Factors in Algae Growth
All types of algae consist of plant cells. The chemical reactions (notably photosynthesis) that drive algae growth are the same as those which drive plant growth. The primary factors in algae growth are: water, carbon dioxide, sunlight, and nutrients. Algae, like other plants, use water, light energy, and carbon dioxide to produce glucose (sugar), oxygen, and water. Nitrogen and other trace nutrients, including potassium and phosphorus, are used to create other compounds essential for cellular structure and processes, including various protein and lipid molecules. This is called autotrophic growth. Some algae species can also be grown without light, but must be supplemented with glucose. This is called heterotrophic growth.
Commercial Algae Cultivation, Harvesting, and Uses
Microalgae cultivation falls into two categories: monoculture and mixed-culture. Monoculture is preferred by most cultivators. Monoculture algae requires significantly different growing methods than does mixed-culture algae in order to prevent contamination with other organisms or algae species. Macroalgae cultivation is more difficult due to the monoculture algae cultivation.
Monoculture algae cultivation is practiced with the goal of maintaining a pure culture of a single algae species, especially a species which is naturally non-dominant. Monoculture is often used for research purposes or when producing a single species of algae is important. Some commercial uses for monoculture algae include food and nutritional supplements for human consumption, medicine, bioplastics, food coloring, biofuel, and scientific research.
Several different methods are utilized to produce a pure algae culture. In serial dilution, cultivators dilute an algae sample with water, then separate it into many small containers. Statistically, at least one of the small containers will contain only the desired species of algae. A second method is to utilize environmental factors such as water salinity to exclude undesired species.
Once a pure algae culture is contained, cultivators take great care to maintain the purity of the crop. The most common method is to utilize a photobioreactor (or PBR) as a closed system in which the algae is grown. Nutrients, lighting, sterile water, and CO2 are introduced into the system in a controlled manner designed to maximize the yield. Generally, a PBR incorporates a translucent or transparent closed growth chamber. PBRs may operate using the batch method wherein the entire system is cleared at each harvest or a continuous cycle in which harvesting and growth occur indefinitely.
Mixed-culture algae cultivation
Mixed-culture algae cultivation is used when maintaining a pure culture is not important, such as when producing feed for mollusks or other farmed seafood species. Since avoiding species contamination is not an important consideration in mixed-culture algaculture, more relaxed cultivation methods may be used, including open pond systems or “raceways.”
Raceway ponds, along with similar methods of mixed-culture production, are typically open to the surrounding environment and thus may serve to support the growth of a variety of algae in the same crop. Raceways and similar open pond environments are also susceptible to other kinds of contamination, such as bacterial or foreign-body contamination (e.g., dust). Open systems, such as raceways, also offer little control over temperature and lighting, like other outdoor agricultural methods.
Open pond systems offer the advantage of being much cheaper to construct and provide much larger production capacities compared to other systems. Thus, while open pond systems are susceptible to various forms of contamination and lack of environmental controls, they can be successful when these items are not high priorities or when other methods exist to account for these parameters, such as inoculation or a method of harvesting only a single type of algae when many species may be present.
Several methods have been successfully utilized to mitigate the limitations of open-pond systems, such as constructing raceways from translucent materials, and incorporating agitators to ensure that all algae cells receive adequate sunlight exposure within the top few inches of water.
Algae Harvesting
Several methods for harvesting algae exist, depending upon the application. The simplest of these is the use of microscreens to filter the algae out of the growth medium (mostly water). A centrifuge can also be used to separate the algae from the growth medium, producing an algae “pellet.” Two other methods, flocculation and froth flotation are more complex. Flocculation involves utilizing coagulant chemicals, such as alum and ferric chloride, to cause the algae to clump up into larger and larger clusters. The main disadvantage of this method is that it becomes difficult to separate the chemicals from the algae once it has been removed from the growth medium. In froth flotation, air is bubbled through a column creating a froth of algae at the surface, which is removed. Sometimes flocculation and froth flotation are used together to improve results. Froth flotation is currently considered too expensive for commercial use.
Commercial Uses
Perhaps surprisingly, there is evidence that humans have used algae for millennia. However, commercial algaculture has only been prominent for the past 50 years or so. This is due to a rapid increase in the number of potential uses for algae. In addition to human food and nutritional supplements, algae products and uses include: animal feed (especially for farmed seafood products), sustainable plastics, fertilizers, skin-care products, pharmaceuticals, inks, dyes and pigments, thickening agents, wastewater pollution control, and biofuel.
Algae as Fuel
Because algae produce fatty acids as a metabolic product, some species of algae, which produce more lipids, have potential for sustainable fuel production as an alternative to fossil fuels and other biofuels, such as corn-based ethanol. The lipids found in algae can be extracted and processed into biodiesel fuel using similar processes as would be used for any other vegetable-based oil. The US Department of Energy estimates that the total land area required for algae production to replace the entire fossil-fuel industry in the U.S. would be approximately 15,000 square miles, which is far smaller than that required for other biofuel sources (Hartman, 2008).
Despite its promise as a sustainable fuel source, economic conditions have hampered development of algae-based biofuels as a serious alternative to fossil fuels or other biofuels. Algae production currently has relatively low cost-efficiency compared to fossil fuels and corn- or soy-based biofuel. The estimated cost to produce one gallon of oil from algae is approximately $7.19.6 Although improved cultivation, harvesting, and processing technology is expected to drastically reduce this cost, the high up-front costs of research and facilities is a major obstacle to the economic viability of biofuels derived from algae.
Relatively low prices for fossil fuel reduce the economic incentive to pursue development of algae-based biofuels. Many companies have begun algae fuel projects but suspended activity in the current economic environment. While the production of fuel from algae may not meet our current energy needs, the science and engineering processes involved in growing/farming algae and extracting/harvesting cellular products will continue to be investigated. The demand for algae-based fuel could change in the event that accessible oil reserves diminish or if political circumstances jeopardize access to foreign oil reserves.
