2/4 Spirulina 101: The Methods

There are tons of methods to grow and harvest spirulina

Closed Bioreactor

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.

Photo of their setup (from 1986) I Science Direct

Open Bioreactor

This is basically the fancy name for growing algae in a pond :)


The most common open bioreactor is the Raceway-type. It’s basically just a giant 0.2–0.4 m deep oval ring.


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.

Not the most appetizing… I Pond Ecosystem Site
A very scientifically accurate representation of what happens


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.

Removable covers over Greenspring Farms’ ponds serve as a hybrid option
Open and in a greenhouse? Best of both worlds 🤩 I EcoWatch

Culture Medium

There are many types of culture medium but the standard, most commonly used medium is the Zarrouk medium.

Recipe for basic Zarrouk medium


There’s a reason chlorine is used in pools…to kill algae!! I LiveScience


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.

If spirulina grew in different temperature drinks instead :)

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.

Some very sad jars of spirulina

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!

The red light setup that worked great in the study

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.

Paddle wheels agitating giant commercial spirulina ponds I Pinterest
Trichomes are found on many plants too: here are floral trichomes on a thyme flower I Britannica


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.

Don’t eat these ones! I Spirulina Society

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! 💡

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.

The negative charge of the cells and the positive charge of the flocculant combine to create a neutral charge
  • 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.
The supernatant is just super thin nutrient medium because all the nutrients & cells have sedimented


This method is increasingly being used to harvest microalgae. There are many spin offs, but here are two basic methods:

The cations from dissolving the anode release into the medium and attach to the spirulina, neutralizing it
To harvest spirulina after electrolytic flocculation, floating cells can just be scooped off of the top

Magnetic Separation

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.

It’s like waving a magnet over a fluffy carpet when you drop a bunch of pins


By far the simplest of options, filtration is just removing solids by intercepting them with a semipermeable barrier, aka filter.

Filtration works for both large and small scale operations


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.

Much more complicated than it needs to be I Thermal Kinetics

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).

A super powerful spray dryer I Envitech Corp
The less flashy, less energy intensive drum dryer I Teycomur
The fanciest option: freeze drying I OpenPR


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.

SpiraKind is one of the few companies that sells spirulina in its pre-milled, nib form



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Klara Zietlow

Klara Zietlow

15 year passionate about the future of food and the environment. Likes animals too :)