylum tricornutum is an especially suitable marine 
species for mass culture, numerous other species could be reproduced by 
means of the present invention. Examples of such species appear in the 
following non-exclusive list: 
I. MARINE/ESTUARINE TYPES 
Fragilaria sublinearis 
Skeletonema costatum 
Cyclotella nana 
Isochrysis galbana 
Pavlova gyrens 
Monochrysis lutheri 
Coccolithus huxleyi 
Nitzschia palea 
Dunaliella tertiolecta 
Prymnesium paruum 
II. FRESHWATER TYPES 
Chlorella spp. 
Chlamydomonas reinhardtii 
Synedra acus 
Scenedesmus spp. 
Asterionella formosa 
Navicula spp. 
Nitzschia spp. 
Fragilaria spp. 
Chrysococcus spp. 
Cyclotella spp. 
Dinobryon spp. 
Freshwater algae may be easier to maintain in a continuous process than 
marine algae because media suitable for marine algae are based on seawater 
which contains a very complex mixture of nutrients, many of which are 
present in minute concentrations. It is more difficult to monitor and 
maintain the chemistry of seawater based media as opposed to media based 
on freshwater. 
While I have shown and described a preferred embodiment of my invention, it 
will be apparent to those skilled in the art that changes and 
modifications may be made without departing from my invention in its 
broader aspects. 
For example, troughs for containing the nutrient medium can be 
substantially straight and can be constructed to sufficient length so that 
algae would grow to their maximum concentration and size during one trip 
through the trough. Nutrients can be injected at intervals along such a 
trough to maintain a desired nutrient content. Heat exchange means can be 
provided at intervals along the trough to regulate the temperature of the 
liquid medium. 
Such a "one-pass" trough can be used to provide multiple crops, by 
withdrawing a portion, e.g. fifty prcent, of the liquid medium containing 
a mature culture and replacing the withdrawn portion with fresh nutrient 
medium prior to the terminus of the trough. This can be repeated at 
intervals along the trough at any location where the culture reaches a 
desired state of maturity. The makeup nutrient medium can be changed at 
each such location, as desired, so that different portions of the trough 
will produce algal cells having different characteristics. 
Another means for obtaining cells of different characteristics from a 
"one-pass" trough would be to branch the trough as shown schematically in 
FIG. 12. Instead of adding makeup medium as described above, each branch 
line can e reduced in size so that it carries only a portion of the total 
culture. The trough 110 of FIG. 12 has an inlet end 114 and three 
discharge ends 116, 118, 120. As liquid medium flows through the trough it 
is divided into separate streams by longitudinal baffles 122. Because the 
different branches are of different lengths, the maturity of the culture 
removed depends on the total channel length between the inlet end and a 
particular discharge end. Algal cells may have different characteristics 
depending upon the maturity of the culture so that algae harvested from 
the various branches may be best suited for differeing uses. 
When culturing Phaeodactylum tricornutum, for example, cells removed at the 
discharge end 116 might best be used as a protein source. Depending on the 
branch spacing, cells taken from the discharge end 118 might be most 
useful as a source of lipids and those which complete the journey to 
discharge end 120 best used for the production of antibiotics. Careful 
regulation of nutrient conditions in each of the separate channels could 
further enchance the variation among algal cells.