This is the second article in a four part series on everything you need to know about spirulina. Read article one (the introduction) here. Or, skip to article three (nutrition & applications) or article four (barriers to adoption).
Since its transition from naturally growing on lakes to being grown on a farm, many methods of cultivating, harvesting, and processing spirulina have emerged.
Still, every method requires some starter culture taken from another bioreactor, water, sunlight, agitation, nutrients, and a place to grow.
Spirulina bioreactors are generally between 1,000 and 5,000 m² in volume but what goes in those bioreactors and how they look can vary significantly. The two broad categories most microalgal bioreactors fall under is open and closed.
Growing in a closed environment enables higher pH and temperature control as well as enables higher photosynthetic efficiency and biomass productivity. Being a closed system allows details to be fine tuned to the needs of the algae. It also means water evaporation and CO2 loss to the atmosphere can be minimized and risk of contamination greatly reduced.
Nevertheless, closed systems have higher construction and operation costs making them difficult to scale up.
There are certain limitations to the size and shape of bioreactors–light still needs to be able to reach the microalgae–but it does allow for more freedom than an open bioreactor would. This is because if it’s constructed with a clear material such as glass or plastic, sunlight can penetrate from all directions, not only the top.
A study found that under central Italy’s climate, biomass harvested from a photobioreactor was nearly 90% greater than when it was grown in open ponds.
This can be explained by a considerable extension of the cultivation period from 175 days with an open pond to 215 days due to significantly higher temperature in the bioreactor than outside.
The main issues the researchers found were high oxygen concentration and overheating during the summertime. Fortunately, this can be slightly alleviated through selective breeding to withstand higher temperatures. With the strain they used, the bioreactor only had to be sprayed with water for 20 days out of the year.
Oxygen concentration can be lowered with better design allowing the culture to degas before tension becomes harmful.
This is basically the fancy name for growing algae in a pond :)
Open systems are widely used because of their lower construction and operational costs. They have the advantage that they’re easier to clean and maintain (think bathtub vs water bottle). They also have direct exposure to sunlight and low accumulation of oxygen, which can become toxic in large concentrations.
The most common open bioreactor is the Raceway-type. It’s basically just a giant 0.2–0.4 m deep oval ring.
Rotating blades attached to an electric motor agitate the medium. One method of constructing Raceway bioreactors is to build directly into the soil with concrete. They can also be excavated into the ground and lined with a plastic coating.
In 2017, a Raceway bioreactor was even successfully built using super thin, <0.5 mm glass fibers.
Today, the cost of constructing a 100,000 m² Raceway bioreactor is around $725,000 USD. This includes earthworks, lining, inlet/outlet, and deflectors. Adding on production costs, the spirulina grown in it would round out to be between $1.8 and $2.0 per kg.
The cost using a closed, tubular photobioreactor in comparison is between $10.3 and $11.4 per kg.
The downside of open systems is that they’re dependent on climatic conditions and more likely to become contaminated.
Contamination is a major issue with outdoor, open microalgae cultivation. SAC, a spirulina farm in Thailand, suffered from heavy rainfall one season which left them with some unpleasant results. Suddenly, their ponds were diluted and teeming with unfavorable organisms such as bacteria, green algae, protozoa, and insects.
This meant they were faced with the challenge of restoring the growth medium back to its original concentrations, preferably in one day–no small task.
Spirulina’s high pH requirement means that usually contamination isn’t a big issue but if the pH is changed, for example with heavy rainfall to dilute it, it’s more tolerable to invading microorganisms.
Occasionally SAC experienced phage-like phenomena that destroyed all their culture at once. It’s unclear why exactly this occurred, but it’s likely due to some sort of viral infection that made its way into the pools and spread from there.
One of the benefits of microalgae is that its culture medium can be continuously recycled. Unfortunately, in outdoor open ponds, this commonly results in green algae and bacteria contamination.
Recycling, while sustainable and cheap, can often result in accumulation of organic matter from the decomposition and death of algae cells. This is an invitation for other microbes to come join the feast.
To prevent green algae and bacteria contamination, measures must be taken to minimize the accumulation of organic matter in the culture.
In nature, spirulina benefits from species higher up on the food chain that selectively eat other single cell green algae, leaving spirulina to flourish without much competition.
In terms of aquatic and ground insect contamination, they’re unavoidable and must be removed by common netting. Luckily, they don’t have a massive effect on the spirulina and are easy to remove during or before harvest.
There are simple ways to reduce the downsides of an open system, for example placing a non-airtight cover over the pool to prevent things from falling in and to trap heat and evaporation.
Or to put open ponds in an enclosure to regulate temperature and prevent contamination.
So in summary, if a tolerant strain of microalgae is being grown, such as spirulina, it currently makes more sense to use an open system but if it’s pretty sensitive, closed bioreactors are the way to go.
Hybrid approaches can be taken to take advantage of the strengths of different methods without their negative side effects.
There are many types of culture medium but the standard, most commonly used medium is the Zarrouk medium.
There are also lots of variations of Zarrouk that tweak parts to achieve a certain nutritional profile or functionality in the algae.
Examples of other mediums include Rao’s medium, CFTRI, OFERR, seawater, and wastewater. Different industrial residues can be used too like CO2, anaerobic digestion effluents, and even molasses.
For spirulina to grow best, the pH of the medium should be somewhere between 9.5 and 11. Being on the higher side comes with the benefit that many contaminants won’t be able to survive.
Regular filtered tap water is the way to go for growing spirulina. Filtered is the key word here because chlorine = dead algae.
If distilled water, rainwater, soft water, or water filtered through reverse osmosis is used, extra minerals need to be added. In addition, alkalized water shouldn’t be used because its pH is completely off.
Spirulina likes things pretty warm, between 30–37 degrees Celsius (89–98.6 F). It will still grow in temperatures below 25 C (77 F), but slowly. If it drops below 15 C (59 F), it won’t grow at all and if it shoots above 42 C (108 F), it’ll die.
Spirulina also prefers a fluctuating temperature cycle, warmer during the day and colder at night.
That being said, as long as things don’t get too extreme, spirulina is pretty hardy and will survive conditions that aren’t perfect. I know the spirulina I’m growing right now is often a little on the cooler side, but that just means it’s a bit sluggish.
In fact, when people who grow spirulina at home go on vacation they’ll often lower the temperature of their growth container so the algae won’t need to be tended to until they get back.
Lighting and Algal Density
Algal density ,despite not being the first thing to come to mind, plays a big role in productivity. The optimal density of spirulina is somewhere between 400–600 mg dry weight per liter.
Any lower than 100 mg dw per liter and photoinhibition (reduced ability to photosynthesize) or even photooxidation due to high light intensity can occur.
When scaling up, this is something key to pay attention to. If the color of the medium has turned yellowish or green-olive, it’s a sign of the signs lysing (breaking down). If it’s yellowish and foaming, lysis has already occurred and the cells are now broken open. The foam produced is the polysaccharides from the cells being released into the medium.
If this happens you should shade your pond and hope for the best. Other reasons you may want to consider shading your pond are if it’s too hot, too cold (traps heat), recovering from problems (avoid adding unnecessary stress), or a new/diluted culture (avoid excess sun exposure).
To get the right amount of sunlight, ponds (unless they’re clear tubes) should be between 15–30 cm deep.
Any deeper than that and light limitation will cause a severe reduction in photosynthesis. Sometimes bioreactors can have a shading effect which must be taken into account during the design process.
Natural vs. Artificial Light
When it comes to which type of lighting to use, natural light is usually 100 times brighter than artificial light. Plus, the sun is free!
Of course, you can’t use natural light if you live in a place that’s overcast or dark in Winter. In that case, artificial lights can be used but should be on a 16h per day cycle to avoid overstressing the algae.
Regarding wavelength, spirulina absorbs warmer colors best. This is because cyanobacteria have a unique set of pigments called phycocyanins and allophycocyanins that absorb more red and orange than blue and green.
Agitation and Flow Speed
Agitation is an integral part of algae growing because any spirulina that doesn’t make it to the top, where the sunlight is optimal, will die.
This doesn’t need to be fancy, it could just be a simple aquarium pump or even a stick/broom handle. Commercially, paddle wheels are a popular way to mix large ponds with minimal energy.
Agitation is definitely a good thing, and high culture flow speeds result in more effective photosynthesis, but speeds too high, above 30 cm/s, can fragment algal trichomes (tiny hairs) making them more difficult to harvest.
They can also increase foaming and contamination in the bioreactor which harms photosynthesis and growth.
It’s normal for clumps to coagulate and float to the top of the growth medium. As long as they’re dark blue or green they’re safe to eat. If they’re clear, white, or yellow they’re dead and unsafe.
Clumps can easily be removed with a strainer and aren’t usually an issue, but if there’s a large buildup of them it could be indicative of the temperature being too hot, minerals being off, or contamination.
Harvesting and Dewatering
The goal of the harvesting and dewatering process is to reduce the amount of water in the spirulina to a concentration between 10 and 20% solids by weight. The processing stage represents between 20–30% of microalgal biomass production costs so is thirsty for innovation! 💡
The reason it’s so expensive to concentrate the cells is because the broth is dilute and microalgal cells are small, with a density close to that of water. In addition, sometimes multiple processes are needed to get to the desired concentration.
Flocculation and Gravity Sedimentation
Flocculation is the process in which cells in the medium are made to agglomerate (collect) into larger particles called flocs. These flocs sediment much more easily than individual cells.
Flocculation can be induced by a flocculating agent or cells might produce their own in the form of metabolites. This is a method often used to treat water and wastewater.
Spirulina cells have a net negative surface charge because of the ionization of the functional groups on their cell wall. With each negatively charged cell repelling from each other, it prevents them from agglomerating and therefore sinking. A flocculant neutralizes the charge of the cells and induces flocs.
There are three types of flocculants:
- Salts of multivalent metals like aluminum and iron: The cations in the multivalent metal neutralize the charge of the cell and bridge them together. Aluminum and iron are often used because of their efficacy, availability, safety, and low cost.
- Cationic polymers containing multiple positive charges: Another effective method for flocculation, these generally only need low dosages to be effective. Because synthetic polymers can sometimes contain toxins, natural biopolymers are preferred. An example of this is chitosan, which is deacetylated chitin.
- Microbial flocculants: These are mainly biopolymers produced by microbes and function in a similar fashion to those above.
Gravity sedimentation combined with flocculation is one of the cheapest techniques for solid-liquid separation. It’s not effective on its own because it requires larger particle sizes, hence the need for flocculation.
After sedimentation, the supernatant (layer of liquid on top) is poured off. The process is sometimes slow and may require another treatment after since it will still contain quite a bit of water, but has the benefit that it will work on most algae.
One scenario where it might not work is if the microalgal flocs have a density close to water or if the cells generate oxygen bubbles in the sunlight, causing them to float instead of sediment.
The most important thing to consider about flocculation is that not all flocculants are safe for food. If the spirulina is meant for animal feed or non-nutritional purposes, more flocculants will be available.
This method is increasingly being used to harvest microalgae. There are many spin offs, but here are two basic methods:
- Electrolytic Coagulation: Uses relatively low energy and has high efficiency. Metal cationic coagulants are generated by dissolving a reactive sacrificial anode. The produced cations neutralize the cells’ charge like in flocculation and cause particles to aggregate. Unlike with conventional flocculation, no anions such as sulfate and chloride are introduced. Plus, cationic coagulants produced through this method are really effective so the overall dosage of metal ions can be less. Finally, pH adjustment isn’t necessary as the alkalinity of the substance doesn’t change during the process.
- Electrolytic Flocculation: Negatively charged cells migrate toward an electrochemically inert anode, meaning it won’t dissolve. There they aggregate in a field due to a neutralized surface charge. Gases produced through water electrolysis push flocs up. This method achieves 95%+ biomass removal for many microalgae and since there’s no added flocculant, the method can be used for food. Due to its high efficiency, low cost, and no need for sacrificial anodes, this method is definitely one of if not the most promising electrolytic based harvesting techniques.
Originally used for mining and metal processing, this method is rapid at removing magnetic particles from suspension using a magnet. For spirulina, iron oxide (or some other magnetic metal) is added to a cell slurry to which cells adhere as a result of electrostatic attraction, thereby becoming magnetic.
This has low cost, high efficiency, and low energy but comes with the downside that the particles will need to be separated from the algal cells after. This can be achieved through a 5–10 min treatment of sulfuric acid at 40 degrees C with ultrasound.
Magnetic separation is not a good method if intended for food.
By far the simplest of options, filtration is just removing solids by intercepting them with a semipermeable barrier, aka filter.
This is how most home growers harvest their spirulina–by passing it through some sort of screen printing cloth or butter muslin.
Of course it’s not foolproof, otherwise we wouldn’t have other methods of doing it. Cells often compress into cakes that resist fluid flow and tend to foul the filters, slowing the process and requiring them to be replaced. Microalgae in particular are difficult to filter because of the extracellular polymers they secrete.
Evaporation can be used, but spirulina’s nutrition is damaged by heat. If intended for a non-food purpose, this isn’t an issue but due to how dilute the growth medium is, evaporation requires way more energy than other processes.
Falling film evaporation, a method used for making milk and juice powder, takes place in a tall vertical tube. Fluid flows, surrounded externally by steam, down its inner wall in a ring shaped film. This causes the water to evaporate but needs more than 1 kg of steam to evaporate 1 kg of water, due to heat loss. This makes it extremely inefficient and rules the algae out for biofuel production since the energy in is greater than the energy out.
Other Methods of Drying
Many of the methods mentioned above will still need a drying step after they’re separated from the medium, unless they’re sold fresh. The most common methods of biomass drying are spray drying (how cheese powder gets made) and drum drying (more like your clothes drying machine).
The water evaporation capacity of a spray dryer is 10x higher than a drum dryer (10,000 kg evaporated water h compared to only 1,000). It also has lower capital and maintenance costs because less units are needed to meet production demand.
On the other hand, drum dryers use less energy than spray dryers which compensates for their higher capital. Their efficiencies are similar overall with both operating at a cost below $0.59 per kg of dried algae.
Freeze-drying is another method that could be used with the added benefit that it doesn’t affect it the nutrition of the spirulina. It’s blocker is just that it’s super expensive.
The method of drying used just depends on the end-product type. Anything including drum drying, spray drying, sun drying, cross-flow drying, vacuum trays, and freeze-drying has successfully been used to dry algae.
Again this depends on the desired end-product but after drying is when it can be milled and encapsulated if for a pill/supplement. If just being sold as a powder, all that’s needed is milling to create a more homogeneous mixture.
Milling could also be skipped all together and the spirulina sold as nibs or granules, but there is less of a market for this and it reduces their sale value.
Now that you know all about the ways spirulina can be grown and harvested, continue the series and learn about the amazing nutritional and functional aspects of spirulina!
If you missed article one, the Introduction, click here.
If you’re more interested in what’s stopping spirulina from taking over the world, here’s the last article.