Microalgae grows much faster than any land plant while remaining a fairly undemanding crop, which is part of its major appeal. Algae can grow in virtually any temperature – colonies are found throughout Antarctica where temperatures dip below -70 degrees Fahrenheit as well as around hot springs as hot as 185 degrees Fahrenheit. Basically, if there is sunlight, carbon dioxide, and a few basic nutrients, some variety of algae will thrive. In general, to maximize productivity, some control of temperature, mixing technology, and supplemental CO2 should be considered. We’ll discuss some systems and best practices in algae farming below.
Where Does Algae Grow?
Algae may seem like one heterogenous body of green slime, but there’s a whole host of wildly different organisms out there, and some algae species are best suited to specific production methods. For example, autotrophic microalgae produce their own food directly from simple inorganic substances through photosynthesis. This easygoing class of algae is typically grown in large open ponds or in enclosed systems called photobioreactors. In these systems, the algae may be supplemented with CO2, which could be obtained from flue gases collected from power plants or other industrial processes. This is an appealing setup because it promotes rapid growth and allows CO2 (a greenhouse gas) to be recycled and effectively reduces total carbon emissions into the atmosphere.
Heterotrophic microalgae don’t produce food directly from inorganic substances, but in fact must absorb key organic compounds from their surroundings. Some heterotrophic types are completely unable to photosynthesize, in fact. Instead, they use ammonia, nitrogen and glucose produced through other organic processes in the environment. This type of algae must be grown directly in large fermenters using sugar or starch. These fermenters are not unlike those already used to convert corn to ethanol.
Macroalgae (seaweeds) are difficult to grow on land because they can be quite large and require substantially more room than microalgae. There are some investigations into the possibility of growing seaweed in the open ocean, but at this time they are typically cultivated in seawater near the shoreline.
Enhancing the Product
Nutrients
We’ve said over and over again that one of the beauties about cultivating algae is their ability to thrive with minimal nutrients. However, algae products cannot produce minerals that were not available to the algae during the growth process, so those must be provided if your goal is to use algae as a nutritional supplement. Similarly, as a broad group, microalgae can produce all known vitamins on its own, with the exception of vitamin B12, which is mostly found in meat, fish, dairy and eggs. However, if algae are cultivated alongside B-12 producing bacteria, algae will readily absorb it from the environment.
In the end, if the ultimate use of the algae produced is intended as a nutrition supplement for humans or livestock, then it makes sense to ensure that key raw ingredients like minerals are readily available.
Wastewater
Algae and wastewater have a terrific relationship, which helps us out when we’re working on strategies for sustainable living. Wastewater, whether it’s municipal sewage in a treatment plant or a mysterious industrial effluent, cannot be simply released into the environment, yet it can be challenging and expensive to treat. In the past, addressing these challenges has relied on expensive, multi-step treatment processes, many of which created new challenges in the next step down the line. In municipal wastewater treatment, for example, aerobic bacteria are employed to digest organic solids dispersed in the water. The bacteria do a great job, but the bacteria leave behind a water that is rich in nutrients, which usually results in a BOD (Biological Oxygen Demand) that doesn’t meet federal standards for release.
This is where algae start to shine. That nutrient rich water left after aerobic digestion is the ultimate banquet for algae. Where wastewater plant managers might have fought to eliminate any sign of algae in their treatment ponds, they now encourage it to grow and sometimes give it a boost with extra CO2. Add a dose of sunlight and the algae will grow lightning fast, consuming up all those troublesome nutrients and creating new algae, all while producing abundant oxygen to support the digesting bacteria. In the end, nutrient levels have been reduced enough that the water meets strict requirements for BOD (biochemical oxygen demand) and the algae can be removed and used for other purposes (fertilizer, feedstock, etc.)
In the same fashion, partially treated wastewater can be used in algae production in order to eliminate reliance on freshwater supplies.
Harvesting Algae
In open pond systems, harvest is usually a continuous process, with a fraction of the pond harvested every day. However, microalgae float freely in water, so it’s not possible to simply scoop out a cup of concentrated algae whenever you want.
Since the microalgae are dispersed, flocculants may be added to the raw water which cause the cells to stick together into clumps for easier removal. Interrupting the flow of carbon dioxide can also trigger a natural flocculation effect. These flocculated clumps can be captured with filters or by using air bubbles to float clumps to the surface where they can be skimmed away, not unlike froth flotation. Centrifugation is another option, but it tends to be expensive. Even flocculation can be prohibitively expensive, especially in saline or brackish water, since greater amounts of expensive flocculant is necessary. More research is underway on these processes - this single step in harvesting algae can represent 25% of the total production cost.
Once biomass has been separated from the water, the algae can be directed to whatever next step is called for according to the ultimate products. Oil extraction for biofuels and bioplastics are one possibility, while biomass may be dried and powdered for use in nutritional supplements or fertilizers.
