While factors like available light, nutrients, and the risk of contamination can pose difficulties, by far the biggest challenge when producing algae will be harvesting it on a commercial level. To get an idea of proportions, in the average raceway or open algae pond, algae will be concentrated at around one gram of plant matter per liter of water. There are systems that will have higher concentrations of algae—for example, bioreactors, which may go as high as 20 grams per liter of water. But in most commercial operations where algae is grown as a food, fertilizer or biofuel product, the concentration will be much lower. This is combined with the added challenge of size. While it is possible to simply strain out macroalgae and seaweed, fine microalgae and phytoplankton prove much more difficult to separate from water. In algae operations where it is being grown for oil production, some companies have reported that 30% of their costs were associated with dewatering and harvesting.
So what are the common methods for harvesting microalgae? Flocculation is one, and there are a couple of methods to make it happen. One can either starve the algae of CO2, or use a powdered agent to treat the pond. This results in algae clumping and settling at the bottom of the pond, where it can more easily be collected. Another method is filtration, which requires using a series of screens made to trap as much algae as possible, gradually using finer and finer screens throughout the process. There are aeration and flotation techniques that rely on air bubbles to force algae to the surface so that it can more easily be skimmed or filtered out. Lastly, there are dewatering systems that rely on things like centrifuges, conveyor belt screening, or electric pulses, each of which is designed to separate water from the algae. Each of these methods uses a lot of energy, which is why harvesting is such a challenge. Even so, algae represents an amazing opportunity for food and fuel production, which is why improvements are being made all the time to make harvesting easier and more efficient.