Harvesting microalgae means removing the microscopic plants from the water in which they grow and concentrating them into a paste. The grower then needs to remove the moisture, leaving a dense biomass. The minute size of microalgae presents a problem when it comes to harvesting.
One method is filtration. The grower can run the water containing the algae through a cellulose membrane whose pores are smaller than the algae cells. This can be difficult because filters quickly fill up with algae and become clogged. Researchers are looking for better ways to efficiently filter algae.
Flocculation, another method of harvest, means getting the algae to clump together. Adding chemicals or types of algae that naturally clump can cause microalgae to form clumps that become easier to gather.
Another way to harvest algae is by flotation. Here, the grower uses compressed air to create a froth of bubbles and algae that brings the tiny plants to the surface where they can be skimmed off.
A centrifuge is yet another harvest method. Spinning a container filled with water and algae causes the algae to collect in one end.
In order to harvest their crops most effectively, algaculture growers sometimes combine these methods. They might use flocculation to form algae clumps, then separate them with flotation or a centrifuge. Coming up with a truly efficient way to harvest microalgae is a key to bringing down the cost of cultivation.
Harvesting macroalgae involves different problems. Gathering wild seaweed is a labor-intensive process. Some types of seaweed grown in controlled conditions can be gathered in nets. Kelp raised on ropes can be hauled out and hung up to dry. Kelp forests in shallow seas can be mowed by machines, taking off the tops of undersea kelp beds.
Once harvested, algae must be drained of its water and dried. A centrifuge can spin water out, but is relatively expensive. Some systems combine harvest and processing, spreading the algae on belt filters that let the water drain through, then removing more water using a capillary medium that draws water out of the biomass of algae.
The next step is to break down the cell walls of the algae in order to extract the oil inside. The algae are put through a screw or piston press. Chemicals, electromagnetic pulses or ultrasound may also be used to break down the cells. When the oil has been drained off, the remaining biomass is compressed into a cake to be used as to supplement animal feed or as a fertilizer.
Algae have found a wide range of uses, the most exciting ones in the energy field.