Biofuel derived from algae is currently a hotly debated topic because its production is one of the more costly processes, which can dictate the sustainability of algae-based biofuel products. There are two major energy and cost constraints to bulk production of microalgae for biofuels: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange, and the cost of harvesting in achieving feasible algal solids concentration.

Because of the dilute algal suspension, the cost of harvesting microalgal biomass accounts for a significant portion of the overall production cost of microalgal biofuels. Certainly, energy-efficient and cost-effective harvesting are two major challenges in the commercializa­tion of biofuels from algae (Dismukes et al., 2008; Reijnders, 2008). The algae must be concen­trated by removing water in an economically viable fashion before further processing such as drying and oil extraction. The lack of cost-effective methodologies for harvesting has been one of the major hurdles for the economic production of algal biofuels, along with challenges associated with variability of microalgae species (e. g., cell size, robustness, surface charge, culture medium constituent, and desired end-product) (Cooney et al., 2009). An effective microalgae separation process should be workable for all microalgae strains, yield a product with a high dry biomass weight, and require moderate cost of operation, energy, and maintenance.

Microalgae harvesting can be a considerable problem because of the small size (3-30 micrometers in diameter) and the stable suspended state of unicellular algal cells. Since the mass fractions in a culture broth are low (typically less than 0.5 kg/m3 dry biomass in some commercial production systems), large volumes of culture need to be processed to order to recover biomass in a feasible quantity (Cooney et al., 2009; Ramanan et al., 2010). In addi­tion, microalgae harvesting is a major bottleneck to microalgae bioprocess engineering owing to its high operating cost, thus reducing the cost of microalgae harvesting is vital. If microalgae can be concentrated about 30-50 times by coagulation-flocculation and gravity sedimentation prior to dewatering, the energy demand for microalgae harvesting could be significantly reduced (Jorquera et al., 2010).

In comparing algae removal using filtration, flotation, centrifugation, precipitation, ion exchange, passage through a charged zone, and ultrasonic vibration, it was concluded that only centrifugation and precipitation can be economically feasible, with centrifugation being marginal (Golueke and Oswald, 1965). In another study examining three different tech­niques of harvesting microalgae involving centrifugation, chemical flocculation followed by flotation, and continuous filtration with a fine-weave belt filter, it was reported that centrifu­gation gave good recovery and a thickened slurry but required high capital investment and energy inputs (Sim et al., 1988). Dissolved-air flotation was more economical, but, if the recovered algae were to be incorporated into animal feed, the use of coagulants such as alum could have undesirable effects on the growth rate of the animals. This problem could be overcome by the use of nontoxic coagulants. The continuous filtration process had significant advantages in terms of energy efficiency, economics, and chemical-free operation. The only drawback of this process was that the efficiency depended on the size and morphology of the algae.

Most of the algae-harvesting techniques present several disadvantages, not only because of the high costs of operation but also due to the frequently low separation efficiencies and the intolerable product quality. Algae separation processes such as sedimentation, centrifu­gation, and filtration involve the use of equipment that could result in deterioration in algal quality due to cell rupture that causes leakage of cell content. Furthermore, in the case of flocculation, the high concentration of metal salts, which is normally used as the coagulant, can have a negative effect on the quality of the final product, as discussed previously (Kim et al., 2005).

High production yields of microalgae have called forth interest due to economic and sci­entific factors, but it is still unclear whether the production of biodiesel is environmentally sustainable and which transformation steps need further adjustment and optimization. A comparative life-cycle assessment (LCA) of a virtual facility has been undertaken to assess the energetic balance and the potential environmental impacts of the whole process chain, from biomass production to biodiesel combustion (Lardon et al., 2009). The outcome vali­dated the potential of microalgae as an energy source but highlighted the imperative neces­sity of decreasing the energy and fertilizer requirements.

From another comparative LCA study to compare biodiesel production from algae with canola and ultra-low sulfur diesel with respect to greenhouse gas emissions and costs, it was concluded that the need for a high production rate is a vital key to make algal biodiesel economically attractive (Campbell et al., 2011). In a separate study, it was concluded that the potential greenhouse gas emissions from microalgae operational activities are likely to be outweighed by the emission reductions associated with the production efficiency and seques­tration potential of microalgae (Williams and Laurens, 2010).

Some commercial interests in large-scale algal-cultivation systems are looking to tie into existing infrastructures, such as coal-fired power plants or sewage treatment facilities. Wastes generated from those infrastructures, such as flue gas (carbon dioxide) and wastewater nu­trients (nitrogen, phosphorous and other micronutrients), can be converted into raw material resources for algal cultivation. While use of carbon dioxide for algal photosynthesis would help attain carbon sequestration, uptake of waste nutrients for algal growth would eliminate use of fertilizers derived from fossil-fuel energy, thus mitigating emissions.

In essence, algal biofuel is currently more expensive than other fuel options, but it is likely to play a major role in the economy in the long run if technology improvements succeed in bringing down costs. The main challenges are to decrease the energy and fertilizer requirements and to accomplish high production rates in order to make algal bio­diesel economically attractive. The potential of anaerobic digestion of waste oilcakes from oil extraction as a way to reduce external energy demand and to recycle part of the mineral fertilizers is to be further explored (Lardon et al., 2009). Algal biofuel production employing renewable substrates may be a potential answer to overcome some of the economic constraints. There is scope to use certain wastewater effluents containing waste nutrients as cultivation broth. Therefore, production as well as unit energy cost of algal biofuel would be reduced.

A rigorous techno-economic analysis is necessary to draw a clearer prospect comparison between algal biofuel and the various other conventional fossil fuels. In addition to benefits that can be quantified from the use of biofuel for clean energy production, intangible benefits such as flue gas carbon dioxide sequestration, uptake of waste nutrients in place of fertilizers, and biogas energy produced from anaerobic digestion of oilcake should also be considered. These benefits would render a potential for claims of certified emission reductions (CERs) under the Kyoto Protocol for reducing emissions that can be estimated through a holistic LCA of algal biofuel production. The potential for claims of CERs to generate revenue and to finance algal biofuel projects under the Kyoto Protocol for reducing emissions of green­house gases appears to be promising. In view of the prospects of technology development and global carbon trading, it may not be an unreasonable expectation that, in the future, algal biofuel will experience a global shift toward employment of energy-efficient algae biofuel production while mitigating greenhouse gas emissions.

5.4 CONCLUSIONS

Algal biofuel is believed to be one of the biofuels for the future in view of its potential to replace depleting fossil fuels. The future role of algal biofuel as a clean fuel producing near­zero emissions and as an energy carrier is increasingly recognized worldwide. Because energy-efficient and cost-effective harvesting are two major hurdles in the commercialization of biofuels from algae, research addressing these challenges should be intensified. Knowl­edge exchange and cooperation between expert groups of various disciplines should be strengthened in order to leapfrog technological development for algal biofuel.