The pond is designed in a raceway configuration, in which a paddlewheel provides circulation and mixing of the algal cells and nutrients Figure 2. The raceways are typically made from poured concrete, or they are simply dug into the earth and lined with plastic to prevent the ground from soaking up the liquid. Baffles in the channel guide the flow around bends in order to minimize space. The system is often operated in a continuous mode — that is, the fresh feed containing nutrients including nitrogen phosphorus and inorganic salts is added in front of the paddle wheel.
Algal broth is harvested behind the paddle wheel after it has circulated through the loop Figure 2. For some marine types of microalgae, seawater or water with high salinity can be used. Although open ponds cost less to build and operate than enclosed photobioreactors, this culture system has its intrinsic disadvantages. Since these are open-air systems, they often experience a lot of water loss due to evaporation. Thus, microalgae growing in an open pond do not uptake carbon dioxide efficiently, and algal biomass production is limited Chisti, Biomass productivity is also limited by contamination with unwanted algal species as well as other organisms from feed.
In addition, optimal culture conditions are difficult to maintain in open ponds, and recovering the biomass from such a dilute culture is expensive Molina Grima et al. Enclosed photobioreactors: Enclosed photobioreactors have been employed to overcome the contamination and evaporation problems encountered in open ponds Molina Grima et al. These systems are made of transparent materials and generally placed outdoors for illumination by natural light.
The cultivation vessels have a large surface area-to-volume ratio. The most widely used photobioreactor is a tubular design, which has a number of clear transparent tubes, usually aligned with the sun rays Figure 3. The tubes are generally less than 10 centimeters in diameter to maximize sunlight penetration Chisti, 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.
The algal biomass is prevented from settling by maintaining a highly turbulent flow within the reactor, using either a mechanical pump or an airlift pump Chisti, A portion of the algae is usually harvested after the solar collection tubes.
In this way, continuous algal culture is possible Chisti, In some photobioreactors, the tubes are coiled spirals to form what is known as a helical tubular photobioreactor, but these sometimes require artificial illumination, which adds to the production cost. Therefore, this technology is only used for high-value products, not biodiesel feedstock. The photosynthesis process generates oxygen. In an open-raceway system, this is not a problem as the oxygen is simply returned to the atmosphere.
However, in the closed photobioreactor, 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 carbon dioxide, which can cause carbon starvation and an increase in pH.
Therefore, carbon dioxide must be fed into the system in order to successfully cultivate the microalgae on a large scale. Photobioreactors may require cooling during daylight hours, and the temperature must be regulated at night hours as well.
This may be done through heat exchangers, located either in the tubes themselves or in the degassing column. The advantages of the enclosed photobioreactors are obvious. They can overcome the problems of contamination and evaporation encountered in open ponds Molina Grima et al. The biomass productivity of photobioreactors can be 13 times greater than that of a traditional raceway pond, on average Chisti, Harvesting of biomass from photobioreactors is less expensive than that from a raceway pond, since the typical algal biomass is about 30 times as concentrated as the biomass found in raceways Chisti, However, enclosed photobioreactors also have some disadvantages.
For example, the reactors are more expensive and difficult to scale up. Moreover, light limitation cannot be entirely overcome since light penetration is inversely proportional to the cell concentration.
Attachment of cells to the tube walls may also prevent light penetration. Although enclosed systems can enhance the biomass concentration, the growth of microalgae is still suboptimal due to variations in temperature and light intensity. Harvesting: After growing in open ponds or photobioreactors, 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 will be removed through solvent extraction and further processed into biodiesel. Depending on the culture systems used opens ponds vs. In closed systems, engineers can precisely regulate algae growth conditions. Closed systems include both photobioreactors for photosynthetic algae strains and traditional bioreactors enclosed tanks such as those used in other microbial growth for those, such as cyanobacteria, that do not require sunlight.
Open pond systems have been used in many settings, but can be sensitive to various environmental factors, such as contamination by other algae strains, or variations in nutrients, heat and light. Testing is expected to begin in The Navy's Defense Energy Support Center has also purchased and begun testing algaederived diesel distillates from Solazyme.
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