Nutrients and growth inputs
Nutrients can be provided through runoff water from adjacent land areas or by channeling the water from sewage/water treatment plants. Microalgae cultivation using sunlight energy can be earned out in open or covered ponds or closed photobioreactors. Algal cultures consist of a single or multiple specific strains optimized for producing the desired product. Water, necessaiy nutrients and CO, are supplied in a controlled way, while oxygen must be removed (Carlsson et al., 2007).
Nutrients like nitrogen (N), phosphorus (P). and potassium (K), are important for plant growth and are essential parts of fertilizers. Silica and non, as well as several trace elements, may also be considered important nutrients, the lack of one can limit the growth or productivity in an area (Arumugam et al., 2013).
Although most algae grow at low rate when the water temperature gets lower, the biomass of algal communities can get large due to the absence of grazing organisms. The modest increases in water current velocity may also affect rates of algae growth since the rate of nutrient uptake and boundary layer diffusion increases with velocity (Greenwall et al., 2009).
Carbon dioxide
Bubbling CO, through algal cultivation systems can greatly increase productivity and yield (up to a saturation point). Typically, about 1.8 tons of CO, will be utilized per ton of algal biomass (dry) produced, though this varies with algae species (Moellering and Benning. 2009).
Nitrogen
This is a valuable substrate that can be utilized in algal growth. Various sources of nitrogen can be used as a nutrient for algae, with varying capacities. Nitrate was found to be the preferred source of nitrogen, regarding the amount of biomass grown. Urea is a readily available source that shows comparable results, making it an economical substitute for nitrogen source in a large-scale culturing of algae (Arumugam et al., 2013). Despite the clear increase in growth in comparison to a nitrogen-less medium, it has been shown that alterations in nitrogen levels affect lipid content within the algal cells. In one study (Moellering and Beiming, 2009), nitrogen deprivation for 72 hours caused the total fatty acid content (on a per cell basis) to increase 2.4-fold. 65% of the total fatty acids were esterified to triacyl glycerides in oil bodies, when compared to the initial culture, indicating that the algal cells utilized de novo synthesis of fatty acids. It is vital for the lipid content in algal cells to be high enough, while maintaining adequate cell division times, so parameters that can maximize both are under investigation.
Wastewater
A possible nutrient source is wastewater from the treatment of sewage, agriculture or flood plain run-off, all currently major pollutants and health risks. However, this wastewater cannot feed algae directly and must first be processed by bacteria, through anaerobic digestion. If wastewater is not processed before it reaches the algae, it will contaminate the algae in the reactor and, at the very least, kill much of the desired algae strain. In biogas facilities, organic waste is often converted to a mixture of carbon dioxide, methane and organic fertilizer. Organic fertilizer that comes out of the digester is liquid, and nearly suitable for algae growth, but it must first be cleaned and sterilized (Pittman et al.. 2011; Chong et al., 2000).
Cultivation
Microalgae cultivation using sunlight energy can be carried out in open or covered ponds or closed photobioreactors, based on tubular, flat plate or other designs. Closed systems are much more expensive than ponds, and present significant operating challenges, and due to gas exchange limitations, among others, cannot be scaled-up much beyond about a hundred square meters for an individual growth unit.
Algae grow much faster than food crops and can produce hundreds of times more oil per unit area than conventional crops, such as rapeseed, palms, soybeans, or jatropha (Atabaui et al., 2012). Due to algae have a harvesting cycle of 1-10 days, their cultivation permits several harvests in a very short timeframe, a strategy differing from that associated with annual crops (Chisti, 2007). In addition, algae can be grown on lauds unsuitable for terrestrial crops, including arid laud and land with excessively salute soil, minimizing competition with agriculture (Schenk et al., 2008).
Closed-loop system
The lack of equipment and structures needed to begin growing algae in large quantities has inhibited widespread mass-production of algae for biofuel production. Maximum use of existing agriculture processes and hardware is the goal (Beuentann et al., 2012).
Closed systems (not exposed to open air) avoid the problem of contamination by other organisms blown in by the air. The problem for a closed system is finding a cheap source of sterile COv
Photobioreactors
Most companies pursuing algae as a source of biofuels pump nutrient-rich water through plastic or borosilicate glass tubes (called “bioreactors”) that are exposed to sunlight (and so-called photobioreactors).
Running a photobioreactor is more difficult than using an open pond, and costlier, but may pror ide a higher level of control and productivity (Chisti, 2007). In addition, a photobioreactor can be integrated into a closed loop cogeneration system much more easily than ponds or other methods.
Open pond
This consists of simple in-ground ponds, which are often mixed by a paddle wheel. These systems have low power requirements, operating costs, and capital costs when compared to closed loop photo bioreactor systems (Lundquist et al., 2010). Nearly all commercial algae producers for high value algal products utilize open pond systems (Baimon and Adey, 2008).
Turf scrubber
The algae scrubber is a system designed primarily for cleaning nutrients and pollutants out of water using algal turfs. Algae surf scrubber mimics the algal turfs of a natural coral reef by taking in nutrient rich water from waste streams or natural water sources and pulsing it over a sloped surface. This surface is coated with a rough plastic membrane or a screen, which allows naturally occurring algal spores to settle and colonize the surface. Once the algae have been established, it can be harvested every 5-15 days, (Adey et al., 2001) and can produce 18 metric tons of algal biomass per hectare per year. In contrast to other methods, which focus primarily on a single high yielding species of algae, this method focuses on natural polycultures of algae. As such, the lipid content of the algae in a turf scrubber system is usually lower, which makes it more suitable for a fermented fuel product, such as ethanol, methane or butanol (Biddy et ah, 2016). Conversely, the harvested algae could be treated with a hydrothermal liquefaction process, which would make biodiesel, gasoline and jet fuel production possible (Sheehan et ah, 1998; Smith et ah, 2010).
Environmental impact
In comparison with terrestrial-based biofuel crops, such as com or soybeans, microalgal production results in a much less significant land footprint due to the higher oil productivity from the microalgae than all other oil crops (Herro. 2008). Algae can also be grown on marginal lauds useless for ordinary crops and with low conservation value and can use water from salt aquifers that is not usefiil for agriculture or drinking (Bullis, 2007; Chaw’s Environmental and Infrastructure Group. 2011). Algae can also grow on the surface of the ocean in bags or floating screens. Thus, microalgae could provide a source of clean energy with little impact on the provisioning of adequate food and water or the conservation of biodiversity. Algae cultivation also requires no external subsidies of insecticides or herbicides, removing any risk of generating associated pesticide waste streams. In addition, algal biofuels are much less toxic, and degrade far more readily than petroleum-based fuels. However, due to the flammable nature of any combustible fuel, there is potential for some environmental hazards if ignited or spilled, as may occur in a train derailment or a pipeline leak (Acien-Femaudez et ah, 2012). This hazard is reduced compared to fossil fuels, due to the ability for algal biofuels to be produced in a much more localized maimer, and due to the lower toxicity overall, but the hazard is still there, nonetheless. Therefore, algal biofuels should be treated in a similar manner to petroleum fuels in transportation and use, with enough safety measures in place always.
Studies have determined that replacing fossil fuels with renewable energy sources, such as biofuels, has the capacity to reduce CO, emissions by up to 80% (Hemaiswarya et ah, 2012). An algae-based system could capture approximately 80% of the CO, emitted from a power plant when sunlight is available. Although this CO, will later be released into the atmosphere when the fuel is burned (Acieu-Femandez et ah. 2012). The possibility of reducing total CO, emissions, therefore, lies in the prevention of the release of CO, from fossil fuels. Furthermore, compared to fuels like diesel and petroleum, and even compared to other sources of biofuels, the production and combustion of algal biofuel does not produce any sulfur oxides or nitrous oxides, and produces a reduced amount of carbon monoxide, unbumed hydrocarbons, and reduced emissions of other harmful pollutants (Kumar et ah, 2010). Due to the fact that terrestrial plant sources of biofuel production simply do not have the production capacity to satisfy the crment energy requirements, microalgae may be the only option to approach complete replacement of fossil fuels (Milano et ah, 2016).
Microalgae production also includes the ability to use saline waste or CO, waste streams as an energy source. This opens a new strategy to produce biofuel in conjunction with wastewater treatment, while being able to produce clean water as a byproduct (Kumar et ah. 2010). When used in a microalgal bioreactor, harvested microalgae will capture significant quantities of organic compounds as well as heavy metal contaminants absorbed from wastewater streams that would otherwise be directly discharged into the surface and groundwater (Herro, 2008). Moreover, this process also allows the recovery of phosphorus fr om waste, which is an essential but scarce element in nature—the reserves of which are estimated to have been depleted in the last 50 years (Zivojnovich, 2010; Dixner, 2013; Blankenship et al., 2011).
Economic viability
There is clearly a demand for sustainable biofuel production, but whether a biofuel will be used ultimately depends not on sustainability but on cost efficiency. Therefore, research is focusing on cutting the cost of algal biofuel production to the point where it can compete with conventional petroleum (Chisti, 2007). The production of several products from algae has been mentioned as the most important factor for making algae production economically viable. Other factors are the implor ing of the solar energy to biomass conversion efficiency (currently 3%, but 5 to 7% is theoretically attainable) and making the oil extraction from the algae easier (Mata et al., 2010).
Use of byproducts
Many of the byproducts produced in the processing of microalgae can be used in various applications, many of which har e a longer history of production than algal biofuel. Some of the products not used in the production of biofuel include natural dyes and pigments, antioxidants, and other high-value bioactive compounds (Pulz and Gross, 2004; Singh et al., 2005; Sporalore et al., 2006). These chemicals and excess biomass har e found numerous uses in other industries. For example, the dyes and oils have found a place in cosmetics, commonly as thickening and water-binding agents (Tokusoglu and Uunal,
2003). Discoveries within the pharmaceutical industry include antibiotics and antifrmgals derived from microalgae, as well as natural health products, which have been growing in popularity over the past few decades. For example, Spirulina contains numerous polyunsaturated fats (Omega 3 and 6), amino acids, and vitamins (Vonshak, 1997) as well as pigments, such as beta-carotene and chlorophyll, that may be beneficial (Berla et al.. 2013).
