Cultivation of microalgae can be done in open systems (lakes, ponds) and in controlled closed systems called photo-bioreactors (PBR).
Open cultivation systems use ponds or lakes with added mechanical equipment to grow microalgae. Open ponds were the first cultivation technology for mass cultivation of microalgae. In this system water levels are kept no less than 15 cm, and algae are cultured under conditions identical to their natural environment. The pond is designed in a raceway configuration, in which a paddlewheel circulates and mixes the algal cells and nutrients.
The raceways are typically made from poured concrete or they are simply dug into the earth and lined with a plastic liner to prevent the ground from soaking up the liquid. Baffles in the channel guide the flow around the bends in order to minimize space. The system is often operated in a continuous mode, where the fresh feed (containing nutrients including nitrogen phosphorus and inorganic salts) is added in front of the paddlewheel, and algal broth is harvested behind the paddlewheel after it has circulated through the loop. Depending on the nutrients required by algal species, several sources of wastewater can be used for algal culture. For some marine-type microalgae, seawater or water with high salinity can be used.
Although open ponds cost less to build and operate than closed systems using PBRs, this culture system has its disadvantages. The ponds can be built on any type of land but need large land areas for considerable biomass yield. Because they are in the open air, the water levels are affected from evaporation and rainfall. Natural CO2 levels in the atmosphere (0.03%-0.06%) are not enough for continuous mass growth of microalgae. Biomass productivity is also limited by contamination with unwanted algal species, organisms that feed on algae or other poisonous particles. Only few species can be grown in normal conditions.
Other types of construction use: 1) circular ponds where circulation is provided by rotating arms; 2) inclined systems where mixing is achieved through pumping and gravity flow.
Closed cultivation systems use PBRs – containers made of transparent materials for optimised light exposure. Enclosed PBRs have been employed to overcome the contamination and evaporation problems encountered in open systems. These systems are generally placed outdoors for illumination by natural light. The cultivation vessels have a large surface area-to-volume ratio. The most widely used PBR is a tubular design, which has a number of clear transparent tubes, usually aligned with the sun’s rays. The tubes are generally less than 10 centimeters in diameter to maximize sunlight penetration. The medium broth is circulated through a pump to the tubes, where it is exposed to light for photosynthesis, and then back to a reservoir. A portion of the algae is usually harvested after it passes through the solar collection tubes, making continuous algal culture possible.
In some PBRs, the tubes are coiled spirals to form what is known as a helical-tubular PBR. These systems sometimes require artificial light for energy, which adds to production costs. Either a mechanical pump or an airlift pump maintain a highly turbulent flow within the reactor, which prevents the algal biomass from settling. The photosynthesis process generates oxygen. In an open raceway system, this is not a problem as the oxygen is simply returned to the atmosphere. In closed PBRS, the oxygen levels will build up until they inhibit and poison the algae. The culture must periodically be returned to a degassing zone—an area where the algal broth is bubbled with air to remove the excess oxygen. Also, the algae use CO2, which can cause carbon starvation and an increase in pH. Therefore, CO2 must be fed into the system in order to successfully cultivate the microalgae on a large scale.
PBRs require cooling during daylight hours, and the temperature must be regulated at night as well. This may be done through heat exchangers located either in the tubes themselves or in the degassing column.
The advantages of enclosed PBRs are obvious. They can overcome the problems of contamination and evaporation encountered in open systems. The biomass productivity of PBRs can average 16 times more than that of a traditional raceway pond. Harvest of biomass from PBRs is less expensive than from raceway ponds, because the typical algal biomass is about 30 times as concentrated as the biomass found in raceways. Controlled conditions in closed systems are suitable for genetic modification of algae cells and enable cultivation of better quality species (e.g. microalgae with higher oil content).
However, closed systems also have disadvantages. Technological challenges with PBRs are: overheating, bio-fouling, oxygen accumulation, difficulty in scaling up, cell damage by shear stress & deterioration and expensive building & maintenance. Light limitation cannot be entirely overcome because light penetration is inversely proportional to the cell concentration. Attachment of cells to the tubes’ walls may also prevent light penetration. Although enclosed systems can enhance biomass concentration, the growth of microalgae is still suboptimal due to variations in temperature and light intensity.
R&D in algae biotechnologies focus on developing innovative PBR designs and materials. Different developed designs are: serpentine, manifold, helical and flat containers. From these elevated reactors can be oriented and tilted at different angles and can use diffuse and reflected (artificial) light for growth. More specific information is available in PBRs section.
After growing in open ponds or PBRs, the microalgae biomass needs to be harvested for further processing. The commonly used harvest method is through gravity settlement or centrifuge. The oil from the biomass is extracted through solvent and further processed into biodiesel.