GROW YOUR OWN SPIRULINA
REVISED ON
August 9, 2005
NOTICE
This is the condensed version of a "Manual of
small scale spirulina culture" written in French and distributed by
Antenna Technology.
This is not one more book on spirulina. Excellent
ones are available*, dealing with such topics as :
- what is spirulina ?
- what is its natural habitat
?
- how did the Aztecs harvest
it and eat it ?
- how was it rediscovered 30
years ago ?
- what nutrients, vitamins,
minerals does it contain ?
- what are its food-grade
specifications ?
- what are its numerous
benefits for your health ?
- how does industry
manufacture and market spirulina ?
- why is spirulina
ecologically friendly ?
- why has it such a brilliant
future ?
The sole purpose of this little manual is to bring
my field experience on small scale spirulina production to those who would need
it : the answers to the above questions are assumed to be well known.
To make
the presentation shorter, easier and more accurate, I decided not to avoid
using common technical terms : in case some would confuse you, look for an
explanation in a chemistry college handbook.
What is called "spirulina" here actually
bears the scientific name of "Arthrospira
platensis", a cyanobacteria. But the common name "spirulina"
is universally used.
* See for instance "Earth Food
Spirulina", by Robert Henrikson, published by Ronore Enterprises, U.S.A.
(1994), and "Spirulina, Production & Potential", by Ripley D. Fox
, Editions Edisud, France (1996), or "Spirulina platensis (Arthrospira) :
Physiology, Cell-biology and Biotechnology", edited by A. Vonshak,
published by Taylor & Francis (1997)
CLIMATIC
FACTORS
Temperature is the most important climatic factor
influencing the rate of growth of spirulina.
Below 20°C, growth is practically nil, but
spirulina does not die. The optimum temperature for growth is 35°C, but above
38°C spirulina is in danger.
Growth only takes place in light (photosynthesis),
but illumination 24 hours a day is not recommended. During dark periods,
chemical reactions take place within spirulina, like synthesis of proteins and
respiration.
Respiration decreases the mass of spirulina
("biomass") ; its rate is much greater at high temperature so cool
nights are better on that account, but in the morning beware that spirulina
cannot stand a strong light when cold (below 15°C).
Light is an important factor but full sunlight may
not be the best rate of illumination : 30% of full sun light is actually
better, except that more may be required to quickly heat up the culture in the
morning.
Individual spirulina filaments are destroyed by
prolonged strong illumination ("photolysis"), therefore it is
necessary to agitate the culture in order to minimize the time they are exposed
to full sunlight.
Rain is beneficial to compensate for evaporation,
but it must not be allowed to cause overflowing of the culture pond.
Wind is beneficial for agitating and aerating the
culture, but it may bring dirt into it.
Artificial light and heating may be used to grow
spirulina, although they are not economical. Fluorescent tubes and halogen
lamps are both convenient. Lamps can illuminate and heat the culture
simultaneously.
PONDS
Spirulina thrives in alkaline, brackish water. Any
water-tight, open container can be used to grow spirulina, provided it will
resist corrosion and be non-toxic. Its shape is immaterial, although sharp
angles should be avoided to facilitate agitation and cleaning. Its depth is
usually 40 cm (twice the depth of the culture itself). It can be as small as 1
m² but 5, 20 or 100 m² are more economical. Dimensions are only limited by the
necessity of accessing for agitation and cleaning. The bottom should have a
slight slope and a recess to facilitate cleaning and emptying. Two ponds are
better than just one, for practical reasons.
The most economical ponds are made of U.V.
resistant plastic film of 0.5 mm
thickness or more (PVC or polyethylene), with sides supported by bricks or a
wooden structure or metal tubes. If termites are present, a layer of dry ash
plus a layer of sand should be placed under the film to protect it, and of
course wood should not be used.
Concrete ponds are a good, durable solution where
experienced labour is available. Before starting the culture, the cement should
be well cured and whitewashed.
A greenhouse over the ponds offers many
advantages, provided it can be aerated and shaded. As a matter of fact,
covering the ponds is necessary in many instances.
Agitation can be manual, with a broom, once every
two hours. If electricity is available, aquarium pumps are practical to agitate
the surface of the culture (one watt/m² is enough). "Raceway" ponds
agitated by paddlewheels are standard in the industry, but somewhat outside the
scope of this manual.
CULTURE MEDIUM
Spirulina can live in a wide range of compositions
of water ; the following is an example of a convenient analysis :
Anions Carbonate 2800
mg/l
Bicarbonate 720
Nitrate 614
Phosphate 80
Sulfate 350
Chloride 3030
Cations Sodium 4380
Potassium 642
Magnesium 10
Calcium 10
Iron 0.8
Urea <
50
Total dissolved solids 12847
Density @ 20°C 1010
g/l
Alcalinity 0.105
N (moles strong base/liter)
pH @ 20°C 10.4
In addition, the solution contains traces of all micronutrients necessary
to support plant life.
Such solution can be obtained by dissolving various combinations of
chemicals ; here is one example convenient for many typical soft waters :
Sodium
carbonate (soda ash) 5 g/l
Sodium chloride, crude 5
Potassium nitrate 2
Sodium
bicarbonate 1
Potassium
sulfate, crystallized 1
Urea 0.02
Monoammonium
Phosphate, crystallized 0.1
Magnesium
sulfate, crystallized, MgSO4, 7 H2O 0.2
Lime 0.02
Ferrous
sulfate, crystallized, FeSO4, 7 H2O 0.005
The water used should be clean or filtered to avoid foreign algae.
Potable water is convenient. Water often contains enough calcium, but if it is
too hard it will cause mud which is more a nuisance than a real problem.
Brackish water may be advantageous but should be analysed for its contents or
tested. Seawater can be used under some very special conditions, outside the
scope of this short manual.
The culture medium described above is used to start new cultures. The
make-up medium should best be as follows : carbonate is replaced by bicarbonate
(8 g/l in total), urea is up to 0.07 g/l, and nitrate can be omitted.
Certain ions can be present in concentrations limited only by the total
dissolved solids which should not be much over 25 g/l ; these are : sulfate, chloride, nitrate, and sodium. Sodium or potassium nitrate can replace
urea, the advantage being a large stock of nitrogen ; urea is more efficient to
supply nitrogen but is highly toxic at too high concentration. Spirulina can
grow on either nitrate or urea alone, but using both together is advantageous.
Phosphate, magnesium and calcium cannot be increased much without
precipitating magnesium or calcium phosphate, possibly leading to imbalances in
the solution.
Potassium concentration can be increased at will, provided it does not
become more than five times the sodium concentration. This makes it possible to
use potash extracted from wood ash to replace sodium carbonate/bicarbonate
should these not be available (let the potash solution absorb CO2 from the air
until its pH has come down to 10.5 before using it).
If fertilizer grade chemicals are
used, they should be of the "soluble" or "crystallized"
type, not of the "slow release", granulated type.
Micronutrients traces contained in the water and in the chemicals are
sufficient to support the initial growth.
In case of necessity ("survival" type situations), nitrogen,
phosphate, sulfate, sodium, potassium and magnesium can all be brought by urine
(from persons or animals in good health, not consuming drugs) at 5 ml/l and
iron by a saturated solution of iron in vinegar (use about 0.1 ml/l).
Solutions of iron should preferably be introduced very slowly and under
agitation into the medium. Dripping is best.
SEEDING
Choose a spirulina strain containing a high proportion of coiled
filaments (less than 25 % straight filaments, and if available 0 %), easy to
harvest, and containing at least 1 % of gamma-linolenic acid (GLA) sed on dry
weight.
Concentrated spirulina seed culture can be obtained either from the
floating layer of an unagitated culture, or by rediluting a freshly filtered
biomass (beware of lumps). A concentration of up to 3 g spirulina (dry) per
liter is permissible if storage and transportation last less than a week's
time, and provided the seed culture be aerated at least two times a day. If
aeration can be continuous, the concentration may be up to 10 g/l (weights of
spirulina always refer to contained dry matter).
It is advisable to maintain the growing culture at a fairly high
concentration in spirulina after each dilution with culture medium, about 0.3
g/l : the "Secchi disk" reading (see Annex 1) should not be above 5
cm, i.e. the color of the culture should stay clearly green (otherwise shading
is mandatory). The rate of growth is about 30 % /day when light
and temperature are adequate and the make-up culture medium is based on
bicarbonate (without carbonate). As the growth is proportional to the area of
the culture exposed to light, it is recommended to maximize this area at all
times (i.e. use the minimum feasible depth during the expanding area period,
generally 5 to 10 cm).
When the final area and depth (10 to 20 cm) are reached in the pond, let
the spirulina concentration rise to about 0.5 g/l (Secchi disk at about 2 cm)
before harvesting.
HARVESTING
When the spirulina is in good condition, separating it from the water
("harvesting") is an easy operation, but when it gets too old and
"sticky" harvesting may become a nightmare ( see § "Taking
care").
The best time for harvesting is early morning for various reasons :
- the cool temperature
makes the work easier,
- more sunshine hours will
be available to dry the product,
- the % proteins in the
spirulina is highest in the morning.
There are basically two steps in harvesting :
- filtration to obtain a
"biomass" containing about 10 % dry matter (1 liter = 100 g dry) and
50 % residual culture medium,
- removal of the residual
culture medium to obtain the "fresh spirulina biomass", ready to be
consumed or dried, containing about 20 % dry matter and practically no residual
culture medium.
Filtration is simply accomplished by passing the culture through a fine
weave cloth, using gravity as the driving force. Synthetic fiber cloth
(especially polyamide or polyester) with a mesh size of about 30 to 50 microns
is the preferred filtering medium. Supporting the filtration cloth by a net
will accelerate somewhat the filtration and protect the cloth against
rupturing, but a simple bag made from the cloth works well also.
The filter can be installed above the pond to directly recycle the
filtrate.
The culture to be harvested should be passed through a sieve (mesh size
about 200 µ) to remove any foreign matter such as insects, larvae, leaves and
lumps of polysaccharide or mud.
When the spirulina floats, which is the normal case without agitation, it
is efficient to scoop out the "cream", using a straight edge pail.
Harvesting the floating layer (generally richer in spiralled spirulina) will
tend to increase the % straight spirulina in the culture. Straight spirulina is
more difficult to harvest. So actually it is not recommended to harvest the
floating layer when both straight and spiralled spirulina are present.
The filtration is accelerated by gently moving or scraping the filter
cloth. When most of the water has filtered through, the biomass will often
agglomerate into a "ball" under the motion, leaving the cloth clean
(this desirable condition happens mostly when the biomass is richer in
spiralled forms and the culture medium is clean). Otherwise it may be necessary
to scrape it out from the cloth.
The final dewatering is accomplished by pressing the biomass enclosed in
a piece of filtration cloth plus a strong cotton cloth, either by hand or in
any kind of press. The simplest is to apply pressure (0.15 kg/cm² is enough) by
putting a heavy stone on the bag containing the biomass. The "juice"
that is expelled comes out first colorless, later it turns green and the
operation must then be discontinued otherwise too much product will be lost.
For the usual thickness of cake (about one inch after pressing), the pressing
time is about 15 minutes. Practically all the interstitial water (culture
medium) is removed, and some rinsing may be effected by the internal juices
from ruptured cells. The pH of the well pressed biomass is near 7 (neutrality
).
This pressing operation effects a more efficient separation of the
residual culture medium than washing the biomass with its weight of water on
the filter. Washing with fresh water may cause rupture of the cell wall of the
spirulina due to osmotic shock, leading to loss of valuable products; it may
also introduce germs contained in the wash water. Washed biomass is a lot more
prone to fermentation than pressed biomass. Pressed biomass contains twice as
much dry matter as unpressed biomass, which reduces the drying time.
When the biomass is too "sticky", for instance 100 % straight
filaments, it may not be possible to dewater it : in such case, it must be
washed.
FEEDING THE CULTURE
The nutrients extracted from the culture medium by the harvested biomass
should be replaced to maintain the fertility of the culture medium.
The main nutrient is carbon, which is spontaneously absorbed by the
medium from the air, as carbon dioxide (CO2), whenever the pH of the medium is
above 10. However the air contains so little CO2 that this absorption is a slow
process, corresponding to a maximum productivity of 4 g spirulina/day/m².
This maximum rate is reached at or above pH = 10.5. Extra CO2 can be introduced
to increase the productivity, either pure CO2 gas (from fermentation or from a
cylinder). The gas is bubbled into the medium, under a piece of floating
plastic film (about 4 % of the total area of the pond).
Another popular, although usually costly, means of feeding carbon is
bicarbonate. Adding bicarbonate is an easy and efficient way of reducing the
pH, but it increases the salinity ; to maintain the salinity, it is mandatory
to drain part of the culture medium from time to time and replace it by new
medium rich in bicarbonate. Disposal of the drained medium may be an
environmental problem and the cost may of the chemicals consumed may be
uneconomical.
The amount of gas or bicarbonate to be fed is adjusted so as to control
the pH at around 10.4. A pH lower than 10.2 may cause an overproduction of
undesirable, but not dangerous, exopolysacharide (EPS). A good practical dose of carbon feed is the
equivalent of 40 % of the spirulina
produced (i.e. about 0.8 kg of CO2 per kg of dry spirulina harvested).
Apart from carbon, spirulina requires the usual major biological
nutrients : N, P, K, S, Mg, Ca, Fe, plus a number of micronutrients. In many
cases, the micronutrients and the calcium need not be fed to the culture, being
supplied as natural impurities contained in the make-up water and the chemicals
used as food for the spirulina. In some locations, the water contains a large
excess of calcium, magnesium or iron, that may become a nuisance by producing
abundant mud.
The major nutrients can be supplied in various ways, preferably in a
soluble form, but even insoluble materials will slowly be dissolved as the
corresponding ions are consumed by the spirulina in the medium. Availability,
quality and cost are the main criterions for selecting the sources of
nutrients, but their content in valuable micronutrients may also affect the
choice.
If fertilizer grade chemicals are
used, they should be of the "soluble" or "crystallized"
type, not of the "slow release", granulated type. Beware of the
contents in "heavy metals" (mercury, cadmium, lead and antimony), as
the spirulina readily absorbs these and strict specifications must be met.
Natural nitrate from Chile, where available, is a good source of
nitrogen, not only on the basis of its low cost but also because it contains
many valuable nutrients apart from nitrogen. But very generally the cheapest source
of nitrogen is urea. Urea, made up of ammonia and CO2, is an excellent nutrient
for spirulina but its concentration in the medium must be kept low (below about
60 mg/liter. Excess urea is
converted either to nitrates or
to ammonia in the medium. A faint smell of ammonia is a sign that there is an
excess of nitrogen, not necessarily harmful ; a strong odour however indicates
an overdose.
Here is a feed formula convenient in most locations, per kg of
harvested spirulina (dry product) :
Urea 300
g
Monoammonium
phosphate 50 g
Potassium sulfate 30 g
(or 40 g if no
potassium nitrate is used in the culture)
Magnesium sulfate*
30 g
Lime 10
g
Iron sulfate* 2.5 g
Micronutrients
solution** 5
ml
* as the usual commercial product, cristallised with 7 molecules of
water.
** supplied by Antenna Technology, Geneva, upon request (the use of this
solution is optional ; it is useful to make the biomass easier to harvest and
also to reduce the need for renewing
the culture medium).
Concentrated pure phosphoric acid may replace the phosphate.
In case of necessity ("survival" type situations), all major
nutrients and micronutrients except iron can be supplied by urine (from persons
or animals in good health, not consuming drugs) at a dose of about 15 to 20
liters/ kg spirulina. Iron can be supplied by a saturated solution of iron in
vinegar (use about 100 ml/kg) plus some lemon juice.
Ferilizers other than urea can be fed every month or so, but urea (or
urine) has to be fed daily, based on the average production expected.
TAKING CARE OF THE CULTURE
Apart from harvesting and feeding, a spirulina culture requires some
attention in order to be kept in good condition.
Agitation is a requisite. Continuous agitation however is not required.
One third of full sun will saturate the photosynthetic capacity of
spirulina, but shading is not required except to reduce the consumption of
water (evaporation) or the temperature (< 38°C) or the pH (< 11.3). The
temperature will practically never be too high, but the pH may soon become too
high if insufficient carbon is supplied.
The depth of culture must be kept between 10 and 20 cm. Evaporation must
be compensated for by adding water. Rains must be compensated for either by
evaporation or by draining part of the medium (in the latter case, add the
chemicals corresponding to the volume of medium drained).
Accumulation of mud may cause some to float due to anaerobic fermentation
gases, and this will disturb the harvesting process. Therefore it is
recommended to agitate the mud layer with a broom from time to time. If too
much mud accumulates at the bottom of the pond, it can be removed by pumping or
siphoning (preferably while the spirulina is floating, in order to reduce the
loss). Add new culture medium to replace the volume removed. Of course another
way to remove the mud is to provisionally transfer the culture into another
pond and clean the bottom.
In large industrial spirulina farms, continuous monitoring of the
elements contained in the culture medium makes the exact make-up of individual
micronutrient possible. But this is too costly for small scale operators, who
then have to rely on renewing the culture medium or on the addition of minor
amounts of a concentrated solution of micronutrients as mentioned above.
Excessive production of exopolysaccharide (EPS) by the spirulina or its
too slow biodegradation will cause "stickiness" of the biomass and/or
a flocculation of spirulina into undesirable aggregates. To control this,
maintain higher pH, nitrogen and iron contents in the culture medium. The pH
should be above 10, preferably above 10.3. Partial or total renewal of the
culture medium also helps remedy the "stickiness" of the biomass.
Excessive turbidity of the filtrate may be reduced by slowing down the
growth of spirulina and/or maintaining agitation during the night. This applies
to the organic mud and EPS also. The culture is an ecosystem inside which
various microorganisms (useful bacteria and zooplankton) live in symbiosis,
resulting in a continuous, but slow, cleaning effect of the medium. If
pollutants are produced more rapidly than this biological cleansing system can
absorb, renewal of the medium will be necessary to keep it clean. Slowing down
the growth may be obtained by shading or by reducing the rate of harvesting.
When stressed by a pH or salinity sudden variation, for instance by a
heavy rain (more than 10% of the culture volume), the spirulina may sink to the
bottom of the pond, where they will be in great danger of dying from
suffocation. In order to facilitate their recovery, agitate the bottom often to
give them more chance to disentangle from the mud.
The culture may become colonized by predators living on spirulina, like
larvae of mosquitoes and Ephydra flies, or amoebas. In our experience these
invaders cause no other trouble than reducing somewhat the productivity. Often
they can be controlled by increased salinity, pH or temperature, or they
disapear by themselves after a few weeks.
If the concentration of spirulina is too low, the culture may be invaded
by chlorella (a unicellular, edible alga). Fortunately, chlorella pass through
the filter during harvesting : so you can harvest all the spirulina, recover
the wet biomass, wash it with some new culture medium and use it to restart a
new tank; The contaminated medium can either be discarded or sterilised. The
same procedure should be applicable to diatoms.
Toxic algae like anabaena, anabaenopsis arnoldii and microcystis do not
grow in a well tended spirulina culture, but for safety's sake it is
recommended to have the culture checked by a microscopic examination at least
once a year. A culture of young artemias can be used to check the absence of
toxic algae : boil a little of the spirulina culture to be checked (10 % of the
artemias culture) during one minute, cool it and mix it with the artemias
culture : observe the small animals ; if they retain their vitality for at
least 6 hours, there is no toxic algae. Artemias eggs are sold by aquariophilic
stores. The culture sample should be boiled one minute
Usual pathogenic bacteria do not survive the high pH (> 9.7) of a
spirulina culture in production ; however a microbiological assay of the
product should be made also at least once a year. Contaminations most generally
occur during or after harvesting.
The color of the culture should be deep green. If it turns yellowish,
this may be due to either a lack of nitrogen or an excess of light (photolysis)
or of ammonia (excess of urea). In the latter two cases recovery is generally
possible within two weeks while resting the culture under shading.
STORING THE PRODUCT
There is no question that freshly harvested, pressed biomass is superior
to any other form of spirulina. However it will not keep more than a few days
in the refrigerator, and no more than a few hours at room temperature.
Adding 10 % salt is a way to extend these keeping times up to several
months, but the appearance and taste of the product change : the blue pigment
(phycocyanin) is liberated, the product becomes fluid and the taste is somewhat
like anchovy's paste.
Freezing is a convenient way to keep fresh spirulina for a long time. It
also liberates the blue pigment, but it does not alter the taste.
Drying is the only commercial way to store and distribute spirulina. If
suitably packaged under vacuum in aluminised heat sealed plastic bags, dry
spirulina is considered good for consumption up to five years. But drying is an
expensive process and it generally conveys the product a different and possibly
unpleasant taste and odour, especially if the product is spray dried at high
temperature as is the case in large scale plants.
DRYING
The industrial type of spirulina dryer is the spray drier which flash
dries fine droplets at very high temperature and yields an extremely fine powder
of low apparent density. This type is outside the reach of artisan producers.
So is freeze drying, the best way of drying but far too expensive and
complicated.
Sun drying is the most popular among small producers, but requires a few
precautions. Direct sun drying must be very quick, otherwise the chlorophyll
will be destroyed and the dry product will appear bluish.
Whatever the source of heat, the biomass to be dried must be thin enough
to dry before it starts fermenting. Basically two types of shapes are used :
thin layers of rather fluid biomass laid on a plastic film, and rods
("spaghetti") laid on a perforated tray. In the former case the air
flows horizontally over the film, while in the latter one it flows either
horizontally or vertically through the tray. The rod shape is theoretically
better as evaporation can take place all around ; rods are obtained by
extrusion to a diameter of 1 to 2 mm. But rods must be sturdy enough to
maintain their shape, so this type of drying is restricted to biomasses that
can be dewatered by pressing into a paste of firm consistency.
Warm, dry air passed over or through the biomass to be dried must have a
high velocity at the beginning of the drying process. Later on in the process
the velocity of the air is less important than its dryness (therefore it is
usual to end up with air heated at 65°C). The total duration of the drying
should not exceed a few hours, preferably 2 hours.
During the drying process as well as afterwards the product must be
protected against contaminations from dust and insects and should not be
touched by hands.
Drying temperature should be limited to 68°C, and drying time to 7 hours.
Incipient fermentation during drying can be detected by smelling during
the drying process as well as afterwards. However it is customary that a rather
strong smell evolves from the biomass at the very beginning of the drying.
The dry chips or rods are usually converted to powder by grinding in
order to increase their apparent density. The best storage is in heat sealed,
aluminised plastic bags.
CONSUMPTION
Those persons who cannot stand the taste and odour of spirulina most
probably were once exposed to a low quality product. Good quality fresh
spirulina is so bland it can replace butter on toasts and can enrich almost any
dish ; cold drinks can be prepared by mixing it with fruit juices. Fresh
spirulina is a paste easily mixed, diluted, extruded, etc.
There are literally thousands of possible recipes making use of spirulina
either fresh, frozen or dry, raw or cooked.
Above 70°C the gorgeous green color often turns brown in the presence of
water. So you can choose your preferred color for soups and sauces.
ANNEX
A1) MEASURING THE CONCENTRATION
IN SPIRULINA WITH THE SECCHI DISK
The "Secchi disk" is a self-made instrument : a piece of white
plastic fixed at the tip of a graduated rod. Dip it vertically into the
spirulina culture until you just cannot see the white piece ; the reading in
centimeters gives an approximate value of the concentration. If the medium
itself (the filtrate) is turbid, use the appropriate curve, after measuring the
turbidity of the filtrate using a black Secchi disk, expressed in cm in the
same way as the concentration.
As the reading depends on the eye of the operator, every one should make
his own graph, based on absolute measurements of the concentration (by
filtering a given amount, drying in the oven and weighing).
The reading also depends on the shape of the filaments.
The following graphs were established by the author for the Lonar
(coiled) and for the Paracas (loosely coiled, almost straight) strains. They
can be used as approximations.
PARACAS
LONAR
A2) MEASURING THE SALINITY OF
THE CULTURE MEDIUM
Use a densitometer calibrated for densities above 1.
Temperature correction :
D
= DT + 0.000325 x (T - 20)
Where D = density at 20 °C, DT = density at T °C, expressed in kg/liter
Salinity SAL is calculated from D by the formulas :
If D > 1.0155, SAL = 1275 x (D - 1) - 0.75, g/liter
Otherwise, SAL = 1087 x
(D-0.998)
A3) MEASURING THE ALCALINITY OF
THE MEDIUM (ALCALIMETRY)
Titrate the medium using normal hydrochloric acid (concentrated acid
diluted 10 times with water). Use pH 4 as the end point.
Alcalinity (moles of strong base/liter) is the ratio of the volume of
acid used to the volume of the sample of medium.
A4) MEASURING THE PH
The pH meter should be calibrated at least once a week. If standard
calibration solutions are not available, self-made solutions can be made for
calibration as follows (pH at 25°C) :
- pH 11.6 : 10.6 g sodium carbonate per liter water
- pH 9.9 : 5.5 g sodium bicarbonate + 1.4 g caustic soda per liter
water, or : 4.2 g sodium bicarbonate + 5.3 g sodium carbonate per liter water ;
maintain in contact with the atmosphere and make up for evaporated water.
- pH 7 : 5.8 g monoammonium phosphate + 11 g sodium bicarbonate
per liter of water ; maintain in a closed bottle.
- pH 2.8 : standard vinegar (6 % acetic acid, density 1.01).
Temperature correction on pH :
pH
at 25°C = pH at T°C + 0.00625 x (T - 25)
A5) COMPARING SPIRULINA SAMPLES
Protein, iron, gamma-linolenic acid, heavy metals contents and the
microbiological analysis can only be performed by a competent laboratory, but a
few home-made tests can give an idea of the quality of a spirulina sample by
comparing with a reference product.
Examination of color, odor and taste may reveal significant differences
between samples. The green color should tend more towards the blue than the
yellow.
The "pH test" reveals the degree of removal of the culture
medium from the biomass. On fresh spirulina simply measure the pH : if near 7,
the biomass is pure. For dry spirulina powder, mix a 4 % suspension in water
and measure the pH : the initial pH should be near 7 (for many commercial
products it is near 9 or even 10), and after 12 hours it usually falls down to
well below 6. For biomasses that were washed with acidified water, the initial
pH may be acidic (< 7).
To assay the blue pigment phycocyanin content proceed as for the pH test
on dry samples, mixing several times the suspension. After 12 hours, take a one
drop sample of the decanted solution and put it on a filter paper (for instance
the "Mellita" filter paper for coffee making) maintained horizontal.
The amount of blue color in the stain is proportional to the concentration of
phycocyanin in the sample. Some spirulina samples require to be heated to 70°C
before the test for the blue pigment to be fully released into the solution.
To assay the carotenoids content, mix the dry powdered sample with twice
its weight of acetone (or of 90 % ethanol) in a closed flask, wait 15 minutes,
and put one drop of the decanted solution on filter paper. The intensity of the
brown-yellow color of the stain is proportional to the concentration of
carotenoids (and hence of
beta-carotene) in the sample. Old samples stored without precautions
contain practically no carotenoïds.
A6) HARVESTING AND DRYING
SPIRULINA
Filtration is done on a 30 µ mesh cloth.
When most of the water has filtered through, the biomass will agglomerate
into a "ball" under motion of the filtering cloth, leaving the cloth
clean (this desirable condition happens when the biomass is richer in spiralled
forms and the culture medium is clean). At this stage the biomass contains 10 %
dry matter and it has a soft consistency ; it will not stick to plastic
materials but glide on it.
Final dewatering of the biomass is accomplished by pressing the biomass
enclosed in a piece of filtration cloth, either by hand or in any kind of
press. The simplest is to apply pressure (0.15 kg/cm² is enough) by putting a
heavy stone on the bag containing the biomass. The "juice" that is
expelled comes out clear and colorless, and the operation must then be
discontinued when no more liquid drops out. For the usual thickness of cake
(about one inch after pressing), the pressing time is about 15 minutes.
Practically all the interstitial water (culture medium) is removed. The pH of
the pressed biomass is near 8 and may even be brought below due to breakage of
some spirulina cells, but it is not advisable to bring it too low.
This pressing operation effects a more efficient separation of the
residual culture medium than washing the biomass.. Washing with fresh water may
cause rupture of the cell wall of the spirulina due to osmotic shock, leading
to loss of valuable products; it may also introduce germs contained in the wash
water.
Pressed biomass contains twice as much dry matter as unpressed biomass,
which reduces the drying time. It has a firm consistency (can be cut by a knife
like cheese). It can be eaten as is.
The biomass to be dried must be thin enough to dry before it starts
fermenting. It is extruded into fine rods ("spaghetti") of a diameter
of 1 to 2 mm onto a plastic perforated tray (or nylon mosquito net). The rods
must be sturdy enough to maintain their shape, so this type of drying is
restricted to biomasses that can be dewatered by pressing into a firm
consistency. In India the "indiappam makker" kitchen instrument can
be used for extruding (the wooden type is preferred to the aluminium one).
During the drying process as well as afterwards the product must be
protected against contaminations from dust and insects and should not be
touched by hands.
Drying temperature should be limited to 68°C, and drying time to 7 hours.
With good ventilation and low charge (1 kg fresh rods/m² of tray) the drying
time may be reduced to 2 hours. The final % water should be less than 9. The
dry product detaches itself easily from the tray.
Incipient fermentation during drying can be detected by smelling during
the drying process as well as afterwards.
The dry rods are usually converted to powder by grinding in order to
increase their apparent density. The best storage is under vacuum in heat sealed,
aluminized plastic bags.
A7) A SIMULATION MODEL FOR THE
CULTURE OF SPIRULINA
Instructions for use of the simulation model (English version)
The models presented
here are freely available for non-commercial uses. They can be run on any PC with
DOS. Create a new folder on your local
disk (C) and name it SPIRUL. In SPIRUL create 4 subfolders and name them SITES,
PERSO, IMPRIM and EXE. Download BSI.EXE, METEO.EXE into the folder named EXE and run METEO.EXE
once before using the models. The main model is SPIRU-E.EXE. The
models can be downloaded into EXE or they can be run directly from their link
(in this case, to the question input path ?, answer C:/SPIRUL/EXE).
If you ask for a printout of the results, go to the
file SPIRU-E.DOC automatically generated in the folder IMPRIM, and print it. To
print graphs use Print Screen.
Other models can be run the same way : SPITFIX.EXE
for simulating laboratory cultures at constant temperature under constant
light, and PRIXSPIR.EXE (French) for the calculation of spirulina cost
prices.]
[Part of the following reproduces a paper given at the First ALGAL
Technology Symposium, Ege University, Izmir, Turkey, October 24-26, 2001]
A
PRACTICAL SIMULATION MODEL FOR SPIRULINA PRODUCTION
JOURDAN
Jean-Paul, Le Castanet, 30140-Mialet, France
Abstract
A model was written to simulate the operation
of a spirulina (Arthrospira platensis)
culture under a greenhouse or in the open air. The rate of photosynthesis is
assumed to be directly proportional to five functions when biomass
concentration is above 0.1 g/l : photosynthesis = k x f(light) x f(temperature)
x f(pH) x f(stirring) x f(salinity). The rate of respiration is assumed to be a
function of the temperature. The solar illumination is calculated from the
sun's position and from local meteorological data; an artificial lighting may
be provided. The culture temperature is calculated from a thermal balance and
the pH from a CO2 balance around the tank. The calculations are carried out for
each hour for a period of up to 600 days on end. The results include a graph of
the daily production and a cost price analysis. In order to optimize the
production about 80 technical parameters can be varied at will, including the
temperature control means (inflatable double plastic roof, air circulation,
shading, night cover, artificial heating). Various fuels are available either for heating or as a source of CO2.
Make-up water may be saline and/or alkaline. Purified culture medium may be
recycled at a lower pH to increase the growth.
The model appears to correctly predict the
operation of a spirulina culture. It is useful to predict trends, optimize
operating conditions, make technical and economic analyses, and as a tutorial
aid.
Keywords : Arthrospira platensis, culture, simulation, model.
INTRODUCTION
The software SPIRU-E.EXE containing the mathematical
model presented here is freely available for non-commercial uses. The program
itself contains all necessary information for use.
The model is based on data from the literature
plus data obtained by the author in the course of ten years of experiments with
spirulina (Arthrospira platensis)
culture. It makes use of basic equations from the solar energy and chemical
engineering fields. It applies to any spirulina culture in an open air tank or
under a greenhouse, in any climate. It also applies to the case of cultures
flowing on inclined planes.
In addition to technical aspects, the model
also calculates a simplified cost price
for the product.
MATERIALS
AND METHOD
Starting from a given set of initial
conditions, the growth of spirulina is calculated hourly for the desired
duration of the culture (up to 18 months), as a batch culture or rather as a
semi-batch culture because of harvesting. The basis is one square meter of
illuminated tank area. Temperature and pH of the culture are obtained by heat
and CO2 balances around the tank and are the basis for the calculation of
growth. The main hypothesis on which the model is based is that the rate of
photosynthesis is assumed to be directly proportional to five functions when
the biomass concentration is above 0.1 g/l :
photosynthesis
= k x f(light) x f(temperature) x f(pH) x f(salinity) x f(stirring)
with the proportinality factor k chosen to best
fit experimental results. This hypothesis may not be scientifically justified,
but it makes the calculation much simpler ans gives acceptable results. This
equation assumes that photosynthesis is not limited by nutrients other than
bicarbonates, and that it is independant of spirulina concentration (which is
largely true as the biomass concentration is maintained above 0.15 g/liter).
The functions of light, temperature, pH and salinity are based on Zarrouk 1966,
adapted to better fit experimental results when necessary. Figs. 1 to 5 (Fig1
, Fig2 , Fig4 , Fig5) show these functions as used in the model. The function of the
stirring rate is largely hypothetical (note : stirring and agitation will be
synonymous in this paper).
For biomass concentrations below 0.1 g/l the
photosynthesis is exponential and is calculated by multiplying the above
equation by the factor (concentration/.01).
The net growth is
calculated as the difference between photosynthesis and respiration. The assumed influence of temperature on the rate of respiration is
illustrated in Fig6, based on Tomaselli et al., 1987 and Cornet 1992, for homogeneous cultures maintained in contact with air.
Ambiant air temperature and solar radiation are
calculated hourly from meteorological data, latitude and altitude of the site,
with formulas used commonly in the solar energy field. The average percent
cloudiness is assumed to be concentrated each month in three series of days
evenly distributed within the month, which are the rainy days of the month. Dew
point and wind velocity are assumed to be constant within each month.
Greenhouses used may be equipped with various
devices to control their internal climates : inflatable double plastic
roof, adjustable ventilation, adjustable shading, fixed shading, infra-red
reflectors (night screen) and night insulation. Various additional options for
greenhouses in cold climates are available including heating by fuel
combustion, night insulation and artificial lighting.
Adjustable and/or fixed shading and night
screen may also be mounted on open air tanks.
Harvesting is done every day (except on 0 to 3
days on end without harvest per week) at a given time of the day, reducing the
spirulina concentration down to a given fixed value, but is limited by the
harvesting capacity. There is no harvest as long as the pH is below a limit
(generally 9.6) in order to minimize the pathogenic germs. At the end of the
culture period a final harvest reduces the concentration down to the initial
value. The average productivity is based on the total duration of the culture
period from inoculation to restarting a new culture, including the idle days.
The pH of the culture is controlled by daily
feeding of CO2 or CO2-evolving compounds (bicarbonate, sugar) or (for
greenhouses) CO2-containing combustion gases. The CO2 contributed by the urea
and by the ventilation air is taken into account in the carbon balance. The
absorption coefficient of CO2 from the air into the culture medium was
experimentally determined as 20 gmoles/hr/m²/atm; this figure may be changed
(in the following examples a figure of 18 was taken). The vapor pressure of CO2
over the medium is calculated using the formula given in Kohl and Riesenfeld
1960. The resulting rate of CO2 absorption from the air is illustrated in Fig7. The experimentally determined graph in Fig8 is
used to relate the amount of CO2 contained in the medium to the pH of the
medium. The CO2 consumption assumed in the examples given below is 1.8 gram per
gram of spirulina grown, but the model allows it to be adjusted to take into
account variations in the exopolysaccharide and other by-products according to
the strain and the culture conditions.
The tank level is allowed to fluctuate between
a minimum and a maximum value, and is controlled either by draining part of the
medium or by adding water (plus the salts needed to maintain the quality of the
medium) depending on the needs. The salinity and alcalinity of the make-up
water are taken into account, but its hardness is neglected. The salinity and
the basicity of the medium are controlled below given maximum values by
replacing part of the medium by water (plus the required salts).
Purified, low pH culture medium may be recycled
with no change in basicity, salinity, level nor temperature in the tank..
The cost price calculated by the model is based
on the following formulas for chemicals usages :
The cost price also includes an adjustable
fixed costs contribution.
The model does not take into account the cost
of treatment of the spent culture medium, but it allows recycling of the
treated medium.
RESULTS
The site of Izmir, Turkey was chosen to give a
series of examples of results obtained using the model with 6 harvesting days
per week.. To facilitate comparison
of the various cases, the same set of data were used in all cases, except the
parameters being varied. A greenhouse of the simplest type is used in all cases,
with no inflatable double roof, no shading, no night insulation nor night
screen, but with adjustable ventilation. The standard duration chosen for the
culture is one year. Table 1 and 2 (Table1 , Table2) show the printout of the standard data used.
The daily results of each simulation come out
both as a graph (Fig9) and as a table (not shown here), while average results over the period
of culture come out as a table (Table3).
The search for the minimum cost price or the
maximum production is effected by varying parameters.
Fig10 gives the relationship between the bicarbonate consumption (used as the
sole pH controlling agent) and the productivity when using the simplest type of
greenhouse (standard case for the examples given here) and when using a fully
equipped, modern greenhouse. At low pH the simplest greenhouse is 25 % better
than without any greenhouse.
Fig11 show typical results obtained by varying the pH control value while
using bicarbonate as the sole pH controlling agent and Fig12 shows the same using liquid CO2. The optimum pH obviously depends on
the cost of the carbon source.
Fig13 shows the negative influence of high biomass concentrations on the
productivity, due to the effect of respiration.
Fig14 shows the negative influence of a high depth of culture on the
productivity, due both to the reduction of the
maximum temperature and to higher respiration.
Fig15 shows the minute influence the air circulation rate has on the
productivity. When an artificial carbon source
is used, the influence is negative due to lower temperatures. Without an
artificial carbon source, it becomes positive due to more CO2 available from
the air, but it remains negligible.
Fig16 shows the influence of the salinity of the make-up water on the
productivity.
Fig17 gives an example where propane fuel is the sole artificial carbon
source. The cost price can be quite low provided the air circulation rate is
kept minimal.
Another use of the model is to evaluate the
economic penalty due to shorter periods between changing the culture medium.
For three changes per year instead of one, the penalty comes out to be 4 % on
productivity and only 1 % on cost price in the example given here. So, as a new
culture is easier to harvest, it is
recommendable to renew the medium several times a year.
DISCUSSION
In spite of taking into account around eighty
parameters, the model is far from comprehending the totality of the factors,
notably biological, that influence the growth and quality of the spirulina
produced in artificial conditions.
Although results from the model fit actual data
generally well, the model has yet to be fully validated. It would be extremely
desirable to compare a number of calculated and observed results, but for such
comparisons to be valid, the data used should closely match the experimental
conditions. Such a close match actually is beyond the scope of this work, but
could constitute interesting thesis subjects for students. It is suggested that
comparisons with actual results be communicated to the author for further
validation or modification of the model. Laboratory studies are often conducted
as batch cultures under constant illumination twelve hours a day. A variant of
the model was developed to facilitate validation from such laboratory studies.
As it is, this model can be useful to predict
trends, optimize operating conditions, make technical and economic analyses,
and as a teachingl aid.
Cornet J.F. 1992. Kinetic and energetic study of a
photobioreactor (in French), Thesis, University of Paris-Orsay
Kohl A.L. and Riesenfeld F.C. 1960. Gas Purification, McGraw-Hill Book Co.
Tomaselli L., Giovanetti L., Pushparaj B. and
Torzillo G. 1987. Biotechnologies for the production of spirulina (in Italian),
IPRA, Monografia 17.
Zarrouk C. 1966. Contribution
to the study of a cyanophycea : influence of various physical and chemical
factors on the growth and photosynthesis of Spirulina
maxima (in French), Thesis, University of Paris
TABLES AND FIGURES
[Note : in this paper, the
coefficient of absorption of CO2, ka, was assumed to be 18 gmole/hour/m²/atm]
Table 2
Example of meteorological data
RESULTS
Nutriments, kg/kg of
harvested spirulina :
bicarbonate (initial medium and drainages included): 9.17
bicarbonate (excluding initial medium) : 8.85
carbonate = 0.00
sugar : 0.00
liquid CO2 : 0.00
Water consumption
(including medium, drainages, evaporation), l/kg = 710
Total rainfall on area equal
to tank area, l/kg = 192
Drainages, average %/day
= 3.35
Fuel consumption, kg/kg
= 0.00
Surplus electricity (sold),
kWh/kg = -3.8
Electricity consumption by
lamps, kWh/kg = 0.0
Electricity consumption by
agitation, kWh/kg = 3.8
Final concentration in
spirulina, g/l = 0.3
Final salinity of medium,
g/l = 17.4
Final basicity of medium,
moles/l = 0.20
Maximum pH (before days
without carbon feed) = 10.32
Maximum tank temperature, °C
= 38.2
Minimum tank temperature, °C
= 4.4
Maximum CO2 concentration in
internal air, vpm = 398
Minimum CO2 concentration in
internal air, vpm = 302
Maximum level in tank, cm
= 10.0
PRODUCTIVITY, gram per day per m² = 6.79
PRODUCTION, kg per m² =
2.48
COST PRICE (present value at day 1), $/kg = 16.86
Table 3
Example of results
Fig. 1 Photosynthesis vs. temperature
Fig. 2 Photosynthesis vs. pH
Fig.3 Photosynthesis vs. Light intensity
Fig.4 Photosynthesis vs. salinity
Fig. 5 Photosynthesis vs. stirring rate
Fig. 6 Respiration rate vs. temperature
Fig. 7 CO2
absorption vs. pH
Fig. 8
CO2/base molar ratio vs. pH
Fig. 9
Daily production
Fig. 10
Production vs. bicarbonate consumption
Fig. 11 Cost
price vs. pH with bicarbonate (@ 0.8 $/kg)
Fig.12 Cost
price vs. pH with CO2 (@ 2 $/kg)
Fig. 13 Productivity vs.
biomass concentration
Fig. 14
Productivity vs. tank level
Fig. 15 Cost
price vs. ventilation
Fig.
16 Productivity vs. salinity of
make-up water ( maximum salinity = 40)
Fig. 17 Cost
price vs. fuel rate
(propane @ 1 $/kg as sole carbon source and
ventilation rate = 0.1 m/h)
LEGENDS OF TABLES AND
FIGURES
Table 1
Example of data
Table 2
Example of weather data
Table 3 Example of results
Fig.1 Photosynthesis vs. temperature
Fig. 2
Photosynthesis vs. pH
Fig. 3
Photosynthesis vs. light intensity
Fig. 4
Photosynthesis vs. salinity
Fig. 5
Photosynthesis vs. stirring rate
Fig. 6
Respiration rate vs. temperature
Fig. 7
CO2 absorption vs. pH
Fig. 8
CO2/base vs. pH
Fig. 9
Daily production
Fig. 10
Productivity vs. bicarbonate consumption
Fig. 11
Cost price vs. pH with bicarbonate (@ 0.8 $/kg)
Fig. 12
Cost price vs. pH with CO2 (@ 4 $/kg)
Fig. 13
Productivity vs. biomass concentration
Fig. 14
Productivity vs. culture depth or level
Fig. 15
Productivity vs. ventilation rate
Fig. 16
Cost price vs. salinity (total disolved salts) of make-up water
Fig. 17
Cost price vs. fuel rate with propane @ 1 $/kg as sole carbon source and
ventilation rate = 0.1 m/hr