Producing microalgal biomass is generally more expensive than growing crops. Pho­tosynthetic growth requires light, carbon dioxide, water, and inorganic salts. Tem­perature must remain generally within a range of 293 to 303 K. To minimize ex­pense, biodiesel production must rely on freely available sunlight, despite daily and seasonal variations in light levels (Chisti 2007).

Algae cultivation has four basic, and equally important, requirements: carbon, water, light, and space. By maximizing the quality and quantity of these require­ments, it is possible to maximize the quantity of oil-rich biomass and the return on investment. In order to maximize algal growth, CO2 needs to be provided at very high levels, much higher than can be attained under natural conditions. Rather than becoming an expense, this need for CO2 fertilization creates a unique opportunity to offset costs by consuming air pollution. The flue gases from industrial processes, and in particular from power plants, are rich in CO2 that would normally be released directly into the atmosphere and thereby contribute to global warming. By divert­ing the CO2 fraction of the flue gas through an algae cultivation facility, the CO2 can be diverted back into the energy stream and the rate of algal production can be greatly increased (Pulz 2007). Although most of the CO2 will ultimately be de­posited in the atmosphere, we can realize a greater energy return for each molecule of carbon.

Water, containing the essential salts and minerals for growth, is the second re­quirement. Fresh water is a valuable resource as are the salts and minerals needed; however, algae cultivation can be coupled to another type of environmental remedi­ation that will enhance productivity while mitigating pollution. High nutrient waste­water from domestic or industrial sources, which may already contain nitrogen and phosphate salts, can be added to the algal growth medium directly (Schneider 2006).

This allows for inexpensive improvement in algae production along with simulta­neous treatment of wastewater. Alternatively, salt water can be used, either from a saline aquifer or sea water. This means that competition for water will be low.

The main advantages of second-generation microalgal systems are that they (1) have a higher photon conversion efficiency, (2) can be harvested batchwise nearly year round providing a reliable and continuous supply of oil, (3) can utilize salt and wastewater streams, thereby greatly reducing freshwater use, (4) can cou­ple CO2-neutral fuel production with CO2 sequestration, and (5) produce nontoxic and highly biodegradable biofuels. Current limitations exist mainly in the harvest­ing process and in the supply of CO2 for high efficiency production (Schenk et al. 2008).

Prior economic-engineering feasibility analyses have concluded that even the simplest open-pond systems, including harvesting and algal biomass processing equipment, would cost at least $ 100,000 per hectare, and possibly significantly more. To this would need to be added operating costs. Algae production requires a site with favorable climate, available water (which can be saline, brackish, or wastewater), a ready and essentially free source of CO2, nearly flat land, and a clay soil or liner, as plastic liners would be too expensive.

Nutrients such as phosphorus must be supplied in significant excess from phos­phates complex with metal ions; therefore, not all the added phosphorus is bioavail­able. Sea water supplemented with commercial nitrate and phosphate fertilizers and a few other micronutrients is commonly used for growing marine microalgae (Molina Grima et al. 1999). Genetic and metabolic engineering is likely to have the greatest impact on improving the economics of production of microalgal diesel (Roessler et al. 1994; Dunahay et al. 1996).

Growth media are generally inexpensive. Microalgal biomass contains approx. 50% carbon by dry weight (Sanchez Miron et al. 2003). All of this carbon is typi­cally derived from CO2. Producing 100 tons of algal biomass fixes roughly 183 tons of carbon dioxide. Feeding controlled in response to signals from pH sensors mini­mizes loss of CO2 and pH variations.

Algae can grow practically anywhere where there is enough sunlight. Some al­gae can grow in saline water. All algae contain proteins, carbohydrates, lipids, and nucleic acids in varying proportions. While the percentages vary with the type of algae, there are algae types whose overall mass is comprised of up to 40% fatty acids (Becker 1994). The most significant distinguishing characteristic of algal oil is its yield and, hence, its biodiesel yield. According to some estimates, the yield (per acre) of oil from algae is over 200 times the yield from the best-performing plant/vegetable oils (Sheehan et al. 1998). Microalgae are the fastest-growing pho — tosynthesizing organisms. They can complete an entire growing cycle every few days. Approximately 46 tons of oil/ha/year can be produced from diatom algae. Different algae species produce different amounts of oil. Some algae produce up to 50% oil by weight. The production of algae to harvest oil for biodiesel has not been undertaken on a commercial scale, but working feasibility studies have been conducted to arrive at the above number. Specially bred mustard varieties can pro­duce reasonably high oil yields and have the added benefit that the meal left over after the oil has been pressed out can act as an effective and biodegradable pesticide (Demirbas 2009c).

Microalgae are very efficient solar energy converters and can produce a great variety of metabolites (Chaumont 1993). A culture of algae can yield 30 to 50% oil. Oil supply is based on the theoretical claims that 47,000 to 308,000L/ha/year of oil could be produced using algae. The calculated cost per barrel would be only $ 20. Currently, a barrel of oil in the US market is selling for over $ 100 per barrel. Despite all the claims and research dating from the early 1970s, none of the projected algae and oil yields has been achieved (Dimitrov 2008; Demirbas 2009c). Algae, like all plants, require large quantities of nitrogen fertilizer and water, plus significant fossil energy inputs for a functioning system (Goldman and Ryther 1977).

Harvesting algae from tanks and separating the oil from the algae is a difficult and energy-intensive process (Pimentel et al. 2004). One difficulty in culturing algae is that the algae shade one another and thus there are different levels of light saturation in the cultures, even under Florida conditions. This influences the rate of growth of the algae. In addition, wild strains of algae invade and dominate algae culture strains, and oil production by the algae is reduced (Biopact 2008). Another major problem with the culture of algae in ponds or tanks is the harvesting of the algae. This problem was observed at the University of Florida where algae were being cultured in managed ponds for the production of nutrients for hogs. After 2 years without success, the algal-nutrient culture was abandoned (Pimentel 2008).

In recent years, there has been increasing interest in greenhouse gas mitigation technologies. As a consequence, there has been renewed interest in microalgae mass culture and fuel production from the perspective of CO2 utilization. This is not a new concept, as Oswald and Golueke (1960) had previously emphasized the potential for microalgae systems to reduce and avoid CO2 emissions and thus reduce the potential for global warming. Indeed, microalgae have a rather unique attribute: they can utilize concentrated CO2 for growth, rather than the atmospheric levels of CO2 used by higher plants. Flue gas can be utilized in algal ponds.

Microalgae wastewater treatment uses less energy, and thus fossil fuels, than con­ventional treatment processes, resulting in a reduction in greenhouse gas emissions. Wastewater treatment processes could provide a near-term pathway to developing large-scale microalgae production processes and could find applications in the real world.

Open ponds can be categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, and raceway ponds. The major capital costs for an open-pond system are tabulated in Table 4.1 (Weissman and Goebel 1987; Shee­han et al. 1998). Polymers can be used in very small amounts, without contributing a major cost to the overall process. The base case (30g/m2/d) capital costs were estimated at almost $ 72,000/ha, without working capital, or almost twice as high as the prior effort (Benemann et al. 1982). This was due to higher costs for many com­ponents, such as earthworks, which were several-fold higher. Among other things, higher costs were assumed for rough and fine (laser) grading, which depends on the type of site available. Also, the 1987 study estimated about $5,000/ha to provide

a 3- to 5-cm crushed-rock layer to reduce the suspension of silt from the pond bot­tom. There is, however, little evidence that such erosion prevention is needed, except perhaps for some areas around the paddlewheel and perhaps the turns. Further, the

Weissman and Goebel (1987) study selected slip form poured concrete walls and di­viders (baffles) as the design of choice. A power generation system can be specified to produce electricity from the methane generated from the algal residues (at about 10% of total costs).

Table 4.2 shows the operating costs for an open-pond system (Sheehan et al. 1998). The operating costs were discussed in terms of mixing, carbon utilization, nutrient, flocculants, salt disposal, maintenance, labor, and the accumulation of pho­tosynthetically produced oxygen (Benemann 2008).

Table 4.3 shows the comparative economics of open ponds and closed photo­bioreactors.