Optimal Harvesting and Post-Harvesting Time
One way to increase revenue, is by optimizing the harvesting and post-harvesting
processes. These include the collection of Spirulina, cleaning and dewatering, drying, and
processing of product. The following analyses demonstrate the cost analysis and utility of the
Secchi Disk, Algae Press, and Cheese Cloth.
Cost and Use of the Secchi Disk
Background
According to Jourdan in “Grow Your Own Spirulina,” the best time for harvesting is early
morning, when the temperature is cooler and the concentration of proteins in Spirulina is the
highest. It is advisable to maintain the growing culture at a fairly high concentration after each
dilution with culture medium. The concentration of a culture is often gauged by the intensity of
its color and a measurement is taken with the Secchi Disk.
The objective of this feasibility analysis is to obtain the cost of fabricating versus buying a
Secchi Disk and understand the correlation between the Secchi depth and the optical density of
the medium.
Tools & Assumptions
Multiple papers were found on the use and application of the Secchi disk for the
production of Spirulina. Please reference the cited work section to find them.
The concentration of a culture can be gauged by the intensity of its color. However, the
Secchi Disk offers a more quantitative and repeatable measurement. The measurement taken
with the Secchi disk is called “Secchi Depth.” It represents the measurement at which the disk is
no longer visible. Furthermore, it is related as a measurement of transparency of the water,
which is related to turbidity - “a measure of water clarity of how much the material suspended in
water decreases the passage of light through the water” (Turbidity). Higher turbidity increases
water temperatures, and decreases concentration of dissolved oxygen and sunlight exposure.
Therefore, there is an optimal quantity at which the Spirulina concentration is good to harvest.
Research & Analysis
The growth of the Spirulina is proportional to the area of the culture exposed to the light.
Harvesting in the morning will provide good production during the day for harvesting the next
day. According to Jourdan, for a tank with a depth of 10-20 cm, a culture is diluted if the Secchi
depth is approximately around 5-6 cm, while a value of 2-3 cm shows a culture ready for
harvesting. Values below 2 cm indicate that the culture should be diluted, or heavy harvesting
should be undertaken. In good conditions, the amount of Spirulina present in a culture doubles
every 2-4 days, until it reaches a max concentration at Secchi depth less than 1.5 cm.
Nevertheless, the productivity of the system should be forced by harvesting approximately 6
grams per square meter per day. A test plan was developed to find the optimal Secchi depth at
which to begin harvesting to maximize production per tank.
Furthermore, a cost analysis was made to understand the benefits of buying versus
building a Secchi Disk. Typically, it is a black and white disk of 20 cm (8 in) diameter. Below is a
list of the materials needed and the assembly process.
Materials:
● a 20cm-diameter, 6mm-thick Plexiglas disk with a hole in the middle (this
may be cut from a square sheet of Plexiglas)
● metal disk with a hole in the middle (to weigh down Secchi disk)
● an eyebolt with nuts and washers to fit it
● rope or cord- try to avoid cotton, as it stretches
● waterproof black and white paint
Assembly:
● Divide the Plexiglas disk into equal quadrants and paint the quadrants
alternating black and white. Using masking tape as a guide often helps
maintain sharp edges to each quadrant. Let the paint dry fully.
● Attach the metal disk to the unpainted side of the Plexiglass disk using the
eyebolt, nuts, and washers.
● Tie the cord securely to the eyebolt. You may want to mark the cord with a
permanent marker in 0.5 or 1 m increments to make measurements easier
to read.
Material
Cost
Cost for 1 Disk
Plexiglass
2ft L x 1ft W x 1/4” t
$23.18
(produces 3 disks)
$7.73
Metal plate
10” x 10” x ¼”
$35.57
$35.57
Eyebolt with nuts and
washers
$7.95
$7.95
Total Cost
$66.70
$51.25
The cost of the materials for construct one Secchi disk is approximately $51.25 while it
was found that NovaTech sells them for $18.72. Therefore, it is better to buy them from a
supplier.
Other Information
Through research it was found that there is potential for variation between users.
Therefore methods need to be standardized as much as possible. Below is the information found
on how to use a Secchi Disk.
HOW TO USE:
● Check to make sure the Secchi Disk is securely attached to the measured line
● When taking measurement block sun glare.
● Take measurements when water is calm, sunny days during the middle of the
day.
● Should be taken between 10am to 2pm. same observer at the same time and
using the same procedure.
○ Approach 1: lower disk beyond a point of disappearance, then raising and
lowering is slightly to set the secchi depth.
○ Approach 2: Record the depth at which the disk disappears, lower another
few feet, and then record the depth at which the disk reappears as it is
slowly brought up. Take the average of the two values.
○ Approach 3: Lower the disk until it disappears from view. Lower it one
third of a meter and then slowly raise the disk until it just reappears.
Move the disk up and down until the exact vanishing point is found.
Attach a clothespin to the line at the point where the line enters the water.
Record the measurement on your data sheet. Repeating the measurement
will provide you with a quality control check.
● Should be taken before adding anything else to the medium.
● The lower the Secchi depth, the higher the algal concentration and lake
productivity.
● Take numerous measurements.
● Take measurements from determined spot.
Algae Press
Background
Currently, the harvested Spirulina is collected and pressed and left to dry. However, the
drying period takes up to 3 days. Therefore, there is room for improvement by decreasing the
drying period and length of the process.
Research & Analysis
Filtration can be 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. The filter can be installed above the pond to
directly recycle the filtrate. Furthermore, 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. 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. Currently, pressure is applied (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.
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 un-pressed biomass, which reduces the drying time. Lastly, there are two
techniques used to further dry the product to avoid fermentation. The first is to lay the
“Spirulina paste” in thin layers on a plastic film. The second is to extrude 1-2 mm rods on a
perforated tray. The rods are theoretically a better option as it allows evaporation all around.
Jeff Lodge provided the team with sketches of an algae press use for oil extraction. Below
are pictures of the system. It is possible to modify this algae press to help with the collection and
dewatering of the algae. A design will be developed in the next phase and a stress model
developed to verify that the Spirulina filaments are not damaged with the force applied.
Materials:
● 6- 750X100X50 wood planks (main frame members)
● 2- 350X100X50 wood planks (end pieces for stability)
● 8- 150mm / M10 plated Coach bolts
● 1- 500X500X38 press base
● 4- 100mm M10 plated Coach screws
● Optional: 2 ton hydraulic jack
● Cheese cloth
● Finer mesh
Bibliography
"5.5 Turbidity." 5.5 Turbidity. N.p., n.d. Web. 19 Oct. 2015.
<http://water.epa.gov/type/rsl/monitoring/vms55.cfm>.
Jourdan, Jean-Paul. "Grow Your Own Spirulina." GROW YOUR OWN SPIRULINA (n.d.): n.
pag. Antenna. 13 Dec. 2001. Web. 17 Sept. 2001.
"SCDSK1." , Secchi Disk, (each). N.p., n.d. Web. 22 Oct. 2015. <http://www.novatechusa.com/SCDSK1?gclid=COi_p6Ozs8gCFUKQHwodbfEFnQ>.